Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
El Niño and the Southern Oscillation
Anderson, M.L., M. L. Kavvas, M. D. Mierzwa (2001). Probabilistic/ensemble forecasting: a case study using hydrologic response distributions associated with El Niño/Southern Oscillation (ENSO). Journal of Hydrology 249 (1-4): 134-147
ABSTRACT: Due to the non-linear processes and interactions of the hydroclimatic system, a given hydroclimatic event such as the El Niño/Southern Oscillation (ENSO) can lead to a range of possible hydrologic system responses described by a probability distribution. This probability distribution changes in space and time reflecting the non-stationary behavior of the hydroclimatic system. An initial approach in quantifying the evolving probability distributions of hydrologic system response utilizes a physically based hemispheric hydrologic model, PBHHM, that incorporates the salient physics of the hydroclimatic system for the midlatitudes of the Northern Hemisphere. The state variables of the model include atmospheric temperature, atmospheric water content, quasi-geostrophic potential vorticity, land hydrologic water storage, and land/sea surface temperature. The model is structured in such a way that characteristics (e.g. sea surface temperature, geopotential anomalies, etc.) of a hydroclimatic event such as ENSO can be incorporated into the model as a forcing event. The hydrologic system response probability distribution is quantified, via the land hydrologic water storage state variable.
As a case study, the hydrologic system response probability distributions of the western continental United States to both the El Niño and La Niña phases of ENSO have been simulated. One hundred realizations were run for each phase using random initial conditions for the state variables in order to reflect differing hydroclimatic conditions during the initiation and evolution of the forcing event. The probability distributions of hydrologic system response and their evolution in space and time are described using relative frequency histograms, cumulative distribution functions, and contour plots of frequency histogram categories. Simulation results of the hydrologic system response probability distribution associated with each phase of the ENSO phenomenon are presented which show a distinct response that varies in space and time. The influence of the number of realizations upon these distributions will be discussed along with a means of incorporating the distributions into a water resources planning scheme.
ABSTRACT: Three different classes of numerical models successfully predicted the occurrence of the El Niño of 1986-87 at lead times of 3 to 9 months. Although the magnitude and timing of predicted ocean surface temperatures were not perfect, these results suggest that routine prediction of moderate to large El Niño events is feasible. The key to the success of the models lies in recognizing or simulating the low-frequency, large-scale changes in the tropical ocean-atmosphere system that give rise to El Niño events.
ABSTRACT: The value of using climate indices such as ENSO or PDO in water resources predictions is dependent on understanding the local relationship between these indices and streamflow over time. This study identifies long term seasonal and spatial variations in the strength of El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) correlations with timing and magnitude of discharge in snowmelt streams in Oregon. ENSO is best correlated with variability in annual discharge, and PDO is best correlated with spring snowmelt timing and magnitude and timing of annual floods. Streams in the Cascades and Wallowa mountains show the strongest correlations, while the southernmost stream is not correlated with ENSO or PDO. ENSO correlations are weaker from 1920 to 1950 and vary significantly depending on whether Southern Oscillation Index (SOI) or Niño 3.4 is used. PDO correlations are strong from 1920 to 1950 and weak or insignificant other years. Although there are not consistent increasing or decreasing trends in annual discharge or spring snowmelt timing, there are significant increases in fractional winter runoff that are independent of precipitation, PDO, or ENSO and may indicate monotonic winter warming.
ABSTRACT: We review forecasts of the future of El Niño and the Southern Oscillation (ENSO), a coupled instability of the ocean–atmosphere system in the tropical Pacific with global impacts. ENSO in the modern world is briefly described, and the physics of the ENSO cycle is discussed. Particular attention is given to the Bjerknes feedback, the instability mechanism which figures prominently in ENSO past and future. Our knowledge of ENSO in the paleoclimate record has expanded rapidly within the last 5 yr. The ENSO cycle is present in all relevant records, going back 130 kyr. It was systematically weaker during the early and middle Holocene, and model studies indicate that this results from reduced amplification in the late summer and early fall, a consequence of the altered mean climate in response to boreal summer perihelion. Data from corals shows substantial decadal and longer variations in the strength of the ENSO cycle within the past 1000 yr; it is suggested that this may be due to solar and volcanic variations in solar insolation, amplified by the Bjerknes feedback. There is some evidence that this feedback has operated in the 20th century and some model results indicate that it will hold sway in the greenhouse future, but it is very far from conclusive. The comprehensive general circulation models used for future climate projections leave us with an indeterminate picture of ENSO's future. Some predict more ENSO activity, some less, with the highly uncertain consensus forecast indicating little change.
ABSTRACT: An important part of the water supply in the western United States is derived from runoff fed by mountain snowmelt. Snow accumulation responds to both precipitation and temperature variations, and forms an interesting climatic index, since it integrates these influences over the entire late fall-spring period. Here, effects of cool season climate variability upon snow water equivalent (SWE) over the western part of the conterminous United States are examined. The focus is on measurements on/around 1 April, when snow accumulation is typically greatest. The primary data, from a network of mountainous snow courses, provides a good description of interannual fluctuations in snow accumulations, since many snow courses have records of five decades or more. For any given year, the spring SWE anomaly at a particular snow course is likely to be 25%–60% of its long-term average. Five separate regions of anomalous SWE variability are distinguished, using a rotated principal components analysis. Although effects vary with region and with elevation, in general, the anomalous winter precipitation has the strongest influence on spring SWE fluctuations. Anomalous temperature has a weaker effect overall, but it has great influence in lower elevations such as in the coastal Northwest, and during spring in higher elevations. The regional snow anomaly patterns are associated with precipitation and temperature anomalies in winter and early spring. Patterns of the precipitation, temperature, and snow anomalies extend over broad regional areas, much larger than individual watersheds. These surface anomalies are organized by the atmospheric circulation, with primary anomaly centers over the North Pacific Ocean as well as over western North America. For most of the regions, anomalously low SWE is associated with a winter circulation resembling the PNA pattern. With a strong low in the central North Pacific and high pressure over the Pacific Northwest, this pattern diverts North Pacific storms northward, away from the region. Both warm and cool phases of El Niño-Southern Oscillation tend to produce regional patterns with out-of-phase SWE anomalies in the Northwest and the Southwest.
ABSTRACT: Decadal (>7- yr period) variations of precipitation over western North America account for 20%–50% of the variance of annual precipitation. Spatially, the decadal variability is broken into several regional [O (1000 km)] components. These decadal variations are contributed by fluctuations in precipitation from seasons of the year that vary from region to region and that are not necessarily concentrated in the wettest season(s) alone. The precipitation variations are linked to various decadal atmospheric circulation and SST anomaly patterns where scales range from regional to global scales and that emphasize tropical or extratropical connections, depending upon which precipitation region is considered. Further, wet or dry decades are associated with changes in frequency of at least a few short-period circulation “modes” such as the Pacific–North American pattern. Precipitation fluctuations over the southwestern United States and the Saskatchewan region of western Canada are associated with extensive shifts of sea level pressure and SST anomalies, suggesting that they are components of low-frequency precipitation variability from global-scale climate processes. Consistent with the global scale of its pressure and SST connection, the Southwest decadal precipitation is aligned with opposing precipitation fluctuations in northern Africa.
ABSTRACT: Frequency distributions of daily precipitation in winter and daily stream flow from late winter to early summer, at several hundred sites in the western United States, exhibit strong and systematic responses to the two phases of ENSO. Most of the stream flows considered are driven by snowmelt. The Southern Oscillation index (SOI) is used as the ENSO phase indicator. Both modest (median) and larger (90th percentile) events were considered. In years with negative SOI values (El Niño), days with high daily precipitation and stream flow are more frequent than average over the Southwest and less frequent over the Northwest. During years with positive SOI values (La Niña), a nearly opposite pattern is seen. A more pronounced increase is seen in the number of days exceeding climatological 90th percentile values than in the number exceeding climatological 50th percentile values, for both precipitation and stream flow. Stream flow responses to ENSO extremes are accentuated over precipitation responses. Evidence suggests that the mechanism for this amplification involves ENSO-phase differences in the persistence and duration of wet episodes, affecting the efficiency of the process by which precipitation is converted to runoff. The SOI leads the precipitation events by several months, and hydrologic lags (mostly through snowmelt) delay the stream flow response by several more months. The combined 6–12-month predictive aspect of this relationship should be of significant benefit in responding to flood (or drought) risk and in improving overall water management in the western states.
Clark, M. P., Serreze, M. C., McCabe, G. J. (2001). Historical effects of El Niño and La Niña events on the seasonal evolution of the montane snowpack in the Columbia and Colorado River basins. Water Resources Research 37 (3): 741-757
ABSTRACT: Snow-water equivalent (SWE) data measured at several hundred montane sites in the western United States are used to examine the historic effects of El Niño and La Niña events on seasonal snowpack evolution in the major subbasins in the Columbia and Colorado River systems. Results are used to predict annual runoff. In the Columbia River Basin, there is a general tendency for decreased SWE during El Niño years and increased SWE in La Niña years. However, the SWE anomalies for El Niño years are much less pronounced. This occurs in part because midlatitude circulation anomalies in El Niño years are located 35° east of those in La Niña years. This eastward shift is most evident in midwinter, at which time, SWE anomalies associated with El Niño are actually positive in coastal regions of the Columbia River Basin. In the Colorado River Basin, mean anomalies in SWE and annual runoff during El Niño years depict a transition between drier-than-average conditions in the north, and wetter-than-average conditions in the southwest. Associations during La Niña years are generally opposite those in El Niño years. SWE anomalies tend to be more pronounced in spring in the Lower Colorado River Basin. Our predictions of runoff reveal modest skill for scenarios using only historic El Niño and La Niña information. Predictions based on the water stored in the seasonal snowpack are, in almost all cases, much higher than those based on El Niño-Southern Oscillation (ENSO) information alone. However, combining observed midwinter snow conditions with information on seasonal snowpack evolution associated with ENSO improves predictions for basins in which ENSO signals exhibit strong seasonality.
ABSTRACT: The El Niño Southern Oscillation (ENSO) system orchestrates a well-documented suite of climate anomalies worldwide. The details of ENSO's extratropical influence vary among events, but this variability has not been described or diagnosed beyond the past few decades, and previous descriptions have looked at inter-event differences rather than decadal patterns. We use a new tree-ring based drought reconstruction for the continental U.S. and instrumental ENSO indices to document systematic decadal changes in the U.S. drought-ENSO relationship since the late 19th century. Significant ENSO-drought correlation occurs consistently in the southwest U.S., but the strength of penetration of moisture anomalies into the continent varies substantially. The most striking change over the past 130 years is the initiation of a "bipolar" ENSO-drought signature around 1920, producing opposite-sign moisture anomalies in the southwest and mid-Atlantic states. Shifts in teleconnection patterns coincide with variations in the strength of ENSO and in a North Pacific mode.
ABSTRACT: The authors have analyzed global station data and created a gridded dataset of monthly precipitation for the 1900-1988 period. Statistical analyses suggest that discontinuities associated with instrumental errors are large for many high-latitude station records although they are unlikely to be significant for the majority of the stations. The first leading EOF in global precipitation fields is an ENSO-related pattern concentrating mostly in the low latitudes. The second leading EOF depicts a linear increasing trend (~2.4 mm/decade) in global precipitation fields during the 1900-1988 period. Consistent with the zonal precipitation trends identified in previous analyses, the EOF trend is seen as a long-term increase mostly in North America, mid- to high-latitude Eurasia, Argentina and Australia. The spatial patterns of the trend EOF and the rate of increase are generally consistent with those of the precipitation changes in increasing CO2 GCM experiments.
The North Atlantic Oscillation accounts for ~10% of December-February precipitation variance over North Atlantic surrounding regions. The mode suggests that during high-NAO-index winters, precipitation is above normal in northern (>50°N) Europe, the eastern United States, northern Africa and the Mediterranean; while below-normal precipitation occurs in southern Europe, eastern Canada and western Greenland.
Wet and dry months of one standard deviation occur at probabilities close to those of a normal distribution in midlatitudes. In the subtropics, the mean interval between two extreme events is longer. The monthly wet and dry events seldom (probability <5%) last longer than 2 months. ENSO is the single largest cause for global extreme precipitation events. Consistent with the upward trend in global precipitation, globally, the averaged mean interval between two dry months increased by ~28% from 1900-1944 to 1945-1988. The percentage of wet areas over the U.S. has more than doubled (from ~12% to >24%) since the 1970s while the percentage of dry areas has decreased by a similar amount since the 1940s. Severe droughts and floods comparable to the midwest U.S. 1988 drought and 1993 flood have occurred 2-9 times in each of several other regions of the world during this century.
ABSTRACT: The instrumental record is too brief for evaluation of the El Niño-Southern Oscillation (ENSO) system and its long-term response to climate forcing. To supplement these data, we use a new reconstruction of December–February Niño-3 sea surface temperatures based on subtropical North American tree-ring records to investigate aspects of ENSO variability over the past six centuries (AD 1408–1978). Spectral analyses reveal that the reconstruction best resolves variability within the “classical” ENSO band of 2–8 years. A low amplitude ENSO epoch in the 17th to 18th centuries broadly coincides with “Little Ice Age” conditions over much of the globe. The detailed behavior shows good agreement with shorter tree-ring reconstructions of ENSO over the past few centuries, but differs at times from other longer coral ENSO records and recent model simulations of past ENSO behavior. We discuss possible reasons for these discrepancies.
ABSTRACT: The overall amount of precipitation deposited along the West Coast and western cordillera of North America from 25° to 55°N varies from year to year, and superimposed on this domain-average variability are varying north–south contrasts on timescales from at least interannual to interdecadal. In order to better understand the north–south precipitation contrasts, their interannual and decadal variations are studied in terms of how much they affect overall precipitation amounts and how they are related to large-scale climatic patterns. Spatial empirical orthogonal functions (EOFs) and spatial moments (domain average, central latitude, and latitudinal spread) of zonally averaged precipitation anomalies along the westernmost parts of North America are analyzed, and each is correlated with global sea level pressure (SLP) and sea surface temperature series, on interannual (defined here as 3–7 yr) and decadal (>7 yr) timescales. The interannual band considered here corresponds to timescales that are particularly strong in tropical climate variations and thus is expected to contain much precipitation variability that is related to El Niño–Southern Oscillation; the decadal scale is defined so as to capture the whole range of long-term climatic variations affecting western North America.
Zonal EOFs of the interannual and decadal filtered versions of the zonal-precipitation series are remarkably similar. At both timescales, two leading EOFs describe 1) a north–south seesaw of precipitation pivoting near 40°N and 2) variations in precipitation near 40°N, respectively. The amount of overall precipitation variability is only about 10% of the mean and is largely determined by precipitation variations around 40°–45°N and most consistently influenced by nearby circulation patterns; in this sense, domain-average precipitation is closely related to the second EOF. The central latitude and latitudinal spread of precipitation distributions are strongly influenced by precipitation variations in the southern parts of western North America and are closely related to the first EOF. Central latitude of precipitation moves south (north) with tropical warming (cooling) in association with midlatitude western Pacific SLP variations, on both interannual and decadal timescales. Regional patterns and zonal averages of precipitation-sensitive tree-ring series are used to corroborate these patterns and to extend them into the past and appear to share much long- and short-term information with the instrumentally based zonal precipitation EOFs and moments.
Dettinger, M.D., D.R. Cayan, G.J. McCabe, J.A. Marengo, H. F. Diaz, V. Markgraf (2000). Multiscale streamflow variability associated with El Niño/Southern Oscillation. Cambridge University Press: 113-146
ABSTRACT: Streamflow responses to the El Niño/Southern Oscillation (ENSO) phenomenon in the tropical Pacific are detectable in many regions. During warm-tropical El Niño and cool-tropical La Niña episodes, streamflows are affected throughout the Americas and Australia, in northern Europe, and in parts of Africa and Asia. In North and South America, correlations between peakflow season streamflows and seasonal Southern Oscillation Indices (SOIs) show considerable persistence. In South America, correlations between flows in other seasons with December-February SOIs also are notably persistent, whereas, in North America, correlations are smaller when other, non-peak season time periods are considered.
At least two modes of streamflow response to ENSO are present in the Western Hemisphere. When interannual North and South American streamflow variations are analyzed together in a single principal components analysis, two of the leading components are found to be associated with ENSO climate variability. The more powerful of these modes corresponds mostly to ENSO responses by the rivers of tropical South America east of the Andes, along with rivers in southern South America and the southwestern United States, with Brazil experiencing less runoff during El Niños and the other regions experiencing more runoff. This streamflow mode is correlated globally with ENSO-like sea surface temperature (SST) patterns on both interannual and interdecadal time scales; indeed, the tropical South American rivers east of the Andes are coherent with SOI on virtually all historical time scales. The second ENSO-related streamflow mode characterizes other parts of extratropical streamflow variation, emphasizing the north-south differences in streamflows in North America during ENSO extremes and (less robustly) streamflow variations along the central Andes. The relation of this extratropical streamflow mode to ENSO seems to be mostly from scattered interannual time scales and, overall, its decadal variations follow North Atlantic SSTs.
On decadal time scales, the most remarkable variation identified in the Western Hemisphere ENSO-streamflow correlations or teleconnections is a decades-long contrast between the teleconnections of recent decades and teleconnections from about the 1920s into the 1950s. Correlations between streamflows and SOI, Niño-3 SSTs, and even global SSTs nearly vanished in many regions of North and South America during the earlier period. The change appears to have been associated with weakening of ENSO and, possibly, a weakening of connections between the atmospheric and oceanic components of ENSO during the earlier period. The development of two ENSO-related principal components of North and South American streamflow, rather than one, may be an artifact of the differences in decadal scale responses of streamflows in the tropics and extratropics to multiscale ENSO forcings.
ABSTRACT: In an earlier study of the teleconnection between streamflow and the warm (El Niño) phase of the El Niño/Southern Oscillation (ENSO) cycle, we found a strong relationship evident in four regions of the United States: the Gulf of Mexico, the Northeast, the North Central, and the Pacific Northwest. In this present study we have examined the same four regions for a relationship between streamflow and the cold (La Niña) phase of the Southern Oscillation (SO). Invariably, we found evidence of strong and consistent streamflow responses to La Niña events within the study regions. In each of the four regions, the strongest La Niña signal occurred at the same time of year as had the El Niño signal in their respective years. The sign of the seasonal streamflow anomaly associated with the La Niña events is the opposite of that associated with the El Niño events. This documents the existence of the biennial tendency related to the SO in the streamflow anomaly, which is expected, since La Niña/El Niño are opposite phases of the ENSO cycle. Finally, the relationships between streamflow and La Niño/El Niño were found to be statistically significant, based on the hypergeometric distribution. The results of this study demonstrate coherent, consistent, and significant midlatitude streamflow responses to the tropical SO phenomenon. This confirms the results of previous climatological studies that have examined the extratropical teleconnections from a hydrological and meteorological perspective.
ABSTRACT: River ecosystems are naturally variable in time and space and this variability is largely determined by climate, geology, and topography. We explore how variability in climate influences rivers. Our specific goals are to discuss (1) the major natural drivers of globalscale climate; (2) variability in temperature, precipitation, and streamflow patterns and how they relate to natural climate oscillations, such as ENSO (El Niño/Southern Oscillation, PDO (Pacific Decadal Oscillation), and AO/NAO (Arctic Oscillation/North Atlantic Oscillation); (3) how human activities influence climate variability; (4) how climate variability influences river systems; and (5) the need to account for climate variability in river restoration activities. Three regional-scale river drainages are explored in detail: the Columbia River in the Pacific Northwest; the Colorado River in the Rocky Mountains and the Southwestern USA; and the Kissimmee–Okeechobee–Everglades drainage in South Florida. As is true for many river drainages, humans have strongly influenced the hydrologic cycle in the three aforementioned basins through land-use practices. Clearing forests, creating urban environments, building dams, irrigating fields and straightening rivers all contribute to hydrologic change, especially river flooding. Rates of climate change and climate variability are now being influenced by human activities. Restoring the connectivity between river channels and floodplains, and “naturalization” of flow regimes of many large river drainages could be a major management action for ameliorating changes due to increased climate variability.
ABSTRACT: El Niño events—anomalous warmings of the tropical Pacific with associated climatic and economic impacts around the globe—have occurred at several-year intervals since before written records began with the logs of Francisco Pizarro in 1525. In this review, the history of El Niño research is traced from its beginnings through the key innovations of Bjerknes and Wyrtki to the unusual 1982-1983 event. Recent research is then reviewed, with detailed discussions of two important processes: instability growth and vacillation between climate states. Throughout the paper there are adjunct discussions of extraregional teleconnections, ecological impacts, and research on El Niño in the ancient record. The final section discusses the present paradigm for vacillations between El Niño and non-El Niño states and speculates on the possibly chaotic nature of El Niño. El Niño and its atmospheric counterpart, the Southern Oscillation, appear to occur as an internal cycle of positive and negative feedbacks within the coupled ocean-atmosphere climate system of the tropical Pacific, although hypotheses based on external forcing also exist. All events are preceded by westerly wind anomalies on the equator near the date line. Baroclinic equatorial Kelvin waves are generated, propagating eastward toward South America where they depress the thermocline and raise sea level, while the deep, upper ocean reservoir of warm water in the western Pacific is depleted. Sea surface temperature (SST) anomalies in the cool eastern Pacific occur primarily because the normal source of cold water is depressed below the reach of mixing and upwelling processes. In the central equatorial Pacific, eastward advection by anomalous zonal flows is the principal mechanism. Nonlinear heat transfer to the lower atmosphere creates a positive ocean-atmosphere feedback resulting in the unstable growth of anomalies along the equator. Much of the present research aims at determining how the ocean-atmosphere system vacillates between the El Niño and non-El Niño states. Coupled models suggest that a longer time scale, negative-feedback process produces the transitions: at the apex of an El Niño development an anomalous atmospheric convection above the areas of maximum SST produces areas of reduced upper layer thickness in the off-equatorial ocean, which slowly propagate westward to the western boundary as Rossby waves and back to the central equatorial Pacific as upwelling Kelvin waves, reestablishing the normal cooling process. A similar negative feedback of opposite sign completes the second half of an oscillation, returning again to the El Niño state. However, the notion that El Niño-Southern Oscillation variability results only from an internal feedback process is still highly contentious, and a number of external forcing mechanisms have been proposed.
ABSTRACT: The area burned in the North American boreal forest is controlled by the frequency of midtropospheric blocking highs that cause rapid fuel drying. Climate controls the area burned through changing the dynamics of large-scale teleconnection patterns (Pacific Decadal Oscillation/El Niño Southern Oscillation and Arctic Oscillation, PDO/ENSO and AO) that control the frequency of blocking highs over the continent at different time scales. Changes in these teleconnections may be caused by the current global warming. Thus, an increase in temperature alone need not be associated with an increase in area burned in the North American boreal forest. Since the end of the Little Ice Age, the climate has been unusually moist and variable: large fire years have occurred in unusual years, fire frequency has decreased and fire–climate relationships have occurred at interannual to decadal time scales. Prolonged and severe droughts were common in the past and were partly associated with changes in the PDO/ENSO system. Under these conditions, large fire years become common, fire frequency increases and fire–climate relationships occur at decadal to centennial time scales. A suggested return to the drier climate regimes of the past would imply major changes in the temporal dynamics of fire–climate relationships and in area burned, a reduction in the mean age of the forest, and changes in species composition of the North American boreal forest.
ABSTRACT: El Niño-Southern Oscillation (ENSO) is one of the most important coupled ocean-atmospheric phenomena to cause global climate variability on interannual timescales. Efforts to understand recent, apparently anomalous ENSO behaviour are hampered by the phenomenon's unstable (non-stationary) nature and the limitations inherent in palaeoclimate records. In this paper, the complexities associated with isolating ENSO signals in observational and palaeoclimate records are reviewed. The utility and limitations of high-resolution (tree-ring, coral, speleothems, ice and documentary) proxy data for ENSO reconstruction are discussed. To overcome the regional biases contained within each palaeoclimatic source, it is necessary to compare complementary signals derived from multiple proxy climate records. To date, there have been limited attempts to reconstruct large-scale ENSO using these multiproxy methodologies. A critique of the complexities associated with previous approaches of reconstructing discrete ENSO events and atmospheric/oceanic indices is provided. Abundant potential remains to better characterise teleconnection patterns, propagation signatures and non-stationary features of large-scale ENSO behaviour. If key uncertainties in ENSO dynamics (such as the response of extreme events to natural/human forcing) are to be adequately assessed, then complementary attempts must be made to model the historic synoptic conditions with apparent changes in reconstructed indices.
ABSTRACT: Interannual time-scale associations between fire occurrence and drought indices, the Southern Oscillation, and other synoptic patterns demonstrate that large-scale, long-term atmospheric features are precursors to regional fire activity. However, our knowledge of fire-climate relations over longer (century) timescales is fragmentary because of the rarity of comparable climate and fire time-series with sufficient resolution, length and regional extent. In this study, we develop reconstructions of wildfire occurrence from tree-ring data collected from northwestern New Mexico to compare with a millennium-length dendroclimatic reconstruction of precipitation. Reconstructions of both wildfires and climate show simultaneous changes since AD 1700 that indicate climate forcing of wildfire regimes on interannual to century timescales. Following a centuries-long dry period with high fire frequency (c. AD 1400-1790), annual precipitation increased, fire frequency decreased, and the season of fire shifted from predominantly midsummer to late spring. We hypothesize that these shifts in fire regimes reflect long-term changes in rainfall patterns associated with changes in synoptic-scale atmospheric circulation patterns and the Southern Oscillation. Our evidence supports century-scale climate forcing of fire regimes in the American Southwest, providing a useful analogue of future wildfire regimes expected under changing global climate conditions.
ABSTRACT: A simple method has been devised to incorporate the El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) climate signals into the well-known extended streamflow prediction forecasting approach. Forecasts of ENSO are currently available up to a year or more in advance, which facilitates forecasting of the streamflow response to this climate signal at interannual forecast lead times. The bimodal phase of the PDO can be identified in real time using a combination of assumed persistence of the existing phase and the tracking of extreme events to identify transitions. The technique makes use of a gridded meteorological data set to drive a macroscale hydrology model at 1° spatial resolution over the Columbia River Basin above The Dalles. A streamflow forecast ensemble is created by resampling from the historical meteorological data according to six predefined PDO/ENSO categories. Given a forecast of the ENSO climate signal for the coming water year and the existing phase of the PDO, these meteorological ensembles are then used to drive the hydrology model based on the initial soil and snow conditions as of the forecast date. To evaluate the technique, a retrospective forecast of the historic record was prepared (1989–1998), using October–September as the forecast period, as well as an ensemble forecast for water years 1999 and 2000 that were prepared on June 1, 1998 and May 10, 1999, respectively. The results demonstrate the increase in lead time and forecast specificity over climatology that can be achieved by using PDO and ENSO climate information to condition the forecast ensembles.
INTRODUCTION: In the mid-1970s, ocean conditions in the North Pacific Ocean underwent a dramatic and abrupt change (Graham 1994). Coincident with the physical regime shift, Alaskan salmonids entered an era of greatly increased production that has persisted into the 1990s (Fig. 1). Throughout their long (over 100 yr) commercial exploitation history, several of the Alaskan salmon species have demonstrated "red noise" variability, wherein periods of high (low) production tend to persist for a lengthy period before abruptly reversing to the opposite state. For example, in the 1930s and early 1940s, salmon landings were high, followed by an era of low catches from the late 1940s to late 1970s. As Alaskan landings increased in the late 1970s, several North American west coast stocks, notably Oregon coho salmon (Oncorhynchus kisutch ; Pearcy 1992), went into a prolonged period of decline.
Much early research on variability in salmon survival (and therefore production and catch) focused on the freshwater stage of their life cycle, attempting to link survival to conditions in their spawning and rearing habitat. The period spent at sea was regarded as relatively unimportant. There is now a growing awareness of the importance of the marine environment in determining salmon production (e.g. Pearcy 1984; Beamish and McFarlane 1989).
Variability in marine survival of salmon is poorly understood (Mathews 1984). Numerous studies have attempted to correlate survival with environmental factors, though few have proven useful in predicting salmon abundance or assisting in management decision making (Pearcy 1992). Part of the difficulty in elucidating the driving factors of survival is that the relationship between the environment and survival is clouded by many factors. Biotic (e.g. intra- and inter-specific competition, prey availability, predation) and abiotic (environmental variables, habitat) factors not only exhibit complex relationships with survival (non-linear, threshold) but are themselves often highly correlated.
Despite these drawbacks, the importance of attempting to understand the causes of variable survival should not be underestimated (Francis and Sibley 1991). In particular, understanding large-scale and long-term variability would benefit both fishery managers and fishermen (Shepherd et al. 1984).
Large marine ecosystems fluctuate in response to physical forcings that occur over a number of time intervals. There appears to be a nested hierarchy of interacting processes occurring on different time scales that are relevant to their dynamics, ranging from relatively discrete processes that occur over times on the order of 1 yr or less (e.g., the 1970 North Pacific winter atmospheric circulation pattern (Hollowed and Wooster 1992)), to processes that persist over long time periods and fluctuate at the inter-century level (Baumgartner et al. 1992). What we are most interested in identifying in this analysis are regimes that define points in time, separated by intervals on the order of decades, where major jumps or shifts in the level of abundance occur in large marine ecosystems. Therefore, in examining the interannual dynamics of various biological components of large marine ecosystems, what we see are responses to these nested hierarchies of interacting processes occurring at different time scales and working synergistically to create pattern. In this analysis, it is pattern at the regime level that we are trying to interpret.
We hypothesize that regional variability in salmon production is driven by large-scale climate change, reflected in North Pacific atmospheric-oceanic regime shifts. Under this hypothesis, salmon populations exhibit two characteristics: relatively stable production while a particular regime persists, followed by a rapid transition to a new production level in response to the physical regime shift. If large-scale salmon production is closely related to North Pacific climate processes, we should find coherent shifts in mean production levels across both species and area.
In addition to the late 1970s regime shift, we surmise that an earlier shift, opposite in character, occurred in the late 1940s. Based on evidence summarized in the Discussion, we tentatively identify the regime shifts as taking place in the winters of 1946-47 and 1976-77. Our hypothesis suggests that two shifts in Alaskan salmon production should be detectable: a decrease in the late 1940s and an increase in the late 1970s.
To test this hypothesis, we proceed by statistically analyzing the historical production dynamics of four major Alaskan salmon stocks: western and central sockeye salmon (Oncorhynchus nerka ), southeast and central pink salmon (Oncorhynchus gorbuscha ). While many researchers have noted the aforementioned swings in production (e.g., Beamish and Bouillon 1994), there remained the possibility that the changes might be either random processes or nonsignificant, in a statistical sense. Owing to the high serial correlation (lack of independence between successive observations), the t-test for equality of means cannot be used to test for production shifts. We utilize a time-series technique known as intervention analysis to identify the significance, magnitude, and form of structural shifts (interventions) in the four time series. We identify and test the timing of the interventions by matching the onset of the physical regimes with the life history of the different species of salmon. Intervention analysis is a relatively recent statistical technique recommended as a method for detecting and quantifying non-random change in an unreplicated experiment (Carpenter 1990).
To test for interventions, we fitted univariate time-series models of the Box-Jenkins (1976) autoregressive integrated moving average (ARIMA) class. These ARIMA models provide a baseline fit to the correlation structure exhibited by the time series. Interventions are subsequently identified by analyzing model residuals. Model parameters are re-estimated incorporating the intervention(s), and the models compared on the basis of several criteria. After identifying the timing and nature of the interventions, we then review the evidence for synchronous large-scale physical regime shifts in the North Pacific.
Hashimoto, H., Nemani, R. R., White, M. A., Jolly, W. M., Piper, S. C., Keeling, C. D., Myneni, R. B., Running, S. W. (2004). El Niño–Southern Oscillation–induced variability in terrestrial carbon cycling. Journal of Geophysical Research 109 (D23110): doi:10.1029/2004JD004959
ABSTRACT: We examined the response of terrestrial carbon fluxes to climate variability induced by the El Niño–Southern Oscillation (ENSO). We estimated global net primary production (NPP) from 1982 to 1999 using a light use efficiency model driven by satellite-derived canopy parameters from the Advanced Very High Resolution Radiometer and climate data from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis project. We estimated a summed heterotrophic respiration and fire carbon flux as the residual between NPP and the terrestrial net carbon flux inferred from an atmospheric inversion model, excluding the impacts of land use change. We propose that for global applications this approach may be more robust than traditional, biophysically based approaches of simulating heterotrophic respiration. NPP interannual variability was significantly related to ENSO, particularly at lower latitudes (22.5°N–22.5°S) but was weakly related to global temperature. Global heterotrophic respiration and fire carbon fluxes were strongly correlated with global temperature (7.9 pgC/°C). Our results confirm the dependence of global heterotrophic respiration and fire carbon fluxes on interannual temperature variability and strongly suggest that ENSO-mediated NPP variability influences the atmospheric CO2 growth rate.
ABSTRACT: Anticipating the consequences of climatic change for fire requires understanding of the causes of variation in historical fire regimes. We assessed the influence of annual and decadal variation in climate on fire regimes of ponderosa pine-dominated forests in eastern Oregon and Washington using existing, annually dated tree-ring reconstructions (1687–1994). In four watersheds, we compared the extent of low-severity fires (total area burned each year) to precipitation and the Southern Oscillation Index, a measure of variation in El Niño-Southern Oscillation (ENSO), which affects weather in this region. At the annual scale, large fires burned during dry years and El Niño years (low SOI) in all watersheds while small fires burned regardless of variation in these climate parameters. Large fires also burned during relatively wet years and La Niña years (high SOI) in one watershed, indicating that local factors can override regional climate controls in some locations. Climate from previous years did not influence current year's fire extent. The influence of ENSO on fire regimes in this region has not previously been demonstrated at these multicentury, regional scales. At the decadal scale, fire extent varied with precipitation, perhaps in response to variation in such climate features as the Pacific Decadal Oscillation. Several decades of low fire extent in the watersheds during the early 1800s was synchronous with a lack of fire at other sites in North and South America, probably in response to a change in the global climate that included a lessening in the frequency and/or intensity of ENSO events.
ABSTRACT: Linkages between tropical Pacific Ocean monthly climatic variables and the Upper Colorado River basin (UCRB) hydroclimatic variations from 1909 to 1998 are analyzed at interseasonal timescales. A study of the changes in these linkages through the years and their relationship to the Pacific Decadal Oscillation (PDO) is also investigated. Tropical Pacific climate variations were represented by atmospheric/oceanic ENSO indicators. For the UCRB, warm season (April–September) streamflow totals at Lee's Ferry, Arizona, and precipitation averages at different periods (cold season: October–March; warm season: April–September; and annual: October–September) were used to study the UCRB's response to tropical Pacific climatic forcing. A basinwide ENSO signature was found in the significant correlations between warm season precipitation in the UCRB and warm season SST averages from the Niño-3 region in most of the stations around the UCRB. This link is more evident during the warm phase of ENSO (El Niño), which is associated with an increase in warm season precipitation. The analysis also showed a link between June to November ENSO conditions and cold season precipitation variations contained in a principal component representing the high-elevation precipitation stations, which are the main source of streamflow. However, the amplitude and coherence of the cold season ENSO signal is significantly smaller compared to the general precipitation variations found in stations around the UCRB. Only when very few stations in the high elevations are considered is the ENSO signal in cold season precipitation in the basin revealed. Interdecadal hydroclimatic variations in the UCRB related to possible PDO influences were also investigated. There are significant shifts in the mean of UCRB's moisture-controlled variables (precipitation and streamflow) coincident with the PDO shifts, suggesting a connection between the two processes. It has been suggested in other studies that this connection could be expressed as a modulation on the predominance of each ENSO phase; that is, strong and consistent winter El Niño (La Niña) patterns are associated with the positive (negative) phase of the PDO. In the UCRB this apparent modulation seems to be accompanied by a general change in the sign of the correlation between ENSO indicators and cold season precipitation in most stations of the basin around 1932/33. From 1909 to 1932 the basin has a predominantly cold season ENSO response characteristic of the northwestern United States (drier than normal associated with tropical SST warming and vice versa); from 1933 to 1998 the response of the basin is predominantly typical of the southwestern United States during winter (wetter than normal associated with tropical SST warming and vice versa). This apparent correlation sign reversal is suggested to be related to interdecadal changes in the boundary of the north–south bipolar response characteristic of the ENSO signal in the western United States during winter.
ABSTRACT: Snow water equivalent anomalies (SWEA) measured around April 1 by stations in the Columbia basin area in British Columbia, Canada, were studied for their interannual variability during the period 1950–1999, particularly in relation to El Niño/La Niña events and to high and low Pacific–North American (PNA) atmospheric circulation patterns. Composites of the SWEA showed that SWEA were negative during El Niño years, positive during La Niña years, negative during high PNA years, and positive during low PNA years. High PNA appeared to have the most impact on the SWEA, followed by La Niña, El Niño, and low PNA. In the Columbia basin area, La Niña effects (relative to El Niño effects) on SWEA decrease northward and eastward but strengthen with elevation. Composites of the Pacific sea surface temperature anomalies (SSTA) during the 10 lowest SWEA years revealed weak signals, with El Niño warm SSTA present only during spring and early summer in the preceding year and the SSTA pattern consistent with a high PNA present by fall and winter. In contrast, composites of the SSTA during the 10 highest SWEA years showed strong La Niña cool SSTA starting around May in the preceding year and lasting onto winter.
ABSTRACT: We present evidence showing that the nonlinear dynamic heating (NDH) in the tropical Pacific ocean heat budget is essential in the generation of intense El Niño events as well as the observed asymmetry between El Niño (warm) and La Niña (cold) events. The increase in NDH associated with the enhanced El Niño activity had an influence on the recent tropical Pacific warming trend and it might provide a positive feedback mechanism for climate change in the tropical Pacific.
ABSTRACT: The relationship between the El Niño/Southern Oscillation (ENSO) and unimpaired streamflow over the contiguous United States is studied. The extreme phases of the Southern Oscillation have been linked to fairly persistent classes of atmospheric anomalies over the low and middle latitudes at regional and global scales. Of particular interest in this investigation is the identification of regions of land that appear to have strong and consistent ENSO-related streamflow signals. The first harmonic extracted from a 24-month ENSO composite at each station is assumed to be the ENSO-related signal appearing in streamflow anomalies. These regions were identified by the similarity in phase of the harmonic vectors. The vectorial display of these harmonics over a map of the United States provides the areal extents of ENSO influence on streamflow. Coherent and significant streamflow responses to hypothesized ENSO forcing are found in four regions of the United States: the Gulf of Mexico, the Northeast, the North Central, and the Pacific Northwest. Once an ENSO event sets in, a long-range forecasting utility may be available for these regions. The results of this analysis, which are consistent with previous studies on precipitation and temperature, demonstrate the mid-latitude hydrologic response to the tropical ENSO phenomena.
ABSTRACT: Recent research has highlighted the persistence of multi-decadal epochs of enhanced/reduced flood risk across New South Wales (NSW), Australia. Recent climatological studies have also revealed multi-decadal variability in the modulation of the magnitude of El Niño/Southern Oscillation (ENSO) impacts. In this paper, the variability of flood risk across NSW is analysed with respect to the observed modulation of ENSO event magnitude. This is achieved through the use of a simple index of regional flood risk. The results indicate that cold ENSO events (La Niña) are the dominant drivers of elevated flood risk. An analysis of multi-decadal modulation of flood risk is achieved using the inter-decadal Pacific Oscillation (IPO) index. The analysis reveals that IPO modulation of ENSO events leads to multi-decadal epochs of elevated flood risk, however this modulation appears to affect not only the magnitude of individual ENSO events, but also the frequency of their occurrence. This dual modulation of ENSO processes has the effect of reducing and elevating flood risk on multi-decadal timescales. These results have marked implications for achieving robust flood frequency analysis as well as providing a strong example of the role of natural climate variability.
ABSTRACT: Records from South America show that modern ENSO (El Nino-Southern Oscillation) did not exist 7000 cal yr B.P. and has developed progressively since then. There has been little information available on oceanic conditions in the eastern equatorial Pacific (EEP) to constrain explanations for ENSO onset. We report quantitative observations on thermocline and mixed-layer conditions in the EEP during ENSO start up. We found important changes in both the thermocline and the mixed layer, indicating increased upwelling of cooler waters since 7000 cal yr B.P. This resulted from change in the source and/or properties of waters supplying the Equatorial Undercurrent, which feeds upwelling along the equator and the Peru margin. Modeling shows that ENSO is sensitive to subsurface conditions in the eastern equatorial Pacific and that the changes in the thermocline we observed were driven by extratropical processes, giving these a role in conditioning the development of ENSO. This is in contrast to models that call for control of equatorial Pacific oceanography by tropical processes only. These infer stronger upwelling and cooler surface waters for the EEP during the mid-Holocene, which is not supported by our results.
ABSTRACT: Using principal component analysis (PCA), cluster analysis, and jackknife analysis, we investigated the spatial and temporal modes that dominate streamflow variability in the western US in response to El Niño-Southern Oscillation (ENSO) events. Spatial variability was investigated with data only from ENSO years and with rotated PCA on 79 streamflow stations in the western United States. Eight regions, or clusters, were thus pinpointed as areas where streamflow tends to co-vary similarly following ENSO events; traditional cluster analysis confirmed the identification of these regions. The ENSO response in streamflow was then further evaluated by forming an aggregate ENSO composite for each region.
Temporal variability of western US streamflow in the PCA-identified regions was evaluated with a `T-mode' PCA that isolated the different responses in streamflow following ENSO events. The T-mode PCA breaks the 13 ENSO events that occurred from 1932 to 1993 into five subsets. It is interesting to note that the events in the dominant mode, PC1(+), occurred before 1976, and next mode, PC2(+), included events prior to 1976.
Finally, we investigated the atmospheric circulation patterns over the North Pacific Ocean and much of North America that are associated with the various US streamflow responses. The circulation patterns vary according to the prescribed ENSO forcing. The results of this study contribute to a better understanding of the varied ENSO-streamflow relationship in the western US and the use of ENSO for long-range streamflow forecasting.
ABSTRACT: The recent El Niño has been by most measures one of the most extreme, and there have been several papers on its thermal signature and associated wind field. There has also been wide coverage of the changes in terrestrial precipitation, with torrential rains in California and devastating fires in Borneo in response to the prolonged drought. Here we complete the picture by examining oceanic precipitation data derived from novel processing of dual-frequency altimetry. An increased area of precipitation, with more frequent and slightly more intense rainfall, is found to mirror the expansion of the western warm pool.
ABSTRACT: Applicable publications, involving five languages, have been reviewed to obtain information on El Niños that occurred over the past four and a half centuries. Since this information refers strictly to El Niño occurrences, a regional manifestation of the large-scale (El Niño-Southern Oscillation (ENSO)) event, it is based primarily on evidence obtained from the west coast region of northern South America and its adjacent Pacific Ocean waters. Authored lists of events were not acceptable without referenced valid information sources. It was desirable to have cross-correlated reports from independent sources. Relative strengths of events are based on such considerations as wind and current effects on travel times of ancient sailing ships, degree of physical damage and destruction, amounts of rainfall and flooding, mass mortality of endemic marine organisms and guano birds, extent of invasion by tropical nekton, rises in sea temperatures and sea levels, affects on coastal fisheries and fish meal production, etc. Emphasis is placed on strong and very strong events. For example, the 1940-1941, 1957-1958, and 1972-1973 events fall into the strong category, whereas the 1891, 1925-1926 and 1982-1983 events are considered very strong. Over our period of study, 47 El Niño events were placed in the strong or very strong categories. Over the period 1800-present, we noted 32 El Niño events of moderate or near moderate intensity. Weak events are not included here. The approach used here caused us to revise many of our earlier evaluations concerning event occurrences and intensities. Our tropical Pacific thickness analyses and cumulative plots of Southern Oscillation index anomalies over the southeast Pacific trade wind zone showed additional evidence as to the unusual strength of the 1982-1983 event. Also, in our investigation we noted several periods of long-term (near decadal or longer) climatic change.
INTRODUCTION: The Southern Oscillation (SO) has been traditionally monitored by indices formed from surface pressure differences across the South pacific (e.g. Chen 1982; McBride and Nicholls 1983; Trenberth 1984). Several of the most-used indices of the SO are those based on Darwin and Tahiti pressures. Recently recoverd station records for Tahiti have allowed us to more than double the length of the Tahiti-Darwin index time series to over 100 years. The purpose of this note is to make those data available to the climate research community and to suggest a rational scheme for identifying the positive and negative extremes of the SO.
ABSTRACT: The quasi-periodic El Niño -Southern Oscillation (ENSO) phenomenon in the tropical Pacific Ocean produces the largest interannual variation in the cold season climate of Canada. The diabatic heating in the eastern tropical Pacific, associated with the warm phase of ENSO (El Niño), triggers Rossby waves which in turn gives rise to the Pacific-North American teleconnection (PNA) over the North American sector. The strongest cell of the PNA pattern lies over western Canada. In most of southern Canada, mean winter temperature distribution is shifted towards warmer values, and precipitation is below normal. The presence of El Niño provides the best opportunity to make skillful long-range winter forecast for Canada. A strong El Niño event, while bringing respite from the otherwise cold winter in Canada, can be expected to cost the Canadian economy two to five billion dollars.
ABSTRACT: An influential 1996 paper presented a statistical analysis showing that the prolonged ENSO warm event of the early 1990's was inconsistent with the historical pattern of ENSO variability and therefore concluded that there had been a shift in ENSO behavior possibly connected to global warming. A fundamental problem with this earlier analysis is that the data used to test for a shift in ENSO behavior were not independent of the data used to identify the hypothetical shift. A new analysis is presented that avoids this problem by using more recent data. The results raise a question about the earlier finding.
ABSTRACT: Fire scar and tree growth chronologies (1700 to 1905) and fire statistics (since 1905) from Arizona and New Mexico show that small areas burn after wet springs associated with the low phase of the Southern Oscillation (SO), whereas large areas burn after dry springs associated with the high phase of the SO. Through its synergistic influence on spring weather and fuel conditions, climatic variability in the tropical Pacific significantly influences vegetation dynamics in the southwestern United States. Synchrony of fire-free and severe fire years across diverse southwestern forests implies that climate forces fire regimes on a subcontinental scale; it also underscores the importance of exogenous factors in ecosystem dynamics.
Thomson, A.M., R.A. Brown, N.J. Rosenberg, R. César Izaurralde, D.M. Legler, R. Srinivasan (2003). Simulated impacts of El Niño/Southern Oscillation on United States water resources. Journal of the American Water Resources Association 39 (1): 137-148
ABSTRACT: The El Niño/Southern Oscillation (ENSO) phenomena alter global weather patterns with consequences for fresh water supply. ENSO events impact regions and their natural resource sectors around the globe. For example, in 1997 and 1998, a strong El Niño brought warm ocean temperatures, flooding, and record snowfall to the west coast of the United States. Research on ENSO events has improved long range climate predictions, affording the potential to reduce the damage and economic cost of these weather patterns. Here, using the Hydrologic Unit Model for the United States (HUMUS), we simulate the impacts of four types of ENSO states (Neutral, El Niño, La Niña, and strong El Niño) on water resources in the conterminous United States. The simulations show that La Niña conditions increase water yield across much of the country. We find that water yield increases during El Niño years across the south while declining in much of the rest of the country. However, under strong El Niño conditions, regional water yields are much higher than Neutral, especially along the West Coast. Strong El Niño is not simply an amplification of El Niño; it leads to strikingly different patterns of water resource response.
ABSTRACT: The utility of a simple index for monitoring the Southern Oscillation signal is explored in detail. Based upon sea level pressure data at the two stations Tahiti (T) and Darwin (D), an optimal index, in the sense that it combines the Southern Oscillation variance into one series is the combination [Tn + Dn ] where the subscript n denotes normalization by the overall standard deviation of each series. A direct measure of the noise due to small-scale or transient phenomena that are not a part of the large-scale coherent Southern Oscillation fluctuations is the index [Tn + Dn ]. It is recommended that this index of noise also should be monitored in order to determine the representativeness of the Southern Oscillation index.
The signal-to-noise ratio is shown to depend upon the cross correlation between Darwin and Tahiti, and can be increased by applying weighted moving average low-pass filters to the data. Monthly data exhibit a signal-to-noise ratio, defined as the ratio of the standard deviations, of 1.44 and this increases to 1.97 for seasonal data. An 11-term low-pass filter is designed that increases the signal-to-noise ratio to 2.70 without adversely reducing the variance in frequencies that are important in the Southern Oscillation. Resulting time series plots are presented.
ABSTRACT: The 1988 summer drought in the United States was the most extensive in many years. Because the drought developed in different places at different times, not all regional effects can be traced to the same cause. Along the West Coast and in the northwestern United States drought conditions developed during 1987 in association with the 1986 to 1987 El Niño in the tropical Pacific Ocean. Record low rainfalls from April to June 1988 led to rapid development of drought in the North Central United States. Strong anticyclonic conditions and a northward displaced jet stream in the upper atmosphere over North America throughout this period were only part of pronounced and distinctive wavetrain of anomalies in the atmospheric circulation that appeared to emanate from the tropical Pacific. Below average sea surface temperatures along the equator in the Pacific in the northern spring of 1988, combined with warmer than normal water from 10° to 20°N, led to a northward displaced but still active intertropical convergence zone (ITCZ) southeast of Hawaii. Results from a steady-state planetary wave atmospheric model indicate that the atmospheric heating anomalies associated with the displaced ITCZ can force an anomalous wavetrain across North America similar to that observed. Land surface processes probably contributed to the severity and persistence of the drought; however, the large-scale atmospheric circulation perturbations associated with natural variations in the coupled atmosphere-ocean system in the tropical Pacific were most likely the primary cause.
ABSTRACT: The tendency for more frequent El Niño events and fewer La Niña events since the late 1970’s has been linked to decadal changes in climate throughout the Pacific basin. Aspects of the most recent warming in the tropical Pacific from 1990 to 1995, which are connected to but not synonymous with El Niño, are unprecedented in the climate record of the past 113 years. There is a distinction between El Niño (EN), the Southern Oscillation (SO) in the atmosphere, and ENSO, where the two are strongly linked, that emerges clearly on decadal time scales. In the traditional El Niño region, sea surface temperature anomalies (SSTAs) have waxed and waned, while SSTAs in the central equatorial Pacific, which are better linked to the SO, remained positive from 1990 to June 1995. We carry out several statistical tests to assess the likelihood that the recent behavior of the SO is part of a natural decadal-timescale variation. One test fits an autoregressive-moving average (ARMA) model to a measure of the SO given by the first hundred years of the pressures at Darwin, Australia, beginning in 1882. Both the recent trend for more ENSO events since 1976 and the prolonged 1990-1995 ENSO event are unexpected given the previous record, with a probability of occurrence about once in 2,000 years. This opens up the possibility that the ENSO changes may be partly caused by the observed increases in greenhouse gases.
ABSTRACT: A comprehensive statistical analysis of how an index of the Southern Oscillation changed from 1882 to 1995 was given by Trenberth and Hoar , with a focus on the unusual nature of the 1990–1995 El Niño-Southern Oscillation (ENSO) warm event in the context of an observed trend for more El Niño and fewer La Niña events after the late 1970s. The conclusions of that study have been challenged by two studies which deal with only the part of our results pertaining to the length of runs of anomalies of one sign in the Southern Oscillation Index. They therefore neglect the essence of Trenberth and Hoar, which focussed on the magnitude of anomalies for certain periods and showed that anomalies during both the post-1976 and 1990–mid-1995 periods were highly unlikely given the previous record. With updated data through mid 1997, we have performed additional tests using a regression model with autoregressive-moving average (ARMA) errors that simultaneously estimates the appropriate ARMA model to fit the data and assesses the statistical significance of how unusual the two periods of interest are. The mean SOI for the post-1976 period is statistically different from the overall mean at <0.05% and so is the 1990–mid-1995 period. The recent evolution of ENSO, with a major new El Niño event underway in 1997, reinforces the evidence that the tendency for more El Niño and fewer La Niña events since the late 1970s is highly unusual and very unlikely to be accounted for solely by natural variability.
ABSTRACT: This study uses proxy climate records derived from paleoclimate data to investigate the long-term behaviour of the Pacific Decadal Oscillation (PDO) and the El Niño Southern Oscillation (ENSO). During the past 400 years, climate shifts associated with changes in the PDO are shown to have occurred with a similar frequency to those documented in the 20th Century. Importantly, phase changes in the PDO have a propensity to coincide with changes in the relative frequency of ENSO events, where the positive phase of the PDO is associated with an enhanced frequency of El Niño events, while the negative phase is shown to be more favourable for the development of La Niña events.
ABSTRACT: The effect of the Southern Oscillation on daily precipitation in the southwestern United States is examined by using the Southern Oscillation Index (SOI) to perturb parameters of a stochastic daily precipitation model. Daily precipitation is modeled with a Markov chain-mixed exponential model and seasonal variability of model parameters is described by Fourier series. The hypothesized linkage between the SOI and the model parameters is of the formG (N, t) =G (t) +b S(N, t - t ) whereG (N, t) is the perturbed parameter i for day t of yearN, G (t) is the annually periodic parameteri for dayt ,b is a coefficient, S is the SOI, andt is a lag in days. Daily precipitation data for 27 stations in California, Nevada, Arizona, and New Mexico were analyzed. Perturbations of the logits of the dry-dry transition probabilities resulted in statistically significant improvements in the log likelihood function for 23 stations and perturbations of the mean daily rainfall resulted in significant increases for 18 stations. The most common lag identified was 90 days, suggesting the possibility of conditional simulations of daily precipitation. Seasonal effects were detected, confirming the results of previous analysis with groups of stations.
Xu, Z. X., K. Takeuchi, H. Ishidaira (2004). Correlation between El Niño-Southern Oscillation (ENSO) and precipitation in South-east Asia and the Pacific region. Hydrological Processes 18 (1): 107-123
ABSTRACT: The relationship between El Niño-Southern Oscillation (ENSO) events versus precipitation anomalies, and the response of seasonal precipitation to El Niño and La Niña events were investigated for 30 basins that represent a range of climatic types throughout South-east Asia and the Pacific region. The teleconnection between ENSO and the hydroclimate is tested using both parametric and non-parametric approaches, and the lag correlations between precipitation anomalies versus the Southern Oscillation Index (SOI) several months earlier, as well as the coherence between SOI and precipitation anomalies are estimated. The analysis shows that dry conditions tend to be associated with El Niño in the southern zone, and part of the middle zone in the study area. The link between precipitation anomalies and ENSO is statistically significant in the southern zone and part of the middle zone of the study area, but significant correlation was not observed in the northern zone. Patterns of precipitation response may differ widely among basins, and even the response of a given river basin to individual ENSO events also may be changeable.