Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
The Pacific Decadal Oscillation
ABSTRACT: The Pacific Decadal Oscillation (PDO) has significant climatological and ecological effects in northwestern North America. Its possible effects and their modification by feedbacks are examined in the forest-tundra ecotone in Glacier National Park, Montana, USA. Tree ring samples were collected to estimate establishment dates in 10 quadrats. Age-diameter regressions were used to estimate the ages of uncored trees. The temporal pattern of establishment and survival was compared to the pattern of the PDO. A wave of establishment began in the mid-1940s, rose to a peak rate in the mid-1970s, and dropped precipitously beginning ca. 1980 to near zero for the 1990s. The period of establishment primarily coincided with the negative phase of the PDO, but the establishment and survival pattern is not correlated with the PDO index. The pattern indicates a period during which establishment was possible and was augmented by positive feedback from surviving trees. Snow may be the most important factor in the feedback, but studies indicate that its effects vary locally. Spatially differentiated analyses of decadal or longer periodicity may elucidate responses to climatic variation.
ABSTRACT: This paper explores the effects of the interaction of anthropogenic trends and climate cycles on salmon declines in the Columbia and Snake river basins. A basic population model, including anthropogenic and environmental factors, is discussed and literature relating decadal scale climate patterns and the response of the North Pacific ecosystem is reviewed. From this background a ratchet-like decline in Columbia and Snake river salmon production resulted from the interactions of human activities and climatic regime shifts. These interactions are illustrated using hundred year patterns in spring chinook salmon (Oncorhynchus tshawytscha ) catch, the Columbia River hydroelectric generating capacity, and a climate index characterizing the shifts between a cool/wet regime favorable to West Coast salmon and a warm/dry regime unfavorable to West Coast salmon. A half century correlation of the climate index and chinook catch suggest that a favorable climate regime counteracted detrimental impacts of hydrosystem development between 1945 and 1977, while an unfavorable climate regime negated beneficial effects of salmon mitigation efforts after 1977. This hypothesis is elaborated by a comparison of changes in the climate index relative to changes in Snake River salmon survival indicators.
Proposed Snake River salmon restoration plans are considered in terms of this counteractive effects hypothesis. The recent declines of salmon stocks have led a number of groups to propose plans that discontinue the present recovery actions, especially transportation of juveniles salmon around the dams. This paper hypothesizes that salmon recovery efforts, in part, have been limited by recent poor climate/ocean conditions. If this hypothesis is true, then eliminating the transportation program could be detrimental to fish. If the hypothesis is false, then eliminating transportation may be a viable recovery measure. In either case resolving the issue of counteracting processes is essential prior to making major changes to the hydrosystem operations.
ABSTRACT: Ongoing drought in the Colorado River Basin, unprecedented urban growth in the watershed, and numerical model simulations showing higher temperatures and lower precipitation totals in the future have all combined to heighten interest in drought in this region. In this investigation, we use principal components analysis (PCA) to independently assess the influence of various teleconnections on Basin-wide and sub-regional winter season Palmer Hydrological Drought Index (PHDI) and precipitation variations in the Basin. We find that the Pacific Decadal Oscillation (PDO) explains more variance in PHDI than El Niño-Southern Oscillation (ENSO), the Atlantic Multidecadal Oscillation (AMO), and the planetary temperature combined for the Basin as a whole. When rotated PCA is used to separate the Basin into two regions, the lower portion of the Basin is similar to the Basin as a whole while the upper portion, which contains the high-elevation locations important to hydrologic yield for the watershed, demonstrates poorly defined relationships with the teleconnections. The PHDI for the two portions of the Basin are shown to have been out of synch for much of the twentieth century. In general, teleconnection indices account for 19% of the variance in PHDI leaving large uncertainties in drought forecasting.
Beamish, R. J., Neville, C. M., Cass, A. J. (1997). Production of Fraser River sockeye salmon (Oncorhynchus nerka ) in relation to decadal-scale changes in the climate and the ocean. Canadian Journal of Fisheries and Aquatic Sciences 54 (3): 543-554
ABSTRACT: The abundance of Fraser River sockeye salmon (Oncorhynchus nerka ) stocks was low in the 1960s, increased to high levels in the 1980s, and possibly entered a period of low abundance in recent years. The abundance changes of the combined stocks can be separated into productivity regimes that correspond to changes in climate trends. The most distinct change occurred when there was a major change in the climate over the Pacific Ocean in the winter of 1976-1977. The existence of natural shifts in abundance trends means that the high returns that occur during periods of high productivity would not be expected to occur during the low-productivity periods. The response of Fraser River sockeye to climate changes may be a specific example of a more general response by a number of species of fishes in the Pacific and perhaps in other oceans. Because the shift from one regime to the other occurred quickly in the 1970s, future shifts could also occur quickly. It is necessary to detect natural shifts in productivity when attempting to manage fishing impacts to ensure that economic expectations are sound and that overfishing does not occur.
Benson, L., B. Linsley, J. Smoot, S. Mensing, S. Lund, S. Stine, A. Sarna-Wojcicki (2003). Influence of the Pacific Decadal Oscillation on the climate of the Sierra Nevada, California and Nevada. Quaternary Research 59 (2): 151-159
ABSTRACT: Mono Lake sediments have recorded five major oscillations in the hydrologic balance between A.D. 1700 and 1941. These oscillations can be correlated with tree-ring-based oscillations in Sierra Nevada snowpack. Comparison of a tree-ring-based reconstruction of the Pacific Decadal Oscillation (PDO) index  with a coral-based reconstruction of Subtropical South Pacific sea-surface temperature  indicates a high degree of correlation between the two records during the past 300 yr. This suggests that the PDO has been a pan-Pacific phenomena for at least the past few hundred years. Major oscillations in the hydrologic balance of the Sierra Nevada correspond to changes in the sign of the PDO with extreme droughts occurring during PDO maxima. Four droughts centered on A.D. 1710, 1770, 1850, and 1930 indicate PDO-related drought reoccurrence intervals ranging from 60 to 80 yr.
ABSTRACT: Climate in the North Pacific and North American sectors has experienced interdecadal shifts during the 20th century. A network of recently developed tree-ring chronologies for Southern and Baja California extends the instrumental record, and reveals decadal-scale variability back to AD 1661. The Pacific Decadal Oscillation (PDO) is closely matched by the dominant mode of tree-ring variability, which provides a preliminary view of multi-annual climate fluctuations spanning the past four centuries. The reconstructed PDO index features a prominent bidecadal oscillation, whose amplitude weakened in the late 1700s to mid-1800s. A comparison with proxy records of ENSO suggests that the greatest decadal-scale oscillations in Pacific climate between 1706 and 1977 occurred around 1750, 1905, and 1947.
ABSTRACT: Prominent and persistent anomalies in the atmospheric flow (troughs and ridges) occur sporadically over the central North Pacific, and can have profound consequences for the weather of North America. We have examined how these events are associated with large scale central North Pacific sea surface temperature (SST) anomalies, using an index for the Pacific Decadal Oscillation (PDO). The anomalies in turbulent air-sea heat fluxes and low-level baroclinity associated with the PDO are manifested differently during troughs than during ridges in their effects on the transient eddies (storms). These effects may help explain why prominent troughs (ridges) occur about 3 (2.5) times more frequently during periods when the PDO is significantly positive (negative) than of opposite sign. Our results suggest that the state of the mid-latitude Pacific Ocean more fundamentally affects the atmosphere than has been thought.
ABSTRACT: Streamflow characteristics in the Yukon River Basin of Alaska and Canada have changed from 1944 to 2005, and some of the change can be attributed to the two most recent modes of the Pacific Decadal Oscillation (PDO). Seasonal, monthly, and annual stream discharge data from 21 stations in the Yukon River Basin were analyzed for trends over the entire period of record, generally spanning 4–6 decades, and examined for differences between the two most recent modes of the PDO: cold-PDO (1944–1975) and warm-PDO (1976–2005) subsets. Between 1944 and 2005, average winter and April flow increased at 15 sites. Observed winter flow increases during the cold-PDO phase were generally limited to sites in the Upper Yukon River Basin. Positive trends in winter flow during the warm-PDO phase broadened to include stations in the Middle and Lower Yukon River drainage basins. Increases in winter streamflow most likely result from groundwater input enhanced by permafrost thawing that promotes infiltration and deeper subsurface flow paths. Increased April flow may be attributed to a combination of greater baseflow (from groundwater increases), earlier spring snowmelt and runoff, and increased winter precipitation, depending on location. Calculated deviations from long-term mean monthly discharges indicate below-average flow in the winter months during the cold PDO and above-average flow in the winter months during the warm PDO. Although not as strong a signal, results also support the reverse response during the summer months: above-average flow during the cold PDO and below-average flow during the warm PDO. Changes in the summer flows are likely an indirect consequence of the PDO, resulting from earlier spring snowmelt runoff and also perhaps increased summer infiltration and storage in a deeper active layer.
Annual discharge has remained relatively unchanged in the Yukon River Basin, but a few glacier-fed rivers demonstrate positive trends, which can be attributed to enhanced glacier melting. A positive trend in annual flow during the warm PDO near the mouth of the Yukon River suggests that small increases in flow throughout the Yukon River Basin have resulted in an additive effect manifested in the downstream-most streamflow station.
Many of the identified changes in streamflow patterns in the Yukon River Basin show a correlation to the PDO regime shift. This work highlights the importance of considering proximate climate forcings as well as global climate change when assessing hydrologic changes in the Arctic.
ABSTRACT: An analysis of ocean surface temperature records show that low frequency changes of tropical Pacific temperature lead global surface air temperature changes by about 4 years. Anomalies of tropical Pacific surface temperature are in turn preceded by subsurface temperature anomalies in the southern tropical Pacific by approximately 7 years. The results suggest that much of the decade to decade variations in global air temperature may be attributed to tropical Pacific decadal variability. The results also suggest that subsurface temperature anomalies in the southern tropical Pacific can be used as a predictor for decadal variations of global surface air temperature. Since the southern tropical Pacific temperature shows a distinct cooling over the last 8 years, the possibility exists that the warming trend in global surface air temperature observed since the late 1970's may soon weaken.
ABSTRACT: Analysis of this century's sea surface temperatures over the Pacific Ocean reveals an interdecadal oscillation with a period of 15-20 years. Our results show that the well-known 1976-77 climate regime shift is not unique, but represents one of several phase transitions associated with this interdecadal oscillation, also found around 1924-25, 1941-42, and 1957-58. The oscillations's striking north-south symmetry across the equator implies strong interactions between tropics and extratropics. A mode with a period of approximately 70 years and an apparently different spatial pattern is also identified tentatively but has to be evaluated further using longer time series.
ABSTRACT: March-August sea surface temperatures (SST) are reconstructed for the Gulf of Alaska (GOA) from 1750-1983 based on tree-ring data from coastal and south-central Alaska and the Pacific Northwest. Some of the trends resemble those documented in other northern instrumental and proxy records, including cooler SSTs in the early and middle 1800s, during the Little Ice Age. There is overall warming in this century, including a positive trend from the mid-1970s to 1980s, following cooler 1960s-1970s. The twentieth century warming exceeds maxima in the reconstructed SSTs back to AD 1750 and is consistent with other evidence for unusual Northern Hemisphere warming. Changes over the period of recorded North Pacific SST have been linked to a pattern of variability known as the Pacific Decadal Oscillation (PDO). Maps comparing the reconstruction to the North Pacific SST field and other analyses suggest that it may reflect variations related to the PDO over several centuries.
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: An integrated program of ecosystem modeling and field studies in the mountains of the Pacific Northwest (U.S.A.) has quantified many of the ecological processes affected by climatic variability. Paleoecological and contemporary ecological data in forest ecosystems provided model parameterization and validation at broad spatial and temporal scales for tree growth, tree regeneration and treeline movement. For subalpine tree species, winter precipitation has a strong negative correlation with growth; this relationship is stronger at higher elevations and west-side sites (which have more precipitation). Temperature affects tree growth at some locations with respect to length of growing season (spring) and severity of drought at drier sites (summer). Furthermore, variable but predictable climate-growth relationships across elevation gradients suggest that tree species respond differently to climate at different locations, making a uniform response of these species to future climatic change unlikely. Multi-decadal variability in climate also affects ecosystem processes. Mountain hemlock growth at high-elevation sites is negatively correlated with winter snow depth and positively correlated with the winter Pacific Decadal Oscillation (PDO) index. At low elevations, the reverse is true. Glacier mass balance and fire severity are also linked to PDO. Rapid establishment of trees in subalpine ecosystems during this century is increasing forest cover and reducing meadow cover at many subalpine locations in the western U.S.A. and precipitation (snow depth) is a critical variable regulating conifer expansion. Lastly, modeling potential future ecosystem conditions suggests that increased climatic variability will result in increasing forest fire size and frequency, and reduced net primary productivity in drier, east-side forest ecosystems. As additional empirical data and modeling output become available, we will improve our ability to predict the effects of climatic change across a broad range of climates and mountain ecosystems in the northwestern U.S.A.
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: A major reorganization of the North-east Pacific biota transpired following a climatic `regime shift' in the mid 1970s. In this paper, we characterize the effects of interdecadal climate forcing on the oceanic ecosystems of the NE Pacific Ocean. We consider the concept of scale in terms of both time and space within the North Pacific ecosystem and develop a conceptual model to illustrate how climate variability is linked to ecosystem change. Next we describe a number of recent studies relating climate to marine ecosystem dynamics in the NE Pacific Ocean. These studies have focused on most major components of marine ecosystems – primary and secondary producers, forage species, and several levels of predators. They have been undertaken at different time and space scales. However, taken together, they reveal a more coherent picture of how decadal-scale climate forcing may affect the large oceanic ecosystems of the NE Pacific. Finally, we synthesize the insight gained from interpreting these studies. Several general conclusions can be drawn.
1 There are large-scale, low-frequency, and sometimes very rapid changes in the distribution of atmospheric pressure over the North Pacific which are, in turn, reflected in ocean properties and circulation.
2 Oceanic ecosystems respond on similar time and space scales to variations in physical conditions.
3 Linkages between the atmosphere/ocean physics and biological responses are often different across time and space scales.
4 While the cases presented here demonstrate oceanic ecosystem response to climate forcing, they provide only hints of the mechanisms of interaction.
5 A model whereby ecosystem response to specified climate variation can be successfully predicted will be difficult to achieve because of scale mismatches and nonlinearities in the atmosphere–ocean–biosphere system.
ABSTRACT: Annual growth increments from trees and coral heads provide an opportunity to develop proxy records of North Pacific climatic variability that extend back in time well beyond the earliest instrumental records, and in regions where records have not been kept. Here we combine five published proxy records of North Pacific climatic variability in order to identify the extent to which these records provide a coherent picture of Pacific Basin climatic variability. This composite chronology is well correlated with the Pacific Decadal Oscillation (PDO) index, and provides a better record of PDO variability than any of the constituent chronologies back to 1840. A comparison of these records suggests that the PDO may not have been an important organizing structure in the North Pacific climate system over much of the 19th century, possibly indicating changes in the spatial pattern of sea-level pressure and consequent surface climate patterns of variability over the Americas.
ABSTRACT: An abrupt change in the large-scale boreal winter circulation pattern over the North Pacific was observed during the mid-1970s. Most notably, this change was marked by a southward shift and intensification of the Aleutian Low and prevailing westerlies over the mid-latitude central and eastern Pacific. Associated changes in diverse North Pacific climatological, hydrological, and biological variables have been noted by numerous researchers. Intriguingly, the timing of these changes in the extra-tropical circulation was coincident with a shift in the background state of the coupled ocean-atmosphere system over the tropical Pacific. These changes include increases in SST over broad regions of the central and eastern tropical Pacific and an eastward displacement of the region of persistent convection in the western Pacific. This paper presents a variety of observed data and model results to describe the climate shift, and to understand some of the links within the coupled climate system that produced it. Five main findings are emphasized: (1) evidence of abrupt, simultaneous, and apparently related changes can be found in many fields and in many model results; the climate shift is not an artifact, (2) over the tropical Pacific the climate change represents a shift in the state of the coupled ocean-atmosphere system, some aspects of which resemble features associated with El Niño episodes. However, the shift in state is not well characterized as due to a change in the frequency of intensity of El Niño episodes; it is better described as a change in background mean state, (3) when forced with observed SSTs, both a very simple atmospheric model and a full general circulation model (GCM) qualitatively simulate aspects of the decadal-scale shift over the tropical Pacific, (4) when forced with observed surface wind stress, two ocean models of the tropical Pacific, in which surface heat fluxes are parameterized as Newtonian damping, reproduce some aspects of the near-equatorial decadal SST signal. However, the models do not reproduce the large changes in SST observed at higher latitudes of the tropical Pacific, suggesting that altered surface heat fluxes dominated in producing these changes, and (5) an important new finding of this study is the success of a GCM in reproducing important aspects of the observed mid-1970s shift in winter northern hemisphere circulation. Comparative analyses of the observed and GCM simulated circulation suggest the altered patterns of tropical Pacific SST and convection were important in forcing the changes in the mid-latitude circulation, a finding corroborated by recent GCM experiments.
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.
Hamlet, A. F., P. W. Mote, M. Clark, D. P. Lettenmaier (2005). Effects of temperature and precipitation variability on snowpack trends in the western United States. Journal of Climate 18 (21): 4545-4561
ABSTRACT: Recent studies have shown substantial declines in snow water equivalent (SWE) over much of the western US in the last half century, as well as trends towards earlier spring snowmelt and peak spring streamflows. These trends are influenced both by interannual and decadal scale climate variability, and also by temperature trends at longer time scales that are generally consistent with observations of global warming over the 20th century.
In this study we examine linear trends in April 1 snow water equivalent (SWE) over the western US as simulated by the Variable Infiltration Capacity hydrologic model implemented at 1/8 degree latitude-longitude spatial resolution, and driven by a carefully quality controlled gridded daily precipitation and temperature data set for the period 1915-2003. The long simulations of snowpack are used as surrogates for observations, and are the basis for an analysis of regional trends in snowpack over the western U.S. and southern British Columbia.
By isolating the trends due to temperature and precipitation in separate simulations, the influence of temperature and precipitation variability on the overall trends in SWE is evaluated. Downward trends in April 1 SWE over the western U.S. from 1916 to 2003, 1947-2003, and for a time series constructed using two warm Pacific Decadal Oscillation (PDO) epochs concatenated together, are shown to be primarily due to widespread warming. These temperature-related trends are not well explained by decadal climate variability associated with the PDO. Trends in SWE associated with precipitation trends, however, are very different in different time periods and are apparently largely controlled by decadal variability rather than longer term trends in climate.
Hanson, R.T., Newhouse, M.W., Dettinger, M.D. (2004). A methodology to assess relations between climatic variability and variations in hydrologic time series in the southwestern United States. Journal of Hydrology 287 (1-4): 252-269
ABSTRACT: A new method for frequency analysis of hydrologic time series was developed to facilitate the estimation and reconstruction of individual or groups of frequencies from hydrologic time-series and facilitate the comparison of these isolated time-series components across data types, between different hydrologic settings within a watershed, between watersheds, and across frequencies. While climate-related variations in inflow to and outflow from aquifers have often been neglected, the development and management of ground-water and surface-water resources has required the inclusion of the assessment of the effects of climatic variability on the supply and demand and sustainability of use. The regional assessment of climatic variability of surface-water and ground-water flow throughout the southwestern United States required this new systematic method of hydrologic time-series analysis.
To demonstrate the application of this new method, six hydrologic time-series from the Mojave River Basin, California were analyzed. The results indicate that climatic variability exists in all the data types and are partially coincident with known climate cycles such as the Pacific Decadal Oscillation and the El Nino–Southern Oscillation. The time-series also indicate lagged correlations between tree-ring indices, streamflow, stream base flow, and ground-water levels. These correlations and reconstructed time-series can be used to better understand the relation of hydrologic response to climatic forcings and to facilitate the simulation of streamflow and ground-water recharge for a more realistic approach to water-resource management.
ABSTRACT: Principal component analysis reveals that Pacific salmon catches in Alaska have varied inversely with catches from the U.S. West Coast during the past 70 years. If variations in catch reflect variations in salmon production, then results of our analysis suggest that the spatial and temporal characteristics of this “inverse” catch/production pattern are related to climate forcing associated with the Pacific Decadal Oscillation, a recurring pattern of pan-Pacific atmosphere-ocean variability. Temporally, both the physical and biological variability are best characterized as alternating 20-to 30-year-long regimes punctuated by abrupt reversals. From 1977 to the early 1990s, ocean conditions have generally favored Alaska stocks and disfavored West Coast stocks. Unfavorable ocean conditions are likely confounding recent management efforts focused on increasing West Coast Pacific salmon production. Recovery of at-risk (threatened and endangered) stocks may await the next reversal of the Pacific Decadal Oscillation. Managers should continue to limit harvests, improve hatchery practices, and restore freshwater and estuarine habitats to protect these populations during periods of poor ocean productivity.
ABSTRACT: Ecological theory asserts that the climate of a region exerts top-down controls on regional ecosystem patterns and processes, across space and time. To provide empirical evidence of climatic controls, it would be helpful to define climatic regions that minimized variance in key climate attributes, within climatic regions—define the periods and features of climatic regimes, and then look for concordance between regional climate and ecosystem patterns or processes. In the past, these steps have not been emphasized. Before we evaluated the recent climate of the northwestern United States, we established a Northwest climatic region by clustering time series of the Palmer Drought Severity Index (PDSI) for the period of 1675–1978, for the western United States. The background climatic regime and anomalies of the recent northwestern U.S. climate were then identified through temporal pattern analysis involving application of correspondence analysis to the same PDSI time series.
Our analysis distinguished 10 distinct periods and four unique types of regimes (climatic signals). Five of the 10 periods (79% of the 300-year record) were marked by mild and equitable moisture conditions (Pacific regime), the “background” climate of the Northwest. The remaining periods were anomalies. Two periods displayed a high-variance, mixed signal marked by switching between severe to extreme annual to interannual dry and wet episodes (High/Mixed regime; 9% of the record). Two more periods displayed a moderate-variance, mixed signal marked by switching between moderate to severe annual to interannual dry and wet episodes (Moderate/Mixed regime; 5%). Only one period was unidirectional and relatively low variance, marked by persistent yet mild to moderate drought (Low/Dry regime, 7%).
Our method distinguished decadal- to interdecadal-scale regimes, defined regime periods, and detected both mixed and unidirectional anomalies from the background climate. The ability to distinguish the variance, direction, and period of sequential climatic regimes provides a plausible basis for examining the role of past climate within terrestrial ecosystems of the Northwest. For example, we found concordance between the period of the Low/Dry anomaly and a period of tree establishment in the Olympic Mountains of Washington, close alignment between tree growth with the Moderate/Mixed and High/Mixed signals in Oregon, and a mixed fire response to mixed climatic signals in northeastern Oregon. Linking historical climatic regimes to particular ecosystem patterns and processes also aids in the prediction of future ecosystem changes by providing evidence of the kinds of interactions that may be anticipated.
ABSTRACT: Historical variability of fire regimes must be understood within the context of climatic and human drivers of disturbance occurring at multiple temporal scales. We describe the relationship between fire occurrence and interannual to decadal climatic variability (Palmer Drought Severity Index [PDSI], El Nino/Southern Oscillation [ENSO], and the Pacific Decadal Oscillation [PDO]) and explain how land use changes in the 20th century affected these relationships.
We used 1701 fire-scarred trees collected in five study sites in central and eastern Washington State (USA) to investigate current year, lagged, and low frequency relationships between composite fire histories and PDSI, PDO, and ENSO (using the Southern Oscillation Index [SOI] as a measure of ENSO variability) using superposed epoch analysis and cross-spectral analysis. Fires tended to occur during dry summers and during the positive phase of the PDO. Cross-spectral analysis indicates that percentage of trees scarred by fire and the PDO are spectrally coherent at 47 years, the approximate cycle of the PDO. Similarly, percentage scarred and ENSO are spectrally coherent at six years, the approximate cycle of ENSO. However, other results suggest that ENSO was only a weak driver of fire occurrence in the past three centuries.
While drought and fire appear to be tightly linked between 1700 and 1900, the relationship between drought and fire occurrence was disrupted during the 20th century as a result of land use changes. We suggest that long-term fire planning using the PDO may be possible in the Pacific Northwest, potentially allowing decadal-scale management of fire regimes, prescribed fire, and vegetation dynamics.
ABSTRACT: Low-frequency (periodicities lower than 20 years) hydrologic variability in the western United States over the past 500 years is studied using available tree-ring reconstructions of Palmer Drought Severity Index (PDSI), streamflow, and climate indices. Leading rotated principal component (RPC) scores of a gridded tree-ring reconstruction of the PDSI from 1525 to 1975 are significantly correlated with indices representing large-scale climate variations from the Pacific and Atlantic Oceans. RPC1 (31%) is related to the influence of North Pacific sea surface temperature (SST) variations, indexed by the Pacific Decadal Oscillation (PDO). RPC2 (24%) is apparently related to North Atlantic SST variations, indexed by the Atlantic Multidecadal Oscillation (AMO). RPC3 (19%) is moderately correlated with a smoothed version of the Southern Oscillation Index. Consistent with recent studies of instrumental data, RPC1 (PDO) and RPC2 (AMO) explain a large part of the multidecadal hydrologic variability of the interior western United States. Western U.S. PDSI variability exhibits significant pentadecadal (and longer) oscillations in the epochs from circa 1525 to 1650 and 1850 to 1975, while bidecadal oscillations are prevalent in the middle epoch from circa 1650 to 1850. The changes in spectral characteristics of western U.S. PDSI were related to similar changes in the PDO (and therefore in RPC1). In contrast, RPC2 had a regular periodicity of 51 years for the past ~500 years. This regularity is intriguing, and although RPC2 was primarily related to the AMO in this study, the influence from Pacific climate cannot be discarded.
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: The magnitude and timing of spring snowmelt floods reflects seasonal snow accumulation and spring temperature patterns. Consequently, interannual variations in regions such as the intermountain West, with snowmelt annual maximum floods, may be related to low-frequency variations in winter/spring large-scale climate variability. Changes in the seasonality of basin precipitation and temperature consequent to slow changes in the baseline climate state (e.g., owing to natural climate variations and/or potential global warming trends) may have significant impacts on such floods. A case study of the Blacksmith Fork River in northern Utah that explores such a hypothesis is presented here. Trends and associations in the magnitude and timing of annual maximum floods are documented, their impact on time-varying estimates of the 100 year flood is assessed, and relationships with known large-scale, quasi-oscillatory patterns of climate variability are explored. Evidence for structured low-frequency variation in flood timing and magnitude and its relation to winter/spring precipitation and temperature and to tropical (El Niño-Southern Oscillation) and extratropical (Pacific Decadal Oscillation) Pacific climate precursors is presented. Mechanisms for these ocean-atmosphere teleconnections to basin precipitation, temperature, and flood potential are discussed.
ABSTRACT: Hydrographic changes in the Northwest Pacific Ocean are examined using data for two time periods: 1945: 1975 and 1976:1998. The largest changes in T/S (2°C,0.2 psu) are within the interfrontal zone between the Kuroshio Extension (KE) and the subarctic (or Oyashio) front near the Shatsky Rise, and are consistent with a southward shift of the Kuroshio Bifurcation Front (KBF). Other major changes seen are a freshening of ~0.04 psu of the newly formed North Pacific Intermediate Water (NPIW); a shoaling over time of the halocline in the center of the Western Subarctic Gyre (WSAG); and a southward shift of the subarctic front between the dateline and 150°W. Because long-term T/S changes near the Shatsky Rise are well-correlated, we have examined subsurface thermal data at 100 and 200-m depth in the region and found that shifts in front locations show a high degree of correlation with the PDO index on an interannual basis, and lagging it by ~1 year at 200-m depth.
ABSTRACT: The cause of decadal climate variability over the North Pacific Ocean and North America is investigated by the analysis of data from a multidecadal integration with a state-of-the-art coupled ocean-atmosphere model and observations. About one-third of the low-frequency climate variability in the region of interest can be attributed to a cycle involving unstable air-sea interactions between the subtropical gyre circulation in the North Pacific and the Aleutian low-pressure system. The existence of this cycle provides a basis for long-range climate forecasting over the western United States at decadal time scales.
ABSTRACT: The dynamics and predictability of decadal climate variability over the North Pacific and North America are investigated by analyzing various observational datasets and the output of a state of the art coupled ocean–atmosphere general circulation model that was integrated for 125 years. Both the observations and model results support the picture that the decadal variability in the region of interest is based on a cycle involving unstable ocean–atmosphere interactions over the North Pacific. The period of this cycle is of the order of a few decades.
The cycle involves the two major circulation regimes in the North Pacific climate system, the subtropical ocean gyre, and the Aleutian low. When, for instance, the subtropical ocean gyre is anomalously strong, more warm tropical waters are transported poleward by the Kuroshio and its extension, leading to a positive SST anomaly in the North Pacific. The atmospheric response to this SST anomaly involves a weakened Aleutian low, and the associated fluxes at the air–sea interface reinforce the initial SST anomaly, so that ocean and atmosphere act as a positive feedback system. The anomalous heat flux, reduced ocean mixing in response to a weakened storm track, and anomalous Ekman heat transport contribute to this positive feedback.
The atmospheric response, however, consists also of a wind stress curl anomaly that spins down the subtropical ocean gyre, thereby reducing the poleward heat transport and the initial SST anomaly. The ocean adjusts with some time lag to the change in the wind stress curl, and it is this transient ocean response that allows continuous oscillations. The transient response can be expressed in terms of baroclinic planetary waves, and the decadal timescale of the oscillation is therefore determined to first order by wave timescales. Advection by the mean currents, however, is not negligible.
The existence of such a cycle provides the basis of long-range climate forecasting over North America at decadal timescales. At a minimum, knowledge of the present phase of the decadal mode should allow a “now-cast” of expected climate “bias” over North America, which is equivalent to a climate forecast several years ahead.
ABSTRACT: Hydrologically sensitive tree-ring chronologies fromPinus flexilis in California and Alberta were used to produce an AD 993–1996 reconstruction of the Pacific Decadal Oscillation (PDO) and to assess long-term variability in the PDO's strength and periodicity. The reconstruction indicates that a ~50 to 70 year periodicity in the PDO is typical for the past 200 years but, was only intermittently a strong mode of variability prior to that. Between AD 1600 and 1800 there is a general absence of significant variability within the 50 to 100 year frequency range. Significant variability within in the frequency range of 50 to 100 years reemerges between AD 1500 and 1300 and AD 1200 to 1000. A prolonged period of strongly negative PDO values between AD 993 and 1300 is contemporaneous with a severe medieval megadrought that is apparent in many proxy hydrologic records for the western United States and Canada.
ABSTRACT: Retrospective analyses of Pacific Basin climate records highlight the existence of a pan-Pacific interdecadal climate oscillation. We find strong evidence for coherent patterns of interdecadal variability in Pacific winds, sea level pressures, and upper ocean temperatures. Collectively, the ocean-atmosphere pattern of variability has been labeled the “Pacific Decadal Oscillation”, or PDO. An index for the PDO has been developed from an empirical orthogonal function (EOF) analysis of north Pacific SST records dating back to 1900.
An analysis of Pacific coast salmon catch records suggests that the dominant pattern of salmon production is driven by low-frequency climate variations associated with the PDO. The characteristics of this salmon production pattern of variability include a preferentially interdecadal time scale of variation that is coherent with our PDO index, and a north-south inverse production pattern in which Alaska stocks tend to be productive while those in the Pacific northwest are relatively unproductive (and vice versa).
ABSTRACT: The Pacific Decadal Oscillation (PDO) has been described by some as a long-lived El Niño-like pattern of Pacific climate variability, and by others as a blend of two sometimes independent modes having distinct spatial and temporal characteristics of North Pacific sea surface temperature (SST) variability. A growing body of evidence highlights a strong tendency for PDO impacts in the Southern Hemisphere, with important surface climate anomalies over the mid-latitude South Pacific Ocean, Australia and South America. Several independent studies find evidence for just two full PDO cycles in the past century: “cool” PDO regimes prevailed from 1890–1924 and again from 1947–1976, while “warm” PDO regimes dominated from 1925–1946 and from 1977 through (at least) the mid-1990's. Interdecadal changes in Pacific climate have widespread impacts on natural systems, including water resources in the Americas and many marine fisheries in the North Pacific. Tree-ring and Pacific coral based climate reconstructions suggest that PDO variations—at a range of varying time scales—can be traced back to at least 1600, although there are important differences between different proxy reconstructions. While 20th Century PDO fluctuations were most energetic in two general periodicities—one from 15-to-25 years, and the other from 50-to-70 years—the mechanisms causing PDO variability remain unclear. To date, there is little in the way of observational evidence to support a mid-latitude coupled air-sea interaction for PDO, though there are several well-understood mechanisms that promote multi-year persistence in North Pacific upper ocean temperature anomalies.
Mantua, N.J., S.R. Hare, Y. Zhang, J.M. Wallace, R.C. Francis (1997). A Pacific decadal climate oscillation with impacts on salmon production. Bulletin of the American Meteorological Society 78 (6): 1069-1079
ABSTRACT: Evidence gleaned from the instrumental record of climate data identifies a robust, recurring pattern of ocean-atmosphere climate variability centered over the mid-latitude Pacific basin. Over the past century, the amplitude of this climate pattern has varied irregularly at interannual-to-interdecadal time scales. There is evidence of reversals in the prevailing polarity of the oscillation occurring around 1925, 1947, and 1977; the last two reversals correspond with dramatic shifts in salmon production regimes in the North Pacific Ocean. This climate pattern also affects coastal sea and continental surface air temperatures, as well as streamflow in major west coast river systems, from Alaska to California.
G. J. McCabe, M. A. Palecki, J. L. Betancourt (2004). Pacific and Atlantic Ocean influences on multidecadal drought frequency in the United States. Proceedings of the National Academy of Sciences 101 (12): 4136-4141
ABSTRACT: More than half (52%) of the spatial and temporal variance in multidecadal drought frequency over the conterminous United States is attributable to the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO). An additional 22% of the variance in drought frequency is related to a complex spatial pattern of positive and negative trends in drought occurrence possibly related to increasing Northern Hemisphere temperatures or some other unidirectional climate trend. Recent droughts with broad impacts over the conterminous U.S. (1996, 1999–2002) were associated with North Atlantic warming (positive AMO) and northeastern and tropical Pacific cooling (negative PDO). Much of the long-term predictability of drought frequency may reside in the multidecadal behavior of the North Atlantic Ocean. Should the current positive AMO (warm North Atlantic) conditions persist into the upcoming decade, we suggest two possible drought scenarios that resemble the continental-scale patterns of the 1930s (positive PDO) and 1950s (negative PDO) drought.
ABSTRACT: The chronology of interdecadal climatic regime shifts is examined, using instrumental data over the North Pacific, North America and the tropical oceans, and reconstructed climate records for North America. In the North Pacific and North America, climatic regime shifts around 1890 and in the 1920's with alternating polarities are detected, whose spatial structure is similar to that of the previously-known climatic shifts observed in the 1940's and 1970's. Sea-surface temperatures in the tropical Indian Ocean-maritime continent region exhibit changes corresponding to these four shifts. Spectra obtained by the Multi-Taper-Method suggest that these regime shifts are associated with 50-70 year climatic variability over the North Pacific and North America.
The leading mode of the empirical orthogonal functions of the air-temperature reconstructed from tree-rings in North America exhibits a spatial distribution that is reminiscent of instrumentally observed air-temperature differences associated with the regime shifts. The temporal evolution of this mode is characterized by a 50-70 year oscillation in the eighteenth and nineteenth centuries. This result, combined with the results of the analyses of the instrumental data, indicates that the 50-70 year oscillation is prevalent from the eighteenth century to the present in North America.
ABSTRACT: The roles of interdecadal oscillations in climate regime shifts, which are observed as rapid strength changes in the Aleutian low in winter and spring seasons, have been analyzed. A regime shift results from simultaneous phase reversals between pentadecadal and bidecadal variations, which synchronize with one another at a relative period of three. The pentadecadal variation, which is observed in both winter and spring seasons, provides the basic timescale of regime shifts, while the bidecadal variation, which is observed only in winter, characterizes the rapidity of the shifts. A Monte-Carlo simulation has shown that the simultaneous phase reversals or resonance between the pentadecadal and bidecadal variations reflect a physical linkage between them and do not coincide accidentally. The role of this synchronization feature for assessing and predicting regime shifts is discussed.
ABSTRACT: This study identified and examined differences in Southeast Alaskan streamflow patterns between the two most recent modes of the Pacific decadal oscillation (PDO). Identifying relationships between the PDO and specific regional phenomena is important for understanding climate variability, interpreting historical hydrological variability, and improving water-resources forecasting. Stream discharge data from six watersheds in Southeast Alaska were divided into cold-PDO (1947–1976) and warm-PDO (1977–1998) subsets. For all watersheds, the average annual streamflows during cold-PDO years were not significantly different from warm-PDO years. Monthly and seasonal discharges, however, did differ significantly between the two subsets, with the warm-PDO winter flows being typically higher than the cold-PDO winter flows and the warm-PDO summer flows being typically lower than the cold-PDO flows. These results were consistent with and driven by observed temperature and snowfall patterns for the region. During warm-PDO winters, precipitation fell as rain and ran-off immediately, causing higher than normal winter streamflow. During cold-PDO winters, precipitation was stored as snow and ran off during the summer snowmelt, creating greater summer streamflows. The Mendenhall River was unique in that it experienced higher flows for all seasons during the warm-PDO relative to the cold-PDO. The large amount of Mendenhall River discharge caused by glacial melt during warm-PDO summers offset any flow reduction caused by lack of snow accumulation during warm-PDO winters. The effect of the PDO on Southeast Alaskan watersheds differs from other regions of the Pacific Coast of North America in that monthly/seasonal discharge patterns changed dramatically with the switch in PDO modes but annual discharge did not.
ABSTRACT: The two leading patterns of Pacific decadal sea surface temperature (SST) variability are strongly linked to large-scale patterns of warm-season drought and streamflow in the United States, recent analysis shows. The predictive potential of this link may contribute to the development of warm-season hydroclimate forecasts in the United States. Understanding of low-frequency variations in drought and streamflow would be important for both agriculture and water resources management.
The two leading patterns are what we call the Pacific Decadal Oscillation (PDO) and the North Pacific mode. Their link with drought and streamflow patterns was notably expressed in the 1960s when severe drought in the northeast (the 1962-66 "Northeastern" drought) and exceptional positive SST anomalies in the North Pacific Ocean (Figures 1a, 1b) both occurred. Analysis of upper tropospheric circulation anomalies showed the North Pacific to be a source region of wave activity affecting the drought area in these summers. The anomalous circulation was vertically coherent and opposed the climatological low-level moisture inflow over the eastern United States associated with the western extension of the Bermuda High.
ABSTRACT: Following a strong El Niño, the climate of the North Pacific underwent a rapid and striking transition in late 1998. Upwelling-favorable winds strengthened over the California Current (CC), and winds weakened in the Gulf of Alaska (GOA). Coastal waters of the CC and GOA cooled by several degrees, and the Pacific Decadal Oscillation (PDO) reversed sign and remained negative through summer 2002. Zooplankton biomass in the northern CC doubled and switched from warm to cold water species dominance, coho and chinook salmon stocks rebounded, and anchovy and osmeriids increased. Persistent changes in atmosphere and upper ocean fields and ecosystem structure suggest a climate regime shift has occurred, similar (opposite) to shifts observed in 1947 (1925 and 1976). If the 1998 regime shift in the northern CC is completely analogous to earlier shifts, then ecosystem structure should have changed in the GOA. Recent surveys indicate this ecosystem has transformed as well.
ABSTRACT: We investigate relationships between climate and wildfire activity between 1929 and 2004 in Pacific coast forests of the United States. Self-Organizing Mapping (SOM) of annual area burned in National Forests (NF) in California, Oregon, and Washington identifies three contiguous NF groups and a fourth group of NF traversed by major highways. Large fire years in all groups are dry compared to small fire years. A sub-hemispheric circulation pattern of a strong trough over the North Pacific and a ridge over the West Coast is characteristic of large fire years in all groups. This pattern resembles the Pacific North American (PNA) teleconnection and positive phase of the Pacific Decadal Oscillation (PDO). A reverse PNA and negative PDO phase characterizes small fire years. Despite the effect of fire suppression management between 1929 and 2004, forest area burned is linked to climatic variations related to large-scale atmospheric circulation patterns.
ABSTRACT: A number of recent studies have reported an ENSO-like EOF mode in the global sea surface temperature (SST) field, whose time variability is marked by an abrupt change toward a warmer tropical eastern Pacific and a colder extratropical central North Pacific in 1976–77. The present study compares this pattern with the structure of the interannual variability associated with the ENSO cycle and documents its time history back to 1900. The analysis is primarily based on the leading EOFs of the SST anomaly and “anomaly deviation” fields in various domains and the associated expansion coefficient (or principal component) time series, which are used to construct global regression maps of SST, sea level pressure (SLP), and a number of related variables. The use of “anomaly deviations” (i.e., departures of local SST anomalies from the concurrent global-mean SST anomaly) reduces the influence of global-mean SST trends upon the structure of the EOFs and their expansion coefficient time series. An important auxiliary time series used in this study is a “Southern Oscillation index” based on marine surface observations.
By means of several different analysis techniques, the time variability of the leading EOF of the global SST field is separated into two components: one identified with the “ENSO cycle-related” variability on the interannual timescale, and the other a linearly independent “residual” comprising all the interdecadal variability in the record. The two components exhibit rather similar spatial signatures in the global SST, SLP, and wind stress fields. The SST signature in the residual variability is less equatorially confined in the eastern Pacific and it is relatively more prominent over the extratropical North Pacific. The corresponding SLP signature is also stronger over the extratropical North Pacific, and its counterpart in the cold season 500-mb height field more closely resembles the PNA pattern. The amplitude time series of the ENSO-like pattern in the residual variability reflects the above-mentioned shift in 1976–77, as well as a number of other prominent features, including a shift of opposite polarity during the 1940s.