Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
ABSTRACT: Observational evidence suggests that river inflows to the Arctic Ocean have increased over the last 30 years. Continued increases have the potential to alter the freshwater balance in the Arctic and North Atlantic Oceans and hence the thermohaline circulation. Simulations with a macroscale hydrological model and climate change scenarios derived from six climate models and two emissions scenarios suggest increases of up to 31% in river inflows to the Arctic by the 2080s under high emissions and up to 24% under lower emissions, although there are large differences between climate models. Uncertainty analysis suggests low sensitivity to model form and parameterization but higher sensitivity to the input data used to drive the model. The addition of up to 0.048 sverdrup (Sv, 106 m3 s-1 ) is a large proportion of the 0.06–0.15 Sv of additional freshwater that may trigger thermohaline collapse. Changes in the spatial distribution of inflows to the Arctic Ocean may influence circulation patterns within the ocean.
O. Arzel, T. Fichefet, H. Goosse, J.-L. Dufresne (2008). Causes and impacts of changes in the Arctic freshwater budget during the twentieth and twenty-first centuries in an AOGCM. Climate Dynamics 30 (1): 37-58
ABSTRACT: The fourth version of the atmosphere-ocean general circulation (AOGCM) model developed at the Institut Pierre-Simon Laplace (IPSL-CM4) is used to investigate the mechanisms influencing the Arctic freshwater balance in response to anthropogenic greenhouse gas forcing. The freshwater influence on the interannual variability of deep winter oceanic convection in the Nordic Seas is also studied on the basis of correlation and regression analyses of detrended variables. The model shows that the Fram Strait outflow, which is an important source of freshwater for the northern North Atlantic, experiences a rapid and strong transition from a weak state toward a relatively strong state during 1990–2010. The authors propose that this climate shift is triggered by the retreat of sea ice in the Barents Sea during the late twentieth century. This sea ice reduction initiates a positive feedback in the atmosphere-sea ice-ocean system that alters both the atmospheric and oceanic circulations in the Greenland-Iceland-Norwegian (GIN)-Barents Seas sector. Around year 2080, the model predicts a second transition threshold beyond which the Fram Strait outflow is restored toward its original weak value. The long-term freshening of the GIN Seas is invoked to explain this rapid transition. It is further found that the mechanism of interannual changes in deep mixing differ fundamentally between the twentieth and twenty-first centuries. This difference is caused by the dominant influence of freshwater over the twenty-first century. In the GIN Seas, the interannual changes in the liquid freshwater export out of the Arctic Ocean through Fram Strait combined with the interannual changes in the liquid freshwater import from the North Atlantic are shown to have a major influence in driving the interannual variability of the deep convection during the twenty-first century. South of Iceland, the other region of deep water renewal in the model, changes in freshwater import from the North Atlantic constitute the dominant forcing of deep convection on interannual time scales over the twenty-first century.
ABSTRACT: The purpose of this paper is to provide an overview of paleoceanography and paleoclimatology as a framework for other papers dealing with The Earth System: Geochemical Perspectives. An introduction to both paleoceanography and paleoclimatology is followed by examples of the temporal changes through the Phanerozoic. The important and interactive role of the biosphere is emphasized. Many important changes in the Earth system have affected the coupled ocean–atmosphere system and many, in turn, have been reflected in biotic events. Many such changes can be tracked through time using geochemical signatures as proxy indicators. Whereas the scale of past paleoceanographic and paleoclimatic changes have been generally appreciated for some time, the recognition of periodic rapid change in the ocean/climate state and the ability to study and measure these precisely is only a recent accomplishment. The potential for such rapid change in the ocean/climate/biosphere of the Earth system raises concerns for events in the near future that may be forced by anthropogenic activities that enhance natural variability.
Bond, G., W. Showers, M. Chaseby, R. Lotti, P. Almasi, P. deMinocal, P. Priore, H. Cullen, I. Hajdas, G. Bonani (1997). Pervasive millennial-scale cycle in North Atlantic Holocene and glacial climates. Science 278 (5341): 1257-1266
INTRODUCTION: Evidence from North Atlantic deep sea cores reveals that abrupt shifts punctuated what is conventionally thought to have been a relatively stable Holocene climate. During each of these episodes, cool, ice-bearing waters from north of Iceland were advected as far south as the latitude of Britain. At about the same times, the atmospheric circulation above Greenland changed abruptly. Pacings of the Holocene events and of abrupt climate shifts during the last glaciation are statistically the same; together, they make up a series of climate shifts with a cyclicity close to 1470 ± 500 years. The Holocene events, therefore, appear to be the most recent manifestation of a pervasive millennial-scale climate cycle operating independently of the glacial-interglacial climate state. Amplification of the cycle during the last glaciation may have been linked to the North Atlantic's thermohaline circulation.
FIRST PARAGRAPH: A DIAGRAM DEPICTING the ocean's "conveyor belt" has been widely adopted as a logo for the Global Change Research Initiative. This diagram (Fig. 1) first appeared as an illustration in an article about the Younger Dryas event that was published in the November 1987 issue of Natural History. It was designed as a cartoon to help the largely lay readership of this magazine to comprehend one of the elements of the deep sea's circulation system. Had I suspected that it would be widely adopted as a logo, I would have tried to "improve" its accuracy. In hindsight such repairs would likely have ruined the diagram both for the readers of Natural History and for use as a logo.
ABSTRACT: Multiproxy data from North Atlantic deep-sea sediment core NEAP18K provide a detailed record of climate through oxygen isotope stage (OIS) 5. Seven distinct, large-scale episodes of ice rafting (C25–C19) were identified between 126 and 70 ka. Global ice-volume reconstructions, based on high-resolution benthicd18 O records, indicate that major ice-rafting events were not confined to ice-volume maxima at OIS 5d and 5b, but also occurred during periods of ice-sheet growth and disintegration. However, iceberg discharges were restricted to times when sea levels were 40–65 m below present values. Ice-rafting episode C25, the first large-scale cooling of mid-Atlantic surface waters after the last interglacial, occurred during the gradual buildup of continental ice sheets at the OIS 5e-5d transition. Major ice-sheet collapses allied to ice-rafting events C24 and C21 were associated with rapid sea-level increases of 20 and 40 m, respectively. Suborbital climatic fluctuations in the NEAP18K sedimentary record, denoted by prominent 7.5, 4.5, and 3 k.y. cyclicities, appear to correlate with both Greenland atmospheric temperatures and changes in thermohaline circulation patterns, inferred from benthicd13 C values, and hence provide clear evidence of a highly interconnected North Atlantic climatic regime during OIS 5.
ABSTRACT: The possibility of a reduced Atlantic thermohaline circulation in response to increases in greenhouse-gas concentrations has been demonstrated in a number of simulations with general circulation models of the coupled ocean–atmosphere system. But it remains difficult to assess the likelihood of future changes in the thermohaline circulation, mainly owing to poorly constrained model parameterizations and uncertainties in the response of the climate system to greenhouse warming. Analyses of past abrupt climate changes help to solve these problems. Data and models both suggest that abrupt climate change during the last glaciation originated through changes in the Atlantic thermohaline circulation in response to small changes in the hydrological cycle. Atmospheric and oceanic responses to these changes were then transmitted globally through a number of feedbacks. The palaeoclimate data and the model results also indicate that the stability of the thermohaline circulation depends on the mean climate state.
ABSTRACT: About 8200 years ago, the climate of much of the Northern Hemisphere cooled abruptly for a period of about 200 years. In their Perspective, Clarke et al. examine the most likely culprit for this cooling: an outburst of fresh water from a vast, ice-dammed glacial lake in North America. The superlake had formed when the kilometers-thick ice sheet covering much of North America disintegrated. When the ice dam became unstable, fresh water flooded from the lake into the North Atlantic. It remains unclear how this fresh water affected ocean circulation or whether the outburst occurred in more than one stage, but the timing points strongly to the outburst flood as the trigger of the 8200-year climate event.
ABSTRACT: Paleoclimatic data are increasingly showing that abrupt change is present in wide regions of the globe. Here a mechanism for abrupt climate change with global implications is presented. Results from a tropical coupled ocean–atmosphere model show that, under certain orbital configurations of the past, variability associated with El Niño–Southern Oscillation (ENSO) physics can abruptly lock to the seasonal cycle for several centuries, producing a mean sea surface temperature (SST) change in the tropical Pacific that resembles a La Niña. It is suggested that this change in SST would have a global impact and that abrupt events such as the Younger Dryas may be the outcome of orbitally driven changes in the tropical Pacific.
ABSTRACT: Abrupt changes in climate, termed Dansgaard-Oeschger and Heinrich events, have punctuated the last glacial period (~100-10 kyr ago) but not the Holocene (the past 10 kyr). Here we use an intermediate-complexity climate model to investigate the stability of glacial climate, and we find that only one mode of Atlantic Ocean circulation is stable: a cold mode with deep water formation in the Atlantic Ocean south of Iceland. However, a 'warm' circulation mode similar to the present-day Atlantic Ocean is only marginally unstable, and temporary transitions to this warm mode can easily be triggered. This leads to abrupt warm events in the model which share many characteristics of the observed Dansgaard-Oeschger events. For a large freshwater input (such as a large release of icebergs), the model's deep water formation is temporarily switched off, causing no strong cooling in Greenland but warming in Antarctica, as is observed for Heinrich events. Our stability analysis provides an explanation why glacial climate is much more variable than Holocene climate.
Harshburger, B., H. Ye, J. Dzialoski (2002). Observational evidence of the influence of Pacific SSTs on winter precipitation and spring stream discharge in Idaho. Journal of Hydrology 264 (1-4): 157-169
ABSTRACT: Forty years of winter precipitation (23 stations) and spring stream flow discharge records (five stations) from across Idaho are analyzed to reveal regional patterns of association with sea surface temperatures (SSTs) in the Pacific Ocean. Results indicate that winter precipitation in the northern Idaho mountains, between 45° and 48°N, is negatively correlated with fall SSTs in the eastern tropical Pacific Ocean (El Nino and La Nina). Winter precipitation north of 45°N, is negatively correlated with winter SSTs in the northern Pacific (Pacific Decadal Oscillation, PDO). Spring stream discharge in Idaho is also negatively correlated with SSTs in the eastern tropical and northern regions of the Pacific Ocean.
The association is asymmetric with stronger responses during negative SSTs for both regions in the Pacific Ocean. Wet and dry conditions are most likely associated with the combination of La Nina–negative PDO and El Nino–positive PDO, respectively. The greatest anomalies occur during the optimal combination of La Nina with negative PDO conditions. The revealed connections are valuable for climatic predictions based on the previous season's SST conditions in the eastern tropical Pacific and slowly evolving SSTs in the northern Pacific Ocean.
ABSTRACT: The 1976 Pacific climate shift is examined, and its manifestations and significance in Alaskan climatology during the last half-century are demonstrated. The Pacific Decadal Oscillation index shifted in 1976 from dominantly negative values for the 25-yr time period 1951–75 to dominantly positive values for the period 1977–2001.
Mean annual and seasonal temperatures for the positive phase were up to 3.1°C higher than for the negative phase. Likewise, mean cloudiness, wind speeds, and precipitation amounts increased, while mean sea level pressure and geopotential heights decreased. The pressure decrease resulted in a deepening of the Aleutian low in winter and spring. The intensification of the Aleutian low increased the advection of relatively warm and moist air to Alaska and storminess over the state during winter and spring.
The regime shift is also examined for its effect on the long-term temperature trends throughout the state. The trends that have shown climatic warming are strongly biased by the sudden shift in 1976 from the cooler regime to a warmer regime. When analyzing the total time period from 1951 to 2001, warming is observed; however, the 25-yr period trend analyses before 1976 (1951–75) and thereafter (1977–2001) both display cooling, with a few exceptions. In this paper, emphasis is placed on the importance of taking into account the sudden changes that result from abrupt climatic shifts, persistent regimes, and the possibility of cyclic oscillations, such as the PDO, in the analysis of long-term climate change in Alaska.
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.
L. D. Keigwin, G. A. Jones, S. J. Lehman, E. A. Boyle (1991). Deglacial meltwater discharge, North Atlantic deep circulation, and abrupt climate change. Journal of Geophysical Research 96 (C9): 16,811–16,826
ABSTRACT: High-resolution paleogeochemical data from the North Atlantic Ocean indicate that in the interval 15,000 to 10,00014 C years before present (B.P.) North Atlantic Deep Water (NADW) production was decreased or eliminated four times: at about 14,500 (and probably older), 13,500, 12,000 and 10,500 years B.P. Each of these changes occurred at the same time as abrupt events of meltwater discharge to the surface ocean (inferred from oxygen isotope studies of planktonic foraminifera and from glacial geological studies on land). In addition, each of these times may be associated with brief episodes of cooler climate in the North Atlantic region, the best example of which is the Younger Dryas cooling of 10,500 years ago. These results support models linking meltwater discharge, decreased NADW production, and decreased North Atlantic heat flux.
T. Kitzberger, P. M. Brown, E. K. Heyerdahl, T. W. Swetnam, T. T. Veblen (2007). Contingent Pacific-Atlantic Ocean influence on multicentury wildfire synchrony over western North America. Proceedings of the National Academy of Sciences 104 (2): 543-548
ABSTRACT: Widespread synchronous wildfires driven by climatic variation, such as those that swept western North America during 1996, 2000, and 2002, can result in major environmental and societal impacts. Understanding relationships between continental-scale patterns of drought and modes of sea surface temperatures (SSTs) such as El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) may explain how interannual to multidecadal variability in SSTs drives fire at continental scales. We used local wildfire chronologies reconstructed from fire scars on tree rings across western North America and independent reconstructions of SST developed from tree-ring widths at other sites to examine the relationships of multicentury patterns of climate and fire synchrony. From 33,039 annually resolved fire-scar dates at 238 sites (the largest paleofire record yet assembled), we examined forest fires at regional and subcontinental scales. Since 1550 CE, drought and forest fires covaried across the West, but in a manner contingent on SST modes. During certain phases of ENSO and PDO, fire was synchronous within broad subregions and sometimes asynchronous among those regions. In contrast, fires were most commonly synchronous across the West during warm phases of the AMO. ENSO and PDO were the main drivers of high-frequency variation in fire (interannual to decadal), whereas the AMO conditionally changed the strength and spatial influence of ENSO and PDO on wildfire occurrence at multidecadal scales. A current warming trend in AMO suggests that we may expect an increase in widespread, synchronous fires across the western U.S. in coming decades.
D. J. Leathers, B. Yarnal, M. A. Palecki (1991). The Pacific/North American teleconnection pattern and United States climate. Part I: regional temperature and precipitation associations. Journal of Climate 4 (5): 517-528
ABSTRACT: The Pacific/North American (PNA) teleconnection index, a measure of the strength and phase of the PNA teleconnection pattern, is related to the variations of the surface climate of the United States from 1947 through 1982 for the autumn, winter, and spring months when the PNA is a main mode of Northern Hemisphere midtropospheric variability. The results demonstrate that the PNA index is highly correlated with both regional temperature and precipitation. The strongest, most extensive correlations between the index and temperature are observed in winter, but large areas of the country show important associations during the spring and autumn as well. Although the centers of highest correlation migrate systematically with changes in the circumpolar vortex over the course of the annual cycle, the southeastern and northwestern parts of the United States possess consistently high index-temperature correlations.
Correlations between the PNA index and precipitation are weaker and less extensive than those for temperature, but large coherent regions of high correlations are observed across the nation. Winter and early spring exhibit the strongest relationships because spatially coherent synoptic-scale systems, related to the long-wave pattern, control precipitation. The late spring and early autumn seasons have the least extensive and weakest correlations due to the importance of less organized smaller-scale convective rainfall events.
ABSTRACT: The ocean's thermohaline circulation has long been recognized as potentially unstable and has consequently been invoked as a potential cause of abrupt climate change on all timescales of decades and longer. However, fundamental aspects of thermohaline circulation changes remain poorly understood.
McCabe, G. J., Betancourt, J. L., Hidalgo, H. G. (2007). Associations of decadal to multidecadal sea-surface temperature variability with upper Colorado River flow. Journal of the American Water Resources Association 43 (1): 183-192
ABSTRACT: The relations of decadal to multidecadal (D2M) variability in global sea-surface temperatures (SSTs) with D2M variability in the flow of the Upper Colorado River Basin (UCRB) are examined for the years 1906-2003. Results indicate that D2M variability of SSTs in the North Atlantic, North Pacific, tropical Pacific, and Indian Oceans is associated with D2M variability of the UCRB. A principal components analysis (with varimax rotation) of detrended and 11-year smoothed global SSTs indicates that the two leading rotated principal components (RPCs) explain 56% of the variability in the transformed SST data. The first RPC (RPC1) strongly reflects variability associated with the Atlantic Multidecadal Oscillation and the second RPC (RPC2) represents variability of the Pacific Decadal Oscillation, the tropical Pacific Ocean, and Indian Ocean SSTs. Results indicate that SSTs in the North Atlantic Ocean (RPC1) explain as much of the D2M variability in global SSTs as does the combination of Indian and Pacific Ocean variability (RPC2). These results suggest that SSTs in all of the oceans have some relation with flow of the UCRB, but the North Atlantic may have the strongest and most consistent association on D2M time scales. Hydroclimatic persistence on these time scales introduces significant nonstationarity in mean annual streamflow, with critical implications for UCRB water resource management.
ABSTRACT: Changing patterns of correlations between the historical average June-November Southern Oscillation Index (SOI) and October-March precipitation totals for 84 climate divisions in the western US indicate a large amount of variability in SOI/precipitation relations on decadal time scales. Correlations of western US precipitation with SOI and other indices of tropical El Niño-Southern Oscillation (ENSO) processes were much weaker from 1920 to 1950 than during recent decades. This variability in teleconnections is associated with the character of tropical air-sea interactions as indexed by the number of out-of-phase SOI/tropical sea surface temperature (SST) episodes, and with decadal variability in the North Pacific Ocean as indexed by the Pacific Decadal Oscillation (PDO). ENSO teleconnections with precipitation in the western US are strong when SOI and NINO3 are out-of-phase and PDO is negative. ENSO teleconnections are weak when SOI and NINO3 are weakly correlated and PDO is positive. Decadal modes of tropical and North Pacific Ocean climate variability are important indicators of periods when ENSO indices, like SOI, can be used as reliable predictors of winter precipitation in the US.
McCabe, G. J., M. D. Dettinger (2002). Primary modes and predictability of year-to-year snowpack variations in the western United States from teleconnections with Pacific Ocean climate. Journal of Hydrometeorology 3 (1): 13-25
ABSTRACT: Snowpack, as measured on 1 April, is the primary source of warm-season streamflow for most of the western United States and thus represents an important source of water supply. An understanding of climate factors that influence the variability of this water supply and thus its predictability is important for water resource management. In this study, principal component analysis is used to identify the primary modes of 1 April snowpack variability in the western United States. Two components account for 61% of the total snowpack variability in the western United States. Relations between these modes of variability and indices of Pacific Ocean climate [e.g., the Pacific decadal oscillation (PDO) and Niño-3 sea surface temperatures (SSTs)] are examined. The first mode of snowpack variability is closely associated with the PDO, whereas the second mode varies in concert with both the PDO and Niño-3 SSTs. Because these atmospheric–oceanic conditions change slowly from season to season, the observed teleconnections between the Pacific Ocean climate and 1 April snowpack may be useful to forecast 1 April snowpack using data that describe the Pacific Ocean climate in the previous summer and autumn seasons, especially for the northwestern United States.
ABSTRACT: Interannual and interdecadal variability in the summertime mean North Pacific storm track is examined in relation to summertime mean sea surface temperature (SST), nimbostratus, and marine stratiform cloudiness (MSC) (stratus, stratocumulus, fog). The storm track is diagnosed by root-mean-squared daily vertical velocity at 500 mb during the summer season (rmsω) obtained from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis. The cloud and SST data are obtained from surface observations. Year-to-year variations in the storm track exhibit significant coupling to variations in cloudiness and SST across the North Pacific. These correspond to coincident latitudinal shifts in the storm track, SST gradient, and MSC gradient. Moreover, both rmsω and nimbostratus show that the storm track moved equatorward and intensified between 1952 and 1995, consistent with the previously documented upward trend in MSC and downward trend in SST. Lead–lag relationships suggest variability in the storm track has a large role in forcing variability in SST. Boundary layer cloudiness responds to and adds a positive feedback to variability in SST.
Weak relationships are observed with the summertime mean large-scale circulation, as diagnosed by sea level pressure. This suggests summertime North Pacific atmosphere–ocean interaction is dominated by local processes operating in the storm track and over the SST gradient, unlike the situation during winter.
ABSTRACT: Oceans cover more than two-thirds of our blue planet. The waters move in a global circulation system, driven by subtle density differences and transporting huge amounts of heat. Ocean circulation is thus an active and highly nonlinear player in the global climate game. Increasingly clear evidence implicates ocean circulation in abrupt and dramatic climate shifts, such as sudden temperature changes in Greenland on the order of 5–10 °C and massive surges of icebergs into the North Atlantic Ocean — events that have occurred repeatedly during the last glacial cycle.
Redmond, K. T., Koch, R. W. (1991). Surface climate and streamflow variability in the western United States and their relationship to large-scale circulation indices. Water Resources Research 27 (9): 2381-2399
ABSTRACT: A statistical analysis was undertaken to determine the nature and magnitude of the relationship of precipitation, temperature and streamflow in the western United States to large-scale atmospheric circulation patterns. The Southern Oscillation Index (SOI) was used as an indicator of the El Niño/Southern Oscillation (ENSO) and the PNA index as an indicator of the Pacific//North America pattern. These indices were correlated with surface climate data and split sample analyses were conducted to determine climate response during the extreme phases of each index. October-March precipitation was shown to be most strongly correlated with SOI averaged over the July-November period. The analysis showed that there are two centers of opposite association with the SOI. During low values of the SOI (ENSO events) precipitation is low in the Pacific northwest and high in the desert southwest. Correlations between SOI and temperature were greatest in the Pacific Northwest. The split sample analysis also revealed statistically significant differences in precipitation occurring during extremes of the SOI. The PNA pattern was related to precipitation and temperature over a concurrent time period. Especially strong associations were noted in the Pacific northwest for both precipitation and temperature. Streamflow showed associations with SOI similar to precipitation.
Stouffer, R.J., J. Yin, J.M. Gregory, K.W. Dixon, M.J. Spelman, W. Hurlin, A.J. Weaver, M. Eby, G.M. Flato, H. Hasumi, A. Hu, J.H. Jungclaus, I.V. Kamenkovich, A. Levermann, M. Montoya, S. Murakami, S. Nawrath, A. Oka, W.R. Peltier, D.Y. Robitaille, A. Sokolov, G. Vettoretti, S.L. Weber (2006). Investigating the causes of the response of the thermohaline circulation to past and future climate changes. Journal of Climate 19 (8): 1365-1387
ABSTRACT: The Atlantic thermohaline circulation (THC) is an important part of the earth's climate system. Previous research has shown large uncertainties in simulating future changes in this critical system. The simulated THC response to idealized freshwater perturbations and the associated climate changes have been intercompared as an activity of World Climate Research Program (WCRP) Coupled Model Intercomparison Project/Paleo-Modeling Intercomparison Project (CMIP/PMIP) committees. This intercomparison among models ranging from the earth system models of intermediate complexity (EMICs) to the fully coupled atmosphere–ocean general circulation models (AOGCMs) seeks to document and improve understanding of the causes of the wide variations in the modeled THC response. The robustness of particular simulation features has been evaluated across the model results. In response to 0.1-Sv (1 Sv 106 m3 s−1 ) freshwater input in the northern North Atlantic, the multimodel ensemble mean THC weakens by 30% after 100 yr. All models simulate some weakening of the THC, but no model simulates a complete shutdown of the THC. The multimodel ensemble indicates that the surface air temperature could present a complex anomaly pattern with cooling south of Greenland and warming over the Barents and Nordic Seas. The Atlantic ITCZ tends to shift southward. In response to 1.0-Sv freshwater input, the THC switches off rapidly in all model simulations. A large cooling occurs over the North Atlantic. The annual mean Atlantic ITCZ moves into the Southern Hemisphere. Models disagree in terms of the reversibility of the THC after its shutdown. In general, the EMICs and AOGCMs obtain similar THC responses and climate changes with more pronounced and sharper patterns in the AOGCMs.
G. D. Thackray (2001). Extensive early and middle Wisconsin glaciation on the western Olympic Peninsula, Washington, and the variability of Pacific moisture delivery to the Northwestern United States. Quaternary Research 55 (3): 257-270
ABSTRACT: Large glaciers descended western valleys of the Olympic Mountains six times during the last (Wisconsin) glaciation, terminating in the Pacific coastal lowlands. The glaciers constructed extensive landforms and thick stratigraphic sequences, which commonly contain wood and other organic detritus. The organic material, coupled with stratigraphic data, provides a detailed radiocarbon chronology of late Pleistocene ice-margin fluctuations. The early Wisconsin Lyman Rapids advance, which terminated prior to ca. 54,00014 C yr B.P., represented the most extensive ice cover. Subsequent glacier expansions included the Hoh Oxbow 1 advance, which commenced between ca. 42,000 and 35,00014 C yr B.P.; the Hoh Oxbow 2 advance, ca. 30,800 to 26,30014 C yr B.P.; the Hoh Oxbow 3 advance, ca. 22,000–19,30014 C yr B.P.; the Twin Creeks 1 advance, 19,100–18,30014 C yr B.P.; and the subsequent, undated Twin Creeks 2 advance. The Hoh Oxbow 2 advance represents the greatest ice extent of the last 50,000 yr, with the glacier extending 22 km further downvalley than during the Twin Creeks 1 advance, which is correlative with the global last glacial maximum. Local pollen data indicate intensified summer cooling during successive stadial events. Because ice extent was diminished during colder stadial events, precipitation—not summer temperature—influenced the magnitude of glaciation most strongly. Regional aridity, independently documented by extensive pollen evidence, limited ice extent during the last glacial maximum. The timing of glacier advances suggests causal links with North Atlantic Bond cycles and Heinrich events.
ABSTRACT: A study of the influence of interdecadal, decadal, and interannual oceanic-atmospheric influences on streamflow in the United States is presented. Unimpaired streamflow was identified for 639 stations in the United States for the period 1951–2002. The phases (cold/negative or warm/positive) of Pacific Ocean (El Niño–Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO)) and Atlantic Ocean (Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation (NAO)) oceanic-atmospheric influences were identified for the year prior to the streamflow year (i.e., long lead time). Statistical significance testing of streamflow, based on the interdecadal, decadal, and interannual oceanic-atmospheric phase (warm/positive or cold/negative), was performed by applying the nonparametric rank-sum test. The results show that in addition to the well-established ENSO signal the PDO, AMO, and NAO influence streamflow variability in the United States. The warm phase of the PDO is associated with increased streamflow in the central and southwest United States, while the warm phase of the AMO is associated with reduced streamflow in these regions. The positive phase of the NAO and the cold phase of the AMO are associated with increased streamflow in the central United States. Additionally, the coupled effects of the oceanic-atmospheric influences were evaluated on the basis of the long-term phase (cold/negative or warm/positive) of the interdecadal (PDO and AMO) and decadal (NAO) influences and ENSO. Streamflow regions in the United States were identified that respond to these climatic couplings. The results show that the AMO may influence La Niña impacts in the Southeast, while the NAO may influence La Niña impacts in the Midwest. By utilizing the streamflow water year and the long lead time for the oceanic-atmospheric variables, useful information can be provided to streamflow forecasters and water managers.
ABSTRACT: This study examines the response of the tropical atmospheric and oceanic circulation to increasing greenhouse gases using a coordinated set of twenty-first-century climate model experiments performed for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The strength of the atmospheric overturning circulation decreases as the climate warms in all IPCC AR4 models, in a manner consistent with the thermodynamic scaling arguments of Held and Soden. The weakening occurs preferentially in the zonally asymmetric (i.e., Walker) rather than zonal-mean (i.e., Hadley) component of the tropical circulation and is shown to induce substantial changes to the thermal structure and circulation of the tropical oceans. Evidence suggests that the overall circulation weakens by decreasing the frequency of strong updrafts and increasing the frequency of weak updrafts, although the robustness of this behavior across all models cannot be confirmed because of the lack of data. As the climate warms, changes in both the atmospheric and ocean circulation over the tropical Pacific Ocean resemble “El Niño–like” conditions; however, the mechanisms are shown to be distinct from those of El Niño and are reproduced in both mixed layer and full ocean dynamics coupled climate models. The character of the Indian Ocean response to global warming resembles that of Indian Ocean dipole mode events. The consensus of model results presented here is also consistent with recently detected changes in sea level pressure since the mid–nineteenth century.
ABSTRACT: Climatologists have identified and started to explain a range of different modes of climatic variability which seem to be essential components of behaviour of the global climatic system. Of potentially high geomorphological importance are oscillations in climate over interannual to century scales. A range of geomorphological impacts of such climatic oscillations has been recognised, such as alterations in streamflow and sediment yield, mass movement frequencies and coastal erosion, some recent findings on which are reviewed here. Geomorphological impacts of interannual, decadal and multidecadal scale climatic variability vary from place to place and time to time, and are often complexly related to impacts of tectonic and human factors. The importance of improved understanding of decadal scale climatic variability for the progress of geomorphology in general is discussed in terms of the development of geomorphic ideas.