Shift Of Storm Tracks Research Articles

Changes in storm tracks and energy transports in a warmer climate simulated by the GFDL CM2.1 model

Storm tracks play a major role in regulating the precipitation and hydrological cycle in midlatitudes. The changes in the location and amplitude of the storm tracks in response to global warming will have significant impacts on the poleward transport of heat, momentum and moisture and on the hydrological cycle. Recent studies have indicated a poleward shift of the storm tracks and the midlatitude precipitation zone in the warming world that will lead to subtropical drying and higher latitude moistening. This study agrees with this key feature for not only the annual mean but also different seasons and for the zonal mean as well as horizontal structures based on the analysis of Geophysical Fluid Dynamics Laboratory (GFDL) CM2.1 model simulations. Further analyses show that the meridional sensible and latent heat fluxes associated with the storm tracks shift poleward and intensify in both boreal summer and winter in the late twenty-first century (years 2081–2100) relative to the latter half of the twentieth century (years 1961–2000). The maximum dry Eady growth rate is examined to determine the effect of global warming on the time mean state and associated available potential energy for transient growth. The trend in maximum Eady growth rate is generally consistent with the poleward shift and intensification of the storm tracks in the middle latitudes of both hemispheres in both seasons. However, in the lower troposphere in northern winter, increased meridional eddy transfer within the storm tracks is more associated with increased eddy velocity, stronger correlation between eddy velocity and eddy moist static energy, and longer eddy length scale. The changing characteristics of baroclinic instability are, therefore, needed to explain the storm track response as climate warms. Diagnosis of the latitude-by-latitude energy budget for the current and future climate demonstrates how the coupling between radiative and surface heat fluxes and eddy heat and moisture transport influences the midlatitude storm track response to global warming. Through radiative forcing by increased atmospheric carbon dioxide and water vapor, more energy is gained within the tropics and subtropics, while in the middle and high latitudes energy is reduced through increased outgoing terrestrial radiation in the Northern Hemisphere and increased ocean heat uptake in the Southern Hemisphere. This enhanced energy imbalance in the future climate requires larger atmospheric energy transports in the midlatitudes which are partially accomplished by intensified storm tracks. Finally a sequence of cause and effect for the storm track response in the warming world is proposed that combines energy budget constraints with baroclinic instability theory.

Changes in synoptic weather patterns and Greenland precipitation in the 20th and 21st centuries: 2. Analysis of 21st century atmospheric changes using self‐organizing maps

Using a three‐model ensemble, predictions of increasing Greenland precipitation over the 21st century are analyzed using self‐organizing maps (SOMs). The models that make up the three‐model ensemble (CCCMA‐CGCM3.1(T63), MIROC3.2(hires), and MPI‐ECHAM5), which are all atmosphere‐ocean global circulation models used in the Intergovernmental Panel on Climate Change Fourth Assessment Report, were chosen due to their ability to best reproduce North Atlantic surface synoptic climatology and Greenland precipitation from ERA‐40. Daily sea level pressure and precipitation data from model simulations for years 1961–1999, 2046–2065, and 2081–2100 are compared, where future simulations are based on the SRES A1B emissions scenario. Results indicate that the North Atlantic storm track is predicted to shift northward through the 21st century and Greenland precipitation is predicted to increase from 35.8 cm yr−1 to 45.8 cm yr−1 by the end of the 21st century, a 27.8% increase. The precipitation change is attributed to changes in atmospheric circulation, which are due to changes in synoptic pattern frequency of occurrence, and changes in the amount of precipitation that occurs when a given synoptic circulation pattern occurs, termed intrapattern variability, during the future time periods. The northward shift in storm track results in less precipitation being produced dynamically over the southeast coast of Greenland, but an increase in precipitation over the remainder of the ice sheet, with largest increases over the southwest coast of Greenland and the eastern region. Intrapattern variability changes, however, dominate the future precipitation changes, accounting for 82.5% of the total change. This is due to an increase in precipitable water in the atmosphere in response to rising temperatures. Changes in sea ice and ocean temperature are also thought to contribute to this change.

Temporal stability in bristlecone pine tree-ring stable oxygen isotope chronologies over the last two centuries

The absolutely dated bristlecone pine ( Pinus longaeva) tree-ring chronology spans almost 9000 years, offering great potential for inferring past environmental change. Existing ring width chronologies have been widely used to produce some of the most influential millennial length temperature reconstructions for the Northern Hemisphere. A recently published δ 18O record from two bristlecone pine trees growing at Methuselah Walk in the White Mountains of California showed a dramatic decrease in δ 18O between AD 1850 and 1920 ( c. 13‰), interpreted as indicating a major shift in Pacific storm tracks over the past 300 years. Here we present new bristlecone pine δ18O time series from 15 trees at three White Mountains sites, including two series from Methuselah Walk. Whilst occasional high interannual variability is observed in our δ 18O series, none of our chronologies exhibit an equivalent pronounced or sustained twentieth-century decrease, suggesting the earlier results are anomalous and may require palaeoclimatic re-interpretation.

Simulation of Present-Day and Twenty-First-Century Energy Budgets of the Southern Oceans

Abstract The energy budget of the modern-day Southern Hemisphere is poorly simulated in both state-of-the-art reanalyses and coupled global climate models. The ocean-dominated Southern Hemisphere has low surface reflectivity and therefore its albedo is particularly sensitive to cloud cover. In modern-day climates, mainly because of systematic deficiencies in cloud and albedo at mid- and high latitudes, too much solar radiation enters the ocean. Along with too little radiation absorbed at lower latitudes because of clouds that are too bright, unrealistically weak poleward transports of energy by both the ocean and atmosphere are generally simulated in the Southern Hemisphere. This implies too little baroclinic eddy development and deficient activity in storm tracks. However, projections into the future by coupled climate models indicate that the Southern Ocean features a robust and unique increase in albedo, related to clouds, in association with an intensification and poleward shift in storm tracks that is not observed at any other latitude. Such an increase in cloud may be untenable in nature, as it is likely precluded by the present-day ubiquitous cloud cover that models fail to capture. There is also a remarkably strong relationship between the projected changes in clouds and the simulated current-day cloud errors. The model equilibrium climate sensitivity is also significantly negatively correlated with the Southern Hemisphere energy errors, and only the more sensitive models are in the range of observations. As a result, questions loom large about how the Southern Hemisphere will actually change as global warming progresses, and a better simulation of the modern-day climate is an essential first step.

Long-term variability and trends of sea level storminess and extremes in European Seas

This paper documents the variability and trends of mean storminess and extreme sea level amplitudes at selected long-term operating tide gauge stations located in the European Seas over different frequency bands, seasons and months. Six stations have been chosen for the analyses–Antalya, Ceuta, Rovinj, Newlyn, Wladyslawowo and Lerwick–which possess at least a half-centurial record of hourly or higher frequency sea level data. The data have been carefully inspected for time shifts and drifts in the record. The analyses included the extraction of sea level amplitudes (envelopes) over four frequency bands: super-diurnal frequencies (0–1 days), small-scale synoptic disturbances (1–3 days), large-scale synoptic disturbances (3–10 days) and planetary-scale disturbances (10–100 days). Interannual variability in sea level amplitudes is occasionally found to coincide with some known variability in the atmosphere. For example, the northern European stations have overall positive sea level storminess and extreme trends, which is opposite from the southern stations, confirming a northward shift in atmosphere storm tracks. Redistribution of sea level amplitudes between different seasons and different frequency bands has been observed at some stations in both variability and trends. The latter may be important for the assessment of a region's total hazard risks and vulnerability, as maximum storminess and extremes may or may not coincide with maximum mean sea level.

Aridity changes in the Temperate-Mediterranean transition of the Andes since ad 1346 reconstructed from tree-rings

The Andes Cordillera acts as regional “Water Towers” for several countries and encompasses a wide range of ecosystems and climates. Several hydroclimatic changes have been described for portions of the Andes during recent years, including glacier retreat, negative precipitation trends, an elevation rise in the 0° isotherm, and changes in regional streamflow regimes. The Temperate-Mediterranean transition (TMT) zone of the Andes (35.5°–39.5°S) is particularly at risk to climate change because it is a biodiversity hotspot with heavy human population pressure on water resources. In this paper we utilize a new tree-ring network of Austrocedrus chilensis to reconstruct past variations in regional moisture in the TMT of the Andes by means of the Palmer Drought Severity Index (PDSI). The reconstruction covers the past 657 years and captures interannual to decadal scales of variability in late spring–early summer PDSI. These changes are related to the north–south oscillations in moisture conditions between the Mediterranean and Temperate climates of the Andes as a consequence of the latitudinal position of the storm tracks forced by large-scale circulation modes. Kernel estimation of occurrence rates reveals an unprecedented increment of severe and extreme drought events during the last century in the context of the previous six centuries. Moisture conditions in our study region are linked to tropical and high-latitude ocean-atmospheric forcing, with PDSI positively related to Nino-3.4 SST during spring and strongly negatively correlated with the Antarctic Oscillation (AAO) during summer. Geopotential anomaly maps at 500-hPa show that extreme dry years are tightly associated with negative height anomalies in the Ross–Amundsen Seas, in concordance with the strong negative relationship between PDSI and AAO. The twentieth century increase in extreme drought events in the TMT may not be related to ENSO but to the positive AAO trend during late-spring and summer resulting from a gradual poleward shift of the mid-latitude storm tracks. This first PDSI reconstruction for South America demonstrates the highly significant hindcast skill of A. chilensis as an aridity proxy.

Ozone hole and Southern Hemisphere climate change

Climate change in the Southern Hemisphere (SH) has been robustly documented in the last several years. It has altered the atmospheric circulation in a surprising number of ways: a rising global tropopause, a poleward intensification of the westerly jet, a poleward shift in storm tracks, a poleward expansion of the Hadley cell, and many others. While these changes have been extensively related with anthropogenic warming resulting from the increase in greenhouse gases, their potential link to stratospheric cooling resulting from ozone depletion has only recently been examined and a comprehensive picture is still lacking. Examining model output from the coupled climate models participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment (AR4), and grouping them depending on the stratospheric ozone forcing used, we here show that stratospheric ozone affects the entire atmospheric circulation in the SH, from the polar regions to the subtropics, and from the stratosphere to the surface. Furthermore, model projections suggest that the anticipated ozone recovery, resulting from the implementation of the Montreal Protocol, will likely decelerate future climate change resulting from increased greenhouse gases, although it might accelerate surface warming over Antarctica.

Glacial to Holocene climate changes in the SE Pacific. The Raraku Lake sedimentary record (Easter Island, 27°S)

Easter Island (SE Pacific, 27°S) provides a unique opportunity to reconstruct past climate changes in the South Pacific region based on terrestrial archives. Although the general climate evolution of the south Pacific since the Last Glacial Maximum (LGM) is coherent with terrestrial records in southern South America and Polynesia, the details of the dynamics of the shifting Westerlies, the South Pacific Convergence Zone and the South Pacific Anticyclone during the glacial–interglacial transition and the Holocene, and the large scale controls on precipitation in tropical and extratropical regions remain elusive. Here we present a high-resolution reconstruction of lake dynamics, watershed processes and paleohydrology for the last 34 000 cal yrs BP based on a sedimentological and geochemical multiproxy study of 8 cores from the Raraku Lake sediments constrained by 22 AMS radiocarbon dates. This multicore strategy has reconstructed the sedimentary architecture of the lake infilling and provided a stratigraphic framework to integrate and correlate previous core and vegetation studies conducted in the lake. High lake levels and clastic input dominated sedimentation in Raraku Lake between 34 and 28 cal kyr BP. Sedimentological and geochemical evidences support previously reported pollen data showing a relatively open forest and a cold and relatively humid climate during the Glacial period. Between 28 and 17.3 cal kyr BP, including the LGM period, colder conditions contributed to a reduction of the tree coverage in the island. The coherent climate patterns in subtropical and mid latitudes of Chile and Eastern Island for the LGM (more humid conditions) suggest stronger influence of the Antarctic circumpolar current and an enhancement of the Westerlies. The end of Glacial Period occurred at 17.3 cal kyr BP and was characterized by a sharp decrease in lake level conducive to the development of major flood events and erosion of littoral sediments. Deglaciation (Termination 1) between 17.3 and 12.5 cal kyr BP was characterized by an increase in lake productivity, a decrease in the terrigenous input and a rapid lake level recovery, inaugurating a period of intermediate lake levels, dominance of organic deposition and algal lamination. The timing and duration of deglaciation events in Easter Island broadly agree with other mid- and low-latitude circum South Pacific terrestrial records. The transition to the Holocene was characterized by lower lake levels. The lake level dropped during the early Holocene (ca 9.5 cal kyr BP) and swamp and shallow lake conditions dominated till mid Holocene, partially favored by the infilling of the lacustrine basin. During the mid- to late-Holocene drought phases led to periods of persistent low water table, subaerial exposure and erosion, generating a sedimentary hiatus in the Raraku sequence, from 4.2 to 0.8 cal kyr BP. The presence of this dry mid Holocene phase, also identified in low Andean latitudes and in Patagonian mid latitudes, suggests that the shift of storm tracks caused by changes in the austral summer insolation or forced by “El Niño-like” dominant conditions have occurred at a regional scale. The palm deforestation of the Easter Island, attributed to the human impact could have started earlier, during the 4.2–0.8 cal kyr BP sedimentary gap. Our paleoclimatic data provides insights about the climate scenarios that could favor the arrival of the Polynesian people to the island. If it occurred at ca AD 800 it coincided with the warmer conditions of the Medieval Climate Anomaly, whereas if it took place at ca AD 1300 it was favored by enhanced westerlies at the onset of the Little Ice Age. Changes in land uses (farming, intensive cattle) during the last century had a large impact in the hydrology and limnology (eutrophication) of the lake.

Sources of CAM3 temperature bias during northern winter from diagnostic study of the temperature bias equation

The Community Atmosphere Model version 3 (CAM3) temperature simulation bias is examined in this paper. We compare CAM3 output with European Centre for Medium-Range Weather Forecasts (ECMWF) 40 year reanalysis (ERA-40) data. We formulate a time mean temperature bias equation then evaluate each term in the equation. Our focus is on the Northern Hemisphere winter time. We group the temperature equation terms into these categories: linear advection terms, nonlinear advection terms, transient eddy terms and diabatic heating, and find that linear advection and diabatic bias are the largest. The nonlinear terms (velocity bias advection of temperature bias) are much smaller than each of the other groups of terms at all levels except near the surface. Linear advection terms have dipolar pattern in the Atlantic (negative NW of positive) which reflects the shift of the CAM3 model North Atlantic storm track (NAST) into Europe, especially in the upper troposphere; opposite sign dipolar structure occurs over Alaska (positive) and the north Pacific storm track (negative). The transient advection terms in middle latitudes are larger in the upper troposphere and generally positive along the Atlantic storm track. Along the north Pacific storm track (NPST), the transient terms are negative in the mid and lower troposphere over much of the NPST (positive in upper troposphere). The diabatic heating bias has large values in the tropics along the Intertropical Convergence Zone (ICZ) and along the midlatitude storm tracks. During this time of year the ICZ is mainly in the Southern Hemisphere, but CAM3 emphasizes an ICZ-like heating in the northern hemisphere of the Atlantic and Pacific Oceans. CAM3 tends to have a weaker ICZ, especially in the Atlantic. In midlatitudes, we find large bias in heating by precipitation and vertically averaged net radiation over the NAST, Europe, and the Middle East.

Scenario changes in the climatology of winter midlatitude cyclone activity over eastern North America and the Northwest Atlantic

The present study explores how midlatitude winter cyclone activity can be modified under warming‐induced conditions due to enhanced greenhouse gas concentrations. We performed simulations with the Canadian Regional Climate Model (CRCM version 3.5) implemented on a domain that covers the Northwest Atlantic and eastern North America. These simulations are driven by control conditions (1975–1994) and high‐CO2 scenario conditions (2040–2059) suggested by the Canadian Climate Centre model, CGCM2 (Second Generation Coupled Global Climate Model), following the IPCC IS92a scenario. Comparisons between model simulations for the control period (1975–1994) and North America Regional analysis (NARR) suggest that both CGCM2 and CRCM reliably reproduce the overall NARR patterns of sea level pressure, tropospheric baroclinicity and Atlantic storm tracks. However, compared to CGCM2 results, CRCM offers an improvement in simulations of the most intense cyclones. Although both models underestimate the track density of intense cyclones, the CGCM2 underestimates are larger than those of CRCM. Under the high‐CO2 climate change scenario, the CRCM and CGCM2 model simulations show similar changes in sea level pressure, surface temperature, and total track density of midlatitude winter cyclones. Although we can see the northwest shift of the dominant Atlantic storm track, it is not statistically significant. Moreover, simulations from both models show a decrease in the total cyclone track density along the Canadian east coast; the decrease is more robust in CRCM simulations than in CGCM2 results. For intense cyclones, CRCM simulations show a slight decrease in the track density, while no such change is found in CGCM2 simulations.

Model projected changes of extreme wind events in response to global warming

The changes in the frequency of occurrence of extreme wind storm events in response to anthropogenic global warming are explored using a multi‐model ensemble of coupled climate model simulations. These changes, diagnosed using several different metrics based on the daily wind fields, indicate that the frequency of the most extreme wind events decreases over the tropics in association with the model‐projected weakening of the large‐scale atmospheric circulation. At higher latitudes, the strongest near‐surface wind events are found to increase in frequency in association with the amplified baroclinicity and poleward shift of the midlatitude storm tracks. The frequency of the heaviest precipitation events increases in all models, despite a systematic reduction of extreme upward vertical velocities, due to the increased moisture content of the lower troposphere. All of these changes are shown to be robust projections of current climate models.

Effect of Climate Change between 1984 and 2007 on Precipitation Chemistry at a Site in Northeastern USA

Climate change predictions for the northeastern US call for an increase in tropical storms and a decrease in extra tropical cyclones including continental storms. We ran 24-h back trajectories for each precipitation event that occurred at the Cary Institute of Ecosystem Studies in southeastern New York, U.S.A. from 1984 to 2007 and analyzed precipitation chemistry as well as air mass position 24 h prior to the onset of each precipitation event. The results showed an increase in marine precipitation and a slight but statistically insignificant decrease in continental precipitation during the 1984-2007 period. The chemistry of precipitation from the two directions was quite different marine storms were higher in Na4 and Cl- but lower in solutes associated with acid precipitation (H+, SO4(2-), NO3-, and NH4+). Annual mean concentrations of acid precipitation solutes declined for storms from both directions during the period. We used a simple mixing model based on the current rates of increase and decrease of marine and continental precipitation respectively to show that chemical changes in precipitation resulting from the shift in storm tracks are small compared to chemical changes due to emissions reductions.

Sea ice drift in the Arctic since the 1950s

Sea ice drift data (from Russian North Pole stations, various ice camps, and the International Arctic Buoy Program) and surface wind stress data from the NCAR/NCEP Reanalysis are analyzed to determine their long‐term trends and causality. The study finds that both parameters (ice drift and wind stress) show gradual acceleration over last 50 years. Significant positive trends are present in both winter and summer data. The major cause of observed positive trends is increasing Arctic storm activity over the Transpolar Drift Stream caused by a shift of storm tracks toward higher latitudes. It is speculated, with some observational evidence, that the increased stirring of the ocean by winds could hasten the transition of the Arctic toward a weakly stratified ocean with a potential for deep convection and a new sink for atmospheric CO2.

The Influence of the El Niño–Southern Oscillation on Cloud-to-Ground Lightning Activity along the Gulf Coast. Part II: Monthly Correlations

Abstract The El Niño–Southern Oscillation (ENSO) cycle is known to influence weather and climate along the Gulf Coast region, causing anomalously high precipitation during El Niño winters. This region is also known for having the highest lightning flash density in the United States. An 8-yr dataset (1995–2002) of cloud-to-ground (CG) lightning flashes was analyzed to determine if the ENSO cycle influences lighting activity along the Gulf Coast region. Simple Pearson’s correlations were computed between concurrent monthly pairings of Niño-3.4 sea surface temperature (SST) and CG lightning flash deviation values from the study area. The correlation results are mapped and analyzed for links to meteorological features. Statistically significant correlation values greater than 0.8 were noted over large swaths of the study area during each winter month. The highest correlations were arranged in banded swaths and associated with regions of low flash densities during December and February. In January, areas of high correlation were spatially coincident with areas of enhanced flash density. Both the enhanced CG flash regions and high correlation values and patterns are indicative of a southerly shift in the midlatitude storm track known to occur during warm ENSO events. During the spring and summer, most of the region has weak correlation with ENSO except for August, which has a large area of negative correlations. These findings indicate that lightning increases during La Niña summers. Correlation patterns in late fall are similar to those of winter. The ENSO–lightning relationship has implications for hazard assessment and can be a useful tool for long-term seasonal planning.

Influence of the persistence of circulation patterns on warm and cold temperature anomalies in Europe: Analysis over the 20th century

Recent studies have pointed out that persistence of the atmospheric circulation over Europe, as measured by residence times of circulation types, has increased since the mid-1980s in all seasons and for most groups of the types. The greater persistence may affect surface climatic anomalies, particularly the frequency and severity of heat and cold waves associated with severe impacts on society and environment. In this paper, relationships between the persistence of circulation types over Europe and extreme surface air temperature anomalies are studied over the 20th century using the Hess–Brezowsky catalogue of large-scale circulation patterns and long-term temperature series at stations covering most of the European continent. Types significantly conducive to heat and cold waves are identified, and temperature anomalies are linked to their persistence. It is shown that more persistent circulation enhances the severity of temperature extremes over the whole area, which is slightly more important for warm than cold temperature anomalies. The changes in both frequencies and residence times of circulation patterns have been supporting sharply rising trends in warm temperature extremes observed over Europe in recent decades, and the circulation changes may also contributed to the fact that trends in cold temperature extremes have been less pronounced or absent in the same period. The findings also emphasize the need for taking into account the persistence of circulation types together with their frequencies when evaluating links between the atmospheric circulation and the surface climate. In global warming context, the effects of the future climate change on the occurrence and severity of temperature extremes may be exacerbated by a more persistent circulation related to a decreased cyclone activity over mid-latitudes and a northward shift of storm tracks.

Possible evidence for wet Heinrich phases in tropical NE Australia: the Lynch's Crater deposit

Unarguably, one of the most significant paleoclimatological discoveries of the last two decades has been that of abrupt climate events (Dansgaard–Oeschger cycles and Heinrich events). Most evidence for these events has originated from the high-latitude Northern Hemisphere, with few records documenting the response of the low latitude Southern Hemisphere. Here we present new data from Lynch's Crater, a unique terrestrial record from NE-Australia that may show evidence for southward propagations of the Intertropical Convergence Zone (ITCZ) during abrupt climate perturbations as a result of alteration of the low latitude air masses. Proxies for precipitation/wetness indicate enhanced rainfall in the region during Heinrich events (H events 1–3) and the 8.2 ka Northern Hemisphere cold event. A fully coupled atmosphere/ocean climate model simulating a 1 Sv freshwater influx to the North Atlantic Ocean produces a scenario which agrees with the climate changes shown by the Lynch's Crater record. The model shows precipitation anomalies that include a southward migration of the ITCZ and a zonal shift in mid-latitude storm tracks over the Southern Hemisphere equatorial region. These data indicate large-scale shifts of the austral summer ITCZ position that is known to control monsoonal precipitation in NE Australia. This terrestrial record from Australia may demonstrate the involvement of the tropical western Pacific Ocean in ITCZ migrations during abrupt climate events of the last glacial period. Defining such past migrations offers insight into the importance and role of the equatorial region in global climate dynamics.

Response of Northern Hemisphere extratropical cyclone activity and associated precipitation to climate change, as represented by the Community Climate System Model

The projected effects of rising CO2 levels on Northern Hemisphere extratropical cyclone activity and cyclone‐associated precipitation are examined for September–May, using output from version 3 of the Community Climate System Model (CCSM3). A cyclone identification algorithm was applied to a five member ensemble of CCSM3 20th and 21st century output, along with a method of isolating precipitation produced by each cyclone. Mean seasonal statistics describing cyclone activity and the character of associated precipitation were calculated over several study regions for 20 a periods. The dominant change in cyclone activity is a marked midlatitude decrease in frequency during autumn, winter, and spring. Few significant shifts in storm tracks or cyclone intensity were identified. Total daily precipitation from these events is found to increase into the 21st century, largely because of increases in available atmospheric moisture with rising temperatures. This thermodynamic increase in precipitation leads to large rises in total seasonal cyclone‐associated precipitation over high latitudes, while over midlatitudes the thermodynamic increase is offset by the dynamic effect associated with decreased cyclone frequency.

Interannual variations and future change of wintertime extratropical cyclone activity over North America in CCSM3

Climatology and interannual variations of wintertime extratropical cyclone frequency in CCSM3 twentieth century simulation are compared with the NCEP/NCAR reanalysis during 1950–1999. CCSM3 can simulate the storm tracks reasonably well, although the model produces slightly less cyclones at the beginning of the Pacific and Atlantic storm tracks and weaker poleward deflection over the Pacific. As in the reanalysis, frequency of cyclones stronger than 980 hPa shows significant correlation with the Pacific/North America (PNA) teleconnection pattern over the Pacific region and with the North Atlantic Oscillation (NAO) in the Atlantic sector. Composite maps are constructed for opposite phases of El Nino-Southern Oscillation (ENSO) and the NAO and all anomalous patterns coincide with observed. One CCSM3 twenty-first century A1B scenario realization indicates there is significant increase in the extratropical cyclone frequency on the US west coast and decrease in Alaska. Meanwhile, cyclone frequency increases from the Great Lakes region to Quebec and decreases over the US east coast, suggesting a possible northward shift of the Atlantic storm tracks under the warmer climate. The cyclone frequency anomalies are closely linked to changes in seasonal mean states of the upper-troposphere zonal wind and baroclinicity in the lower troposphere. Due to lack of 6-hourly outputs, we cannot apply the cyclone-tracking algorithm to the other eight CCSM3 realizations. Based on the linkage between the mean state change and the cyclone frequency anomalies, it is likely a common feature among the other ensemble members that cyclone activity is reduced on the East Coast and in Alaska as a result of global warming.

Climate change projections for the Mediterranean region

We present a review of climate change projections over the Mediterranean region based on the most recent and comprehensive ensembles of global and regional climate change simulations completed as part of international collaborative projects. A robust and consistent picture of climate change over the Mediterranean emerges, consisting of a pronounced decrease in precipitation, especially in the warm season, except for the northern Mediterranean areas (e.g. the Alps) in winter. This drying is due to increased anticyclonic circulation that yields increasingly stable conditions and is associated with a northward shift of the Atlantic storm track. A pronounced warming is also projected, maximum in the summer season. Inter-annual variability is projected to mostly increase especially in summer, which, along with the mean warming, would lead to a greater occurrence of extremely high temperature events. The projections by the global and regional model simulations are generally consistent with each other at the broad scale. However, the precipitation change signal produced by the regional models shows substantial orographically-induced fine scale structure absent in the global models. Overall, these change signals are robust across forcing scenarios and future time periods, with the magnitude of the signal increasing with the intensity of the forcing. The intensity and robustness of the climate change signals produced by a range of global and regional climate models suggest that the Mediterranean might be an especially vulnerable region to global change.

Storm Tracks and Climate Change

Abstract Extratropical and tropical transient storm tracks are investigated from the perspective of feature tracking in the ECHAM5 coupled climate model for the current and a future climate scenario. The atmosphere-only part of the model, forced by observed boundary conditions, produces results that agree well with analyses from the 40-yr ECMWF Re-Analysis (ERA-40), including the distribution of storms as a function of maximum intensity. This provides the authors with confidence in the use of the model for the climate change experiments. The statistical distribution of storm intensities is virtually preserved under climate change using the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A1B scenario until the end of this century. There are no indications in this study of more intense storms in the future climate, either in the Tropics or extratropics, but rather a minor reduction in the number of weaker storms. However, significant changes occur on a regional basis in the location and intensity of storm tracks. There is a clear poleward shift in the Southern Hemisphere with consequences of reduced precipitation for several areas, including southern Australia. Changes in the Northern Hemisphere are less distinct, but there are also indications of a poleward shift, a weakening of the Mediterranean storm track, and a strengthening of the storm track north of the British Isles. The tropical storm tracks undergo considerable changes including a weakening in the Atlantic sector and a strengthening and equatorward shift in the eastern Pacific. It is suggested that some of the changes, in particular the tropical ones, are due to an SST warming maximum in the eastern Pacific. The shift in the extratropical storm tracks is shown to be associated with changes in the zonal SST gradient in particular for the Southern Hemisphere.