Sea Surface Temperature Forcing Research Articles

Variability of seasonal-mean fields arising from intraseasonal variability. part 2, application to nh winter circulations

A study has been made, using the National Centers for Environmental Prediction and National Center for Atmospheric Research re-analysis 500 hPa geopotential height data, to determine how intraseasonal variability influences, or can generate, coherent patterns of interannual variability in the extratropical summer and winter Southern Hemisphere atmospheric circulation. In addition, by separating this intraseasonal component of interannual variability, we also consider how slowly varying external forcings and slowly varying (interannual and longer) internal dynamics might influence the interannual variability of the Southern Hemisphere circulation. This slow component of interannual variation is more likely to be potentially predictable. How sea surface temperatures are related to the slow components is also considered. The four dominant intraseasonal modes of interannual variability have horizontal structures similar to those seen in both well-known intraseasonal dynamical modes and statistical modes of intraseasonal variability. In particular, they reflect intraseasonal variability in the high latitudes associated with the Southern Annular Mode, and wavenumber 4 (summer) and wavenumber 3 (winter) patterns associated with south Pacific regions of persistent anomalies and blocking, and possibly variability related to the Madden-Julian Oscillation (MJO). The four dominant slow components of interannual variability, in both seasons, are related to high latitude variability associated with the Southern Annular Mode, El Nino Southern Oscillation (ENSO) variability, and South Pacific Wave variability associated with Indian Ocean SSTs. In both seasons, there are strong linear trends in the first slow mode of high latitude variability and these are shown to be related to similar trends in the Indian Ocean. Once these are taken into account there is no significant sea surface temperature forcing of these high latitude modes. The second and third ENSO related slow modes, in each season, have high correlations with tropical sea surface temperature variability in the Pacific and Indian Oceans, both contemporaneously and at one season lag. The fourth slow mode has a characteristic South Pacific wave structure of either a wavenumber 4 (summer) or wavenumber 3 (winter) pattern, with strongest loadings in the South Pacific sector, and an association simultaneously with a dipole SST temperature gradient in the subtropical Indian Ocean.

Circulation Regimes and SST Forcing: Results from Large GCM Ensembles

Abstract The purpose of this paper is to study the influence of tropical sea surface temperature (SST) on the midlatitude circulation regime behavior in the Pacific–North American region. Toward this end, a cluster analysis has been applied to 55-member ensembles of winter seasonal simulations of the Center for Ocean–Land–Atmosphere Studies general circulation model for 18 winters. The ensemble members for each winter utilize the same prescribed, observed weekly varying SST for that winter. The cluster analysis includes all fluctuations with time scales longer than 10 days except for the ensemble mean seasonal cycle. Using a partitioning algorithm separately applied to each winter's ensemble, clusters are found in the 200-hPa height field that are signicant (vis-a-vis a suitable Gaussian background), reproducible (in half-length datasets), and consistent (with clusters obtained from the 200-hPa u wind) for all winters except the strong El Nino events of 1982/83, 1986/87, and 1997/98. One cluster found con...

Remote Response of the Indian Ocean to Interannual SST Variations in the Tropical Pacific

Remote forcing of sea surface temperature (SST) variations in the Indian Ocean during the course of El Nino- Southern Oscillation (ENSO) events is investigated using NCEP reanalysis and general circulation model (GCM) experiments. Three experiments are conducted to elucidate how SST variations in the equatorial Pacific influence surface flux variations, and hence SST variations, across the Indian Ocean. A control experiment is conducted by prescribing observed SSTs globally for the period 1950-99. In the second experiment, observed SSTs are prescribed only in the tropical eastern Pacific, while climatological SSTs are used elsewhere over the global oceans. In the third experiment, observed SSTs are prescribed in the tropical eastern Pacific, while a variable-depth ocean mixed layer model is used at all other ocean grid points to predict the SST. Composites of surface fluxes and SST over the Indian Ocean are formed based on El Nino and La Nina events during 1950-99. The surface flux variations in the eastern Indian Ocean in all three experiments are similar and realistic, confirming that much of the surface flux variation during ENSO is remotely forced from the Pacific. Furthermore, the SST anomalies in the eastern tropical Indian Ocean are well simulated by the coupled model, which supports the notion of an ''atmospheric bridge'' from the Pacific. During boreal summer and fall, when climatological winds are southeasterly over the eastern Indian Ocean, remotely forced anomalous easterlies act to increase the local wind speed. SST cools in response to increased evaporative cooling, which is partially offset by increased solar radiation associated with reduced rainfall. During winter, the climatological winds become northwesterly and the anomalous easterlies then act to reduce the wind speed and evaporative cooling. Together with increased solar radiation and a shoaling mixed layer, the SST warms rapidly. The model is less successful at reproducing the ENSO-induced SST anomalies in the western Indian Ocean, suggesting that dynamical ocean processes contribute to the east-west SST dipole that is often observed in boreal fall during ENSO events.

The Scatter in Tropical Average Precipitation Anomalies*

Tropical mean precipitation anomalies ^P9& for 3-month averages appear quite scattered in relation to tropical average sea surface temperature (SST) anomalies ^& , based on examination of a number of observational T9 datasets and of atmospheric general circulation model (GCM) results. Even though SST is locally important for determining precipitation, for a given warm SST anomaly, the tropical average precipitation anomalies can be of either sign due to the near cancellation of positive against negative values. No simple relation is found between ^P9& and ^& . On the other hand, tropical average tropospheric temperature anomalies ^T ˆ9& are approximately T9 linearly related to SST forcing. The scatter of ^P9& versus ^& and ^T9& challenges the prevailing view that T9 s tropical tropospheric temperature anomalies are proportional to tropical convective heating anomalies (i.e., precipitation anomalies), while the latter are governed by SST forcing. A simple analytical model shows that convective heating anomalies are more strongly influenced by dry static-energy transports into or out of the Tropics and by nonlinearities within the Tropics than are the tropospheric temperature anomalies. As convection maintains a quasi balance between surface and tropospheric temperature, the tropical average convective heating (i.e., precipitation) anomalies react to oppose any processes that would tend to cool the tropical troposphere. On interannual timescales, the integral constraint on tropical average precipitation is dominated by the dry static energy transport into or out of the Tropics. Thus while tropical average tropospheric temperature is closely related to SST, tropical‐midlatitude transports can create large scatter in tropical average precipitation.

Coupled variability and air-sea interaction in the South Atlantic Ocean

A total of 52 years of data (1949–2000) from the NCEP/NCAR reanalysis are used to investigate mechanisms involved in forcing and damping of sea surface temperature (SST) variability in the South Atlantic Ocean. Organized patterns of coupled ocean–atmosphere variability are identified using EOF and SVD analyses. The leading mode of coupled variability consists of an SST pattern with a strong northeast–southwest gradient and an SLP monopole centered at 15°W, 45°S. The anomalous winds associated with this monopole generate the SST pattern through anomalous latent heat flux and mixed layer deepening. Other heat flux components and anomalous Ekman transport play only a secondary role. Once established, the SST pattern is attenuated through latent heat flux. The higher SST modes are also induced by anomalous winds and destroyed by latent heat flux. It thus appears that the coupled variability in the South Atlantic Ocean consists of atmospheric circulation anomalies that induce SST anomalies through anomalous latent heat fluxes and wind-induced mixed layer deepening. These SST anomalies are destroyed by latent heat flux with no detectable systematic feedback onto the atmospheric circulation. Atmospheric variability in the South Atlantic is found to be largely independent of that elsewhere, although there is a weak relation with ENSO (El Nino-Southern Oscillation).

Atmospheric Response to Zonal Variations in Midlatitude SST: Transient and Stationary Eddies and Their Feedback*

Midwinter storm track response to zonal variations in midlatitude sea surface temperatures (SSTs) has been investigated using an atmospheric general circulation model under aquaplanet and perpetual-January conditions. Zonal wavenumber-1 SST variations with a meridionally confined structure are placed at various latitudes. Having these SST variations centered at 30°N leads to a zonally localized storm track, while the storm track becomes nearly zonally uniform when the same SST forcing is moved farther north at 40° and 50°N. Large (small) baroclinic energy conversion north of the warm (cold) SST anomaly near the axis of the storm track (near 40°N) is responsible for the large (small) storm growth. The equatorward transfer of eddy kinetic energy by the ageostrophic motion and the mechanical damping are important to diminish the storm track activity in the zonal direction. Significant stationary eddies form in the upper troposphere, with a ridge (trough) northeast of the warm (cold) SST anomaly at 30°N. Heat and vorticity budget analyses indicate that zonally localized condensational heating in the storm track is the major cause for these stationary eddies, which in turn exert a positive feedback to maintain the localized storm track by strengthening the vertical shear near the surface. These results indicate an active role of synoptic eddies in inducing deep, tropospheric-scale response to midlatitude SST variations. Finally, the application of the model results to the real atmosphere is discussed.

Barometric Pressure Variations Associated with Eastern Pacific Tropical Instability Waves*

Abstract Barometric pressure, surface temperature, and wind time series in the eastern equatorial Pacific are analyzed to determine if oceanic tropical instability wave (TIW) sea surface temperature variations cause barometric pressure gradients large enough to influence the atmospheric boundary layer. During the study period from April 2001 to September 2002, 11 TIWs propagated westward past 110°W, causing a spectral peak at 20–30 days in the sea surface temperature (SST) meridional difference between 2°N, 110°W and 0°, 110°W. Likewise, the meridional pressure difference also had a spectral peak in the 20–30-day TIW band. Cross-spectral analysis shows that within the TIW band, SST-induced pressure variations were roughly −0.1 hPa °C−1 in magnitude. The resulting pressure gradient force is comparable in magnitude to other terms in the meridional momentum balance. Implications about the role of the boundary layer capping in the adjustment to SST forcing are discussed.

Assessment of regional seasonal rainfall predictability using the CPTEC/COLA atmospheric GCM

This is a study of the annual and interannual variability of regional rainfall produced by the Center for Weather Forecasts and Climate Studies/Center for Ocean, Land and Atmospheric Studies (CPTEC/COLA) atmospheric global climate model. An evaluation is made of a 9-member ensemble run of the model forced by observed global sea surface temperature (SST) anomalies for the 10-year period 1982–1991. The Brier skill score and, Relative Operating Characteristics (ROC) are used to assess the predictability of rainfall and to validate rainfall simulations, in several regions world wide. In general, the annual cycle of precipitation is well simulated by the model for several continental and oceanic regions in the tropics and mid latitudes. Interannual variability of rainfall during the peak rainy season is realistically simulated in Northeast Brazil, Amazonia, central Chile, and southern Argentina–Uruguay, Eastern Africa, and tropical Pacific regions, where the model shows good skill. Some regions, such as northwest Peru–Ecuador, and southern Brazil exhibit a realistic simulation of rainfall anomalies associated with extreme El Nino warming conditions, while in years with neutral or La Nina conditions, the agreement between observed and simulated rainfall anomalies is not always present. In the monsoon regions of the world and in southern Africa, even though the model reproduces the annual cycle of rainfall, the skill of the model is low for the simulation of the interannual variability. This is indicative of mechanisms other than the external SST forcing, such as the effect of land–surface moisture and snow feedbacks or the representation of sub-grid scale processes, indicating the important role of factors other than external boundary forcing. The model captures the well-known signatures of rainfall anomalies of El Nino in 1982–83 and 1986–87, indicating its sensitivity to strong external forcing. In normal years, internal climate variability can affect the predictability of climate in some regions, especially in monsoon areas of the world.

Simulation and Sensitivity in a Nested Modeling System for South America. Part II: GCM Boundary Forcing

Abstract A regional climate model driven at the lateral boundaries by ensemble integrations of a general circulation model (GCM) is used to investigate 1) the large-scale circulation anomalies associated with tropical sea surface temperatures (SSTs) that lead to extreme rainfall anomalies during January–May of 1983 (dry) and 1985 (wet) in tropical South America, and 2) the sensitivity of the nested model results to the choice of domain. The nested model is composed of the Regional Climate Model (RegCM) and the Community Climate Model version 3 (CCM3), both developed at the National Center for Atmospheric Research. In Part I of this study, the RegCM's ability to simulate circulation and rainfall observed in the two extreme seasons was demonstrated when driven at the lateral boundaries by reanalyzed forcing. Seasonal integrations with the RegCM driven by GCM ensemble–derived lateral boundary forcing demonstrate that the nested model responds well to the SST forcing, by capturing the major features of the ci...

Model study of the North Atlantic region atmospheric response to autumn tropical Atlantic sea‐surface‐temperature anomalies

AbstractLead–lag Maximum Covariance Analysis (MCA) between National Centers for Environmental Prediction re‐analysis sea surface temperature (SST) and 500 hPa geopotential‐height fields shows that autumn tropical Atlantic SST anomalies are significantly linked with the following‐winter North Atlantic Oscillation (NAO). The ability of the Météo‐France atmospheric general circulation model ARPEGE to reproduce this relationship is tested, by forcing it with autumn tropical SST anomalies derived from lead–lag MCA analysis results. The autumn SST forcing induces a strong wave‐like simultaneous response in October and November. The occurrence of the autumn weather regimes is also affected, in agreement with the significant spatial correlation of the midlatitude part of the wave response with the NAO pattern. By coupling the model with a slab ocean in midlatitudes, we show that the thermal coupling between the ocean and the atmosphere allows a better representation of the midlatitude part of the response. A negative autumn tropical SST anomaly triggers an interaction between the midlatitude SST, the low‐frequency circulation and the storm‐track activity, which reinforces and maintains a positive phase of the NAO until winter. Copyright © 2003 Royal Meteorological Society

Sensitivity of Tropical Tropospheric Temperature to Sea Surface Temperature Forcing*

Abstract During El Niño, there are substantial tropospheric temperature anomalies across the entire tropical belt associated with the warming of sea surface temperatures (SSTs) in the central and eastern Pacific. The quasi-equilibrium tropical circulation model (QTCM) is used to investigate the mechanisms for tropical tropospheric temperature response to SST forcing. In both observations and model simulations, the tropical averaged tropospheric temperature anomaly 〈T̂′〉 is approximately linear with the tropical mean SST anomaly 〈T′s〉 for observed SST forcing. Regional SST anomaly experiments are used to estimate regional sensitivity measures and quantify the degree of nonlinearity. For instance, SST anomalies of 3°C in the central Pacific would give a nonlinear 〈T̂′〉 response about 15% greater than a linear fit to small SST anomaly experiments would predict, but for the maximum observed SST anomaly in this region the response differs by only 5% from linearity. Nonlinearity in 〈T̂′〉 response is modest even when local precipitation response is highly nonlinear. While temperature anomalies have large spatial scales, the main precipitation anomaly tends to be local to the SST anomaly regions. The tropical averaged precipitation anomalies 〈P′〉 do not necessarily have a simple relation to tropical averaged tropospheric temperature anomalies or SST forcing. The approximate linearity of the 〈T̂′〉 response is due to two factors: 1) the strong nonlinearities that occur locally tend to be associated with the transport terms, which become small in the large-area average; and 2) the dependence on temperature of the top-of-atmosphere and surface fluxes has only weak nonlinearity over the range of 〈T̂′〉 variations. Analytical approximations to the QTCM suggest that the direct impact of climatological SST, via flux terms, contributes modestly to regional variations in the sensitivity α of 〈T̂′〉 to 〈T′s〉. Wind speed has a fairly strong effect on α but tends to oppose the direct effect of SST since cold SST regions often have stronger climatological wind, which would yield larger slopes. A substantial contribution to regional variation in α comes from the different reaction of moisture to SST anomalies in precipitating and nonprecipitating regions. Although regions over climatologically warm water have a slightly higher sensitivity, subregions of El Niño SST anomalies even in the colder eastern Pacific contribute substantially to tropospheric temperature anomalies.

The Nature and Causes for the Delayed Atmospheric Response to El Niño

Abstract Remarkable among the atmospheric phenomena associated with El Niño–Southern Oscillation (ENSO) is the lag in the zonal mean tropical thermal anomalies relative to equatorial east Pacific sea surface temperatures (SSTs). For the period 1950–99, the maximum correlation between observed zonal mean tropical 200-mb heights and a Niño-3.4 (5°N–5°S, 120°–170°W) SST index occurs when the atmosphere lags by 1–3 months, consistent with numerous previous studies. Results from atmospheric general circulation model (GCM) simulations forced by the monthly SST variations of the last half-century confirm and establish the robustness of this observed lag. An additional feature of the delay in atmospheric response that involves an apparent memory or lingering of the tropical thermal anomalies several seasons beyond the Niño-3.4 SST index peak is documented in this study. It is characterized by a strong asymmetry in the strength of the zonal mean tropical 200-mb height response relative to that peak, being threefold stronger in the summer following the peak compared to the preceding summer. This occurs despite weaker Niño-3.4 SST forcing in the following summer compared to the preceding summer. The 1–3-month lag in maximum correlation is reconciled by the fact that the rainfall evolution in the tropical Pacific associated with the ENSO SST anomalies itself lags one season, with the latter acting as the immediate forcing for the 200-mb heights. This aspect of the lagged behavior in the tropical atmospheric response occurs independent of any changes in SSTs outside of the tropical east Pacific core region of SST variability related to ENSO. The lingering of the tropical atmospheric thermal signal cannot, however, be reconciled with the ENSO-related SST variability in the tropical eastern Pacific. This part of the tropical atmospheric response is instead intimately tied to the tropical ocean's lagged response to the equatorial east Pacific SST variability, including a warming of the tropical Indian and Atlantic SSTs that peak several seasons after the Niño-3.4 warming peak.

Influence of sea surface temperature anomalies in the Gulf of California on North American monsoon rainfall

Linkages between sea surface temperatures (SSTs) in the Gulf of California and the North American monsoon rainfall were examined using observational data and regional model experiments. SSTs in the Gulf start to increase in late June and reach a maximum in August. Monsoon onset dates in the Southwest were determined from the gridded rainfall analysis. Monsoon onsets can occur before SSTs in the Gulf reach a maximum. There is no relationship between monsoon rainfall onset dates or seasonal total rainfall over the Southwest and SSTs in the Gulf. Regional model experiments were then performed to study the impact of local SSTs on monsoon rainfall using the NCEP regional spectral model (RSM). RSM experiments were performed with the observed SSTs for four summers (July–September 1997–2000). The experiments were repeated with the same initial and boundary conditions but with the climatological SSTs in the Gulf of California and its vicinity. The model is able to capture the general features of monsoon rainfall and the diurnal cycle. Warm (cold) SSTs in the Gulf are responsible for more (less) rainfall along the western slopes of the Sierra Madre Occidental. Over the Southwest, the impact of local SSTs is small. Large‐scale flow has more influence on monsoon rainfall than SSTs in the Gulf of California because the SST forcing does not produce significant changes in the low level flow needed to influence rainfall over the Southwest.

Sea surface temperature forcing of the upward trend in U.S. extreme precipitation

Retrospective climate simulations using the Goddard Institute for Space Studies SI2000 global climate model were used to examine possible causes of the observed upward trend in the frequency of U. S. extreme precipitation events. Three experiments were performed for the period 1951–1997, all with identical external radiative forcing but with different ocean treatments. One experiment used the “Q flux” ocean model. The second experiment imposed observed time‐varying sea surface temperatures (SSTs). The third experiment imposed observed time‐varying SSTs for the tropical Pacific only and imposed climatological SSTs elsewhere. The Q flux ocean experiment did not reproduce the observed upward trend. Both experiments using observed time‐varying SSTs produced an upward trend qualitatively similar to the observed trend for 7‐day duration events, although agreement was not as good for 1‐day duration events. However, the experiment with time‐varying SSTs only in the tropical Pacific was found to be overly sensitive to the increasing frequency of El Niño events. Changes in atmospheric water vapor content for the two imposed‐SST experiments were consistent with what might be expected to accompany increases in extreme precipitation. These results suggest that the spatial and temporal variations of SSTs on a global scale may be one direct and proximate cause of the recent observed upward trend in U. S. extreme multiday‐duration precipitation events.

Effect of Tropical SST on the Northwest Pacific Subtropical Anticyclone. Part I: Linear Rossby Wave Propagation

The development of anomalous subtropical anticyclone around East Asia, influences the regional climate of Japan and Korea in the summer. The effect of tropical sea surface temperature (SST) is studied using a regional atmospheric model (MM5 ver. 3.5) with realistic lateral, and surface boundary forcing. The following results were obtained. (1) An uniform increases of SST around the Philippine Islands generates the propagation of a Rossby wave, resulting in the formation of an anomalous subtropical anticyclone around Japan/Korea. (2) This Rossby wave propagation is linear in that the model response to negative SST anomalies reverses polarity. The linearity suggests that the response to the SST forcing around the Philippine Islands is a deterministic problem, with its magnitude depending on the size of the SST anomaly. Nonlinearity increases if there is a tropical depression, or typhoon in the model domain. (3) There are a zonally weakened upper-level jet, and a meridionally intensified low-level jet, when the Rossby wave is excited by SST changes. A positive SST anomaly intensifies the meandering of the upper-level jet. (4) The time scale for the atmospheric circulation to respond to such an SST forcing, is about 5 days. (5) In the vertical, the model response is barotropic over 40°N∼55°N, 100°E∼160°E, and is baroclinic to the south over 10°N∼40°N, 100°E∼140°E. (6) The effect of SST forcing is about four times stronger than model internal variability. (7) Only positive SST forcing affects the precipitation pattern around Japan and Korea, causing rainfall to decrease and increase in these regions, respectively. Both positive and negative SST forcing affect the precipitation around the Philippine Islands; a positive SST forcing increases precipitation, and vice versa. (8) The response of above Rossby wave is sensitive to the geographical location of SST forcing, reaching a maximum in the region (110°E∼140°E, 10°N∼20°N) among the experiments carried out. The study results of meridionally propagating Rossby wave induced anomalous subtropical anticyclone contribute to the understanding of regional climate around East Asia in summer.

The atmospheric response to North Atlantic SST anomalies in seasonal prediction experiments

Seasonal forecasts performed over a 26 yr period as part of the Historical Seasonal Forecasting Project (HFP) are used to analyze the influence of North Atlantic sea surface temperature (SST) anomalies on the atmospheric circulation, its seasonality, and model dependence. The signals related to the El Nino events are first removed from both the SST and the atmospheric data. The North Atlantic SST and the ensemble mean forecast are then correlated over the 26 yr to identify the model response to the SST forcing. The signal-to-noise ratio shows that in spring there is a significant forecast signal that is related to the SST anomaly in the North Atlantic. In that season the two models used in the HFP yield responses to the SST anomaly that are both similar to each other and to the observed response. For the other seasons the agreement between the responses and the observed atmospheric anomalies is poor. In winter the response is very sensitive to the model used.

Tropical Atlantic Sea Surface Temperature Forcing of Quasi-Decadal Climate Variability over the North Atlantic–European Region

Abstract Effects of Atlantic sea surface temperature (SST) anomalies on the North Atlantic low-frequency atmospheric variability are examined by analyzing two ensembles of integrations of the ARPEGE general circulation model (GCM) forced with differently configured observed SSTs and sea ice extents (SIE) over the 1948–98 period. An attempt is made to separate the forced atmospheric response from internal atmospheric variability by using a signal-to-noise maximizing empirical orthogonal function (EOF) analysis. This method yields an estimate of the most detectable common forced response given the knowledge of internal variability provided by the ensemble. Applying the algorithm to North Atlantic atmospheric model data leads to an atmospheric response associated with a tripole pattern in North Atlantic SSTs. The spatial structure of the forced response, which is most consistent in winter, shows a dipole pattern in mean sea level pressure projecting onto the North Atlantic Oscillation. Examination of other a...

Impact of CO2 concentration changes on the biosphere‐atmosphere system of West Africa

AbstractVegetation dynamics plays a critical role in causing the decadal variability of precipitation over the Sahel region of West Africa. However, the potential impact of changes in CO2 concentration on vegetation dynamics and precipitation variability of this region has not been addressed by previous studies. In this paper, we explore the role of CO2 concentration in the regional climate system of West Africa using a zonally symmetric, synchronously coupled biosphere‐atmosphere model. We first document the response of precipitation and vegetation to incremental changes of CO2 concentration; the impact of CO2 concentration on the variability of the regional biosphere‐atmosphere system is then addressed using the second half of the twentieth century as an example. An increase of CO2 concentration causes the regional biosphere‐atmosphere system to become wetter and greener, with the radiative effect of CO2 and improved plant‐water relation dominant in the Sahelian grassland region and the direct enhancement of leaf carbon assimilation dominant in the tree‐covered region to the south. Driven by the observed sea surface temperature (SST) of the tropical Atlantic Ocean during the period 1950–97 and with CO2 concentration prescribed at a pre‐industrial level 300ppmv, the model simulates a persistent Sahel drought during the period of 1960s−1990s. The simulated drought takes place in the form of a transition of the coupled biosphere‐atmosphere system from a wet/green regime in the 1950s to a dry/barren regime after the 1960s. This climate transition is triggered by SST forcing and materialized through vegetation‐climate interactions. The same SST forcing does not produce such a persistent drought when a constant modern CO2 concentration of 350ppmv is specified, indicating that the biosphere‐atmosphere system at higher CO2 level is more resilient to drought‐inducing external forcings. This finding suggests that the regional climate in Sahel, which tends to alternate between dry and wet spells, may experience longer (or more frequent) wet episodes and shorter (or less frequent) dry episodes in the future than in the past. Our study has significant implications regarding the impact of climate change on regional socio‐economic development.

Sensitivity of seasonal climate forecasts to persisted SST anomalies

Most estimates of the skill of atmospheric general circulation models (AGCMs) for forecasting seasonal climate anomalies have been based on simulations with actual observed sea surface temperatures (SSTs) as lower boundary forcing. Similarly estimates of the climatological response characteristics of AGCMs used for seasonal-to-interannual climate prediction generally rest on historical simulations using SST forecasts. This work examines the errors and biases introduced into the seasonal precipitation response of an AGCM forced with persisted SST anomalies, which are generally considered to constitute a good prediction of SST in the first three-month season. The added uncertainty introduced by the persisted SST anomalies weakens, and in some cases nullifies, the skill of atmospheric predictions that is possible given perfect SST forcing. The use of persisted SST anomalies also leads to changes in local signal-to-noise characteristics. Thus, it is argued that seasonal-to-interannual forecasts using AGCMs should be interpreted relative to historical runs that were subject to the same strategy of boundary forcing used in the current forecast in order to properly account for errors and biases introduced by the particular SST prediction strategy. Two case studies are examined to illustrate how the sensitivity of the climate response to predicted SSTs may be used as a diagnostic to suggest improvements to the predicted SSTs.

Atmospheric Response Patterns Associated with Tropical Forcing

Abstract Atmospheric response patterns associated with tropical forcing are examined with general circulation models driven by global sea surface temperature (SST) variations during 1950–99. Specifically the sensitivity of midlatitude responses to the magnitude and position of tropical SST anomalies is explored. This controversial problem, spanning more than a quarter century now, centers on whether response patterns over the Pacific–North American region are affected or changed by inter–El Nino variability in tropical forcing. Ensemble methods are used in this study to reliably identify the signals related to various tropical SST forcings, and the sensitivity is determined from analysis of four different climate models. First, the fraction of Pacific–North American (PNA) wintertime 500-hPa height variability that is potentially predictable and is linked to interannual variations in the global SSTs is identified. This SST-forced component accounts for as much as 20%–30% of the total seasonal mean height v...