Sea Surface Temperature Forcing Research Articles

Atlantic air–sea interaction and seasonal predictability

AbstractWe analyse patterns of North Atlantic air–sea interaction in observations and model output throughout the annual cycle with the dual aims of validating natural variability in climate models, and quantifying seasonal predictability of the atmosphere and of ocean surface temperatures.The Met Office's HadCM3 coupled ocean–atmosphere model is shown to represent well the atmospheric forcing of sea surface temperature (SST) in the North Atlantic region through the action of latent‐heat fluxes. Winter ocean temperature anomalies in both observations and model appear to be preserved below the shallow summer mixed layer and to re‐emerge in the following autumn.We find observational evidence from the last half of the twentieth century for SST forcing of the atmospheric circulation in the North Atlantic region. Results validate our atmospheric model (HadAM2b) and confirm the levels of potential predictability of the North Atlantic Oscillation (NAO) that have been found in recent modelling studies. We suggest that the ultimate correlation skill of a winter NAO hindcast in this period is in the range 0.45 to 0.63.Analysis of observational and atmospheric model data from the first half of the twentieth century identifies very little predictability of the winter NAO. We suggest that the change in seasonal predictability is genuine and may be related to the strength of decadal oscillations.The ocean–atmosphere model, which incorporates a different atmospheric component (HadAM3), fails to show a causal link between North Atlantic temperatures and the atmospheric circulation even during periods where low‐frequency variability is as strong as in the recent observations.A brief observational investigation suggests that any implied predictability of the winter NAO based on a knowledge of South Atlantic SSTs depends on the existence of secular trends in the data. Tropical Pacific SSTs appear to influence the winter climate of the North Atlantic region, but not via trends in the NAO. © Crown copyright, 2002.

On the surface and atmospheric temperature changes following the 1991 Pinatubo volcanic eruption: A GCM study

Ensemble numerical simulations are performed using an atmospheric general circulation model to study the responses of the atmosphere to volcanic aerosol forcing and sea surface temperature (SST) forcing during the two years following the Pinatubo eruption. The simulated surface air temperature (SAT) anomalies, forced respectively and jointly by the Pinatubo aerosol and the observed SST anomalies, are compared to those derived from observations using a singular value decomposition (SVD) model. The simulated land SAT anomalies, forced only by the observed SST anomalies, match well in magnitude and time the leading mode of SAT anomalies from the SVD analysis, which represents the ENSO‐forced SAT variations. When forced by both the Pinatubo aerosol and the observed SST anomalies, the model captures the observed surface cooling over land in June–July–August and September–October–November 1992 but misses the surface warming over northern Eurasia in December–January–February (DJF) 1991–1992 and DJF 1992–1993. In the atmosphere the simulated temperature anomalies in the troposphere (stratosphere) forced by the Pinatubo aerosol are sensitive (insensitive) to the initial conditions and the type of prescribed SST. The signal of SST forcing is stronger in the troposphere and near the surface than in the stratosphere, while the signal of the Pinatubo aerosol forcing is strongest in the lower stratosphere. The simulated global‐mean stratospheric temperature anomalies induced by the Pinatubo aerosol are generally 1°C to 1.5°C larger than observed. Empirical data analyses and numerical model simulations showed that this discrepancy is explained in part by the influences of the quasi‐biennial oscillation and the observed ozone depletion, which the ensemble numerical simulations did not resolve.

Observed and SST‐forced seasonal rainfall variability across tropical America

AbstractThree experiments starting from different initial conditions have been made with the ECHAM‐4 atmospheric General Circulation Model (GCM) integrated at T30 resolution forced with the observed sea‐surface temperature (SST) over the period 1960–1994. The tropical America modes of seasonal rainfall anomalies whose time variation is most accurately simulated by the GCM have been searched for using Singular Value Decomposition Analyses (SVDA) and Canonical Correlation Analysis (CCA) between observed and model fields. The leading modes revealed by SVDA and CCA are highly similar, even though the ordering of the modes showed some fluctuation. A first skilful rainfall anomaly mode has weights of the same sign almost everywhere in tropical America, except along the western coast and the sub‐tropical margins. This mode appears in all of the four seasons assessed. A second major skilful mode is usually a bipolar north–south (N–S) rainfall anomaly pattern (clear in December–March, DJFM; March–May, MAM; and June–September, JJAS).A large portion of the skill of the first rainfall anomaly mode (same sign anomalies across tropical America except small patches along the western coast) is through variance that is in common with the Southern Oscillation Index (SOI). In addition to forcing from the central/eastern tropical Pacific SST, there also appears a contribution from contrasting SST anomalies in the tropical Atlantic. This rainfall mode is usually a regional portion of a more large‐scale mode encompassing at least the whole tropical zone (especially in DJFM, MAM and September–November, SON). Analysis of the relationship of this mode with GCM circulation features reveals that a rainfall deficit (respectively excedent) over the main rainbelt of the tropical America region is associated with strengthening (respectively weakening) of the sub‐tropical westerly jet streams, a global warming (respectively cooling) of the tropical atmosphere, an anomalous divergence (respectively convergence) in the lower levels and an anomalous convergence (respectively divergence) in the upper levels over tropical America and in the region of the Atlantic Inter‐tropical Convergence Zone (ITCZ). Such global features are not so apparent for the dominant mode of JJAS, even though the correlations with El Niño–Southern Oscillation (ENSO) indicators (as SOI or NINO3 SST index) are as high as for the other seasons.The bipolar N–S rainfall anomaly mode in tropical America is mostly related to anomalous N–S gradient of SST anomalies in the tropical Atlantic. The atmospheric circulation anomalies emphasize changes in 850 hPa meridional winds in the tropical Atlantic. However, there is also interannual variance of this rainfall mode in both the model and observations that is unexplained by tropical Atlantic SSTs, but which is explained by central/eastern tropical Pacific SSTs and, potentially, SSTs from other tropical and extratropical areas. This is especially true in MAM.Some differences in the details of the model and observed teleconnection patterns are noted. Such differences can be used to statistically adjust the model simulations using the CCA or SVDA modes as basis patterns. Both statistical approaches have been applied and the results are consistent between the two. The increase of skill is stronger when temporal correlation (the pattern correlation) between the model and observed pattern is high (low) as for JJAS. The skill is moderate to high around the whole Amazon basin, but remains relatively low inside the Amazon basin, though reliability of the observations themselves may influence this result. Averaged over all the seasons, about 15–35% (35–55%) of the interannual grid‐box (regional) seasonal rainfall variance is skilfully simulated from the observed SST forcing. Copyright © 2001 Royal Meteorological Society

Teleconnections between global sea-surface temperatures and the interannual variability of observed and model simulated rainfall over southern Africa

Sea-surface temperatures (SSTs) for the thirty-year period 1961–1990 are prescribed as boundary forcing for a series of atmospheric general circulation model (AGCM) experiments. The results of five separate multidecadal AGCM runs, each forced with the same SSTs, but different initial conditions, have been used to form an ensemble. The interannual variability of AGCM simulated rainfall over South Africa and Namibia (southern Africa) are compared with observations. Empirical orthogonal function (EOF) analyses revealed a strong correlation between the dominant observed austral summer season (October–March) and mid-summer season (December and January) rainfall patterns over southern Africa and the corresponding model results. It is shown that the AGCM simulated austral summer season rainfall variability in the dominant amplitude time series (first principle component or PC1 time series) compare well with SST fluctuations over the equatorial Pacific and tropical western Indian Oceans. Although the associated dominant observed rainfall time series accounts for a larger fraction of the total variability (relative to the AGCM), it also compares well with equatorial Pacific and tropical western Indian Ocean SST fluctuations. To a certain degree these results accentuates the model's ability to capture the major summer seasonal rainfall variability as a response of global SST forcing, which might make the AGCM suitable for future use in seasonal rainfall forecasting research. Similar associations could not be found for the austral winter season (May–August).

Local and remote forcing of sea surface temperature in the coastal upwelling system off Chile

An analysis is presented of records of sea surface temperature (SST), sea level and winds collected during the period 1991–1995 along the coast of northern and central Chile (sea level also from the Peru coast in 1991–1994), and of SST from remote sensing during the austral summers of 1991 and 1992 off northern Chile. Large, interannual fluctuations of SST and sea level off northern Chile are linked to vertical displacements of the equatorial thermocline during El Niño‐La Niña events. In the intraseasonal, 50‐day band, SST and sea level are intimately related all along the coast, whereby SST anomalies lag sea level perturbations by about 12 days. Such sea level fluctuations are due to free, coastal‐trapped waves, ultimately forced via equatorial Kelvin waves by 50‐day wind events in the western and central tropical Pacific. These results and those of the remote sensing analysis suggest significant remote forcing of SST fluctuations in the coastal zone off Chile by the action of the coastal‐trapped waves. Off central Chile, SST is also related to local wind stress in the intraseasonal band. Simulations with a simple, local, wind‐driven upwelling/entrainment model capture rather well observed synoptic and intraseasonal oscillations of SST there. Off northern Chile, however, this simple model is not successful in simulating observed SST. An attempt to include effects of coastal‐trapped waves into this simple, one‐dimensional model did not improve model fit to data at either location. An explanation for the apparent strong influence of these waves on SST along this coast should therefore be sought in other effects not considered by this model such as cross‐shelf and alongshore advection.

Impacts of sea surface temperature in the tropical Pacific on interannual variability of Madden-Julian Oscillation in precipitation

The Madden-Julian Oscillation (MJO) is investigated in two sets of 11-year records of observed precipitation, the daily mean Microwave Sounding Units (MSU) oceanic rainfall (Spencer, 1993) data and the pentad Climate Prediction Center Merged Analysis of Precipitation (CMAP) data (Xie and Arkin, 1997). Obvious interannual variability is found in the MJO in the tropical Pacific. MJO is limited to the west of dateline in normal years while extends more east during the year of warm sea surface temperature (SST) appeared in the eastern Pacific (i.e., El Nino years of 1982–1983, 1986–1988, 1991–1992) and manifested in the central-eastern Pacific for several months. The most significant correlation between interannual variability of MJO in the central-eastern Pacific and SST was found in the vicinity of the Nino3 region. Forced by observed SST, CCM3 presents a realistic trend of interannual variability to MJO in the 11 years, with a smaller magnitude than that from the observation. Comparison between the two realizations of the CCM3 simulation, which are forced by weekly and monthly mean SST respectively, showed that the MJO activities resemble each other in central-eastern Pacific while there is discrepancy in the western Pacific. It is suggested that the interannual variability of MJO is controlled, to certain extent bythe powerful interannual variability of SST in the central-eastern Pacific. In the western Pacific, however, there were remarkable impacts of the intraseasonal oscillation of SST on the MJO, where there was active MJO around the year. The notable disagreement between simulated and observed MJO in the western Pacific may come from the lack of high frequency variation of SST force, or from the shortage of air sea interaction for the intraseasonal time scale. It might be of importance to the MJO which is unable to be represented in the atmospheric model.

Coherent Modes of Global SST and Summer Rainfall over China: An Assessment of the Regional Impacts of the 1997–98 El Niño

In this paper, we have identified three principal modes of summertime rainfall variability over China and global sea surface temperature (SST) for the period 1955-1998. Using these modes, we have assessed the impact of the El Nino/La Nina on major drought and flood occurrence over China during 1997-1998. The first mode can be identified with the growth phase of El Nino superimposed on a linear warming trend since the mid-1950s. This mode strongly influences rainfall over northern China. The second mode comprises of a quasi-biennial tendency manifested in alternate wet and dry years over the Yangtze River Valley (YRV) of central China. The third mode is dominated by a quasi-decadal oscillation in eastern China between the Yangtze River and the Yellow River. Using a mode-by-mode reconstruction, we evaluate the impacts of the various principal modes on the 1997 and 1998 observed rainfall anomaly. We find that the severe drought in northern China, and to a lesser degree the flood in southern China, in 1997 is likely a result of the influence of anomalous SST forcing during the growth phase of the El Nino. In addition, rainfall in southern China may be influenced by the decadal or long-term SST variability. The severe flood over the Yangtze River Valley in 1998 is associated with the biennial tendency of basin scale SST during the transition from El Nino to La Nina in 1997-98. Additionally, the observed prolonged drought over northern China and increasing flooding over the YRV since the 1950s may be associated with a long-term warming trend in the tropical Indian and western Pacific ocean. During 1997, the El Nino SST exacerbated the drought situation over northern China. In 1998, the drought appeared to get temporary relief from the La Nina anomalous SST forcing.

On possible impact of the Indian Summer Monsoon on the ENSO

The Indian summer monsoon (ISM) could influence the El Niño and Southern Oscillation (ENSO) only if it could induce significant surface wind anomalies in the active regions of central and eastern equatorial Pacific. Using 50‐year NCEP reanalysis, it is shown that observed surface winds in the central and eastern Pacific associated ‘purely’ with ISM and unrelated to ENSO are very weak (∼0.5m.s−1). Strong surface winds in the central and eastern Pacific following a ‘strong’ or ‘weak’ ISM, noted in some earlier composite analyses, are related not to ISM but to the concurrent sea surface temperature (SST) forcing associated with the ENSO. A long run of an atmospheric general circulation model (AGCM) without inter‐annual SST forcing also show that a ‘pure’ ISM induces only very weak surface winds in the equatorial central and eastern Pacific. Thus, we conclude that the ISM by itself is unlikely to influence the ENSO in a significant way.

The Sustained North American Warming of 1997 and 1998

North America experienced sustained and strong surface warming during 1997 and 1998. This period coincided with a dramatic swing of the El Niño–Southern Oscillation (ENSO), with El Niño in 1997 rapidly replaced by La Niña in 1998. An additional aspect of the sea surface temperatures (SSTs) was the warmth of the world oceans as a whole for the entire period, with unprecedented amplitudes within the recent instrumental record. Using a suite of dynamical and empirical model simulations, this study examines the causes for the North American warming, focusing on the role of the sea surface boundary conditions. Two sets of atmospheric general circulation model experiments, one forced with the observed global SSTs and the other with the tropical east Pacific portion only, produce similar North American–wide warming during fall and winter of 1997. The GCM results match empirical estimates of the canonical temperature response related to a strong El Niño and confirm that east equatorial Pacific SST forcing was a major factor in the continental warming of 1997. Perpetuation of that warming from spring through fall of 1998 is shown to be unrelated to equatorial east Pacific SSTs and thus cannot be attributed to the ENSO cycle directly. Yet, simulations using the observed global SSTs are shown to reproduce realistically the continuation of North American warming throughout 1998. The continental warmth occurs in tandem with a warming of the troposphere that, initially confined to tropical latitudes during El Niño’s peak in 1997, spreads poleward and covers the entire globe in 1998. This evolutionary aspect of the global circulation anomalies during 1997 and 1998 is found to be a response to global SSTs and not linked directly to ENSO’s evolution. Results presented here demonstrate that a significant fraction of the North American warming in 1997 and 1998 is explainable as the forced response to sea surface boundary conditions. The hand-over in the impact of those SSTs, with a classic ENSO driven signal in 1997 but an outwardly independent signal in 1998 related to the disposition of global SSTs outside the ENSO region is emphasized. The high potential predictability of North American climate during this 2-yr period raises new questions on the role of global SSTs in climate variability and the ability to predict them skillfully.

Pacific Sea Surface Temperature Forcing Dominates Orbital Forcing of the Early Holocene Monsoon

Orbital forcing is known to play a primary role in regulating the strength of the south Asian monsoon circulation. In this study, a comparison is made between orbital forcing and Pacific sea surface temperature (SST) forcing of the monsoon through a sequence of atmospheric general circulation model experiments configured for 6,000 and 9,000 yr B.P. Early–mid Holocene orbital parameters are shown to increase continental seasonality as well as the meridional mean, the zonal mean, and the summer monsoon circulations. Winds in the southeast Asian monsoon are weakened by warm Pacific SST to such an extent that the increase in strength caused by early Holocene orbital parameters is offset. These results imply that SSTs are potentially as important as orbital parameters in governing the monsoon and that more data—particularly from the equatorial Pacific—are crucial to deciphering Holocene climate.

Instability and large‐scale circulations in a two‐column model of the tropical troposphere

AbstractA two‐column model of the tropical atmosphere is developed. The two columns are kept in buoyancy equilibrium by mass exchange driven by the inter‐column pressure gradient, and diabatic processes are parametrized by highly simplified schemes. This model is used to investigate whether spontaneous large‐scale circulations will develop in the tropical atmosphere when (he sea surface temperature and solar radiative forcing are uniform. Such circulations do indeed develop in the model, with ascent in one column and descent in the other, when one of the columns is slightly perturbed from the initial state of radiative‐convective equilibrium. A key element of the circulation dynamics is that increased equivalent potential temperature in a column leads to enhanced convection and rainfall in that column, which further increases the equivalent potential temperature there. The latter effect arises because convection enhances surface heat fluxes and decreases outgoing long‐wave radiation by virtue of the increase in stratiform cloudiness. These results form an attractive explanation for the observed patchiness of deep convection over warm tropical oceans and suggest that further modelling and observational work be directed towards understanding the budget of equivalent potential temperature in the tropical atmosphere and its relationship to rainfall.

Changes in the Spread of the Variability of the Seasonal Mean Atmospheric States Associated with ENSO

Abstract For a fixed sea surface temperature (SST) forcing, the variability of the observed seasonal mean atmospheric states in the extratropical latitudes can be characterized in terms of probability distribution functions (PDFs). Predictability of the seasonal mean anomalies related to interannual variations in the SSTs, therefore, entails understanding the influence of SST forcing on various moments of the probability distribution that characterize the variability of the seasonal means. Such an understanding for changes in the first moment of the PDF for the seasonal means with SSTs is well documented. In this paper the analysis is extended to include also the impact of SST forcing on the second moment of the PDFs. The analysis is primarily based on ensemble atmospheric general circulation model (AGCM) simulations forced with observed SSTs for the period 1950–94. To establish the robustness of the results and to ensure that they are not unduly affected by biases in a particular AGCM, the analysis is ba...

Maintenance of Tropical Intraseasonal Variability: Impact of Evaporation–Wind Feedback and Midlatitude Storms

An intraseasonal tropical oscillation with a period of 20–80 days is simulated in the Neelin–Zeng Quasi-Equilibrium Tropical Circulation Model. This model is an intermediate-level atmospheric model that includes primitive equation nonlinearity, radiative– convective feedbacks, a simple land model with soil moisture, and a Betts–Miller-type moist convective adjustment parameterization. Vertical temperature and moisture structures in the model are based on quasi-equilibrium profiles taken from deep convective regions. The tropical intraseasonal variability is reasonably broadband. The eastward propagating 20–80-day variability is dominated by zonal wavenumber 1, shows features similar to an irregular Madden–Julian oscillation (MJO), and exhibits amplitude and phase speeds that vary both seasonally and between events. At higher wavenumbers, the model has a distinction between the low-frequency MJO-like band and the moist Kelvin wave band, similar to that found in observations. In the model, it is conjectured that this arises by interaction of the wavenumber-1 moist Kelvin wave with the zonally asymmetric basic state. Experiments using climatological sea surface temperature forcing are conducted using this model to examine the effects of evaporation–wind feedback and extratropical excitation on the maintenance of intraseasonal variability, with particular attention paid to the low wavenumber mode in the 20–80-day band. These experiments indicate that evaporation–wind feedback partially organizes this intraseasonal variability by reducing damping, but it is not by itself sufficient to sustain this oscillation for the most realistic parameters. Excitation by extratropical variability is a major source of energy for the intraseasonal variability in this model. When midlatitude storms are suppressed, tropical intraseasonal variability is nearly eliminated. However, the eastward propagating intraseasonal signal appears most clearly when midlatitude excitation is aided by the evaporation–wind feedback.

Impact of global sea surface temperature on summer and winter temperatures in Europe in a set of seasonal ensemble simulations

AbstractIn the PRediction Of climate Variations On Seasonal to interannual Time‐scales (PROVOST) ensemble of seasonal simulations, performed with the European Centre for Medium‐Range Weather Forecasts (ECMWF) atmospheric general‐circulation model forced by observed sea surface temperature (SST) for the 1979–1993 period, the consistency between individual members of the ensemble of simulated temperature at 850 hPa (T850) in Europe varies considerably from year to year. This interannual variability is assumed to be a consequence of slowly varying lower‐boundary forcings, most notably the SST forcing, plus a random part from the internal, chaotic atmospheric variability. In those years when the ensemble members show a particularly consistent pattern of T850 anomalies in Europe, the consistency is significantly higher than would be expected from the interannual variability, indicating that the SST forcing has a significant impact on the model simulated temperature. Composites of global SST indicate that the El Niño/Southern Oscillation (ENSO) might be linked to the high internal consistency of some of the winter season ensembles of T850 anomaly patterns in Europe. However, in some ENSO years the ensemble shows no consistent signal in Europe, so the occurrence of an ENSO event is not sufficient to ensure high internal ensemble consistency in Europe.In a separate approach, an attempt to linearly relate global SST anomaly patterns to the ensemble mean of T850, using canonical correlation analysis, also indicates a link between simulated T850 anomaly patterns in Europe and ENSO events. Mature El Niño (La Niña) conditions are associated with positive (negative) T850 anomalies in the model in southern Europe and negative (positive) anomalies in northern Scandinavia and over the Barents Sea in the January‐February season. A weaker relation is found between onsetting ENSO conditions and T850 anomalies in the model in the south‐eastern and westernmost parts of Europe in the July‐August season.Validations against re‐analysed observations of T850 show poor skill in Europe for the ensemble mean of simulated T850, and no obvious relation is found between internal ensemble consistency and skill. The link that is found in the model between ENSO and simulated T850 in Europe, is not evident between ENSO and re‐analysed T850.

The Effect of Local Sea Surface Temperatures on Atmospheric Circulation over the Tropical Atlantic Sector

The effects of tropical Atlantic sea surface temperature (SST) anomalies on atmospheric circulation are examined by analyzing several ensembles of integrations of an atmospheric general circulation model (AGCM) forced with differently configured SSTs. An attempt is made to separate the atmospheric response to local SST forcing from internal atmospheric variability, using various statistical analyses. The analyses reveal a robust pattern of atmospheric response to SST forcing. The dominant response is largely confined within the tropical Atlantic sector and may be associated with the variation in location and intensity of the intertropical convergence zone (ITCZ) in response to changes in SST gradient near the equator. Within the deep Tropics, particularly in the western tropical Atlantic warm pool region, there is an indication of a positive feedback between surface heat flux and SST anomalies. In this warm SST region, the latent heat flux tends to dominate surface heat flux variability, and the positive feedback takes place between the wind-induced flux and SST. Outside the deep Tropics, surface heat flux generally tends to dampen the SST, indicating a negative feedback. A strong negative feedback is found off the coast of west Saharan Africa, where the mean SST is cold and the surface heat flux variability is largely induced by the air‐sea temperature difference. The forced response exhibits a seasonal dependence. The structure throughout the year bears a close resemblance to the winter pattern, suggesting that the anomalies during the boreal winter contribute to much of the year-to-year variability in the tropical Atlantic sector. An ENSO-like response in the tropical Atlantic is also identified. It appears as the second dominant forced response and has its strongest manifestation during the boreal fall. By comparing the responses of different sets of ensemble experiments, it is found that the dominant near-surface atmospheric response in the tropical Atlantic sector primarily comes from the local SST forcing. A significant remote influence of the Pacific ENSO on the tropical Atlantic variability is also noted. The strongest remote influence from the tropical Pacific occurs during the boreal spring. The companion paper by Saravanan and Chang further explores the effect of ENSO using an index-based regression analysis.

Impact of initial conditions on seasonal simulations with an atmospheric general circulation model

AbstractMany previous studies have examined the use of very long integrations of atmospheric general circulation models (AGCMs) forced by observed sea surface temperatures (SSTs) as proxies for seasonal atmospheric predictions. These long simulations explore a boundary‐value problem in which significant deviations from the model's long‐term climatology must be a result of the SST forcing. Seasonal lead simulations starting with observed initial conditions (ICs) for the atmosphere and land surface while retaining observed SST forcing are an intermediate step between the pure boundary‐value problem and the pure initial‐value forecast problem in which SSTs are also predicted. As part of the Dynamical Seasonal Prediction (DSP) experiment, an ensemble of AGCM integrations with observed atmospheric ICs and model climatology land surface ICs was integrated from mid‐December through March for 16 years. These DSP simulation ensembles are compared to ensembles of long boundary‐value simulations from the same AGCM in a perfect‐model setting (no comparisons of simulations to observations are attempted). Significant differences must be due to the impact of the DSP ICs. Surprisingly large and long‐lived differences are found in both the mean and the variance of the ensembles. Many appear to occur because the ICs of the DSP runs are inconsistent with the AGCM climatology; an extended period of model ‘spin‐up’ is the result. Some differences are related to local impacts of the land surface ICs while others, like shifts in the distribution of tropical precipitation and a cooling of the northern hemisphere, are less obviously related to the ICs. The results suggest that care will be needed when inserting observed ICs into seasonal predictions in order to avoid the long‐term effects of model spin‐up.

Multidecadal Simulations of Australian Rainfall Variability: The Role of SSTs

Australian rainfall variability and its relationship with the Southern Oscillation index (SOI) and global sea surface temperature (SST) variability is considered in both observational datasets and ensembles of multidecadal simulations using two different atmospheric general circulation models forced by observed SSTs and sea ice extent. Monthly and seasonal time series have been constructed to examine the observed and modeled relationships. The models show some success in the Australian region, largely reproducing the observed relationships between rainfall, the SOI, and global SSTs, albeit better in some seasons and geographical regions than others. A partition of the rainfall variance into components due to SST forcing and internal variability, suggests that both models have too much internal variability over the central eastern half of the continent, especially during austral winter and spring. Consequently, the strength of the SOI and SST relationships tend to be underestimated in this region. The largest impact of SST forcing is seen over the tropical and western parts of the continent. A principal component analysis reveals two dominant rotated modes of rainfall variability that are very similar in both the observed and modeled cases. One of these modes is significantly correlated with SST anomalies to the north-northwest of Australia (in the case of the models) and the SST gradient between the Indonesian archepelago and the central Indian Ocean (in the observed case). The other mode is significantly correlated with

Influences of Sea Surface Temperature and Ground Wetness onAsian Summer Monsoon

Abstract The authors have conducted a series of experiments with a general circulation model to understand the influences of sea surface temperature (SST) and ground wetness (GW) (measured by snow amount and soil moisture content) on the Asian summer monsoon. The experiments are designed to illustrate the dominant features of monsoon response to SST and GW forcings and to delineate the relative importance of each forcing function in contributing to the variability of the monsoon. Results indicate that ocean basin-scale SST anomalies exert a stronger control on the interannual variability of the monsoon compared to GW anomalies. The impact of SST anomalies on the monsoon appears nonlinear with respect to warm and cold events. The monsoon is weakened during the warm events but changes less noticeably during the cold events. The diminution of monsoon circulation associated with the warm SST anomalies is accompanied by a broad-scale reduction in water vapor convergence and monsoon rainfall. Results also indic...

Annual Cycle of Pacific–North American Seasonal Predictability Associated with Different Phases of ENSO

Abstract The potential for seasonal mean predictability over the Pacific and North American regions is evaluated as a function of the amplitude of equatorial Pacific sea surface temperature forcing, the phase of that forcing, and the phase of the annual cycle. The potential predictability is measured as the ratio of the seasonal mean SST-forced signal and the internally generated seasonal mean noise. The authors’ assessments are derived from the output of ensemble atmospheric general circulation model experiments forced with observed monthly SSTs for 1950–94. Using a perfect prognostic approach, results are presented on the predictability of upper-tropospheric circulation, North American land temperature, and precipitation. Seasonal predictability is shown to depend on the amplitude of the SST-forced signal, whereas the background noise is largely independent of SSTs. To zero order, that signal grows linearly with the amplitude of anomalous SSTs. An important departure from this is with respect to the pha...

Mechanisms of monsoon-Southern Oscillation coupling: insights from GCM experiments

The relative roles of internal atmospheric dynamics, land surface evaporation and sea surface temperature (SST) forcings on the coupling between the Asian monsoon (AM) and the Southern Oscillation (SO) are investigated in a series of GCM experiments. Results confirm previous studies indicating that the characteristic large-scale pattern of the SO is due primarily to SST anomaly (SSTA) forcing. The AM circulation anomalies are coupled to the SO via a characteristic upper level circulation couplet over the equatorial central Pacific. This couplet acts as a radiating node for teleconnection signals originating from the AM region to the extratropics. Generally, a weak AM is associated with warm SST over the eastern equatorial Pacific, concomitant with the negative phase of the SO, i.e., low (high) surface pressure over Tahiti (Darwin). The reverse holds for strong AM. Two wavetrains associated with the AM fluctuation have been identified: one arcing over northeastern Asia via the Aleutians to North American, and another emanating from northwestern Europe, via Siberia to northern India. Internal dynamics appear to underpin the origin of these wavetrains, which are strongly tempered by SSTA forcing and to a lesser degree by interactive land processes. Regionally, land-atmosphere interaction seems to have the strongest impact over East Asia/Indochina and the adjacent oceanic region of the South China Sea. Here, land-atmosphere interaction is responsible for the enhancement of a subseasonal scale see-saw oscillation in precipitation between land and the adjacent oceans. A local land-atmosphere feedback mechanism involving strong coupling between the hydrologic and energy cycles is identified. It is suggested that the interaction among precipitation, moisture convergence and land surface turbulent heat fluxes and radiation processes play key roles in determining the fast (subseasonal and shorter scales) response of the AM. On these time scales, the occurrences of cool/wet and hot/dry states associated with the precipitation seesaw appear to be chaotic. However, the preferred occurrence of a given state and the abrupt transition between states are dependent on the large-scale circulation and radiation forcings induced by the SO. One of the more provocative findings here is that effects of land-atmosphere interaction do not seem to alter the basic planetary scale features of the AM-SO system. As a result, the interannual variability of the coupled AM-SO is relatively small in the absence of anomalous SST forcing. Yet, the local effect of land-atmosphere interaction on AM is quite pronounced and dependent upon the large-scale forcings related to SO.