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Gabriel Cazes-Boezio
,
Dimitris Menemenlis
, and
Carlos R. Mechoso

Abstract

The impact of ocean-state estimates generated by the consortium for Estimating the Circulation and Climate of the Ocean (ECCO) on the initialization of a coupled general circulation model (CGCM) for seasonal climate forecasts is examined. The CGCM consists of the University of California, Los Angeles, Atmospheric GCM (UCLA AGCM) and an ECCO ocean configuration of the Massachusetts Institute of Technology GCM (MITgcm). The forecasts correspond to ensemble seasonal hindcasts for the period 1993–2001. For the forecasts, the ocean component of the CGCM is initialized in either early March or in early June using ocean states provided either by an unconstrained forward ocean integration of the MITgcm (the “baseline” hindcasts) or by data-constrained ECCO results (the “ECCO” hindcasts). Forecast skill for both the baseline and the ECCO hindcasts is significantly higher than persistence and compares well with the skill of other state-of-the art CGCM forecast systems. For March initial conditions, the standard errors of sea surface temperature (SST) anomalies in ECCO hindcasts (relative to observed anomalies) are up to 1°C smaller than in the baseline hindcasts over the central and eastern equatorial Pacific (150°–120°W). For June initial conditions, the errors of ECCO hindcasts are up to 0.5°C smaller than in the baseline hindcasts. The smaller standard error of the ECCO hindcasts is, in part, due to a more realistic equatorial thermocline structure of the ECCO initial conditions. This study confirms the value of physically consistent ocean-state estimation for the initialization of seasonal climate forecasts.

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Rafael Terra
,
Carlos R. Mechoso
, and
Akio Arakawa

Abstract

This paper examines the impact of orographically induced mesoscale heterogeneities on the macroscopic behavior of planetary boundary layer (PBL) stratiform clouds, and implements and tests a physically based parameterization of this effect in the University of California, Los Angeles (UCLA), atmospheric general circulation model (AGCM). The orographic variance and associated thermal circulations induce inhomogeneities in the cloud field that can significantly alter the PBL evolution; an effect that has been largely ignored in existing climate models. The impact of this effect on AGCM simulations is examined and the mechanisms at work are studied by analyzing a series of Cloud System Resolving Model (CSRM) simulations.

Both the CSRM and AGCM results show that, in the absence of the orographic effect, the continental PBL tends to be in one of two regimes: the solid regime characterized by a cold and overcast PBL and the broken regime characterized by a low time-mean cloud incidence and a large-amplitude diurnal cycle. Without the orographic effect, the PBL may lock in the convectively stable solid regime, with deep convection displaced to the surrounding oceans and subsidence induced over land further contributing to the persistence of the cloud deck. The inclusion of the orographic effect weakens the feedback between the cloud's albedo and the ground temperature responsible for the existence of the two regimes and, therefore, conspires against the persistence of the solid regime rendering the behavior of the PBL–ground system less bimodal. The parameterization featured in this paper also increases the amplitude of the diurnal cycle in the AGCM and reduces the excessive seasonality in PBL cloud incidence, resulting in an improved simulation of convective precipitation over regions where the solid regime was spuriously dominating.

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Laura Zamboni
,
Carlos R. Mechoso
, and
Fred Kucharski

Abstract

The existence of a significant simultaneous correlation between bimonthly mean precipitation anomalies over southeastern South America (SESA) and either the first or the second (depending on season) leading mode of interannual variability of upper-level wind over South America (SA) is demonstrated during all seasons except winter. The pattern associated with these modes of variability is similar during all seasons and consists of a continental-scale vortex centered over the eastern coast of subtropical SA. The vortex has a quasi-barotropic structure during all seasons, and its variability modifies moisture transport from the South American low-level jet and the western tropical Atlantic to SESA thus creating precipitation anomalies in this region. During spring (October–November) and summer (January–February) the circulation creates a second center of precipitation anomalies over the South Atlantic convergence zone that are of opposite sign to those over SESA, while during fall (April–May) precipitation anomalies are primarily confined to SESA. On the basis of the correlation between upper-level winds and precipitation, an empirical method to produce long-range forecasts of bimonthly mean precipitation over SESA is developed. Method tests in hindcast mode for the period 1959–2001 show a potential for reliable predictions during the southern spring, summer, and fall. The method is further tested in an experimental mode by using Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER) wind hindcasts. Forecasts obtained in this way are skillful during spring only, with highest skill during El Niño–Southern Oscillation years. During summer and fall, the DEMETER forecasts of wind anomalies limit the method’s ability to make reliable real predictions.

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Tomoaki Ose
,
Carlos R. Mechoso
, and
David Halpern

Abstract

Simulations with the UCLA atmospheric general circulation model (AGCM) using two different global sea surface temperature (SST) datasets for January 1979 are compared. One of these datasets is based on COADS (SSTs) at locations where there are ship reports, and climatology elsewhere; the other is derived from measurements by instruments onboard NOAA satellites. In the former dataset (COADS SST), data are concentrated along shipping routes in the Northern Hemisphere; in the latter dataset (HIRS SST), data cover the global domain. Ensembles of five 30-day mean fields are obtained from integrations performed in the perpetual-January mode. The results are presented as anomalies, that is, departures of each ensemble mean from that produced in a control simulation with climatological SSTs.

Large differences are found between the anomalies obtained using COADS and HIRS SSTs, even in the Northern Hemisphere where the datasets am most similar to each other. The internal variability of the circulation in the control simulation and the simulated atmospheric response to anomalous forcings appear to be linked in that the pattern of geopotential height anomalies obtained using COADS SSTs resembles the fist empirical orthogonal function (EOF 1) in the control simulation. The corresponding pattern obtained using HIRS SSTs is substantially different and somewhat resembles EOF 2 in the sector from central North America to central Asia.

To gain insight into the reasons for these results, three additional simulations are carried out with SST anomalies confined to regions where COADS SSTs are substantially warmer than HIRS SSTS. The regions correspond to warm pools in the northwest and northeast Pacific, and the northwest Atlantic. These warm pools tend to produce positive geopotential height anomalies in the northeastern part of the corresponding oceans. Both warm pools in the Pacific produce large-scale circulation anomalies with a pattern that resembles that obtained using COADS SSTs as well as EOF 1 of the control simulation; the warm pool in the Atlantic does not. These results suggest that the differences obtained with COADS SSTs and HIRS SSTs are mostly due to the differences in the datasets over the northern Pacific.

There was a blocking episode near Greenland in late January 1979. Both simulations with warm SST anomalies over the northwest and northeast Pacific show a tendency toward increased incidence of North Atlantic blocking; the simulation with warm SST anomalies over the northwest Atlantic shows a tendency toward decreased incidence. These results suggest that features in both SST datasets that do not have a counterpart in the other dataset contribute significantly to the differences between the simulated and observed fields.

The results of this study imply that uncertainties in current SST distributions for the world oceans can be as important as the SST anomalies themselves in terms of their impact on the atmospheric circulation. Caution should be exercised, therefore, when linking anomalous circulation and SST patterns, especially in long-range prediction.

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Alvaro F. Diaz
,
Caarem D. Studzinski
, and
Carlos R. Mechoso

Abstract

This study focuses on precipitation in Uruguay and the Brazilian state of Rio Grande do Sul, which extend along the Atlantic coast of southern South America. The present paper has two principal goals: 1) to describe the annual cycle of precipitation and 2) to investigate the relationships between its anomalies and those in sea surface temperature (SST) in the Pacific and Atlantic oceans. The dataset is provided by 40 rainfall stations almost evenly distributed in space and covers the period 1917–80. The tools used in support of this research include principal component and canonical correlation analyses.

It is found that total precipitation tends to be evenly distributed during the year. The largest spatial variability in the monthly deviations from the annual mean appears as a west–east (inland–coastal) dipole with the largest positive values in the west during early fall and midspring, and in the east along the Atlantic coast during winter. The second mode of rainfall variability appears as a north–south dipole with the largest positive values in the south during late summer and late fall, and in the north during early spring and early summer. The third mode appears primarily as a north–south dipole along the western boundary with the largest positive values in the southwest during fall and in the northwest during early spring. These modes explain 60%, 19%, and 8% of the total variance. Five subregions are identified according to similarities between the characteristics of the annual cycles in their rainfall stations.

It is shown that there are significant relationships between anomalies in rainfall and in SST in the Pacific and Atlantic oceans. Some of these relationships confirm the results of previous studies, such as the links between the El Niño–Southern Oscillation phenomenon in the equatorial Pacific Ocean and rainfall anomalies in Uruguay during late spring–early summer and late fall–early winter. Other relationships have not been reported before, such as the links between SST anomalies in the southwestern Atlantic Ocean and rainfall anomalies in the entire region during October–December and April–July. It is also found that when SST anomalies are considered in both oceans simultaneously, their links with rainfall anomalies are in some cases enhanced and in others weakened.

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Andrew W. Robertson
,
Carlos R. Mechoso
, and
Young-Joon Kim

Abstract

The influence of Atlantic sea surface temperature (SST) anomalies on the atmospheric circulation over the North Atlantic sector during winter is investigated by performing experiments with an atmospheric general circulation model. These consist of a 30-yr run with observed SST anomalies for the period 1961–90 confined geographically to the Atlantic Ocean, and of a control run with climatological SSTs prescribed globally. A third 30-yr integration with observed SSTs confined to the South Atlantic is made to confirm present findings.

The simulated interannual variance of 500-hPa wintertime geopotential heights over the North Atlantic attains much more realistic values when observed Atlantic SSTs are prescribed. Circulation patterns that resemble the positive phase of the North Atlantic oscillation (NAO) become more pronounced in terms of the leading EOF of winter means, and a cluster analysis of daily fields. The variance of an interannual NAO index increases by fivefold over its control value. Atlantic SST variability is also found to produce an appreciable rectified response in the December–February time mean.

Interannual fluctuations in the simulated NAO are found to be significantly correlated with SST anomalies over the tropical and subtropical South Atlantic. These SST anomalies are accompanied by displacements in the simulated summer monsoonal circulation over South America and the cross-equatorial regional Hadley circulation.

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Gabriel Cazes-Boezio
,
Andrew W. Robertson
, and
Carlos R. Mechoso

Abstract

The El Niño–Southern Oscillation (ENSO) has an established impact on precipitation in Uruguay during austral spring (October–December). This impact is absent in peak summer (January–February), and returns weakly in fall–winter (March–July). Interannual and intraseasonal variability of the atmospheric circulation over South America and the South Pacific is investigated using the NCEP–NCAR reanalysis data for these seasons. The leading empirical orthogonal function (EOF) of seasonally averaged 200-hPa winds over South America is found to be associated with ENSO through a pronounced Walker cell component in all seasons. However, during spring, this pattern acquires an extratropical teleconnection that links the circulation over southeastern South America (SESA) with ENSO. This extratropical teleconnection disappears in summer, when the circulation over SESA is dominated by variability in the South Atlantic convergence zone. In fall, extratropical South America again becomes affected by a wavelike pattern that extends over the South Pacific, but it is uncorrelated with ENSO.

On intraseasonal timescales, a cluster analysis of daily geopotential height fields over the South Pacific sector reveals three wave train–like circulation regimes with similar structures in all seasons. During the transition seasons (but not summer), the frequencies of occurrence of two of these regimes are found to be significantly different from normal in years when the interannual wavelike EOFs are pronounced. Interannual anomaly patterns are constructed from the intraseasonal regimes according to these changes in their frequency of occurrence, and shown to resemble quite closely the interannual EOFs over the South Pacific sector. These results provide evidence that the interannual teleconnection patterns seen over the South Pacific in austral spring and fall–winter are due to changes in the frequency of occurrence and amplitude of intraseasonal circulation regimes. The Rossby wave source composited over ENSO years suggests that ENSO heating anomalies are able to trigger these changes in regime occurrence and amplitude during October–December through Rossby wave propagation, leading to the known ENSO teleconnection in austral spring. By contrast, the interannual teleconnection over the South Pacific during fall–winter appears to be due to essentially random changes in the frequency of occurrence of the intraseasonal circulation regimes, which are found to be much larger than during austral summer when no extratropical teleconnection pattern exists.

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Andrew W. Robertson
,
John D. Farrara
, and
Carlos R. Mechoso

Abstract

The sensitivity of the atmospheric circulation to sea surface temperature (SST) anomalies in the tropical and subtropical South Atlantic Ocean is studied by means of simulations with an atmospheric general circulation model (GCM). Two types of prescribed SST anomalies are used, motivated by previous analyses of data. The first occurs during austral summers in association with a strengthening of the South Atlantic convergence zone (SACZ) and consists of cold SST anomalies over the subtropical South Atlantic. The second is the leading seasonally varying empirical orthogonal function of SST, consisting of warm basin-scale anomalies with maximum amplitude in the subtropics during January–March and at the equator in June. An ensemble of about 10 seasonal simulations is made using each type of anomaly, focusing on the January–March period in the first case and the January–June seasonal evolution in the second.

During January–March both experiments yield a statistically significant baroclinic response over the subtropical Atlantic with dipolar SACZ-like anomalies. Some evidence of positive feedback is found. The response is shown to be fairly similar in pattern as well as amplitude to the linear regression of observed interannual low-level wind anomalies with subtropical SST anomalies. However, in the first experiment with cold SST anomalies, the simulated response contrasts with the leading interannual mode of observed SACZ variability.

Warm basin-scale anomalies are found to have their largest impact during boreal summer, with a strong statistically significant equatorial baroclinic response and positive rainfall anomalies over the equatorial ocean. The latter do not extend appreciably into the adjacent continents, although there are significant positive rainfall anomalies over the Sahel in April–June and negative anomalies over the western Indian Ocean. In the upper troposphere, a statistically significant wave train extends southwestward to southern South America and northeastward to Europe in April–June, while there is some linkage between the tropically and subtropically forced responses during January–March.

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Ingo Richter
,
Carlos R. Mechoso
, and
Andrew W. Robertson

Abstract

The South Atlantic anticyclone is a major feature of the austral winter climatology. An atmospheric general circulation model (AGCM) is used to study the dynamics of the South Atlantic anticyclone by means of control simulations and experiments to investigate sensitivity to prescribed orography, sea surface temperatures, and soil wetness. The South Atlantic anticyclone in the first control simulation is unrealistically zonally elongated and centered too far west—errors typical of coupled ocean–atmosphere GCMs. Results of the sensitivity experiments suggest that these deficiencies are associated with another family of systematic model errors: the overprediction of convection over the tropical land surfaces, particularly over eastern tropical Africa and India, and the concurrent large-scale westward shift in the divergence center at upper levels and the convergence center at lower levels. The results also confirm the important role of South American and African orography in localizing the South Atlantic anticyclone over the ocean. Other factors, however, like the regional zonal gradients of sea surface temperatures, are found to have only a minor impact on the anticyclone. To further substantiate these findings, the wintertime anticyclone is examined using a revised version of the atmospheric GCM. Improvements are found in both the anticyclone as well as the Asia–African summer monsoon circulations. The results demonstrate the existence of links between intensity and structure of the wintertime South Atlantic anticyclone and the major summer monsoons in the Northern Hemisphere.

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Chunzai Wang
,
Sang-Ki Lee
, and
Carlos R. Mechoso

Abstract

The Atlantic warm pool (AWP) is a large body of warm water comprising the Gulf of Mexico, Caribbean Sea, and western tropical North Atlantic. The AWP can vary on seasonal, interannual, and multidecadal time scales. The maximum AWP size is in the boreal late summer and early fall, with the largest extent in the year being about 3 times the smallest one. The AWP alternates with the Amazon basin in South America as the seasonal heating source for circulations of the Hadley and Walker type in the Western Hemisphere. During the boreal summer/fall, a strong Hadley-type circulation is established, with ascending motion over the AWP and subsidence over the southeastern tropical Pacific. This is accompanied by equatorward flow in the lower troposphere over the southeastern tropical Pacific, as dynamically required by the Sverdrup vorticity balance.

It is shown by analyses of observational data and NCAR community atmospheric model simulations that an anomalously large (small) AWP during the boreal summer/fall results in a strengthening (weakening) of the Hadley-type circulation with enhanced descent (ascent) over the southeastern tropical Pacific. It is further demonstrated—by using a simple two-level model linearized about a specified background mean state—that the interhemispheric connection between the AWP and the southeastern tropical Pacific depends on the configuration of the background mean zonal winds in the Southern Hemisphere.

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