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

Abstract

The impact of sea surface temperature (SST) anomalies on the extratropical circulation during the El Niño winter of 1997–98 is studied through atmospheric general circulation model (AGCM) integrations. The model’s midlatitude response is found to be very robust, of the correct amplitude, and to have a fairly realistic spatial structure. The sensitivity of the results to different aspects of the anomalous distributions of SST is analyzed. It is found that the extratropical circulation in the North Pacific–North American sector is significantly different if SST anomalies over the Indian Ocean are included. Using a comparison of observed and simulated 200-hPa streamfunction anomalies, it is argued that the modeled midlatitude impact of Indian Ocean SST anomalies is largely realistic. However, while the local sensitivity of the atmosphere to small differences in SST anomalies in the tropical Pacific can be substantial, the remote sensitivity in midlatitudes is small. Consistently, there is little difference between the simulated extratropical circulation anomalies obtained using SSTs predicted by the National Centers for Environmental Prediction in October 1997 and those obtained using observed tropical Pacific SSTs. Neither is there any detectable atmospheric signal associated with SST anomalies over the North Pacific.

Analyses of the results presented here suggest that the influence of SST anomalies in the Pacific and Indian Oceans during the selected ENSO event can be interpreted as the quasi-linear superposition of Rossby wave trains emanating from the subtropics of each ocean. An inspection of intraseasonal weather regimes suggests that the influence of tropical SST anomalies can also be described as a shift in the frequency of occurrence of the model’s modes of intrinsic variability and a change in their amplitude. These findings suggest the potential utility of SST forecasts for the tropical Indian Ocean.

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Dennis L. Hartmann
,
Carlos R. Mechoso
, and
Koji Yamazaki

Abstract

Variations of zonal mean and eddy properties in the Southern Hemisphere during the winter of 1979 are studied. Several periods of enhanced wave activity and rapid zonal-mean changes are observed. During these periods of rapid change both the actual zonal wind tendency and the Eliassen-Palm flux divergence have a dipole pattern in the upper stratosphere, with positive values in high latitudes and negative values in middle and low latitudes. The positive values of E-P flux divergence near the pole are particularly interesting, since they suggest a possible source of wave activity in this region. The stationary component of wavenumber 1 and the eastward traveling component of wavenumber 2 contribute most to the dipole pattern of wave driving in the stratosphere.

A nearly uniform deceleration of the mean flow in the troposphere is contributed to by all zonal wavenumbers from 1 to 10. The lower wavenumbers contribute most to the driving in high latitudes, while the driving in lower latitudes is contributed mostly by high wavenumbers. Wave forcing of the mean flow as measured by the E-P flux divergence and actual changes in the mean flow are correlated with each other, but the actual mean flow tendencies are often significantly smaller than the eddy driving. The largest correlations between E-P flux divergence and the observed zonal mean wind changes exceed 0.5 and occur both in the upper stratosphere near the jet core and near the tropopause across a broad range of latitudes.

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Carlos R. Mechoso
,
Dennis L. Hartmann
, and
John D. Farrara

Abstract

The climatology and interannual variability of wave–mean flow interaction in the Southern Hemisphere (20–80°S, 0–55 km) is described for the winter months of June–September based on a sample of four years, 1979–82. The stratospheric jet stream shifts downward and poleward over the course of the winter in response to seasonal variations in thermal forcing. The shift occurs at different times in different years, however, so that the months of July and August show substantial interannual variability of monthly mean zonal winds. The poleward and downward shift of the jet axis in an individual year is usually abrupt and occurs in association with a burst of upwardly propagating planetary waves. The driving of the mean flow in the stratosphere generally has a dipolar structure with easterly accelerations near 40°S and westerly accelerations in polar latitudes. The structure of the wave driving is consistent with the structure of the observed mean flow accelerations.

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John D. Farrara
,
Michael Fisher
,
Carlos R. Mechoso
, and
Alan O'Neill

Abstract

The early winter (mid-April to mid-July) circulation in the Southern Hemisphere stratosphere is studied. Emphasis is placed on the evolution of strong disturbances with structures dominated by the zonal wavenumber-1 component of the flow (wave 1). The approach to this investigation is based on analysis of 12 years (1979–90) of observational data and comparative analyses of control and hypothesis-testing simulations with a three-dimensional primitive equation model of the stratosphere and mesosphere.Considerable interannual variability is found in both the intensity and timing of wave-1 amplification during early winter. Though usually quasi-stationary, there are six extended periods in the dataset when wave 1 travels steadily eastward and is of large amplitude. Two of these periods (June 1980 and June 1985) are examined in detail. The evolution of the circulation in these two cases resembles that during Canadian warmings in the Northern Hemisphere in several ways. First, there is a large, eastward-moving disturbance with a nearly equivalent barotropic structure, with the largest amplitude in the lower and middle stratosphere. Second, temperature increases are smaller than those observed during final warmings in the Southern Hemisphere. Third, irreversible buckling of contours of Ertel's potential vorticity takes place in a region well away from the zero-wind line. Owing to their geographical preference for development over the South Pacific, wave-1 events in the southern stratosphere during early winter are referred to as South Pacific warmings.The hypothesis-testing simulations suggest that the development of South Pacific warmings is connected with the amplification of wave 1 at 100 mb and that the eastward propagation of the disturbances requires eastward propagation of wave 1 at 100 mb. In addition, the results suggest that development of stratospheric disturbances in the southern stratosphere during early winter depends more on the intensity of wave 1 at 100 mb than on the structure of the zonal-mean flow in the stratosphere.

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Carlos R. Mechoso
,
John D. Farrara
, and
Michael Ghil

Abstract

The intraseasonal variability of the Southern Hemisphere stratosphere and troposphere is studied using multilevel geopotential height data for nine winters (1979–87). The study uses empirical orthogonal function (EOF) analysis of unfiltered data at five tropospheric and five stratospheric levels.

The four leading EOFs at all tropospheric levels exhibit the patterns previously detected at 500 mb. Study of the corresponding principal components (PCs) at each level shows that the quasi-stationary anomalies associated with the leading EOFs are equivalent barotropic and exhibit no preference for early, middle or late winter.

The five leading EOFs in the stratosphere fall into two classes. The first three EOFs at all levels form the first class. This class represents anomalies that are dominated by zonal wavenumber one (wave 1), exhibit strong westward tilt with height and travel slowly eastward or remain stationary. Most cases of large, persistent PC values for this class occur in early winter. The fourth and fifth EOFs form the other class. This class represents anomalies that are dominated by wavenumber two, and tilt noticeably, but less strongly than the first class, westward with height. These anomalies tend to develop mostly in late winter and to travel eastward more rapidly. The intraseasonal variability in the stratosphere resides therewith, as expected, in structures dominated by the longest planetary waves.

No systematic connections between tropospheric and stratospheric persistent anomalies are apparent in the dataset.

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Carlos R. Mechoso
,
J. David Neelin
, and
Jin-Yi Yu

Abstract

The realistic simulation of El Niño–Southern Oscillation (ENSO) by the University of California, Los Angeles (UCLA), coupled atmosphere–ocean general circulation model (CGCM) is used to test two simple theoretical models of the phenomenon: the recharge oscillator model of Jin and the delayed oscillator model of Schopf, Suarez, Battisti, and Hirst (SSBH). The target for the simple models is provided by the CGCM results prefiltered with singular spectrum analysis to extract the leading oscillatory mode. In its simplest form, the Jin model can be reduced to two first ordinary differential equations. If the parameters of the model are fit in this reduced form, it appears to capture the period of the CGCM oscillatory mode. If the Jin model is instead fit using the individual physical balances that are used to derive it, substantial misfits to the CGCM are encountered. The SSBH model can likewise be expressed either in a condensed form or a larger set of individual physical balances with highly analogous results.

It is shown that the misfits in both simple models can be greatly reduced by introducing a spinup timescale for wind stress relative to eastern equatorial Pacific SST. In the CGCM, this spinup time appears to be associated with a combination of atmospheric and ocean mixed layer processes in a way consistent with the “mixed mode” regime discussed by Syu and Neelin, which is not included in the Jin and SSBH models. These appear indistinguishable in this analysis, although the latter is more sensitive to fitting.

This paper provides a bridge between work on ENSO by theoreticians and numerical modelers. The CGCM results validate the conceptual framework of the simple models by demonstrating that they can provide a plausible representation of ENSO with realistic sets of parameters. The results also suggest that, in terms of realistic ENSO variability, the framework of the simple models can be made substantially more complete by including the adjustment time between wind stress and eastern Pacific SST required by the coupled spinup of the atmosphere and the ocean mixed layer processes outside this region.

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Annarita Mariotti
,
Carlos R. Mechoso
,
Bernard Legras
, and
Vincent Daniel

Abstract

The ozone evolution in the lower stratosphere of the Southern Hemisphere during the period 5–10 August 1994 is analyzed. The analysis focuses on the ozone “collar” (the band of maximum values in ozone mixing ratio around the Antarctic ozone “hole” at these altitudes) and the development of “collar filaments.” Ozone mixing ratios provided by the Microwave Limb Sounder (MLS) on board the Upper Atmosphere Research Satellite and by an ER-2 aircraft participating in the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft campaign are compared with values at corresponding locations in high-resolution isentropic maps obtained by using the numerical scheme of “contour advection with surgery” (CAS).

The CAS reconstructed ozone maps provide a view of the way in which air masses are exported from the outskirts of the collar to form the “tongues” of higher mixing ratios observed at lower latitudes on MLS synoptic maps. There is an overall consistency between the datasets insofar as the collar location is concerned. This location seems to be primarily defined by the local properties of the flow. Nevertheless the CAS reconstructed collar tends to become weaker than that depicted by MLS data. By means of radiative calculation estimates, it is argued that diabatic descent may be responsible for maintaining the ozone concentration approximately constant in the collar while filaments isentropically disperse collarlike mixing ratios from this region toward lower latitudes.

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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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Gabriel Pisciottano
,
Alvaro Díaz
,
Gabriel Cazess
, and
Carlos R. Mechoso

Abstract

The relationships between rainfall over Uruguay (in southeastern South America) and the El Niño-Southern Oscillation phenomenon are investigated. Long time series of data from a dense network of rainfall stations are analyzed using an empirical method based on that proposed by Ropelewski and Halpert. The spatial patterns of the relationships and their temporal variability for the entire region and four subregions are studied in detail.

It is found that years with El Niño events tend to have higher than average rainfall, especially from November to the next January. Further, years with high values of the Southern Oscillation index (501) tend to have lower than average rainfall, especially from October through December. These findings are in general agreement with previous studies. It is also found that the period from March through July tends to have higher than average rainfall after El Niño years and lower than average rainfall after high-SOI years. For the southern part of Uruguay, the wet anomalies during El Niño events are relatively weak, but the dry anomalies during high-SOI events are significant for the two periods identified. The dry anomalies disappear, and even revere, during January and February after high-SOI years. This feature does not have a symmetric counterpart during January and February after El Niño years.

This study, therefore, provides both a verification and an extension of other studies that have emphasized southeastern South America but have used data from only a very few stations in the region.

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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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