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  • Author or Editor: Carlos R. Mechoso x
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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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Carlos R. Mechoso
,
Steven W. Lyons
, and
Joseph A. Spahr

Abstract

The response of the tropical atmosphere to the sea surface temperature (SST) anomalies in the Northern Hemisphere spring of 1984 is investigated. The methodology for investigation consists of comparing simulations with and without the global distribution of SST anomalies in the boundary conditions of the UCLA General Circulation Model (GCM). At low levels, the response includes weaker southeast trade winds over the Atlantic, increased precipitation off the northeast coast of Brazil, and reduced precipitation west of this region. The increased precipitation is due to enhanced convergence of moisture advected by the southeast trade winds, although the trades themselves are weaker. The results for the western equatorial Atlantic am in apparent agreement with the observed anomalous southern migration of the ITCZ in years with warm SST anomalies in the southern tropical Atlantic. There are strong anomalous trade winds over the Pacific extending east of the date line and weak wind anomalies over the maritime continent, in broad agreement with the observed.

The sensitivity of the simulated atmospheric response over an ocean basin to using the SST anomalies confined to the basin or in the global ocean is analyzed. It is found that there can be notable local differences in the results obtained using those procedures. In particular, the simulation with the SST anomalies confined to the Pacific shows weak anomalous trade winds over the western part of this ocean basin and strong westerly anomalies over the maritime continent unlike that with the anomalies in the global ocean.

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

Abstract

Interannual variability in the southern and equatorial Atlantic is investigated using an atmospheric general circulation model (AGCM) coupled to a slab ocean model (SOM) in the Atlantic in order to isolate features of air–sea interactions particular to this basin. Simulated covariability between sea surface temperatures (SSTs) and atmosphere is very similar to the observed non-ENSO-related covariations in both spatial structures and time scales. The leading simulated empirical coupled mode resembles the zonal mode in the tropical Atlantic, despite the lack of ocean dynamics, and is associated with baroclinic atmospheric anomalies in the Tropics and a Rossby wave train extending to the extratropics, suggesting an atmospheric response to tropical SST forcing. The second non-ENSO mode is the subtropical dipole in the SST with a mainly equivalent barotropic atmospheric anomaly centered on the subtropical high and associated with a midlatitude wave train, consistent with atmospheric forcing of the subtropical SST.

The power spectrum of the tropical mode in both simulation and observation is red with two major interannual peaks near 5 and 2 yr. The quasi-biennial component exhibits a progression between the subtropics and the Tropics. It is phase locked to the seasonal cycle and owes its existence to the imbalances between SST–evaporation and SST–shortwave radiation feedbacks. These feedbacks are found to be reversed between the western and eastern South Atlantic, associated with the dominant role of deep convection in the west and that of shallow clouds in the east. A correct representation of tropical–extratropical interactions and of deep and shallow clouds may thus be crucial to the simulation of realistic interannual variability in the southern and tropical Atlantic.

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

Abstract

The southern subtropical anticyclones are notably stronger in austral winter than summer, particularly over the Atlantic and Indian Ocean basins. This is in contrast with the Northern Hemisphere (NH), in which subtropical anticyclones are more intense in summer according to the “monsoon heating” paradigm. To better understand the winter intensification of southern subtropical anticyclones, the present study explores the interhemispheric response to monsoon heating in the NH during austral winter. A specially designed suite of numerical model experiments is performed in which summer monsoons in the NH are artificially weakened. These experiments are performed with both an atmospheric general circulation model and a simple two-layer model. The highlight of the findings presented here is that during the boreal summer enhanced tropical convection activity in the NH plays important roles in either maintaining or strengthening the southern subtropical anticyclones. Enhanced NH convection largely associated with the major summer monsoons produces subsidence over the equatorial oceans and the tropical Southern Hemisphere via interhemispheric meridional overturning circulations and increases the sea level pressure locally. In addition, suppressed convection over some regions of climatological subsidence produces stationary barotropic Rossby waves that propagate far beyond the tropics. These stationary barotropic Rossby waves and those forced directly by the summer heating in the NH are spatially phased to strengthen the southern subtropical anticyclones over all three oceans. The interhemispheric response to the NH summer monsoons is most dramatic in the South Pacific, where the subtropical anticyclone nearly disappears in the austral winter without the influence of the NH.

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Xuan Ji
,
J. David Neelin
,
Sang-Ki. Lee
, and
Carlos R. Mechoso

Abstract

The mechanisms that control the interhemispheric teleconnections from tropical heat sources are investigated using an intermediate complexity model [a quasi-equilibrium tropical circulation model (QTCM)] and a simple linear two-level model with dry dynamics. Illustrating the interhemispheric teleconnection process with an Atlantic warm pool principal case, the heat source directly excites a baroclinic response that spreads across the equator. Then, three processes involving baroclinic–barotropic interactions—shear advection, surface drag, and vertical advection—force a cross-equatorial barotropic Rossby wave response. An analysis of these processes in QTCM simulations indicates that 1) shear advection has a pattern that roughly coincides with the baroclinic signal in the tropics and subtropics, 2) surface drag has large amplitude and spatial extent and can be very effective in forcing barotropic motions around the globe, and 3) vertical advection has a significant contribution locally and remotely where large vertical motions and vertical shear occur. The simple model is modified to perform experiments in which each of these three mechanisms may be included or omitted. By adding surface drag and vertical advection, and comparing each to shear advection, the effects of the three mechanisms on the generation and propagation of the barotropic Rossby waves are shown to be qualitatively similar to the results in QTCM. It is also found that the moist processes included in the QTCM can feed back on the teleconnection process and alter the teleconnection pattern by enlarging the prescribed tropical heating in both intensity and geographical extent and by inducing remote precipitation anomalies by interaction with the basic state.

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Chung-Chun Ma
,
Carlos R. Mechoso
,
Akio Arakawa
, and
John D. Farrara

Abstract

The sensitivity of a coupled ocean–atmosphere general circulation model to parameterizations of selected physical processes is studied. The parameterizations include those of longwave radiation and surface turbulent fluxes in the atmospheric model, and those of vertical turbulent mixing and penetration of solar radiation in the ocean model. It is shown that the performance of the coupled model is highly sensitive to the parameterization of longwave radiation. This sensitivity is not solely due to the difference in surface radiative flux but involves interactions among radiation, convection, and large-scale dynamics of the atmosphere and ocean. It is concluded that differences in parameterizations can have large impacts on the performance of the coupled model, and these impacts can be very different from what may be expected from uncoupled model simulations.

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Chung-Chun Ma
,
Carlos R. Mechoso
,
Andrew W. Robertson
, and
Akio Arakawa

Abstract

Extensive and persistent stratus cloud decks are prominent climatic features off the Peruvian coast. They are believed to play a key role in the coupled atmosphere-ocean processes that determine the sea surface temperature (SST) throughout the eastern tropical Pacific. This notion is examined and further developed using a coupled ocean-atmosphere general circulation model (GCM): a control simulation, in which the simulated amount of Peruvian stratus clouds is unrealistically low, is compared with an experiment in which a stratus cloud deck is prescribed to persistently cover the ocean off the Peruvian coast.

Beneath the prescribed cloud deck SSTs are reduced by up to 5 K, as expected from decreased solar radiation reaching the surface. In addition, there is significant cooling over much of the eastern tropical Pacific south of the equator, and even along the equator well into the central Pacific. The prescribed stratus deck largely alleviates the coupled GCM's warm bias in SST in the southeastern Pacific, which is common to most contemporary coupled GCMS, and produces a distribution of SST with more realistic interhemispheric asymmetries.

Examination of differences between SST evolutions in the enhanced stratus experiment and the control circulation reveals that the remote ocean cooling is not due to a single mechanism. The cooling immediately to the west and north of the region with the prescribed stratus deck is primarily associated with increased evaporation as the southeast trades strengthen. The cooling along the equator in the central Pacific is mainly due to increased oceanic cold advection.

The results of this study suggest that the Peruvian stratus clouds are important in modulating the circulation of the tropical Pacific. The “double ITCZ” syndrome of the coupled GCM, however, does not appear to be solely due to underpredicted stratus cloud cover and requires consideration of other processes in the coupled GCM.

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Belen Rodríguez-Fonseca
,
Elsa Mohino
,
Carlos R. Mechoso
,
Cyril Caminade
,
Michela Biasutti
,
Marco Gaetani
,
J. Garcia-Serrano
,
Edward K. Vizy
,
Kerry Cook
,
Yongkang Xue
,
Irene Polo
,
Teresa Losada
,
Leonard Druyan
,
Bernard Fontaine
,
Juergen Bader
,
Francisco J. Doblas-Reyes
,
Lisa Goddard
,
Serge Janicot
,
Alberto Arribas
,
William Lau
,
Andrew Colman
,
M. Vellinga
,
David P. Rowell
,
Fred Kucharski
, and
Aurore Voldoire

Abstract

The Sahel experienced a severe drought during the 1970s and 1980s after wet periods in the 1950s and 1960s. Although rainfall partially recovered since the 1990s, the drought had devastating impacts on society. Most studies agree that this dry period resulted primarily from remote effects of sea surface temperature (SST) anomalies amplified by local land surface–atmosphere interactions. This paper reviews advances made during the last decade to better understand the impact of global SST variability on West African rainfall at interannual to decadal time scales. At interannual time scales, a warming of the equatorial Atlantic and Pacific/Indian Oceans results in rainfall reduction over the Sahel, and positive SST anomalies over the Mediterranean Sea tend to be associated with increased rainfall. At decadal time scales, warming over the tropics leads to drought over the Sahel, whereas warming over the North Atlantic promotes increased rainfall. Prediction systems have evolved from seasonal to decadal forecasting. The agreement among future projections has improved from CMIP3 to CMIP5, with a general tendency for slightly wetter conditions over the central part of the Sahel, drier conditions over the western part, and a delay in the monsoon onset. The role of the Indian Ocean, the stationarity of teleconnections, the determination of the leader ocean basin in driving decadal variability, the anthropogenic role, the reduction of the model rainfall spread, and the improvement of some model components are among the most important remaining questions that continue to be the focus of current international projects.

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Siegfried D. Schubert
,
Ronald E. Stewart
,
Hailan Wang
,
Mathew Barlow
,
Ernesto H. Berbery
,
Wenju Cai
,
Martin P. Hoerling
,
Krishna K. Kanikicharla
,
Randal D. Koster
,
Bradfield Lyon
,
Annarita Mariotti
,
Carlos R. Mechoso
,
Omar V. Müller
,
Belen Rodriguez-Fonseca
,
Richard Seager
,
Sonia I. Seneviratne
,
Lixia Zhang
, and
Tianjun Zhou

Abstract

Drought affects virtually every region of the world, and potential shifts in its character in a changing climate are a major concern. This article presents a synthesis of current understanding of meteorological drought, with a focus on the large-scale controls on precipitation afforded by sea surface temperature (SST) anomalies, land surface feedbacks, and radiative forcings. The synthesis is primarily based on regionally focused articles submitted to the Global Drought Information System (GDIS) collection together with new results from a suite of atmospheric general circulation model experiments intended to integrate those studies into a coherent view of drought worldwide. On interannual time scales, the preeminence of ENSO as a driver of meteorological drought throughout much of the Americas, eastern Asia, Australia, and the Maritime Continent is now well established, whereas in other regions (e.g., Europe, Africa, and India), the response to ENSO is more ephemeral or nonexistent. Northern Eurasia, central Europe, and central and eastern Canada stand out as regions with few SST-forced impacts on precipitation on interannual time scales. Decadal changes in SST appear to be a major factor in the occurrence of long-term drought, as highlighted by apparent impacts on precipitation of the late 1990s “climate shifts” in the Pacific and Atlantic SST. Key remaining research challenges include (i) better quantification of unforced and forced atmospheric variability as well as land–atmosphere feedbacks, (ii) better understanding of the physical basis for the leading modes of climate variability and their predictability, and (iii) quantification of the relative contributions of internal decadal SST variability and forced climate change to long-term drought.

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