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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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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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Carlos R. Mechoso
,
Akio Kitoh
,
Shrinivas Moorthi
, and
Akio Arakawa

Abstract

The atmospheric response to a sea surface temperature anomaly over the equatorial eastern Pacific Ocean (SSTA) is investigated using the UCLA General Circulation Model. The SSTA used is an idealization of that compiled by Rasmusson and Carpenter for the mature phase of El Niño. Two simulations over seasons, one without and the other with the SSTA, are performed and their results are compared for the Northern Hemisphere winter season.

In the tropics the SSTA enhances precipitation over the central and eastern equatorial Pacific, while it decreases precipitation over the adjacent regions. The anomalous precipitation is predominantly balanced by the anomalous moisture flux convergence, which has comparable magnitude in the planetary boundary layer (PBL), and in the free atmosphere with quite different geographical distribution. This suggests that the anomalous precipitation, and hence the anomalous tropical cumulus heating, cannot be related exclusively to either flow anomalies in the PBL or in the free atmosphere.

In the midlatitudes, it is found that the SSTA results in a more zonal flow over the Pacific with an intensification of the upper-tropospheric westerlies. Associated with this intensification, synoptic-scale transient baroclinic waves become more active. This is consistent with interannual differences in observed spectral distributions of transients for five winters, two of which were El Niño winters. Geographically, the increase in baroclinic wave activity occurs in a zonal bell extending from the northeastern Pacific to the northern Atlantic.

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Carlos R. Mechoso
,
Koji Yamazaki
,
Akio Kitoh
, and
Akio Arakawa

Abstract

The predictability of the stratospheric warming events during the winter of 1979 is investigated by performing a series of 10-day forecasts using the UCLA general circulation model. In general, those events are predictable from several days in advance. The accuracy of the prediction, however, can be sensitive to the starting date and such model characteristics as the horizontal resolution. This sensitivity seems to arise because relatively small errors in the predicted tropospheric zonal mean wind can produce large differences in the characteristics of upward wave propagation and thereby large errors in the stratospheric forecast.

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Celal S. Konor
,
Gabriel Cazes Boezio
,
Carlos R. Mechoso
, and
Akio Arakawa

Abstract

This paper presents the basic features of a newly developed planetary boundary layer (PBL) parameterization, and the performance assessment of a version of the University of California, Los Angeles (UCLA), Atmospheric General Circulation Model (AGCM) to which the parameterization is incorporated. The UCLA AGCM traditionally uses a framework in which a sigma-type vertical coordinate for the PBL shares a coordinate surface with the free atmosphere at the PBL top. This framework facilitates an explicit representation of processes concentrated near the PBL top, which is crucially important especially for predicting PBL clouds. In the new framework, multiple layers are introduced between the PBL top and earth’s surface, allowing for predictions of the vertical profiles of potential temperature, total water mixing ratio, and horizontal winds within the PBL. The vertically integrated “bulk” turbulent kinetic energy (TKE) is also predicted for the PBL. The PBL-top mass entrainment is determined through an equation including the effects of TKE and the radiative and evaporative cooling processes concentrated near the PBL top. The surface fluxes are determined from an aerodynamic formula in which the velocity scale depends both on the square root of TKE and the grid-scale PBL velocity at the lowermost model layer. The turbulent fluxes within the PBL are determined through an approach that includes the effects of both large convective and small diffusive eddies. AGCM simulations with the new formulation of PBL are analyzed with a focus on the seasonal and diurnal variations. The simulated seasonal cycle of stratocumulus over the eastern oceans is realistic, as are the diurnal cycles of the PBL depth and precipitation over land. The simulated fluxes of latent heat, momentum, and shortwave radiation at the ocean surface and baroclinic activity in the middle latitudes show significant improvements over the previous versions of the AGCM based on the single-layer PBL.

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Carlos R. Mechoso
,
Chung-Chun Ma
,
John D. Farrara
,
Joseph A. Spahr
, and
Reagan W. Moore

Abstract

The distribution of a climate model across homogeneous and heterogeneous computer environments with nodes that can reside at geographically different locations is investigated. This scientific application consists of an atmospheric general circulation model (AGCM) coupled to an oceanic general circulation model (OGCM).

Three levels of code decomposition are considered to achieve a high degree of parallelism and to mask communication with computation. First, the domains of both the gridpoint AGCM and OGCM are divided into subdomains for which calculations an carded out concurrently (domain decomposition). Second, the model is decomposed based on the diversity of tasks performed by its major components (task decompositions). Three such components are identified: (a) AGCM/physics which computes the effects on the grid-scale flow of subgrid-scale processes such as convection and turbulent mixing; (b) AGCM/dynamics, which computes the evolution of the flow governed by the primitive equations; and (c) the OGCM. Task decomposition allows the AGCM/dynamics and OGCM calculations to be carried out concurrently. Last, computation and communication are organized in such a way that the exchange of data between different tasks is carded out in subdomains of the model domain (110 decomposition). In a dedicated computer network environment, the wall-clock time required by the resulting distributed application is reduced to that for the AGCMJ physics, with the other two components and interprocess communications running in parallel.

The network bandwidth requirements for the distributed application are analyzed. It is assumed that the wall-clock time required to run the AGCM/physics for the model atmosphere in a dedicated computer environment is fixed at a value corresponding to high network efficiency. The analysis shows that, for computer environments based an nodes equivalent to the Intel Touchstone Delta, a bandwidth approaching that of the Gigabit Network is required for an efficient operation of the distributed application with model resolution double that used in current studies of the climate system if output is visualized in real time.

It is argued that distribution of a climate model based on domain, task, and 110 decomposition has the potential for significant and eventually superlinear speedup in model execution, which will facilitate performance of the long integrations required by climate studies.

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Sajal K. Kar
,
Richard P. Turco
,
Carlos R. Mechoso
, and
Akio Arakawa

Abstract

A splitting method is presented for eliminating the need to directly solve for a two-dimensional Helmholtz-type difference equation in a semi-implicit scheme for a global gridpoint shallow-water model. In the proposed method, the model equations are split so that the gravity-oscillation terms are integrated implicitly in two locally one-dimensional steps. It is required that such splitting must preserve the irrotationality properties of the gradients of pressure in finite-difference form so that no spurious sources of vorticity and divergence are introduced into the flow. The semi-implicit scheme, thus derived, provides a locally one-dimensional method of solving the two-dimensional Helmholtz-type difference equation. This method requires the solutions of two linear tridiagonal systems of equations, which can be solved more easily and efficiently than the original two-dimensional Helmholtz-type equation. Using idealized large-scale flows on the sphere, it is shown that the scheme provides stable and accurate model solutions at considerable computational economy.

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Alvaro de la Cámara
,
Ana M. Mancho
,
Kayo Ide
,
Encarna Serrano
, and
Carlos R. Mechoso

Abstract

Transport in the lower stratosphere over Antarctica has been studied in the past by means of several approaches, such as contour dynamics or Lyapunov exponents. This paper examines the problem by means of a new Lagrangian descriptor, which is referred to as the function M. The focus is on the southern spring of 2005, which allows for a comparison with previous analyses based on Lyapunov exponents. With the methodology based on the function M, a much sharper depiction of key Lagrangian features is achieved and routes of large-scale horizontal transport across the vortex edge are captured. These results highlight the importance of lobe dynamics as a transport mechanism across the Antarctic polar vortex.

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Joon-Hee Jung
,
Celal S. Konor
,
Carlos R. Mechoso
, and
Akio Arakawa

Abstract

The principal goal of this paper is to gain further insight into the dynamical processes during the stratospheric major warming of February and early March 1979, with a special emphasis on the recovery stage. To achieve this goal, first the entire evolution of the warming event is numerically simulated using an isentropic vertical coordinate model. Then the results from the following complementary points of view are quantitatively analyzed: wave effects on the mean flow, potential enstrophy conversion and transport, and potential vorticity redistribution on a synoptic chart.

There is an indication that wavenumber 1 during the recovery stage amplifies through a mechanism within the stratosphere and propagates downward. The simulated Eliassen–Palm flux field shows that the amplified wave 1 is responsible for the mean flow acceleration in the recovery stage. It is therefore concluded that the in situ amplification mechanism for wave 1 plays a crucial role in the dynamics of the flow recovery. In order to examine the mechanism, detailed analyses of the simulated eddy potential enstrophy budget are performed. It is found that there is an “anticascade” of potential enstrophy in the recovery stage through which wave 1 amplifies in the midstratosphere. This result is consistent with a synoptic description of the event in terms of potential vorticity distribution on isentropic surfaces.

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