Abstract

A July integration of a GFDL spectral general circulation model is repeated after eliminating from the model the topographic elevations. Zonally averaged. mean fields, regions of frequent Cyclogenesis and cyclone tracks, and the standing waves in geopotential at 500 mb simulated for the Southern Hemisphere in both integrations are compared. It is argued that the differences in the results support the interpretation advanced by Mechoso (1980) concerning the influence of Antarctica an the general circulation of the Southern Hemisphere.

Abstract

Surface frontal structure during cyclogenesis, and the sensitivity of this structure to surface friction, is examined. The approach is based on the analyses of simulations using a primitive equation model, with the domain restricted to a sector of one hemisphere, and the physics reduced to surface drag, horizontal diffusion, and dry convective adjustment. The model horizontal resolution is 1.2° latitude × 1.5° longitude, and there are 21 layers in the vertical. The drag coefficient is varied in simulations with midlatitude jet streams as initial conditions. The extent to which simulations in the adiabatic framework or with highly simplified representations of physical processes succeed in producing features of cyclone evolution emphasized by recent observational analyses is evaluated.

Shallow bent-back warm fronts develop in simulations with surface drag coefficients that are zero or representative of ocean surfaces. Horizontal advection, first in strong easterly and later in strong northerly winds, is primarily responsible for the resulting bent-back structure of the warm front.

The effect of surface drag on simulated lower-tropospheric wind speeds and frontogenesis is nonuniform. Warm frontogenesis is enhanced in simulations with relatively low surface drag through a feedback process involving vorticity, deformation, convergence, and warm-air advection. Surface drag tends to inhibit warm frontogenesis by decreasing the low-level wind speed and reducing the contribution of warm advection to the feedback. Consistently, a distinct warm front does not develop in the simulation with a surface drag coefficient representative of continental surfaces. Cold frontogenesis, on the other hand, is not very sensitive to surface drag.

Further simulations with doubled horizontal resolution (0.6° latitude × 0.75° longitude), slightly higher baroclinity at lower levels in the initial conditions, and small surface drag produce bent-back fronts that spiral around the surface pressure minimum. These results suggest that there are important differences in the structure of surface fronts associated with marine and continental cyclogenesis.

Abstract

The characteristics of subseasonal circulation variability over the South Pacific are examined using 10-day lowpass-filtered 700-hPa geopotential height NCEP–NCAR reanalysis data. The extent to which the variability in each season is characterized by recurrent geographically fixed circulation regimes and/or oscillatory behavior is determined. Two methods of analysis (a K-means cluster analysis and a cross-validated Gaussian mixture model) both indicate three to four geographically fixed circulation regimes in austral fall, winter, and (to some extent) spring. The spatial regime structures are found to be quite similar in each season; they resemble the so-called Pacific–South American (PSA) patterns discussed in previous studies and often referred to as PSA 1 and PSA 2. Oscillatory behavior is investigated using singular spectrum analysis. This identifies a predominantly stationary wave with a period of about 40 days and a spatial structure similar to PSA 1; it is most pronounced in winter and spring and exhibits a noticeable eastward drift as it decays. The power spectrum of variability is otherwise well approximated by a red spectrum, together with enhanced broader-band 15–30-day variability.

The results presented herein indicate that low-frequency variability over the South Pacific is not dominated by a propagating wave whose quadrature phases are PSA 1 and PSA 2, as hitherto described. Rather, it is found that the variability is well described by the occurrence of three to four geographically fixed circulation regimes, with a (near) 40-day oscillation that is predominantly stationary in space. The potential subseasonal predictability implied by this duality is discussed. Only during austral spring is a strong correlation found between El Niño and the frequency of occurrence of the circulation regimes.

Abstract

Interannual variations of the summertime (January–March) atmospheric circulation over subtropical South America are examined during the period 1958–97 using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data. It is found from an empirical orthogonal function analysis that an anomalous upper-tropospheric large-scale stationary eddy in the lee of the Andes tends to accompany a dipole in anomalous vertical motion. An anomalous cyclonic (anticyclonic) eddy accompanies an intensified (diffuse) South Atlantic convergence zone (SACZ), with anomalous descent (ascent) to the southwest. The cold-core equivalent barotropic vertical structure of the anomalous cyclonic eddy and the 200-hPa vorticity balance are both characteristic of a stationary Rossby wave; the tendency for the eddy to be advected downstream by the mean westerlies is compensated by meridional advection of planetary vorticity and stretching associated with vertical motion. The anomalous cyclonic flow at low levels reinforces the thermally direct circulation associated with the SACZ. A weak funneling of submonthly Rossby wave activity into this descent region is also identified.

The interannual time series of the eddy is significantly correlated with north–south dipolar sea surface temperature (SST) anomalies over the southwest Atlantic; one standard deviation 200-hPa wind speed anomalies of up to 5 m s⁻¹ are accompanied by SST anomalies of up to 0.3°C. A near-cyclic 15-yr component is identified, which the authors corroborate from independent analyses of southwest Atlantic SSTs and river flows; both are found to exhibit very similar oscillatory components. When the SACZ is intensified, the Paraná and Paraguay rivers in southern Brazil tend to swell, while the Uruguay and Negro rivers to the south tend to ebb; this north–south contrast in streamflow anomalies is most marked on the interdecadal timescale.

Abstract

Basic issues regarding upper-level frontogenesis addressed in this paper are: (i) simulated frontogenesis influenced by the initial flow, (ii) upper-level frontogenesis as essentially a two-dimensional process, and (iii) frontal-scale positive feedback between vertical advection of momentum and vorticity advection by the ageostrophic wind, which is important for the intensification of upper-level frontal zones. The methodology for investigation is based on analysis of simulated upper-level frontogenesis with a three-dimensional primitive-equation model. The model is a simplified version of the UCLA GCM, with 21 layers in the vertical, horizontal resolution of 1.2° lat × 1.5° long; a 60° sector of one hemisphere as periodic domain, and physics reduced to horizontal diffusion and dry convective adjustment. Simulations initialized with jet streams symmetric about the latitude of maximum wind at each pressure level produce—in the middle troposphere—the strongest frontal zones downstream of the trough of growing baroclinic waves. Strongest upper-level frontal zones originating upstream of the wave trough, as observed, are produced when initial jet streams and perturbations are chosen so that the growing waves have small meridional phase tilt in the initial stages.

In the simulations, tilting associated with divergence of the across-jet ageostrophic flow is the dominant frontogenetical process upstream of the wave trough. Further, tilting associated with divergence of the ageostrophic wind along the jet also contributes to frontogenesis, but to a lesser extent. The former result is similar to that obtained with two-dimensional models in which frontogenetical vertical motions are associated with divergence of the ageostrophic wind across the front.

No definitive evidence is found proving that the simulated frontogenesis is enhanced by a positive-feedback process involving vertical advection of momentum and vorticity advection by the ageostrophic wind. It is found, however, that both of these processes are nonnegligible contributors to the frontal intensification.

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.

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.

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.

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.

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.