John D. Farrara

Simulations of the Atmospheric Response to South Atlantic Sea Surface Temperature Anomalies

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 o...

Climatology and Interannual Variability of Wave, Mean-Flow Interaction in the Southern Hemisphere

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.

Parameterization of Cloud-Radiation Processes in the UCLA General Circulation Model

Abstract A contemporary radiation parameterization scheme has been implemented in the University of California, Los Angeles (UCLA), atmospheric GCM (AGCM). This scheme is a combination of the delta-four-stream method for solar flux transfer and the delta-two-and-four-stream method for thermal infrared flux transfer. Both methods have been demonstrated to be computationally efficient and at the same time highly accurate in comparison with exact radiative transfer computations. The correlated-k distribution method for radiative transfer has been used to represent gaseous absorption in multiple-scattering atmospheres. The single-scattering properties for ice and water clouds are parameterized in terms of ice/liquid water content and mean effective size/radius. In conjunction with the preceding radiative scheme, parameterizations for fractional cloud cover and cloud vertical overlap have also been devised in the model in which the cloud amount is determined from the total cloud water mixing ratio. For radiati...

The behavior of wave 2 in the Southern Hemisphere stratosphere during late winter and early spring

Abstract A detailed analysis is presented of the behavior of the zonal wavenumber 2 component of the flow (wave 2) for July through October in the Southern Hemisphere stratosphere. Wave 2 in the stratosphere is characterized by a broad meridional structure peaking between 55° and 65°S, and regular eastward propagation, with periods ranging from 5 to 40 days. Maximum geopotential height amplitudes for a year range from approximately 600 to 1000 m. Examination of vertical structure suggests that during episodes of largest growth, wave 2 propagates upward from the upper troposphere. Regular eastward propagation is, however, evident only within the stratosphere. There are also episodes of wave 2 growth that do not appear connected with the troposphere; in general, the wave 2 amplitude is not as large in these cases. There are several years when wave 1 and wave 2 amplitudes are strongly anticorrelated in time during September and October. There are also years with strong positive correlation during August and ...

Ocean Roles in the TBO Transitions of the Indian–Australian Monsoon System

Abstract This study uses a series of coupled atmosphere–ocean general circulation model (CGCM) experiments to examine the roles of the Indian and Pacific Oceans in the transition phases of the tropospheric biennial oscillation (TBO) in the Indian–Australian monsoon system. In each of the three CGCM experiments, air–sea interactions are restricted to a certain portion of the Indo-Pacific Ocean by including only that portion of the ocean in the ocean model component of the CGCM. The results show that the in-phase TBO transition from a strong (weak) Indian summer monsoon to a strong (weak) Australian summer monsoon occurs more often in the CGCM experiments that include an interactive Pacific Ocean. The out-of-phase TBO transition from a strong (weak) Australian summer monsoon to a weak (strong) Indian summer monsoon occurs more often in the CGCM experiments that include an interactive Indian Ocean. The associated sea surface temperature (SST) anomalies are characterized by an ENSO-type pattern in the Pacific...

Sensitivity of a Coupled Ocean–Atmosphere Model to Physical Parameterizations

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.

Performance of a distributed memory finite difference atmospheric general circulation model

Abstract A new version of the UCLA atmospheric general circulation model suitable for massively parallel computer architectures has been developed. This paper presents the principles for the codes design and examines performance on a variety of distributed memory computers. A two dimensional domain decomposition strategy is used to achieve parallelism and is implemented by message passing. This parallel algorithm is shown to scale favorably as the number of processors is increased. In the fastest configuration, performance roughly equivalent to that of multitasking vector supercomputers is achieved.

Simulations of the February 1979 Stratospheric Sudden Warming: Model Comparisons and Three-Dimensional Evolution

Abstract The evolution of the stratospheric flow during the major stratospheric sudden warming of February 1979 is studied using two primitive equation models of the stratosphere and mesosphere. The United Kingdom Meteorological Office Stratosphere-Mesosphere Model (SMM) uses log pressure as a vertical coordinate. A spectral, entropy coordinate version of the SMM (entropy coordinate model, or ECM) that has recently been developed is also used. Both models produce similar successful simulations through the peak of the warming, capturing the splitting of the vortex and the development of small-scale structures, such as narrow baroclinic zones. The ECM produces a more realistic recombination and recovery of the polar vortex in the midstratosphere after the warming, due mainly to better conservation properties for Rossby-Ertel potential vorticity in this model. Another advantage of the ECM is the automatic increase in vertical resolution near baroclinic zones. Comparison of SMM simulations with forecasts perf...

South Atlantic Variability Arising from Air–Sea Coupling: Local Mechanisms and Tropical–Subtropical Interactions

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 sub...

Ensembles of AGCM Two-Tier Predictions and Simulations of the Circulation Anomalies during Winter 1997–98

The impact of sea surface temperature (SST) anomalies on the extratropical circulation during the El Nino winter of 1997-98 is studied through atmospheric general circulation model (AGCM) integrations. The models 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 models modes of intrinsic variability and a change in their amplitude. These findings suggest the potential utility of SST forecasts for the tropical Indian Ocean.