ALBERT

Notes: Abstract.  This study evaluates simulations of the East Asian winter monsoon in eight GCMs that participated in the Atmospheric Model Intercomparison Project (AMIP). In addition to validating the mean state of the winter monsoon, the cold surge and its transient properties, which includes the frequency, intensity, preferred propagation tracks, and the evolution patterns of the surges, are examined. GCM simulated temporal distribution of the Siberian high and cold surges is also discussed. Finally, the forcing of the cold surges on the tropical surface wind and convection, along with their interannual variation is analyzed. The mean state of the winter monsoon is generally portrayed well in most of the models. These include the climatological position of the Siberian high, the 200 hPa divergent center, and the large-scale wind patterns at the surface and the 200 hPa. Models display a wide range of skill in simulating the cold surge and its transient properties. In some of the models, the simulated cold surge trajectory, intensity, frequency, propagation patterns and source regions are in general agreement with those from the observed. While in others, the models cannot adequately capture these observed characteristics. The temporal distribution of the Siberian high and cold surges were realistically reproduced in most GCMs. Most models were able to simulate the effect of the cold surges on the tropical surface wind, although a few models unrealistically generated subtropical southerly wind in the mid-winter. The relationship between cold surges and the tropical convection was not satisfactorily simulated in most models. The common discrepancies in the winter monsoon simulation can be attributed to many factors. In some models, the reason is directly related to the improper location of the large-scale convective center near the western Pacific. The satisfactory simulations of the monsoon circulation and the cold surges are partly due to the topographical characteristics of the East Asian continent, i.e., the Tibetan Plateau to the west and the oceans to the east. The correct simulation of the interannual variation of the surface wind near the South China Sea (SCS) and the maritime continent is a demanding task for most of the models. This will require adequate simulations of many aspects, including tropical convection, the Siberian cold dome, the extratropical-tropical linkage, and the air-sea interaction. The discrepancies noted here furnish a guide for the continuing improvement of the winter monsoon simulations. Improved simulations will lead to an adequate delineation of the surface wind and convection near the maritime continent, which is essential for portraying the winter monsoon forcing in a coupled model.

Notes: Abstract The ability of 15 atmospheric general circulation models (AGCM) to simulate the tropical intraseasonal oscillation has been studied as part of the Atmospheric Model Intercomparison Project (AMIP). Time series of the daily upper tropospheric velocity poential and zonal wind, averaged over the equatorial belt, were provided from each AGCM simulation. These data were analyzed using a variety of techniques such as time filtering and space-time spectral analysis to identify eastward and westward moving waves. The results have been compared with an identical assessment of the European Centre for Medium-range Weather Forecasts (ECMWF) analyses for the period 1982–1991. The models display a wide range of skill in simulating the intraseasonal oscillation. Most models show evidence of an eastward propagating anomaly in the velocity potential field, although in some models there is a greater tendency for a standing oscillation, and in one or two the field is rather chaotic with no preferred direction of propagation. Where a model has a clear eastward propagating signal, typical periodicities seem quite reasonable although there is a tendency for the models to simulate shorter periods than in the ECMWF analyses, where it is near 50 days. The results of the space-time spectral analysis have shown that no model has captured the dominance of the intraseasonal oscillation found in the analyses. Several models have peaks at intraseasonal time scales, but nearly all have relatively more power at higher frequencies (〈 30 days) than the analyses. Most models underestimate the strength of the intraseasonal variability. The observed intraseasonal oscillation shows a marked seasonality in its occurrence with greatest activity during northern winter and spring. Most models failed to capture this seasonality. The interannual variability in the activity of the intraseasonal oscillation has also been assessed, although the AMIP decade is too short to provide any conclusive results. There is a suggestion that the observed oscillation was suppressed during the strong El Niño of 1982/83, and this relationship has also been reproduced by some models. The relationship between a model's intraseasonal activity, its seasonal cycle and characteristics of its basic climate has been examined. It is clear that those models with weak intraseasonal activity tend also to have a weak seasonal cycle. It is becoming increasingly evident that an accurate description of the basic climate may be a prerequisite for producing a realistic intraseasonal oscillation. In particular, models with the most realistic intraseasonal oscillations appear to have precipitation distributions which are better correlated with warm sea surface temperatures. These models predominantly employ convective parameterizations which are closed on buoyancy rather than moisture convergence.

Notes: Abstract. The ability of 15 atmospheric general circulation models (AGCM) to simulate the tropical intraseasonal oscillation has been studied as part of the Atmospheric Model Intercomparison Project (AMIP). Time series of the daily upper tropospheric velocity poential and zonal wind, averaged over the equatorial belt, were provided from each AGCM simulation. These data were analyzed using a variety of techniques such as time filtering and space-time spectral analysis to identify eastward and westward moving waves. The results have been compared with an identical assessment of the European Centre for Medium-range Weather Forecasts (ECMWF) analyses for the period 1982–1991. The models display a wide range of skill in simulating the intraseasonal oscillation. Most models show evidence of an eastward propagating anomaly in the velocity potential field, although in some models there is a greater tendency for a standing oscillation, and in one or two the field is rather chaotic with no preferred direction of propagation. Where a model has a clear eastward propagating signal, typical periodicities seem quite reasonable although there is a tendency for the models to simulate shorter periods than in the ECMWF analyses, where it is near 50 days. The results of the space-time spectral analysis have shown that no model has captured the dominance of the intraseasonal oscillation found in the analyses. Several models have peaks at intraseasonal time scales, but nearly all have relatively more power at higher frequencies (〈30 days) than the analyses. Most models underestimate the strength of the intraseasonal variability. The observed intraseasonal oscillation shows a marked seasonality in its occurrence with greatest activity during northern winter and spring. Most models failed to capture this seasonality. The interannual variability in the activity of the intraseasonal oscillation has also been assessed, although the AMIP decade is too short to provide any conclusive results. There is a suggestion that the observed oscillation was suppressed during the strong El Niño of 1982/83, and this relationship has also been reproduced by some models. The relationship between a model's intraseasonal activity, its seasonal cycle and characteristics of its basic climate has been examined. It is clear that those models with weak intraseasonal activity tend also to have a weak seasonal cycle. It is becoming increasingly evident that an accurate description of the basic climate may be a prerequisite for producing a realistic intraseasonal oscillation. In particular, models with the most realistic intraseasonal oscillations appear to have precipitation distributions which are better correlated with warm sea surface temperatures. These models predominantly employ convective parameterizations which are closed on buoyancy rather than moisture convergence.

Notes: Abstract. Interannual and interdecadal variabilities in the Pacific are investigated with a coupled atmosphere-ocean GCM developed at MRI, Japan. The model is run for 70 years with flux adjustments. The model shows interannual variability in the tropical Pacific which has several typical characteristics shared with the observed ENSO. A basin-scale feature of the principal SST variation for the ENSO time scale shows negative correlation in the central North Pacific with the tropical SST, similar to that of the observed one. Associated variation of the model atmosphere indicates an intensification of the Aleutian Low and a PNA-like teleconnection pattern as a response to the tropical warm SST anomaly. The ENSO time scale variability in the midlatitude ocean consists of the westward propagation of the subsurface temperature signal and the temperature variation within the shallow mixed layer forced by the anomalous atmospheric heat fluxes. For the interdecadal time scale, variation of the SST is simulated realistically with a geographical pattern similar to that for the ENSO time scale, but it has a larger relative amplitude in the northern Pacific. For the atmosphere, spatial structure of the variation in the interdecadal time scale is also similar to that in the ENSO time scale, but has smaller amplitude in the northern Pacific. Long oceanic spin-up time (〉∼10 y) in the mid-high latitude, however, makes oceanic response in the interdecadal time scale larger than that in the ENSO time scale. The lagged-regression analysis for the ocean temperature variation relative to the wind stress variation indicates that interdecadal variation of the ocean subsurface at the mid-high latitudes is considered as enhanced ocean gyre spin-up process in response to the atmospheric circulation change at the mid-high latitudes, remotely forced by the interdecadal variation of the tropical SST.

Notes: Abstract  As a part of the Atmospheric Model Intercomparison Project (AMIP), the behaviour of 15 general circulation models has been analysed in order to diagnose and compare the ability of the different models in simulating Northern Hemisphere midlatitude atmospheric blocking. In accordance with the established AMIP procedure, the 10-year model integrations were performed using prescribed, time-evolving monthly mean observed SSTs spanning the period January 1979–December 1988. Atmospheric observational data (ECMWF analyses) over the same period have been also used to verify the models results. The models involved in this comparison represent a wide spectrum of model complexity, with different horizontal and vertical resolution, numerical techniques and physical parametrizations, and exhibit large differences in blocking behaviour. Nevertheless, a few common features can be found, such as the general tendency to underestimate both blocking frequency and the average duration of blocks. The problem of the possible relationship between model blocking and model systematic errors has also been assessed, although without resorting to ad-hoc numerical experimentation it is impossible to relate with certainty particular model deficiencies in representing blocking to precise parts of the model formulation.