This study compares the tropical intraseasonal oscillation (TISO) variability in the Geophysical Fluid Dynamics Laboratory (GFDL) coupled general circulation model (CGCM) and the stand-alone atmospheric general circulation model (AGCM). For the AGCM simulation, the sea surface temperatures (SSTs) were specified using those from the CGCM simulation. This was done so that any differences in the TISO that emerged from the two simulations could be attributed to the coupling process and not to a difference in the mean background state. The comparison focused on analysis of the rainfall, 200-mb velocity potential, and 850-mb zonal wind data from the two simulations, for both summer and winter periods, and included comparisons to analogous diagnostics using NCEP–NCAR reanalysis and Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) rainfall data. The results of the analysis showed three principal differences in the TISO variability between the coupled and uncoupled simulations. The first was that the CGCM showed an improvement in the spatial variability associated with the TISO mode, particularly for boreal summer. Specifically, the AGCM exhibited almost no TISO variability in the Indian Ocean during boreal summer—a common shortcoming among AGCMs. The CGCM, on the other hand, did show a considerable enhancement in TISO variability in this region for this season. The second was that the wavenumber–frequency spectra of the AGCM exhibited an unrealistic peak in variability at low wavenumbers (1–3, depending on the variable) and about 3 cycles yr−1 (cpy). This unrealistic peak of variability was absent in the CGCM, which otherwise tended to show good agreement with the observations. The third difference was that the AGCM showed a less realistic phase lag between the TISO-related convection and SST anomalies. In particular, the CGCM exhibited a near-quadrature relation between precipitation and SST anomalies, which is consistent with observations, while the phase lag was reduced in the AGCM by about 1.5 pentads (∼1 week). The implications of the above results, including those for the notions of “perfect SST” and “two tier” experiments, are discussed, as are the caveats associated with the study's modeling framework and analysis.
This study examines systematic biases in sea surface temperature (SST) under the stratus cloud deck in the southeast Pacific Ocean and upper-ocean processes relevant to the SST biases in 19 coupled general circulation models (CGCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The 20 years of simulations from each model are analyzed. Pronounced warm SST biases in a large portion of the southeast Pacific stratus region are found in all models. Processes that could contribute to the SST biases are examined in detail based on the computation of major terms in the upperocean heat budget. Negative biases in net surface heat fluxes are evident in most of the models, suggesting that the cause of the warm SST biases in models is not explained by errors in net surface heat fluxes. Biases in heat transport by Ekman currents largely contribute to the warm SST biases both near the coast and the open ocean. In the coastal area, southwestward Ekman currents and upwelling in most models are much weaker than observed owing to weaker alongshore winds, resulting in insufficient advection of cold water from the coast. In the open ocean, warm advection due to Ekman currents is overestimated in models because of the larger meridional temperature gradient, the smaller zonal temperature gradient, and overly weaker Ekman currents.
[1] This study examines the contribution of ocean dynamics to sea surface temperature (SST) biases in the eastern Pacific cold tongue region in fifteen coupled general circulation models (CGCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Twenty years (1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999) of the twentieth-century (20C3m) climate simulations from each model are analyzed. An excessive and narrow SST cold tongue that extends too far west into the western Pacific in comparison to observations is a common bias in CGCMs. This feature is found in CGCMs analyzed here and in many previous studies. The heat budget analysis indicates that errors in both net surface heat flux and total upper ocean heat advection significantly contribute to the excessive cold tongue in the equatorial Pacific. The stronger heat advection in the models is caused by overly strong horizontal heat advection associated with too strong zonal currents, and overly strong vertical heat advection due to excessive upwelling and the vertical gradient of temperature. The Bjerknes feedback in the coupled models is shown to be weaker than in observations, which may be related to the insufficient response of surface zonal winds to SST in the models and an erroneous subsurface temperature structure. A hypothesis that describes how the cold tongue bias is possibly developed in the CGCMs is provided based on the results of our analysis.
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