89th American Meteorological Society Annual Meeting

Sunday, 11 January 2009
State of tropical instability waves in a current generation coupled climate model
Phoenix Convention Center
Balachandrudu Narapusetty, George Mason University, Calverton, MD
In eastern equatorial Pacific, the so-called tropical instability waves (TIWs) potentially alter the interactions between sea surface temperature (SST) and atmospheric boundary layer (ABL) and produce significant shifts in atmospheric low-level circulation. We do not know whether the state-of-the-art coupled climate models produce the TIWs with allowable phase and strength as in the real observations. We should understand the potential nonlinear rectification effects posed by TIWs in the current generation coupled models to better address the key climate change projections.

In this study the main goal is to understand the structure of existence and evolution of TIWs in state-of-the-art Community Climate System Model (CCSM) of Intergovernmental Panel on Climate Change (IPCC) Assessment Report 4 (AR4). For this purpose, the data obtained by CCSM AR4 simulations for various climate projection scenarios is selected. The data years are separated based on strong and weak El Niņo and La Niņa and neutral years by the variations in Niņo 3.4 index, and a space-time spectral analysis on the respective composite years is performed. To check the results against the observations, we perform the similar spectral analysis on Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature (SST) data. To compare the effect of TIWs extracted from a typical climate scenario to that of observational estimates, we also run an Atmospheric General Circulation Model (AGCM) driven with TIWs in both the instances.

While the general hierarchy of strength of TIWs in different La Niņa and El Niņo composites is well produced in all the considered IPCC scenarios, two major drawbacks identified as (1) weak simulated TIWs and (2) too quick moving SST fronts characterizing the TIWs. There are no systematic differences in the strength of TIWs among the tested different AR4 scenarios. The differences in strengths of TIWs in different composite years vary marginally.

The performed AGCM simulations show that intraseasonal variability produced by driving a typical climate scenario produced TIWs is very weak and hence produce super diffused three dimensional temperature, wind profile, and specific humidity profiles with in atmospheric boundary layer.

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