Role of Two-Way Ocean-Atmosphere Interaction on Extratropical Climate Variability

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The role of ocean-atmosphere interactions on climate variability in extratropical latitudes, both within the season and during transitions between seasons, will be examined. The focus of the proposed research will be on assessing the two-way interactive relationship between temperature anomalies produced in the ocean and synoptic scale transient activity (e.g. locations and paths of high and low pressure systems) and related low frequency flow patterns produced in the atmosphere. The results will aid in understanding the causes of climate variability and the potential for predictability on seasonal time scales. This research builds upon (1) past studies which investigated the one-way forced effects of SST anomalies (prescribed) on atmospheric transient eddy processes and (2) past work which investigated the two-way interaction between mid-latitude SST and atmospheric circulation anomalies as deduced in long multi-year seasonal integrations of a low resolution atmospheric model coupled to a constant depth mixed layer ocean. For these studies a higher resolution atmospheric model (NCAR CCM1) will be used so that the climatic behavior of transient eddy processes can be simulated more realistically. The simple mixed layer ocean model has been chosen to permit examination of the ways that the important ocean-atmosphere energy exchange influences the evolution of middle latitude SST anomalies. Three experiments are planned: (1) a 100-year seasonal integration of the atmospheric model (CCM1) using prescribed climatological SSTs, (2) a 100-year seasonal integration of CCM1 coupled to a constant 50 m depth mixed layer ocean (CCM1CO) and (3) a 40-50 year seasonal integration of CCM1 coupled to a constant 50 m depth mixed layer ocean but with prescribed forcing of observed tropical SST variability (CCM1CO/TROPSET). The variability of transient eddy activity (storm tracks) and abrupt seasonal shifts in circulation in the absence of SST variability wil be investigated in Experiment 1. The two-way influence between model produced extratropical anomalies and storm tracks (and the associated time mean atmospheric circulation) will be examined in Experiment 2. The modifying influence of tropical SST variability (which includes El Nino cycles) on middle latitude SST anomalies and variability of extratropical atmospheric transient eddy activity will be assessed by comparing the results of Experiments 2 and 3. The behavior of synoptic scale storm tracks will be characterized by examination of model variation in RMS time-filtered 1000 mb and 500 mb heights and in the processes of baroclinic eddy momentum and heat fluxes; the determination of abrupt seasonal changes in atmospheric circulation will include an analysis of model energetics. Dominant SST anomaly patterns and their relationship to atmospheric processes will be examined using various statistical and composite analyses including EOF, lag correlation, singular value decomposition and canonical correlation methods.