13C.1
A simple model of tropical Atlantic decadal climate variability
Yochanan Kushnir, Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY; and R. Seager and J. C. H. Chiang
The tropical Atlantic climate system displays perplexing decadal variability in which SST, surface winds, and the location of the ITCZ, vary together in a coherent manner. Experiments with an ocean GCM driven with observed, historical wind data, suggest that the SST variability in the region is governed by two major players: surface windspeed fluctuations that generate SST anomalies via surface heat exchange and advection by the mean meridional ocean current that damps those anomalies. Based on these results we construct a simple linear model of the atmosphere and ocean mixed layers that successfully simulates the evolution of zonally averaged SST anomalies, given observed variations in zonally averaged windspeed.
Observations and GCM models suggest that the location of the ITCZ is determined by the gradient of SST across the equator. Based on this assumption, we devise a statistical model that given the anomalous SST distribution across the equator predicts the anomalous ITCZ position and the associated changes in meridional winds within the “deep tropics”. We then force our ocean model with random, “white-noise” windspeed variability off the equator, in the trade wind regions, generating SST anomalies north and south of the equator. These anomalies invoke ITCZ shifts and additional windspeed changes close to the equator, which can feedback on SST. The coupling in the ITCZ region leads to a considerable "reddening" of the SST spectrum with large power on decadal time scales. We use the simple model to explore the necessary conditions for such spectral reddening and the prospects for predicting decadal variability in the region.
Session 13C, Ocean Atmosphere Interaction II (Parallel with Sessions 13A, 13B, and 13D)
Thursday, 2 May 2002, 11:00 AM-12:30 PM
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