83rd Annual

Monday, 10 February 2003
Effect of climatological ITCZ on interhemispheric coherence of climate variability
Hideki Okajima, SOEST, Univ. of Hawaii, Honolulu, HI; and S. P. Xie and A. Numaguti
Interannual variability in the latitude of the Atlantic ITCZ is sensitive to the cross-equatorial SST gradient (CESG), which is in turn a result of changes in the ITCZ and wind. While such an air-sea interaction mechanism generates a preferred mode of SST variability with a dipolar structure in the meridional direction, observed interhemispheric correlation is very weak in the tropical Atlantic. This paper explores the factors that affect such interhemispheric interaction.

We use a hybrid coupled atmospheric GCM (HaGCM) that consists of a full-physics atmospheric GCM and an intermediate ocean model. The thermocline depth varies in space and shoals eastward on the equator. Its temporal variations are suppressed, however, to filter out the zonal Bjerknes feedback and allow for a focus on the meridional interaction.

Under equatorial symmetric conditions, the model climate is symmetric on long-term average. On interannual to decadal timescales, the model displays pronounced variability, with the ITCZ moving back and forth across the equator. SST and wind anomalies are strongly negatively correlated across the equator, consistent with the so-called wind-evaporation-SST feedback.

In a second experiment when the eastern boundary of the ocean is tilted, the climatological ITCZ becomes locked to the north of the equator. The model still shows strong interannual variability but its interhemispheric coherence is marked reduced to a level statistically insignificant. A simple-model calculation explains this reduction in interhemispheric correlation, indicating that the departure of the climatological ITCZ from the geographic equator (f=0) weakens the atmospheric feedback onto a meridional SST dipole.

Our results suggest that the configuration of the climatological ITCZ is a key factor for the strength of interhemispheric air-sea interaction, a conclusion consistent with available observations in the tropical Atlantic; the observed interannual CESG variance peaks in the boreal spring when the ITCZ is close to the equator but reaches a seasonal minimum in the boreal fall when the ITCZ is far away from the equator.

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