Interhemispheric Coherence of Tropical Climate Variability: Effect of the Climatological ITCZ

抄録

The intertropical convergence zone (ITCZ) is the rising branch of the global Hadley circulation and often considered as the climatic axis of symmetry. A full-physics atmospheric general circulation model is coupled with an intermediate ocean model to investigate the effect of ITCZ's meridional configuration on the space-time structure of climate variability. In the control experiment where the model settles into a north-south symmetric climatology, strong interhemispheric interaction takes place and sea surface temperature (SST) anomalies are organized into an anti-symmetric dipole pattern with the equator as the nodal line. The trade winds intensify (weaken) over the anomalously cold (warm) side of the equator, indicative of a positive feedback between surface wind, evaporation and SST (WES). When the mean ITCZ is displaced into the Northern Hemisphere by perturbing the shape of continents, SST variability is significantly reduced at low-frequencies, especially with periods greater than 5 years, as a result of reduced interhemispheric interaction. The nodal line now coincides with the northward-displaced ITCZ, and the SST correlation across this nodal line is greatly reduced to a statistically insignificant level. Calculations with a simple baroclinic model of the atmosphere indicate that the departure of the climatic axis of symmetry from the geographic equator weakens the WES feedback and hence interhemispheric interaction.
Implications for tropical Atlantic variability are discussed. In particular, our result of ITCZ’s modulation of interhemispheric interaction is consistent with the seasonality of cross-equatorial SST gradient variability: it peaks in boreal spring when the mean ITCZ is nearly symmetric about the equator and is significantly reduced in other seasons when the Atlantic ITCZ is displaced into the Northern Hemisphere.