Coupled dynamics of the North Equatorial Countercurrent and Intertropical Convergence Zone with relevance to the double-ITCZ problem

The Intertropical Convergence Zone (ITCZ) is a salient feature of tropical atmospheric circulation characterized by intense convective clouds and rainfall north of the equator. Climate models, however, commonly experience the double-ITCZ problem—the tendency to produce another strong precipitation band but south of the equator. Here, we describe coupled interaction between the ITCZ and the North Equatorial Countercurrent (NECC) that is relevant to this problem. This current is a major component of the tropical Pacific upper-ocean circulation, which flows against easterly trade winds and transports warm water from the western Pacific eastward. Its core follows the latitudinal position of the ITCZ. Trade winds converge toward the ITCZ, creating a local minimum in zonal winds and generating positive wind stress curl that maintains an eastward current despite westward winds. We show that interaction between the ITCZ and the NECC involves positive feedback: a stronger NECC advects warm water from the western Pacific to the colder east thus increasing sea surface temperature (SST) along its path, intensifying convection within the ITCZ and hence strengthening wind stress curl, further strengthening the NECC. To demonstrate this wind stress curl-advection-SST-precipitation (WASP) feedback, we conduct climate model experiments in which we progressively strengthen the surface Ekman component of the NECC and observe the intensification of the ITCZ and the entire NECC. Consequently, a weak NECC leads to a weak ITCZ, which can contribute to the double-ITCZ problem in climate models, since weak wind convergence north of the equator enables stronger convergence in the south.