The roles of tropical and subtropical wind stress anomalies in the El Nino Modoki onset

The El Nino Modoki is separated into two types, El Nino Modoki I and El Nino Modoki II, which are characterized by substantially different mechanism. According to comparisons between the evolution of air-sea coupled processes and heat budget analyses, this study discovered the distinct dynamical mechanism of the onset of El Nino Modoki II compared to that of El Nino Modoki I. For El Nino Modoki I, westerly wind anomalies in the western Pacific appear earlier than warm sea surface temperature (SST) anomalies in the central tropical Pacific; in addition, zonal advection is responsible for changes in mixed-layer temperature in the Nino-4 region during the developing phase in spring and summer. However, warm SST anomalies accompany easterly anomalies in the central tropical Pacific, and there are no westerly anomalies in the western tropical Pacific during the developing phase in spring and summer for El Nino Modoki II. For El Nino Modoki II, Ekman feedback is the major contributor to changes in mixed-layer temperature. According to reanalysis data, the 2.5-layer model results and coupled model simulations, anomalous westerly wind stress in the subtropical North Pacific and easterly wind stress in the central-eastern tropical Pacific can lead to equatorward and westward oceanic current anomalies, thus enhancing meridional and zonal convergences at upper layer. Because anomalous convergence at upper layer can inhibit upwelling motion in the central tropical Pacific, SST anomalies in the central tropical Pacific tend to be warm, even without anomalous westerlies in the western Pacific.