Oceanic meridional transports and their roles in warm water volume variability and ENSO in the tropical Pacific

The fluctuation of the subsurface ocean heat condition along the equatorial Pacific is associated with the mass/heat exchanges between the equatorial and off-equatorial regions, which is the main cause of the phase transitions during the El Nino-Southern Oscillation (ENSO) cycle. In this work, the connection between the meridional transport convergences (MTCs) along the equatorial Pacific and variations of the warm water volume in the equatorial Pacific and their connections with the ENSO cycle are investigated. It is noted that the Sverdrup MTC induced by the wind stress curl has a significant impact on the thermocline fluctuation in nearly the entire equatorial Pacific but the impacts of its components vary with longitude. The component induced by the Ekman currents has a significant contribution from 150 degrees W eastward to the coast, as well as the far-western Pacific, while the geostrophic component has a significant contribution in the central Pacific. There is a strong compensation between the surface wind stress-induced Ekman MTC and the Ekman pumping-induced geostrophic MTC which is confined in the central Pacific. Furthermore, the geostrophic component facilitates the phase transition of the ENSO cycle, while the Ekman component compensatively hinders it. The longitudinally varying component of the MTC enhances the anomalous thermocline tilting during the ENSO growth and maturing phases. These results may benefit the understanding, monitoring, and forecasting of ENSO evolution.

Automated summary

This study investigates the connection between meridional transport convergences (MTCs) along the equatorial Pacific and variations in warm water volume, as well as their relationship with the El Nino-Southern Oscillation (ENSO) cycle. The results show that the Sverdrup MTC induced by wind stress curl has a significant impact on the thermocline fluctuation in the equatorial Pacific, with varying contributions from different components. The findings enhance our understanding and forecasting of ENSO evolution.