Seasonal fluctuations of the surface North Equatorial Countercurrent (NECC) across the Pacific basin

Abstract

Based on multisatellite-derived sea surface current, sea surface height anomaly, and geostrophic current data, the annual variability of the surface NECC across the Pacific basin is investigated. Inferred from the surface current data, the NECC axis east and west of the dateline moves northward along its pathway in the latter half of a year and the first half of a year, respectively. The sea surface height anomaly in the NECC region and the meridional movement of NECC are well reproduced by a linear wind-driven first-mode baroclinic Rossby wave model. Model experiments by setting the phase speed of baroclinic Rossby wave or the Ekman pumping to zero in different regions are carried out to clarify the influence of Rossby wave and local Ekman pumping on the movement of NECC. East of the dateline, both Ekman pumping and Rossby wave are important in determining the northward movement of the NECC axis. However, the NECC axis west of the dateline moves northward primarily due to the westward transmission of positive sea surface height anomalies (or downwelling Rossby waves) that originate in the east of the dateline. The local Ekman pumping plays a minor role in controlling the migration of the NECC axis west of the dateline.