El Nino - Southern Oscillation (ENSO) events are accompanied by an anomalous zonal sea surface temperature (SST) gradient over the west Pacific Ocean, defined hereafter as the west Pacific SST gradient (WPG). While the direction of the WPG follows ENSO cycles, the magnitude of the gradient varies considerably between individual El Nino and La Nina events. In this study, El Nino and La Nina events are grouped according to the magnitude of the WPG, and tropical SST, circulations and precipitation are examined in addition to an analysis of Northern Hemisphere wintertime circulation and precipitation for the period of 1948-2011.
The WPG is defined as the difference between area-averaged SST over the central Pacific Ocean (Nino4 region) and west Pacific Ocean (0N-10N; 130E-150E). Until the 1980s the WPG showed little trend as the west and central Pacific warmed at similar rates; however, the west Pacific has recently warmed faster than the central Pacific, which has resulted in an increased (diminished) WPG during La Nina (El Nino) events.
The temporal evolution and distribution of Pacific SST as well as the near-surface tropical Pacific zonal wind, tropical divergence and vertical velocity are considerably different during ENSO events partitioned according to the strength of the WPG. Modifications to the tropical circulation result in changes to the Indo-west Pacific precipitation and vertically integrated energy budgets and are linked to strong and consistent circulation and precipitation modifications throughout the Northern Hemisphere during winter.
