Equatorial Pacific Thermostad response to El Niño

Abstract

El Ninos are characterized by a shift of warm surface water from the western to eastern equatorial Pacific due to weakening of easterly trade winds. This shift is associated with the pycnocline (or thermocline), the large vertical density gradient beneath the surface mixed layer, shoaling in the west and deepening in the east, inducing a redistribution of ocean heat with global impacts. Here the response of the Pacific Equatorial Thermostad, a layer of low vertical stratification below the pycnocline, to El Nino is investigated using a monthly Argo float climatology and Argo float deep velocity data. A mean, seasonal cycle, trend, and time-lagged linear response to the Nino3.4 index are fit by least squares to temperature and salinity at each gridpoint as well as to deep float velocities (omitting the trend). The results of these fits are used to characterize the response of physical properties in the Thermostad, including layer thickness and velocity, to El Nino by comparing the mean properties following neutral conditions (Nino3.4 = 0°C) versus those following a moderate El Nino (Nino3.4 = 1°C). Following an El Nino, a strengthening of the westward-flowing Equatorial Intermediate Current of about 2.7 × 106 m3 s−1 shifts about 97 × 1012 m3 of thermostad water from the east to the west, allowing conservation of volume within the Thermostad as the pycnocline above deepens in the east and shoals in the west. This transport and volume change imply a 14-month time scale, consistent with El Nino. This article is protected by copyright. All rights reserved.