Deep Ocean Motion: Pacific Equatorial Thermostad Response to El Niño

El Niños 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 of warm water causes the thermocline, the large vertical temperature gradient beneath the surface mixed layer, to shoal in the west and deepen in the east, inducing a redistribution of ocean heat that affects global mean values. The effects of El Niños on the ocean beneath the thermocline are important for a comprehensive understanding of the impact of these events. Hence we examine the response of the Pacific Equatorial Thermostad, a layer of low vertical stratification below the thermocline, to El Niño using a monthly Argo float climatology from January 2004 through June 2016 and Argo float deep velocity data. We fit a mean, seasonal cycle, trend, and linear response to the Niño3.4 index to temperature and salinity at each gridpoint and to deep float velocities to characterize physical properties in the thermostad, including layer thickness and velocity. We compare these properties for neutral conditions (Niño3.4 = 0) versus those during a moderate El Niño (Niño3.4 = 1). During an El Niño, a strengthening of the westward-flowing Equatorial Intermediate Current of order 3 x 106 m3 s-1 shifts about 7 x 1013 m3 of thermostad water from the east to the west, allowing conservation of volume within the thermostad as the thermocline above deepens in the east and shoals in the west. This transport and volume change imply a 9-month time scale, consistent with El Niño.