A global analysis of ENSO synchrony: The oceans' biological response to physical forcing

A global-scale Empirical Orthogonal Function (EOF) analysis of physical (sea surface temperatures, sea level height anomalies, atmospheric sea level pressure, photosynthetically active radiation, zonal surface currents and wind-driven upwelling velocities) and biological (surface chlorophyll concentrations and primary production) variables shows synchronous variations from 1993 to 2010 in the first mode of variability associated with El Nino Southern Oscillation (ENSO). The first EOF of vertical temperature structure along the equatorial Pacific shows identical temporal patterns. The ENSO-driven biological changes are explained both qualitatively and quantitatively from a subset of the physical variables. During the strong 1997-1998 El Nino a global new production decrease of similar to 0.6-0.9 PgC yr(-1) is estimated from changes in the depth of the nutricline and wind-driven upwelling. This is consistent with the 3.0 PgC yr(-1) decrease in global primary production observed by satellite remote sensing. A simple two-layer model of chlorophyll and primary production driven by changes in nitrate and light reproduces the patterns and magnitude of changes observed by satellite. Changes in the depth of the nutricline are found to be the primary driver of the biological anomalies. The ENSO mode of zonal currents in the equatorial Pacific shows that horizontal advection is responsible for changes in chlorophyll in the central Pacific not explained by the two-layer model.