Spatial distribution and the interdecadal change of leading modes of heat budget of the mixed-layer in the tropical Pacific and the association with ENSO

Using heat budget diagnosis of ocean mixed layer from the Global Ocean Data Assimilation System, the spatial distribution of the leading modes of the heat budget was examined. The analysis was for the tropical Pacific in 1979–2013 and was based on combined empirical orthogonal function (CEOF) analysis. The interdecadal changes of the leading modes and their associations with El Niño-Southern Oscillation (ENSO) were also analyzed. The first leading CEOF mode (CEOF1) corresponds to the ENSO mature phase. The contribution from the zonal advection was relatively small along the equator, except the region near the Pacific coast of Central America. The vertical entrainment and diffusion (surface heat flux) had pronounced maxima with positive (negative) values along the equatorial central and eastern Pacific. The meridional advection displayed a different spatial pattern with large positive values on both sides of the equator and smaller values along the equator. The total meridional advection anomaly was mainly determined by advection of anomalous temperature by climatological current responsible for broadening of the ENSO SSTA pattern meridionally. The zonal advection varied almost simultaneously with the tendency of ocean temperature anomaly in the mixed layer. The second leading CEOF mode (CEOF2) included contribution to SSTA tendency during the ENSO developing phase. The distribution pattern and amplitude of the zonal advection in the eastern Pacific in CEOF2 was similar to but with opposite sign to that in CEOF1. The amplitudes of the other dynamical and thermodynamical terms were smaller than that in CEOF1 and spatial distributions displayed an opposite variation between the Pacific coast of Central America and central and eastern tropical Pacific in CEOF2. A comparison of two periods (1979–1999 and 2000–2013) suggested that coupling in the tropical Pacific weakened at ENSO time scales and shifted to a relatively higher frequency regime (from 2 to 4 years averaged in 1979–1999 to 1.5–3 years) after 2000.