Interannual Variation of Mixed-layer Heat Balance Inferred From the ECCO Ocean Data Assimilation System

Variation of mixed-layer heat balance associated with El Nino-Southern Oscillation (ENSO), Indian-Ocean Dipole (OID), and Pacific Decadal Oscillation (PDO) during the period of 1997-2001 are studied using an ECCO assimilation product (http://www.ecco-group.org). The product is generated by fitting a near-global ocean general circulation model to satellite and in-situ observations through optimal adjustment of initial and surface boundary conditions. In the eastern and central equatorial Pacific, the warming and cooling are caused by the combined effect of oceanic advection and diffusion. Surface heat flux opposes the change. The opposite is true for the western equatorial Pacific. The relative contribution of different components of oceanic advection is spatially dependent. Zonal advection is dominant in the central equatorial Pacific where zonal temperature gradient associated with the warm-pool edge is large. Vertical advection is more important in the eastern equatorial Pacific where upwelling and vertical temperature gradient are large. Diffusion has a smaller magnitude than advection, but is comparable to the temporal change of mixed-layer temperature (MLT). Advection of anomalous temperature gradient by mean flow and advection of mean temperature gradient by anomalous flow have comparable magnitude, both being in-phase with the change of MLT. Nonlinear tendency (advection of anomalous temperature gradient by anomalous flow) is sizable and counteracts the two linear tendencies above. The role of oceanic advection is similar in MLT changes associated with ENSO, IOD, and PDO. However, that of surface heat flux is different.