Tropical Pacific Ocean Mixed Layer Heat Budget: The Pacific Cold Tongue

Abstract

Data from satellite-tracked drifting buoys and VOS/XBT profiles for the years 1979-95 were used to evaluate the seasonal cycle of how major oceanic processes redistribute heat in the cold tongue region of the tropical Pacific. The most active processes for the annual cycle are local heat storage and heat export by entrainment of upwelling and by mean meridional advection. Heat export by zonal advection, however, is not negligible, and meridional eddy heat fluxes associated with tropical instability waves effect a negative feedback that offsets a considerable fraction of that produced by the mean meridional advection. All of these processes mimic the essentially one cycle per year of the surface wind stress, as do those of the depths of both the bottom of the surface mixed layer and the thermocline. Because it is associated with poleward Ekman transports, upwelling, and baroclinic adjustment near the equator, the zonal wind stress component appears to be the more important. The meridional wind stress, while weaker in the annual mean, has a larger annual variation and, therefore, has equal influence on the annual variation of the scalar stress and perhaps the mixed layer thickness. The Monin-Obukov length is found to underestimate the mixed layer thickness considerably. Finally, the authors produce the first estimates of the seasonal cycle of eddy heat flux convergence, which plays a significant role in the evolution of the cold tongue, and show that the eddy heat flux convergence can be quantitatively modeled as eddy diffusion with a diffusivity derived from single-particle buoy statistics.

Diapycnal heat flux and mixed layer heat budget within the Atlantic Cold Tongue

“…The vertical temperature/velocity covariance involves the correlation of the possible departures of horizontal velocity and temperature from their vertically and temporally averaged values in the ML. Swenson and Hansen (1999) estimated the heat flux due to this term considerably smaller than the other contributions and hence this term will be neglected in the following as also described in Foltz et al (2003). The evaluation of the individual terms closely follows the procedures described by Foltz et al (2003).…”

“…Mean horizontal advection is determined by multiplying the monthly mean velocities (calculated from the combined ARGO and drifter velocities) with climatological longterm SST gradients from the TMI satellite observations. The eddy term of horizontal advection is estimated indirectly from the residual between mean horizontal advection described above and total horizontal advection estimated (Swenson and Hansen 1999). The total time derivative is obtained from drifter SSTs, whereas the local derivative is estimated from the average monthly TMI SSTs.…”

Measuring surface currents with Surface Velocity Program drifters: the instrument, its data, and some recent results

“…These data have been used to show that tropical instability waves, zonal advection, storage and entrainment all play a major role in the heat budget of the Pacific (Hansen and Paul, 1987;Swenson and Hansen, 1999) and Atlantic (Foltz et al, 2003) equatorial cold tongues.…”

SMOSSea Surface Salinity signals of tropical instability waves

“…They are also larger during La Niña conditions when the cold tongue and the related current shear are more intense than those during a normal year. TIWs are found to play important roles in mixing and cross-equator transport of sea water [Jochum and Murtugudde, 2006;Menkes et al, 2006], the modulation of the heat budget of the ocean mixed layer [Swenson and Hansen, 1999;Vialard et al, 2003], air-sea coupling [Liu et al, 2000;Yu and Liu, 2003], and marine biogeochemistry [Boutin et al, 1999;Gorgues et al, 2005;Evans et al, 2009].…”