A Lagrangian Description of the Western Equatorial Pacific Response to the Wind Burst of December 1992: Heat Advection in the Warm Pool

During the Tropical Oceans Global Atmosphere (TOGA) Coupled Ocean–Atmosphere Response Experiment (COARE) intensive observing period (IOP), sustained westerly winds were observed between 20 December 1992 and 10 January 1993 in the area between 155°E and 180°. The oceanic response to this event was monitored by 33 Lagrangian mixed layer drifters, six of which were equipped with SEACAT salinity sensors. The drifters were distributed over several hundred kilometers meridionally and over a zonal extent of 2400 km. During the wind event, the drifters accelerated eastward and formed a strong equatorial jet that was relatively independent of longitude. Following the drifters, the water parcels cooled and became more saline; Sea surface temperature (SST) maps suggest that evaporative cooling occurred. In order to consider the dynamics and thermodynamics of this jet in more detail, wind stress and buoyancy forcing along the track of each individual drifter were constructed from the TOGA COARE European Centre for Medium-Range Weather Forecasts analysis. The mixed layer depth scale and the zonal pressure gradient were calculated from a linear regression between the acceleration and the wind stress. In the meridional direction, the wind stress was smaller and not coherent with the acceleration at any period. During the December wind burst, the entire western equatorial Pacific cooled and a large-scale zonal temperature gradient with cooler water to the west was established west of the date line. Cooled water was advected to the east during this episode. A 4-yr-long TOGA–Tropical Atmosphere Ocean (TAO) current meter record at 165° E and the historical dataset from 250 drifters in the western Pacific within 3° of the equator, together with temperature gradients computed from the National Meteorological Center (renamed the National Centers for Environmental Prediction) SST analysis along the equator, were used to compute a time ensemble average heat advection. On average, cooled water was advected along the equator eastward from the “warm” pool, and this occurred when equatorial currents were to the east.