School of Ocean and Earth Science and Technology,
University of Hawaii, Honolulu, HI 96822
When the winds became calm on 4 January 1993, the eastward jet began to rapidly spin down, and by 10 January it had disappeared (Fig. 3). Part of the spin down process was the generation of an energetic, surface-intensified, cyclonic submesoscale eddy along the southern flank of the jet, centered near the middle of the IFA (Fig. 4).
The velocity field of this eddy was observed by a hull-mounted acoustic Doppler current profiler on R/V Wecoma which made repeated observations along a butterfly-shaped track with legs 130 km in length (Fig. 1). Simultaneously, the temperature and salinity field was observed with a Seabird CTD mounted in a Seasoar towed body cycling over the upper 300 m. Because the shipboard survey sampling did not resolve the energetic internal semidiurnal tides, analysis of the low-frequency evolution of the velocity structure required application of an inverse model. This model assumes that the currents vary smoothly on time scales of days, and that horizontal variations can be described by linear trends over the scale of the repeat survey (see companion poster by Lukas et al.). First order velocity derivatives were computed as part of the fitting of the model to the observations, but because the eddy was nearly centered in the IFA, they average over the eddy. Thus, the model-based analysis can describe the evolution of the large-scale flow field in which the eddy was embedded. The model was sampled at 12 hour intervals for purposes of presentation.
The upper pycnocline upwelled as the eddy spun up (Fig. 5), with some isopycnals becoming nearly vertical, perhaps making vertical mixing more efficient. The upwelling carried cooler, relatively salty water close to the surface. However, initially there was no obvious signature of this upwelling at the sea surface because of the relatively fresh (and very stable) layer that was created by heavy rains starting on 4 January. As the eddy became more well-developed, the effects of the upwelling reached to the surface (Fig. 6). (This north-south section combined with the sequence of east-west sections clearly shows that the doming of isopycnals is that characteristic of the closed circulation in an eddy, rather than in an elongated frontal feature.) The upwelling perturbation appeared to be tilted towards the east, and the eddy center moved eastward, but more slowly than might be expected from advection; this might be a manifestation of -drift, but may also be related to the vertical shear. The upwelling appeared to precede the development of the eddy circulation, but it is difficult to isolate the eddy signal from the other sources of strong circulation variability related to the WWB.
The spin down of the eastward equatorial jet was basically inertial, with being largely balanced by -fV (Fig. 7). This is not unexpected, though equatorially-trapped wave motions with significant contributions from horizontal pressure gradients might obscure this. The nonlinear momentum advection terms were comparable to the local acceleration terms, and the relative vorticity was as large as f in the upper 50 m during the development of the eddy. During the late stages of the wind event, and until the eddy developed, the upper ocean flow in the IFA was convergent.