Spinup of a Submesoscale Eddy in the TOGA COARE Intensive Flux Array During the Spindown of an Intense Eastward Jet

Roger Lukas and Peter Hacker

School of Ocean and Earth Science and Technology,
University of Hawaii, Honolulu, HI 96822

Introduction

During the Intensive Observation Period (IOP) of TOGA COARE, a Yoshida jet-like eastward surface current was spun up in the upper 70 m by a strong westerly wind burst (WWB) which occurred from mid- to late-December 1992. The maximum observed speed was about 1 m s¯ ¹ near the equator (Delcroix et al., 1994), comparable to previously observed upper ocean response to WWBs. This eastward flow extended well south of the equator; peak eastward flow was about 0.8 m s¯ ¹ in the Intensive Flux Array (IFA) centered near 2°S, 156°E (Fig. 1; see also companion poster by Lukas et al.). A north-south section of zonal velocity observed from R/V Le Noroit is shown in Fig. 2 during the spinup of the jet. This eastward jet was associated with an intensified westward flow immediately below, so that the vertical shear of the zonal current was large, with estimated Richardson numbers below 1/4 in the layer between 40 and 70 m.

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.

Discussion

This submesoscale eddy spun up over a period of about 5 days (Fig. 4); unfortunately, we did not observe the evolution of the eddy beyond this point. The radius of the eddy was about 40 km, and a typical velocity associated with the eddy was 0.5 m s¯ ¹. Thus, the Rossby number, Ro=Vo/fL=2.5. The peak speeds in the northern limb of the eddy were about 60 cm s¯ ¹ on 10 January (Fig. 3); the surface intensified flow was matched by another maximum eastward flow centered at 170 m extending to at least 300 m. The core of this eastward flow was centered at 1.7°S. There was a relative minimum in the eastward flow between 70 and 110 m.

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 beta-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 partdiff(U)/partdiff(t) 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.

Conclusions

The observation of this submesoscale eddy during the spin-down of the strong eastward jet forced by the December 1992 westerly wind burst suggests the importance of the nonlinear terms in the upper ocean momentum balance associated with strong wind events over the near-equatorial western Pacific warm pool. Without further observations, it is impossible to determine the net impact of such eddies on the long-term heat, salt and momentum budgets of the warm pool, though the impact may be quite large during brief intervals. Models which incorporate the necessary physics and resolution may be useful for making an estimation. The relative frequency and spatial distribution of such eddies must also be determined in order to estimate their integral impact. However, they are likely to be important only in conjunction with the WWB-driven jets.

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