Cyclonic Eddies in the Gulf of Mexico: Observations by Underwater Gliders and Simulations by Numerical Model

Headnote

ABSTRACT

Circulation in the Gulf of Mexico (GoM) is dominated by the Loop Current (LC) and by Loop Current eddies (LCEs) that form at irregular multimonth intervals by separation from the LC. Comparatively small cyclonic eddies (CEs) are thought to have a controlling influence on the LCE, including its separation from the LC. Because the CEs are so dynamic and short-lived, lasting only a few weeks, they have proved a challenge to observe. This study addresses that challenge using underwater gliders. These gliders' data and satellite sea surface height (SSH) are used in a four-dimensional variational (4DVAR) assimilation in the Massachusetts Institute of Technology (MIT) general circulation model (MITgcm). The model serves two purposes: first, the model's estimate of ocean state allows the analysis of four-dimensional fields, and second, the model forecasts are examined to determine the value of glider data. CEs have a Rossby number of about 0.2, implying that the effects of flow curvature, cyclostrophy, to modify the geostrophic momentum balance are slight. The velocity field in CEs is nearly depth independent, while LCEs are more baroclinic, consistent with the CEs origin on the less stratified, dense side of the LCE. CEs are formed from water in the GoM, rather than the Atlantic water that distinguishes the LCE. Model forecasts are improved by glider data, using a quality metric based on satellite SSH, with the best 2-month GoM forecast rivaling the accuracy of a global hindcast.

(ProQuest: ... denotes formulae omitted.)

1. Introduction

Circulation in the Gulf of Mexico (GoM) is dominated by the Loop Current (LC), an extension of the western boundary current system of the North Atlantic Ocean that loops into the GoM. The LC enters the Gulf through the Yucatan Channel and exits through the Florida Straits. The LC occasionally extends far northward into the GoM, approaching the northern shelf break. This long extension of the loop will eventually separate to form an anticyclonic Loop Current eddy (LCE). Formation of an LCE by separation from the LC happens irregularly every several months, and there can be a number of LCEs in the GoM at one time. Prediction of LCE formation events is central to accurate forecasting of GoM circulation.

Cyclonic eddies (CEs), sometimes called Loop Current frontal eddies (Walker...