Indicators of mean currents and variability in the Gulf of Mexico based on model results
Authors: Howard M.K. , Reid R.O. , Nowlin W.D. , Jochens A.E. , Choi J.-K. 
Published: October 2008

Abstract
Discussed are mean vectors and variance ellipses for currents at selected levels, from the sea surface to 2000 m, as well as near bottom in the Gulf of Mexico. The fields presented are based on record-length mean currents from a circulation model hindcast for model years 1993-1998.

Background
We have examined more than eight million hours of current meter records from the deep water region of the Gulf of Mexico (water depths greater than 200 m). However, the spatial and temporal coverage represented by that information is woefully inadequate to characterize the general circulation of the region. Here we characterize the principal features of the mean circulation based on the output from a numerical circulation model for the Gulf of Mexico developed by Dr. Lakshmi Kantha and colleagues at the University of Colorado with partial support from an industry-sponsored study on Climatology and Simulation of Eddies (CASE).
The model is a version of the Princeton Ocean Model adapted for the Gulf of Mexico, referred to by the acronym CUPOM. The horizontal resolution is 1/12° and the vertical resolution is 24 sigma levels. The model hindcast used was for the years 1993 through 1998. The model assimilates altimeter data for the region with water depth of 1000 m or more. It also assimilates satellite sea surface temperature data, but uses climatological sea surface salinity. The 6-hourly wind stresses from the European Centre for Medium-range Weather Forecasting at 1.125° resolution are used for the wind forcing. The inflow boundary is at 21.333°N across the Yucatan Channel, with a physically balanced inflow prescribed using typical monthly temperature and salinity profiles. The outflow boundary is at the Florida Straits; the boundary condition is set to be balanced and in phase with the inflow boundary. The data assimilation module is the same as in Horton et al. (1997) and Clifford et al. (1997). Details of the specifics with respect to the Gulf of Mexico can be found in Kantha et al. (1999).

Results
Using the CUPOM hindcasts for 1993-1998, we produced time series animations and record-length mean fields of current vectors and variance ellipses at the sea surface, 500 m, 1000 m, 2000 m, and the first sigma level above the sea bed. Fig. 1 shows record-length current vectors at the sea surface for the Gulf of Mexico. The principal feature in the surface field is the Loop Current, and within the loop there is a closed anticyclone just northwest of Cuba. In contrast to observations, no southward flow to the Caribbean Sea is seen west of Cape San Antonio, Cuba, because the boundary conditions just to the south specify only flow into the Gulf. Strong flow is also seen over the continental shelves of the southern and western Gulf as well as within the deepwater portion of the western basin. The variance ellipses (not shown) for the surface currents are largest for the region of the Loop Current and indicate highest variability in a direction normal to the inflowing and outflowing limbs of that current.