Loop Current rings and the deep circulation in the Gulf of Mexico

Abstract

A detailed examination of the three‐dimensional circulation associated with Loop Current rings in the Gulf of Mexico has been made using the Modular Ocean Model with 15 levels in the vertical and ⅛° horizontal grid spacing. Monthly mean wind stress and a seasonal boundary condition on temperature and salinity fields are applied at the surface. The vertical shear and geostrophic transport of the Yucatan Current also vary seasonally. The upper layer seasonal circulation, and the formation, structure, and westward migration of Loop Current rings in the model compare favorably with observations. An anticyclone‐cyclone pair develops in the deep layer beneath the Loop Current as a ring is forming in the eastern Gulf. The model circulation beneath the Loop Current reverses from anticyclonic to cyclonic because the anticyclone leads the pair in the deep layer as the ring separates and migrates westward in the surface layer. The deep anticyclone‐cyclone pairs are guided by the bottom bathymetry as they migrate westward in tandem with the rings in the surface layer. The deep anticyclone decays more quickly than the cyclone in the western Gulf, and the cyclone dominates the deep circulation beneath the ring. Cyclonic flow in the deep Gulf of Mexico has been observed using current meters, inferred from hydrographic data, and produced in other numerical and theoretical models. Although the formation and westward migration of deep anticyclone‐cyclone pairs in conjunction with rings has not been directly observed, the results of this modeling study agree with other published reports on the deep circulation in the Gulf of Mexico.