The Bay of Campeche in the southern Gulf of Mexico (GoM) is characterized by a semi-permanent cyclonic circulation commonly referred to as the Campeche Gyre (CG). Several studies, documenting its upper layer structure, have suggested a relationship between its seasonal occurrence and the wind, and have proposed that non-seasonal variability arises mainly from interaction of the gyre with Loop Current Eddies that arrive in the region. Other studies have shown that the topography of the region is such that an equivalent-barotropic flow can develop, confining the CG to the west of the bay. Nevertheless, a partition of the contributions of these forcings to the circulation of the gyre in a statistically consistent manner is still needed. In this study, the wind-and eddy-driven circulation are examined with a set of long-term numerical simulations of the GoM using HYCOM. Our results show that, in the absence of eddies, the wind is able to sustain a seasonal-modulated circulation in the CG, confined within the upper 600 m. When LCEs are taken into consideration, the gyre appears to extend below 1000 m, however this behavior results from the presence of the cyclonic bottom boundary current in the southern GoM. Interaction with eddies impose high fluctuations in the circulation of the gyre at intraseasonal time scales, leading to reversals in the current if the event is strong. Additionally , we provide evidence of a northward flux of cyclonic vorticity out of the bay during eddy-gyre interaction events. Finally, we found that the role of topography manifests similarly among these different dynamic conditions, resulting in closed geostrophic contours to the west of the bay that confine an upper-layer, symmetric, equivalent-barotropic CG.

Erick Olvera-Prado

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The study of the relationship between the upper and lower layers in the Gulf of Mexico (GoM) has experienced a lot of progress in recent years. Nevertheless, an examination of this coupling for the entire GoM in a statistically consistent manner is still needed. Layer thickness data from a GoM 21-year free-running simulation is used to examine the coupling between the upper (<250 m) and lower (>1000 m) layers with focus on the dominant modes of variability through a Hilbert Empirical Orthogonal Functions (HEOFs) analysis. The three leading modes of the upper layer are associated with the Loop Current’s (LC) lifecycle and LC eddy (LCE) shedding, with lower-layer variability vertically corresponding and intensified along certain bathymetric features. These modes are periodic, indicating recurrence of circulation patterns in agreement with observations. The fourth mode of the upper layer is associated with the translation of LCEs and their dissipation in the northwestern GoM, which significantly contributes to a richer lower-layer variability in a vast area of the western GoM. The lower mode describes the deep anticyclone-cyclone accompanying the LCEs, the strengthening of the circulation along the Sigsbee Gyre western branch, and the variability over the Sigsbee plain. It does not show cyclicity, suggesting persistence of the associated circulation patterns with a dominant timescale of 14 months. Evidence and corroboration of recently observed lower-layer circulation features are provided. The application of the HEOFs technique used here can complement the three-dimensional oceanic assimilation methods by projecting surface information to depth in a physically consistent manner.