Dominant Circulation Patterns of the Deep Gulf of Mexico

Abstract

The large-scale circulation of the bottom layer of the Gulf of Mexico is analyzed, with special attention to the historically least studied western basin. The analysis is based on 4 years of data collected by 158 subsurface floats parked at 1500 and 2500 m and is complemented with data collected by current meter moorings in the western basin during the same period. Three main circulation patterns stand out: a cyclonic boundary current, a cyclonic gyre in the abyssal plain, and the very high eddy kinetic energy observed in the eastern Gulf. The boundary current and the cyclonic gyre appear as distinct features, which interact in the western tip of the Yucatan shelf. The persistence and continuity of the boundary current is addressed. Although high variability is observed, the boundary flow serves as a pathway for water to travel around the western basin in approximately 2 years. An interesting discovery is the separation of the boundary current over the northwestern slope of the Yucatan shelf. The separation and retroflection of the along-slope current appears to be a persistent feature and is associated with anticyclonic eddies whose genesis mechanism remains to be understood. As the boundary flow separates, it feeds into the westward flow of the deep cyclonic gyre. The location of this gyre-named the Sigsbee Abyssal Gyre-coincides with closed geostrophic contours, so eddy-topography interaction via bottom form stresses may drive this mean flow. The contribution to the cyclonic vorticity of the gyre by modons traveling under Loop Current eddies is discussed.

Energetics of the Deep Gulf of Mexico

“…The upper-layer circulation is dominated by the Loop Current (LC), and is covered fairly well in the literature (Alvera-Azcárate et al 2009;Hamilton et al 1999;Oey et al 2005), while the deep layer has attracted less attention because it is less easily observed. Numerical studies present a general cyclonic mean flow of the deep Gulf (Oey and Lee 2002;Lee and Mellor 2003;Chang and Oey 2011;Maslo et al 2020), which has been supported by observations (Weatherly et al 2005;DeHaan and Sturges 2005;Pérez-Brunius et al 2018). Observations indicate that the deep GoM is a surprisingly energetic environment, where the recorded speeds in the northern Gulf can exceed 90 cm s 21 (Hamilton and LugoFernandez 2001).…”

Deep Eddies in the Gulf of Mexico Observed with Floats

“…However, all deep current observations from moorings in the Gulf show either depth-independent flow below 800-1200 m or slightly near-bottom intensified (e.g., Hamilton 2009;Donohue et al 2016;Tenreiro et al 2018), consistent with weak stratification in the lower layer. Also, below 1000 m, density barely varies with depth (Hamilton et al 2018), so little geostrophic shear is expected in the lower layer. No surface expression was found in simultaneous sea surface height observations.…”

“…In August 2013, LCE Kraken pinched off from the LC and drifted westward until December 2013 (Hamilton et al 2018) when it broke apart and dissipated on the western boundary. The behavior of the deep floats during this event is depicted in a multipanel time series, from 1 August 2013 through 15 January 2014, that is, from the time Kraken first entered the western Gulf to when the CEEs were formed (Fig.…”

“…Unlike the three other examples of subsurface anticyclonic eddies listed above, the vertical structure of the CEEs is completely unknown at this point: we checked through all historically available profile data in the HydroBase database, the Argo database, and APEX profiles collected as part of this project (Hamilton et al 2018) and could find no vertical profiles coincident with the CEE observations. However, all deep current observations from moorings in the Gulf show either depth-independent flow below 800-1200 m or slightly near-bottom intensified (e.g., Hamilton 2009;Donohue et al 2016;Tenreiro et al 2018), consistent with weak stratification in the lower layer.…”

“…The dynamics of the GOM can be largely explained by an upper layer (,1000 m) dominated by the surfaceintensified LC and mesoscale eddies and the bottom layer (.1200 m) with nearly depth-independent currents and density fields and near-bottom-intensified flows along the boundaries due to the presence of topographic Rossby waves (TRWs) (e.g., Sheinbaum et al 2007;Hamilton 2009;Hamilton et al 2016aHamilton et al , 2018Donohue et al 2007Donohue et al , 2008Donohue et al , 2016Tenreiro et al 2018). Until recently, the basinwide deep-layer circulation has been mostly unexplored owing to a lack of observations, although more recent observational programs have studied the upper-lower-layer dynamical coupling, resulting in deep vortices in the abyssal plain, as well as the propagation of TRWs along the boundaries (e.g., Hamilton 1990;Hamilton et al 2016a;Donohue et al 2016;Kolodziejczyk et al 2011;Tenreiro et al 2018).…”