This study describes in detail the water masses of the Gulf of Mexico (GoM) west of 88°W based on their thermohaline properties and dissolved oxygen concentration. The existent historical information is complemented with new data from 14 cruises, Argo floats, and over one year of continuous glider monitoring. The results describe the general hydrography of the central and western GoM with focus on the difference between the water properties inside and outside Loop Current Eddies (LCEs). Caribbean Surface Water, Subtropical Underwater, and 18 °C Sargasso Sea Water (18SSW) are exclusive of the LCEs, and they are found along the LCEs preferred path between 23°N and 27°N. Outside the LCEs, the prominent characteristics of these water masses erode, and the Gulf Common Water is ubiquitous in the subsurface. It is shown that the water masses in the GoM need to be described in the frame of the dominant mesoscale features that take place there and that the dissolved oxygen is a key variable to identify some water masses of Caribbean origin as the Tropical Atlantic Central Water and the 18SSW. The previous potential temperature and salinity limits of the water masses within the GoM were revised and redefined in terms of absolute salinity and conservative temperature in the frame of the Thermodynamic Equation of Seawater, 2010 (TEOS-10). While temperature values after conversion have little variation compared to the previous ones, the absolute salinity is in average 0.2 units greater than the former practical salinity.
High-resolution hydrographic measurements reveal the presence of three Intra-Thermocline Eddies (ITEs) embedded within a Loop Current Eddy (LCE). ITEs are lenticular bodies of nearly homogeneous water which contrasts with the well stratified surrounding water. Their radii and thickness ranged between [19-32] km and [150-250] m. Negative relative vorticity within their cores (down to -0.85 times the Coriolis frequency), along with a large negative stratification anomaly, result in low Ertel Potential Vorticity (PV) and intense negative Ertel Potential Vorticity Anomalies (PVA). Vortex stretching and relative vorticity have comparable contributions to PVA, resulting in Burger numbers of order unity. The similarity of thermohaline properties within the ITE?s cores and the surrounding LCE water, suggests that these ITEs likely form by intense mixing events followed by Rossby adjustment.
The vertical structure of a recently detached Loop Current Eddy (LCE) is studied using in-situ data collected with an underwater glider from August to November 2016. Altimetry and Argo data are analyzed to discuss the context of the eddy shedding and evolution as well as the origin and transformation of its thermohaline properties. The LCE appeared as a large body of nearly homogeneous water between 50 and 250 m confined between the seasonal and main thermoclines. A temperature anomaly relative to surrounding Gulf's water of up to 9.7 ºC was observed within the eddy core. The salinity structure had a double core pattern. The subsurface fresh core had a negative anomaly of 0.27 psu while the deeper saline core's positive anomaly reached 1.22 psu. Both temperature and salinity maxima were stronger than previously reported. The saline core, of Caribbean origin, was well conserved during its journey from the Yucatan Basin to the Loop Current and at least 7 months after eddy detachment. The fresher homogeneous core resulted from surface diabatic transformations including surface heat fluxes and mixing within the top 200 m during the winter preceding eddy detachment. Heat and salt excess carried by the LCE were large and require important negative heat fluxes and positive fresh water input to be balanced. The geostrophic velocity structure had the form of a subsurface intensified vortex ring.
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