Thermohaline processes in a tropical coastal zone

Abstract

The detailed thermohaline structure of the northern Yucatan coastal zone was obtained for the first time in order to gain an insight into the interactions between various processes in this complex tropical environment of extreme evaporation and high precipitation rates. From the continent, it has water exchange with numerous coastal lagoons (ranging from brackish to hypersaline) and receives intense submarine groundwater discharges (SGD). In the summer of 2006 a high-resolution (500m cross-shore and 5km along-shore) oceanographic campaign was performed starting at Holbox Island down to the mouth of Celestun Lagoon. CTD profiles were measured at 1020 stations along 69 coastal cross-shore transects. Additionally, CTD data from 2 wider surveys, covering the continental shelf (Campeche Bank) and the southern Gulf of Mexico respectively were used to complement the results. From the thermohaline properties, two main water masses were identified: (a) the Caribbean Subtropical Underwater (CSUW), upwelled from the Caribbean, which was observed at the bottom very close to the coast in more than 260km (from the upwelling region near Cape Catoche to approximately 89.5 W during the summer of 2006) and (b) the second dominant group was a mass of warm hypersaline water which originates in Yucatan due to the high temperature and evaporation rates. We call this water mass the Yucatan Sea Water (YSW) after finding evidence of its presence in various field campaigns both in the Yucatan Sea and further to the west in the southern Gulf of Mexico. All the water masses present in the Yucatan coastal zone showed pronounced variations with important dilution and salinisation effects. The permeable karstic geology of the region prevents the continental water from discharging into the ocean through surface rivers and instead the rainfall permeates directly to the aquifer and travels through caves and fractures towards the sea. Three main regions showed evidence of continental discharges from the coast: the central eastern region (~21.6 N, 87.75 W), around Dzilam Bravo (~21.4 N, 89 W) and, to a lesser degree, the western region of the coast (~21.1 N, 90.3 W). The latter two regions are the eastern and western intersections of the Chicxulub crater ring with the coast respectively.