Inputs of subthermocline waters and nitrate onto the Campeche Bank

Abstract

Geostrophic tilting of isopycnal surfaces in the Yucatan Current raises the thermocline and nitrate nutricline (approximately the 22°C surface) above the level of the shelf break along the eastern margin of the Campeche Bank. Along the northern and western margins of the bank, these features lie 20–30 m below the shelf break. Cool nitrate-rich water can move laterally from the Yucatan Current along isopycnal surfaces beneath the thermocline onto the Campeche Bank. Drogues launched on the bank moved westward, corroborating transport away from the Yucatan Current. Near-bottom water on the bank is denser than water at similar depths in the Gulf of Mexico and Campeche Gulf. This dense near-bottom water is trapped within the euphotic zone on the bank for periods between days and weeks. Anomalies of temperature and chlorophylla at 30 m depth on the bank were directly proportional to density anomalies, indicating the mixing of subthermocline water with surface water. The development of surface patches of phytoplankton or elevated nitrate levels on the bank results from the vertical mixing of the stratified water column. During the summer, lateral transport of subthermocline water from the Yucatan Current appears to be an important source of nitrate nitrogen for the Campeche Bank.