A New Characterization of the Upper Waters of the central Gulf of México based on Water Mass Hydrographic and Biogeochemical Characteristics

Abstract. In the Gulf of Mexico (GoM) at least three near-surface water masses are affected by mesoscale processes that modulate the biogeochemical cycles. Prior studies have presented different classifications of water masses where the greater emphasis was on deep waters and not on the surface waters (σ_θ < 26 kg m⁻³), as in this work. Here presents a new classification of water masses in the GoM, based on thermohaline properties and dissolved oxygen (DO) concentration using data from a total of five summer and winter cruises carried out primarily in the central GoM. The reclassification includes an adjustment to the spatial range of Caribbean Surface Water (CSW), which is detected only during the summer. This water mass extends from the surface to ≈ 90 m and features warm waters (27–32 °C), high salinities (up to ≈ 36.8), non-detectable nitrate concentration, and negative values of the apparent oxygen utilization (AOU) of ≈ −27 μmol kg⁻¹. Below the CSW, the deeper Gulf Common Water (GCW) was also redefined and characterized by a subsurface DO maximum, with values ≈ 50 μmol kg⁻¹ higher than that found in surface waters. In winter, a replacement of the CSW by the GCW affected the biogeochemical composition of surface water as observed from an increase in nitrate concentrations, positives values of AOU (≈ 90 μmol kg⁻¹) and a decrease in surface temperatures (< 27 °C). This is because during winter, the Tropical Atlantic Central Water (TACW) that lies below the GCW is closer to the surface and contributes nutrients and low DO via strong vertical mixing induced by the windy Nortes season. CARS2009 analysis supports the formation of the subsurface maximum of DO during the summer and disappears in winter. In this work also named surface water that is characterized by a low salt content (≈ 33.1) from 0 to 20 m as Freshwater Influenced Surface Water (FISW).