The seasonal variations and the interactions of the water masses in the tropical Pacific off central Mexico (TPCM) and four surrounding areas were examined based on an extensive new hydrographic database. The regional water masses were redefined in terms of absolute salinity (S A ) and conservative temperature (Q) according to the Thermodynamic Equation of Seawater 2010 (TEOS-10). Hydrographic data and the evaporation minus (precipitation 1 runoff) balance were used to investigate the origin and seasonality of two salinity minima in the area. The shallow (50-100 m) salinity minimum originates with the California Current System and becomes saltier as it extends southeastward and mixes with tropical subsurface waters while the surface salinity minimum extends farther north in the TPCM in summer and fall because of the northward advection of tropical surface waters. The interactions between water masses allow a characterization of the seasonal pattern of circulation of the Mexican Coastal Current (MCC), the tropical branch of the California Current, and the flows through the entrance of the Gulf of California. The seasonality of the MCC inferred from the distribution of the water masses largely coincides with the geostrophic circulation forced by an annual Rossby wave.

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 ∘ C 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 practical salinity unit, while the deeper saline core's positive anomaly reached 1.22 practical salinity unit. 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.

Interannual variability of Ocean Heat Content (OHC) is intimately linked to ocean water mass changes. Water mass characteristics are imprinted at the ocean surface and are modulated by climate variability on interannual to decadal time scales. In this study, we investigate the water mass change and their variability using an isopycnal decomposition of the OHC. For that purpose, we address the thickness and temperature changes of these water masses using both individual temperature‐salinity profiles and optimal interpolated products from Argo data. Isopycnal decomposition allows us to characterize the water mass interannual variability and decadal trends of volume and OHC. During the last decade (2006–2015), much of interannual and decadal warming is associated with Southern Hemisphere Subtropical Mode Water and Subantarctic Mode Water, particularly in the South Pacific Eastern Subtropical Mode Water, the Southeastern Indian Subantarctic Mode Water, and the Southern Pacific Subantarctic Mode Water. In contrast, Antarctic Intermediate Water in the Southern Hemisphere and North Atlantic Subtropical Mode Water in the Northern Hemisphere have cooled. This OHC interannual variability is mainly explained by volume (or mass) changes of water masses related to the isopycnal heaving. The forcing mechanisms and interior dynamics of water masses are discussed in the context of the wind stress change and ocean adjustment occurring at interannual time scale.