Water Masses and Circulation in the Tropical Pacific off Central Mexico and Surrounding Areas

Abstract

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.

Surface Salinity Balance in the Tropical Pacific Off Mexico

“…The sum of the mean plus the annual advection produce, mainly in Manzanillo, a net poleward salt advection during most of the year (Figures and ), which means that tropical waters are being advected poleward by the annual MCC mainly during summer. This result is consistent with the summer/fall surface salinity minimum and the saltier winter/spring characteristics of the water masses observed in the TPOM by Portela et al ().…”

“…Relatively intense upwelling has been also reported around Cabo Corrientes, near the domain of Manzanillo, mostly in the first half of the year (Kurczyn et al, ). However, the differences between the salinity at surface and in deeper layers of the water column in this locality are small compared to those in summer and fall (Portela et al, ) when, in turn, there is no significant upwelling. This is in agreement with the lack of importance of the entrainment and nonlinear terms in Manzanillo, where salinity tendency is largely reproduced by the annual linear model.…”

“…In addition, strong Ekman pumping velocity and upwelling events have also been reported in the vicinity of the GoT. In Acapulco and Huatulco, where there is a strong vertical salinity gradient (fresh waters overlay saltier subsurface waters) between 30 and 75 m the whole year (Portela et al, ), upwelling could be important (Chapa‐Balcorta et al, ; Kessler, ). This process could potentially smooth the salinity tendency, which is in agreement with the effect that the entrainment term has in Acapulco (Figure , blue line).…”

“…for example, southern). In contrast, in the AM region the salinity increases poleward across a strong front caused by the confluence of the fresh surface waters from the tropical area with the saltier waters that outflow from the Gulf of California (Portela et al, ).…”

Approach for estimating the dynamic physical thresholds of phytoplankton production and biomass in the tropical‐subtropical Pacific Ocean

“…This mean flow shows that the subtropical zone is influenced primarily by the relatively cold subarctic waters transported by the California Current [ Durazo , ]. On the other hand, the tropical zone is influenced by the warmer and saltier tropical and subtropical waters from the south and southwest, respectively [ Lavín et al , ; Godínez et al , ; Kurczyn et al , ; Portela et al , ].…”