The Mexican Coastal Current: A subsurface seasonal bridge that connects the tropical and subtropical Northeastern Pacific

We used a three-dimensional numerical model to analyze the seasonal variability of the coastal circulation off SW Mexico. In agreement with previous research, our model reproduced a Mexican Coastal Current (MCC) that dominates the regional poleward circulation. The modeled dynamics evidenced an energetic semiannual component that governed the subsurface seasonal variability of this poleward flow. Below the thermocline the MCC was stronger during spring and fall, when it reached subsurface seasonal-averaged velocities of ∼10 cms−1 and flowed continuously from the Gulf of Tehuantepec to the entrance of the Gulf of California. There, the subsurface MCC bifurcated in one branch that continued along the coast of mainland Mexico and a second branch that crossed the gulf and joined the California Undercurrent. Instead of the local wind, the semiannual MCC variability was induced by the transit of equatorial Kelvin waves whose upwelling (downwelling) phase propagation strengthen (weakened) the subsurface poleward circulation along the Tropical Pacific off Mexico. The MCC dynamics reported in this study accounts for the, previously reported, semiannual variability of the alongshore transport and salinity content in the southern Gulf of California. Moreover, the subsurface bridge between the MCC and the California Current System represents an external source of momentum that helps to explain the intensification of the California Undercurrent during spring and fall.