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 (Θ) according to the Thermodynamic Equation of Seawater 2010 (TEOS-10). Hydrographic data and the evaporation minus (precipitation + 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 surface circulation around the entrance to the Gulf of California is described from satellite altimetry supported by 10 conductivity‐temperature‐depth (CTD) surveys. The sea surface height calculated from the 1/4° World Ocean Database 2001 climatology plus the Archiving, Validation, and Interpretation of Satellite Oceanographic altimeter data (October 1992 to January 2008) were RMS adjusted to the dynamic height calculated from CTD data; the 27.0 kg m⁻³ isopycnal provided the optimum reference. In the mean, the surface circulation shows a branch of the California Current heading landward toward the Gulf of California entrance, where it splits into two subbranches; these are separated by a cyclonic circulation attached to the coast south of Cabo Corrientes. This feature is produced by Sverdrup dynamics and is the first observational indication that the Mexican Coastal Current is generated locally by the wind stress curl, as previously suggested by numerical models. The global variance of the surface circulation can be separated into seasonal (explained variance 35%), interannual (explained variance 35%), and mesoscale (explained variance 30%) components. The seasonal signal, which shows the interplay of the poleward Mexican Coastal Current and the equatorward branch of the California Current, can be explained by a long Rossby wave model forced by the annual wind and by radiation from the coast. The interannual component is dominated by the El Niño‐Southern Oscillation, which induces in the gulf entrance an anticyclonic (cyclonic) circulation during El Niño (La Niña); this circulation includes a poleward‐flowing branch (during El Niño) parallel to the Pacific coast of the Baja California peninsula. The mesoscale variability is caused by intense eddy activity.

For the first time on the basis of direct observations (June 2003 and June 2005), the characteristics of the summer‐time coastal poleward current off SW Mexico are reported. Only the surface evidence of this coastal current has previously been described, from ship drift compilations. In June 2003 the current was 90–180 km wide, 400 m deep, with speed ∼0.3 ms⁻¹ and transport ∼4 Sv (1 Sv = 10⁶ m³ s⁻¹). In June 2005, its width was ∼90 km, it was 250–300 m deep, with mean speed ∼0.15 m s⁻¹ and transport ∼2.5 Sv. California Current water (CCW) and equatorward flow were found further offshore. Mesoscale eddies significantly affected the coastal current, and transported CCW into the coastal zone.

Surface drifters released in the Gulf of California between June 2004 and August 2006 are used to describe the surface circulation in late spring and summer. In the June to September mean, there was a poleward coastal current on the shelf and slope of the mainland side of the Gulf, with mean speed ∼0.3 m/s; it reached the northern Gulf and joined the cyclonic circulation typical of this zone in summer. In the western half of the southern Gulf, the drifters presented recirculating currents that are due to mesoscale eddies that appear to dominate the surface circulation in summer. In June 2004, the coastal current presented an enhancement event with mean speed around 0.60 m/s and maximum ∼0.80 m/s. It took ∼20 days for a particular drifter to travel the 1000 km from the Gulf entrance to the head. This strengthening of the coastal current was apparent in chlorophyll a and SST satellite images, the drifters following closely the intrusion of warm, chlorophyll‐poor surface water from outside the Gulf. The drifters and the satellite images suggest that the current‐enhancement event lasted less than a month. This mesoscale event was linked with a mesoscale remote forcing in the tropical Pacific coast and with a mesoscale local forcing of the wind. These events seem to occur every year, and are probably important in carrying organisms and properties from the entrance to the whole length of the Gulf.