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.

Seasonal variability at the entrance to the Gulf of California is described using high resolution CTD data from a section across the entrance to the gulf that was occupied eight times between 1992 and 1998. Variability decreased rapidly below 100 dbar, so that standard deviations exceeding 0.5ºC and 0.05 for temperature and salinity, respectively, were confined to the upper 150 dbar. The mean salinity field wasasymmetric, with fresher waters (S < 34.6) between 25 and 75 dbar next to Sinaloa and saltier waters (S > 34.8) above 50 dbar and between 100 and 150 dbar near Baja California. A seasonal thermocline developed in the upper 50 dbar, increasing from 21ºC in February surface temperatures to 31ºC in August. Below 100 dbar, temperature at a given pressure was typically colder in the center of the section, indicating cyclonic flow (flow into the gulf next to Sinaloa and out of the gulf next to Baja California). The subsurface mid-gulf cooling (cyclonic flow) was strongest during winter and spring when the exchange of heat between the gulf and the Pacific Ocean is large. Subsurface mid-gulf heating (corresponding to anticyclonic flow) was not observed. Temporal and spatial distribution of saline water masses suggests that heat exchanges take place by alternating the inflow and outflow of warm (surface) and cold (subsurface) water masses. The 1997–1998 El Niño resulted in a 50 dbar deepening of the thermocline in November 1997, as well as a freshening of 0.1 to 0.2 of waters above the thermocline.

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.

Most of the available hydrographic data (1939–1988) from the Gulf of California are used to describe the seasonal cycle of heat storage in the upper 400 m of water. The seasonal cycle of surface heat flux is obtained by using the sea surface temperature from the hydrographic data bank and meteorological data from coastal stations along the gulf. Monthly values of surface heat flux and heat storage are fitted to a mean value plus an annual harmonic. The longitudinal (i.e., along‐gulf) heat flux is then obtained by integrating the difference between the rate of change of heat storage and the surface heat flux. It is found that the surface heat flux has a positive (into the sea) annual mean all along the gulf; the average annual net surface heat flux for the whole gulf being ∼118 W m⁻². The southern part of the gulf gains more heat (∼130 W m⁻²) than the northern part (∼100 W m⁻²), owing to the larger loss by evaporation in the latter (∼0.6 m/yr versus ∼1 m/yr) caused by lower humidity and stronger winds. These values are in agreement with previous estimates for the northern gulf. The amplitude of the seasonal signal shows a maximum of ∼230 W m⁻² in the central part. Winter heat losses occur only from the middle of the gulf, the Guaymas Basin, to its head. There is a strong annual signal superimposed on the average longitudinal heat flux, whose amplitude is larger than the annual harmonic of the surface heat flux. Input of heat from the Pacific Ocean occurs from mid‐March to mid‐July, with a maximum at the mouth in May (21×10¹² W). Attempts to explain this flux by diffusive processes were unsuccessful. The proposition that advection is the main longitudinal heat carrier is congruent with the generally accepted circulation patterns.

The hydrodynamic characteristics of mesoscale eddies of the southern Gulf of California are described from observations made in August 2004. Vertical profiles to ~1000 m were made with a conductivity‐temperature‐depth (CTD) probe with dissolved oxygen and fluorescence sensors, and a lowered acoustic Doppler profiler. The four sampled eddies were aligned along the Gulf axis and had alternating sense of rotation. The Okubo‐Weiss parameter was used to calculate the eddy core radii, and it also revealed that the current field of almost all the sampled area was eddy dominated. The mean radii of the eddy cores varied between 32 and 36 km. Maximum surface velocities were 0.4–0.5 m/s. The anticyclonic eddies were 500–700 m deep, while the cyclones were 450–500 m deep. The depression of the isolines associated with the anticyclonic eddies was ~100 m at depths between 200 and 400 m; however, the thermocline/pycnocline, which spanned from ~20 to 75 m, was domed by ~10 m. The cyclonic eddies lifted the isolines by ~70 m at depths above 500 m, but the isolines were flat above ~60 m. This suggests that different dynamics operate in the layer above the strong and shallow pycnocline. In particular, the wind stress curl can affect circulation and pycnocline topography and, therefore, the patterns that appear in chlorophyll satellite images. In our case, the eddies were detectable in those images because of the chlorophyll‐enhanced streamers along their edges and, for anticyclones, probably because the chlorophyll maximum found at 40–50 m depth was domed following the pycnocline.