Abstract. The Gulf of California (GC) presents several oceanographic features that make it unique among semienclosed seas of similar latitude and dimensions, the most important being strong tidal mixing, some of it close to deep stratification. Three-dimensional numerical model results suggest that tidal mixing may be more important than the thermohaline circulation in causing the long-term residual circulation, which consists of outflow in the upper 200 m and inflow below, plus a seasonally-reversing surface layer. The GC is an evaporative basin, but in the mean it gains heat through the surface. Lacking a sill at the point of connection with the Pacific Ocean (PO), the GC is constantly shaken by a wide spectrum of signals coming from the PO, including tides, subinertial trapped waves of various frequencies and El Niño. The seasonal dynamics and thermodynamics of the GC are dominated by the PO, not by local wind or buoyancy flux. Local processes are important at shorter time scales and in altering the thermohaline characteristics of the upper-layer waters. Tidal currents generate internal tides, packets of solitons, and sea surface temperature fronts from which jets may form. Coastal upwelling also seems to generate jets that separate from capes, especially on the mainland coast. The mesoscale off-shore circulation in the GC consists of a series of basin-wide geostrophic gyres that reach below 1000 m; their effect on the mean and seasonal circulation and thermodynamics of the GC remains to be studied. During summer, the currents in the mainland continental shelf are due to coastal trapped waves, while during winter they are wind-driven. The most important interannual anomalies in the GC are due to El Niño.

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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.

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[1] Mesoscale eddies in the northeastern Pacific tropical-subtropical transition zone (16 N-30 N; 130 W-102 W) are analyzed using nearly 18 years of satellite altimetry and an automated eddy-identification algorithm. Eddies that lasted more than 10 weeks are described based on the analysis of 465 anticyclonic and 529 cyclonic eddy trajectories. We found three near-coastal eddy-prolific areas: (1) Punta Eugenia, (2) Cabo San Lucas, and (3) Cabo Corrientes. These three areas are located in places where the coastal morphology changes abruptly and strong surface current intensification occurs at some phase of the seasonal cycle. Although mesoscale eddies in these areas have been previously reported, this study provides their first statistically supported characterization. Punta Eugenia showed the highest eddy production (with more cyclones generated), followed by Cabo Corrientes (also with more cyclones) and Cabo San Lucas (with more anticyclones). Cabo Corrientes eddies showed the highest mean values in propagation speed, swirling speed and eddy kinetic energy, whereas Punta Eugenia eddies showed the lowest values. Cyclonic eddies increased their distance traveled and duration from south to north; in contrast anticyclonic eddies increased from north to south. In average, anticyclones tend to travel faster than cyclones in all the subregions. These long-lived eddies were mainly nonlinear and therefore can redistribute coastal waters relatively far into the open ocean. The peaks in the seasonal signal of eddy generation can be associated with the peaks in the strength of the offshore currents and/or in the Coastal Upwelling Index. No clear relationship could be established between El Niño events and eddy generation.

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.

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