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The Gulf of California, or, more poetically, the Sea of Cortez, is a fascinating and complex semienclosed sea of the eastern North Pacific. Surrounded by arid and mountainous regions, the Gulf loses moisture to the atmosphere, creating a high-salinity, relatively warm water mass, and giving rise to a vertical thermohaline circulation. Although there is an excess of evaporation over precipitation in the Gulf, there is a net gain of heat from the atmosphere, of sufficient magnitude to reverse the evaporative buoyancy loss. This combination of air-sea heat and moisture fluxes requires an annual average of deep inflow and shallower outflow, which is opposite to the exchange between the Mediterranean Sea and the Atlantic Ocean or the Red Sea and the Indian Ocean. Water mass transformation in the northern Gulf reflects this inverted system: limited convection and extensive tidal mixing combine the saline surface waters of the far northern Gulf and the fresh, deep, inflowing Pacific Intermediate Water into a distinct water mass, Gulf Water, exchanged with the Pacific. Deep inflow to the Gulf provides enhanced nutrients and may well be responsible for the high productivity observed in the Gulf.

Tides are strong in the Gulf because of co-oscillation with the Pacific and resonance of the semidiurnal component. Significant tidal energy also distinguishes the Gulf from other midlatitude marginal seas, providing energy for enhanced mixing. Wind forcing of circulation in the Gulf appears to be of secondary importance, even over the shelf. Remote effects, including coastally trapped waves, dominate shelf circulation on the eastern side of the Gulf, at least during summer. Although energetic currents are observed on the western boundary of the Gulf, they do not appear to correlate with the local wind nor with the shelf waves on the eastern coast. The forcing mechanisms responsible for observed basinwide gyres and intense currents along the western boundary have yet to be identified. Interannual variability in the Gulf is dominated by effects of the global phenomenon labeled El Niño-Southern Oscillation (ENSO). In the Gulf, productivity is greatly enhanced during ENSO years, unlike the open coast of California, where productivity is reduced. ENSO conditions in the eastern Pacific bring warm, fresh Tropical Surface Water well into the Gulf, where it is normally not found. Water mass formation by winter convection appears to be more widespread during ENSO years, possibly reflecting changes in the water mass flowing into the northern Gulf during ENSO winters. Changes in hydrography during ENSO years extend to the bottom of the 1500-m-deep channels of the northern Gulf and suggest annual renewal of those deep, oxygen-rich waters.

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