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In this work we present the exchange flows between a semi-enclosed Bay and the adjacent ocean as measured by a real-time coastal observing system in Baja California, Mexico. Several sources of information as numerical modeling, currents profile data, high frequency radar observations and drifters tracking, all together show an interesting three-dimensional structure to the exchange flows. The northern entrance (relatively shallow, ∼40 m at its deepest portion) showed an estuarine-like exchange pattern, with a net outflow at the shallow parts and inflow at the deepest, suggesting that the frictional effect is more important than the earth's rotation, as typically observed in relatively small bodies of water like estuaries and coastal lagoons. On the other hand, the southern entrance (with a ∼400 m deep canyon) is more complex, with recurrent anticyclonic exchange pattern with inflows at the western limit and outflows at the east. We suggest that the vertical structure of this circulation may be sustained by the deep canyon which lowers the thermocline, but further investigation is needed.

Climatological data indicate that Gulf of California (GC) waters are warmed in spring by the exchange of waters with the Pacific Ocean. To better understand this exchange, hydrographic observations were collected across the entrance to the GC during 19–21 April 2013. Results indicated an anticyclonic exchange with the Pacific Ocean. Strong outflow of Gulf of California Water (GCW) occurred over the outer continental shelf and slope of Sinaloa with maximum velocities of 0.5 m·s–1 at ~60 m depth and reaching ~500 m depth. Inflow close to the Baja California Peninsula was weaker (0.1–0.2 m·s–1) and transported both California Current Water and GCW. Satellite altimeter data indicated a possible reason for this flow pattern at the entrance to the GC: a couple of eddies to the east blocked the usual path of California Current flow toward Sinaloa. Horizontal transports integrated across the gulf were calculated with observed velocity and geostrophic balance. Transports were greatest in the upper 200 m, with outflow above 500 m depth, implying a net cooling of these upper layer waters in the gulf. Observations were compared with data collected in early May 1992. April 2013 exhibited much greater thermohaline variability in the upper ocean than May 1992, when a well developed cyclonic flow was observed at the entrance to the GC. The transports estimated with Pegasus for April–May 1992 were very different from those estimated for April 2013, when inflow (and heat gain) dominated above 500 m depth. The contrast between the 2013 and 1992 measurements is an example of how mesoscale eddies can reverse normal seasonal exchanges at the entrance to the GC.

Closely spaced XBT and CTD data were used to study the sub-surface structure of features observed in satellite AVHRR sea-surface temperature (SST) images of the Gulf of California (GC), such as fronts, jets and gyres. The lowest SSTs in the GC are found around the midriff islands, and especially over the sills, because of strong mixing from tidal currents and breaking internal waves. This minimum SST area is limited to the south and north by sharp SST fronts, which frequently show convolutions, gyres, and filaments. A vertical section of temperature along the Gulf length shows that about 300 km south of the sills, the isotherms begin to spread out in the vertical, reducing stratification. The thermocline isotherms begin surfacing about 100 km south of the sill, forming the SST minimum area, which has a maximum temperature difference of ∼6 °C with waters to the south and a horizontal SST gradient of ∼0.05 °C km −1 . The evolution and subsurface structure of a cool filament, apparently originated as an instability of the SST front, was intensely sampled. Satellite images show that its head advanced due south at about 0.5 ms −1 , along the edge of a pool of warm surface water about 110 km in diameter. The thermocline below the warm pool was raised at its edge, and some isotherms reached the surface, producing the cool filament. The tilt of the temperature, salinity and density isolines at the edge of the warm pool produced a geostrophic jet, adjacent to the cool filament and just inside the warm pool, with maximum speed ∼0.5 ms −1 and extending down to ∼80 m. The filament lasted about 10 days, its width was ∼30 km and it reached a maximum length of ∼120 km.The volume transport of the jet in the top 100 m was around 0.7 Sv.