Shelf circulation in the Gulf of California: A description of the variability

Abstract

The first long‐term measurements (1983–1984) of winds, temperature, bottom pressure, and currents made in the Gulf of California are used to describe the dynamically important spatial and temporal scales and to compare and contrast the shelf circulation on opposite sides of the Guaymas Basin. In addition, a qualitative assessment of possible forcing mechanisms of gulf shelf circulation and an account of an energetic coastal‐trapped wave event are presented. Winds over the gulf are weak and variable with maximum wind speeds of 10–15 m/s and sustained wind events of less than a week. Low‐frequency (<3 cpd) fluctuations in the dominant along‐gulf component of the wind field, which are largely coherent over the study area, are directed upgulf during summer and downgulf for the remainder of the year. Low‐frequency winds are not significantly correlated (95% confidence level) with currents, temperature, and pressure, in part because of weak local winds and nonlocally forced coastal‐trapped waves. On longer time scales, seasonal upwelling on both sides of the gulf and Guaymas Basin mean currents are related to local winds. Low‐frequency current fluctuations on the two facing shelf regions, Guaymas on the Mexican mainland and Santa Rosalia on the Baja California peninsula, are not significantly correlated. Along‐gulf currents in the basin, however, are negatively correlated with alongshelf currents at Guaymas (−0.54), due to sporadically occurring eddylike features, and Santa Rosalia (−0.61), due to seasonal flow patterns. Low‐frequency fluctuations on the mainland shelf are significantly correlated during the summer (e.g., 0.67 in temperature) between Topolobampo and Guaymas (separation of 375 km) such that Topolobampo leads Guaymas by approximately 1.5 days. This lagged signal is associated with coastal‐trapped waves that are generated by tropical storms and hurricanes south of the gulf and propagate poleward along the mainland shelf at speeds of 200–250 km/day near Guaymas. Current speeds of approximately 50 cm/s were measured at the 100‐m isobath on the mainland shelf during an event with a 20‐cm coastal sea level rise. The offshore decay of these waves is of the order of 50 km. On the Guaymas shelf, low‐frequency temperature (or equivalently density) and sea level fluctuations are correlated (0.78), such that a 10‐cm rise in sea level corresponds to a 20‐m drop in isotherm depths over much of the water column. This sea surface‐isotherm relationship is such that the amplitude of pressure fluctuations decays rapidly with depth (e‐folding scale = 160 m). A similar relationship is only weakly apparent (correlation = 0.39) on the Santa Rosalia shelf. Cross‐gulf geostrophy is observed with 35% of the along‐gulf current, averaged across the gulf, related to dp/dx measured between Guaymas and Santa Rosalia. On the Guaymas and Santa Rosalia shelves, dp/dx (measured across each shelf) explains approximately 20% of the variability in alongshelf currents. In the alongshelf momentum balance, dp/dy accounts for approximately 40% of the low‐frequency variability in dv/dt at the Guaymas shelf break, consistent with the dynamics of freely propagating coastal‐trapped waves. The same balance is only weakly apparent (<10% of variance explained) at Santa Rosalia. At higher frequencies, temperature fluctuations at Guaymas are coherent and in phase to 200‐m depth, coherent (0.74) with the alongshelf near‐surface current at the shelf break (M8), and exhibit a spring‐neap modulation consistent with internal tides.