Hydrography and geostrophic currents in the Northern Gulf of California during the 1997–1998 El Niño

Conductivity, temperature and depth (CTD) data taken during July and November 1997 and March 1998 are used to analyze the hydrographic conditions in the Northern Gulf of California during the 1997–98 El Niño. The CTD data are compared to data from other years and an anomalous high salinity, lower density water is found near the bottom, whereas a relatively fresh surface layer is found on the eastern side of the gulf during July. The relatively fresh surface water is attributed to advection of tropical waters from the Pacific Ocean. During November and March, positive surface temperature anomalies are identified. Geostrophic currents are calculated across an entire cross section of the gulf passing through the northern tip of Angel de la Guarda Island using either salt or mass conservation as constraints. The most salient feature of the geostrophic currents is the existence of relatively intense bottom currents. The bottom current on the eastern side of the island is southeastward, toward the mouth of the gulf, during July, whereas it is toward the head of the gulf on the western side of the island. This pattern is inverted during November and March and, in all cases, suggests a near-bottom circulation between both basins separated by the island. The near-bottom current toward the head of the gulf on the eastern side of the island appears to be a permanent feature associated with the exchange of the northern gulf. Therefore, the high near-bottom salinities during July, coupled with a current toward the mouth of the gulf, are attributed to an anomalous condition created by the El Niño event. This condition is consistent with a subsidence of Gulf of California waters, flowing toward the mouth. Fluxes of salt and heat estimated from geostrophic currents are compared to seasonal estimates. The July heat flux is about three times greater than the seasonal value, and there appears to be an overall phase shift with respect to the seasonal cycle.