Effects of the 1997–1998 El Niño on the exchange of the northern Gulf of California

We analyze sea level and transports in the Gulf of California from two large‐scale, high‐resolution numerical models: the Naval Research Laboratory Layered Ocean Model and the Hybrid Coordinate Ocean Model. Sea level from both models correlates well with sea surface height from a nearby TOPEX track and subsurface pressure measured in the northern gulf during 1997–1998. An 8‐year mean (1993–2000) section of the modeled meridional velocity at the mouth of the gulf is in good qualitative agreement with observations, showing a localized near‐surface outflow on the Baja California side. The 8‐year mean, laterally integrated modeled near‐surface transport at the mouth of the gulf shows an opposite direction for both models. During the 1997–1998 El Niño, however, both models show a similar structure with increased inflow in the upper 100 m and an increased outflow in the underlying layer to about 500 m depth. The modeled surface transport also shows increased inflow in the northern gulf. The modeled incoming surface transport from both models during El Niño is negatively correlated to near‐bottom currents measured at a sill in the northern gulf. This measured near‐bottom current has previously been shown to constitute part of the strong, localized incoming flow from the Pacific Ocean. The negative correlation strongly suggests that the anomalous incoming surface flow at the mouth of the gulf inhibited the normal near‐surface outflow of the northern Gulf of California water. This, in turn, inhibited the normal deeper inflow, entering the northern gulf from the Pacific Ocean. These anomalies apparently increased the residence time of the waters in the northern gulf. They are consistent with anomalously high near‐bottom salinities found during 1997–1998. Hydrographic data for July 1997 also show an anomalous low salinity surface water, which is most likely associated with advection of tropical waters from the Pacific Ocean.