Water Masses and Ocean Currents Over the Continental Slope off Northern Baja California

Ocean currents and thermohaline properties over the continental slope off northern Baja California are analyzed with data collected between 2008 and 2012. The data set is composed of 3-year-long time series of two moorings with current meter profilers and one chain of conductivity-temperature-depths, as well as data collected during 13 synoptic hydrographic surveys. The upper layer (<100m) was characterized by low salinity Subarctic Waters. Geostrophic velocities suggest that waters of the lighter layer are advected equatorward by near-surface flows. Below 50m, poleward flow dominates, so the denser Subarctic Waters recirculates back north along the slope. The combination of geostrophic velocities derived from the hydrographic surveys and the current meter data confirm the presence of the California Undercurrent (CUC), a narrow subsurface poleward flow with a high-velocity core confined to the continental slope. Mean core speeds were 12-15cm/s at an average depth of similar to 150m. Below the CUC, the core of spicy North Pacific Equatorial Water (NPEW) was found at a mean depth of similar to 200m. The analysis of the continuous time series provides strong evidence of the seasonal signal in speed, spiciness, and depth of the CUC and NPEW cores, with a time resolution unprecedented to date. Most of the variance of water mass and flow properties is explained by the annual cycle. The phase shift between the depth of the NPEW core and the strength of the CUC suggests that remotely generated Kelvin waves are responsible for the seasonal variability observed off northern Baja California. Plain Language Summary The California Undercurrent flows along the North American Pacific coast. It brings salty and nutrient-rich waters from the equator toward the subpolar region, which tend to surface near the coast when the winds blow southward, resulting in the high biological productivity that sustains the fisheries along the Pacific coasts of Baja California and the United States. Its main core is found over the continental slope, and it extends from about 50- to 600-m depths. It is characterized by a subsurface maximum in speed and salinity at about 150-m depth. In this study, measurements allowed to estimate the horizontal and vertical velocity and salinity fields of the Undercurrent off Baja California, as well as to obtain the annual variation in the depth of the maximum salinity and the intensity of the velocity of the core. The results show that these maxima do not coincide in time: they differ by 4-5months. This indicates that the forcing responsible for the annual variation observed could be very long waves that travel along the coast all the way from the equator, similar to the waves that bring the El Nino conditions to higher latitudes.