Triggering mechanisms of the Yucatan upwelling

The main physical processes involved with the origin, development, intensity, and seasonality of Yucatan up-welling events are investigated using 7 years of direct (in situ) observations of the near-bottom temperature and velocity profiles of the Yucatan Current. Time series from three moorings deployed over the shelf break in the western part of the Yucatan Channel between February/2008 and July/2014 were analyzed, together with one year (May/2010 to May/2011) of temperature and velocity data from two moorings in the northeastern part of the Yucatan shelf. In particular, current measurements 10 m above the sea bed are used to calculate bottom Ekman transport on the shelf break. Low temperature events occur throughout the year in the upwelling origin area, but spring-summer events (March-August) are stronger and present dynamical features quite different from those in fall-winter (September-February). During spring-summer, bottom Ekman transport towards the shelf triggers upwelling events, which are followed by changes in sea level anomalies along and across the Yucatan Channel. The offshore movement of the Yucatan Current favors the development of cyclonic vorticity and divergence that help maintain the upwelling for long periods of time. Occasionally, in the northern part of the channel where the isobaths diverge (an area called the "notch"), cooling temperatures associated with cyclonic circulation also lead to persistent upwelling conditions. By contrast, during fall-winter, the bottom Ekman transport decreases the temperature over the Yucatan shelf, but the cooling is now strongly modulated by variability at the 6-10 days period band, linked to the passage of Coastal Trapped Waves that limit the duration of the upwelling events and their impact on shelf waters. Our study provides a plausible explanation of the seasonal strengthening of upwelling in the Yucatan system driven by current-topography interactions occurring throughout the year.