Surface Current Velocity Observations of the Yucatan Channel Using High-Frequency Radar

We report on surface current velocity observations from two 5 MHz coastal radars deployed in Quintana Roo, Mexico (Puerto Morelos and Isla Contoy) on the west side of the Yucatan Channel. The instrument systems are funded by the National Academy of Sciences Understanding the Gulf Ocean System (UGOS) program (Knap et al. 2023). Deployed in June 2022, the radar system provides hourly estimates with 6-km spatial resolution of the surface current velocity field between the west coast of Cuba and the east coast of the Yucatan Peninsula. The data are used to assess the current velocity structure found in multiple operational numerical ocean models operated by federal agencies (RTOFS, GOFS, CMEMS and AMSEAS), university researchers (CNAPS), and industry (TOPS). The radar observations are also part of a larger field campaign of the southeastern Gulf of Mexico that targets adaptive sampling strategies for the Yucatan in-flow region to improve prediction skill of numerical models of the Loop Current System (DiMarco et al. 2023). Maximum speed of the along-channel flow can reach 2 m/s. Model/data comparison metrics were established along a cross-Strait line through the highest quality data in the HF Radar field. Metrics include the maximum downstream velocity and its distance from the Yucatan coast, along with the total downstream surface transport. The variability of the Yucatan Current in space and time reveals how the speed core position changes with Loop Current orientation and state. For example, when the Loop Current is in a retracted state (i.e., the current enters the Gulf of Mexico and turns eastward and flows along the northern Cuban coast and exits the Gulf through the Florida Straits), the Yucatan Current speed core is close to the channel midpoint. As the Loop Current extends into the Gulf of Mexico, the speed core tends to move to the western edge of the Channel. In February of 2023, the HF Radar network observed a rapid transition from a high transport offshore mode to a lower transport nearshore mode. This corresponds to a time when the Loop Current itself was transitioning from a fully retracted state flowing eastward along the coast of Cuba to the Florida Straits with little interaction with the Gulf to a deeply extended state where it flowed northward along the Yucatan Escarpment and interacted with a previously formed Loop Current Eddy. When the Loop Current is in the nearshore mode, a double peak in the downstream speed is often observed, with the local minimum between the two peak currents located downstream of the island of Cozumel. Tidal current variability is also evident in the observational record as diurnal current variability dominates the tidal bands. Variations in the easterly wind speed on scales of 1–2 weeks are correlated with variations in the downstream transport. On the western side of the Yucatan Channel, frequent current reversals of southward flow from the western Cuban coast to the Caribbean Sea are observed; this current reversal has been documented previously in cross-channel transects, however, the magnitude, persistence and variance of the reversals and the spatial extent can now be quantified for the first time. The HF-radar observations are compared against sparse historical observations from the 19th and 20th century.