Residence times and exchange pathways of surface waters in the southern Gulf of Mexico derived from drifters

The circulation in the Bay of Campeche is characterized by a semi-permanent cyclonic gyre in the deep western basin, and a weaker, less structured flow in the shallower eastern basin. Past studies suggest that the gyre acts as a barrier to the exchange of water between the western shelf and deep waters, while the potential for exchange is higher between the eastern shelf and the eastern basin. Observations of fish larvae composition in the region show a large relative abundance of oceanic species in the western basin, and a higher contribution of shelf dwelling species in the eastern basin. These results suggest that the Campeche Gyre acts as a retention zone with relatively large residence times. In this study, we quantify the retention and exchange of particles between the eastern and western basins of the Bay of Campeche, as well as the exchange and transit times of particles between the Bay of Campeche and the rest of the Gulf of Mexico. The analysis is performed using data from surface drifters that were deployed monthly within the Bay of Campeche between 2007 and 2014. The results show that the western basin has a high degree of retention, with more than 50% of particles trapped there after 40 days, and 20% still present after 100 days. In addition, particle exchange occurs preferably from the eastern to the western basin, so the Campeche Gyre tends to act as a sink. Residence times of particles within the Bay of Campeche vary between 10 to 100 days, with 50% of the particles leaving the region in more than 30 days. The preferred path for particles leaving the Bay of Campeche is along the western boundary, mostly driven by the interaction of the Campeche Gyre with Loop Current eddies arriving to the region. The importance of northern Gulf eddies in the renewal of surface waters in the Bay of Campeche merits further research, given the relevance this has for the time evolution of water properties and chemical tracers in this region.