Oceanographic conditions over the co ntinental shelf of f Magdalena Bay (Mexico) in 20112012

The hydrophysical results obtained during six oceanographic cruises in 2011 (February, April, July, and October) and 2012 (February and April) over a 50-station samp ling grid in front of the Magdalena-Almeja s lagoon system (Mexico) are presented. MO DIS-Aqua sensor data (1.1 km) of sea surfa ce temperature (SST) and chlorophyll a , and geostrophic current fields we re analyzed during the sampling periods. In the northern part of the study area the pattern of the thermohaline structure shows subsurface intrusion of modifie d subarctic water, especially during February and April of both years. To the south, in July and Octobe r 2011, evident indications of subtropical subsurface water were found. The geostrophic current fields in February and April show a surface southward flow along the coast, which changes d irection towards the North Pole in July and October. In spring, the sate llite-derived SST showed negative anomalies of up to 4.5 ºC in th e coastal zone, corresponding to cold upwelled wa ter rising to the surface from a depth of 6070 m. Based on the re sults, the study area can be considered the southern boundary of the transition zone betw een the subarctic and subtropi cal waters of the northeast ern Pacific Ocean, with m arked seasonality.