INTERANNUAL VARIABILITY OF MIXED LAYER DYNAMICS IN THE ECUADORIAN OCEAN

Long time series spanning up to 22 years of monthly CTD profiles are used to examine upper water column ocean temperature interannual variability near the Ecuador coastline along the equator. The sampling program instituted by Ecuador’s National Institute of Oceanography of the Navy (INOCAR) began in 1992 occupying two stations about 8 nm from the coast, and then expanding to an additional 3 stations in 2004 by Ecuador’s National Fisheries Institute (INP) and National Institute of Meteorology and Hydrology (INAMHI). The stations are located near 80-81° W and extend from 2° S to 1° N. Climatological (seasonal) signals are removed from the data to focus on the interannual variability. Characterizations of the anomaly time series include the 20°C isotherm depth, mixed layer depth (greater than 5 m depth), heat content (above 100 m), and an EOF decomposition of the temperature profile data set. The data show marked interannual variations with distinct characteristics associated with ENSO indices that delineate periods of El Niño, La Niña, and “normal” conditions. Heat content and the 20°C isotherm depth are both largest during El Niño periods, and weakest during La Niña periods. Mixed layer depths are largely invariant during the various periods, likely due to the dependence of the characterization on details of individual temperature profile. The first mode EOF decomposition of the anomaly long record coastal station data represents bulk variations (71%) of the thermocline depth, and has temporal variability coupled (R = 0.65) to ENSO 3.4 and 1+2 indices. Coastal observations are compared with temperature observations obtained over the same time period from the offshore TAO/Triton buoys located along 95° W from 2° S to 2° N, to the west of the Galápagos Islands outside the geologic delineations of the Ecuadorian Sea. The EOF decomposition of TAO buoy anomaly time series shows similar spatial EOF structure, with the first mode representing bulk changes to the thermocline (about 71% of the variance). The first mode EOF amplitude time series from coastal and TAO station decomposition is correlated (R = 0.86), showing that the dominant variability of the upper water column thermal structure near the coast is coupled to variations along the equator and seaward of the Galápagos Islands. Comparisons of wind fields derived from ECMWF reanalysis in the regions shows that upwelling favorable winds to the north (in the Colombian basin) and to the south (along the coast of Perú) but away from the equator (where earth rotation impacts are minimal), produces a surface convergence at the equator close to shore that has variations that are strongly correlated with ENSO indices. The surface convergences are stronger during ENSO periods, and result in an increase in upper water column heating synchronous with the larger-scale ENSO variability