Revisiting the CMIP5 Thermocline in the Equatorial Pacific and Atlantic Oceans

The thermocline is defined as the ocean layer for which the vertical thermal gradient is maximum. In the equatorial ocean, observations led to the use of the 20 degrees C isotherm depth (z20) as an estimate of the thermocline. This study compares z20 against the physical thermocline in the equatorial Atlantic and Pacific Oceans, using Simple Ocean Data Assimilation reanalysis and fifth phase of the Coupled Model Intercomparison Project preindustrial control simulations. Our results show that z20 is systematically deeper and flatter than the thermocline and does not respond correctly to surface wind stress variations. It is also shown that the annual cycle of z20 is much weaker than that of the physical thermocline. This happens in both equatorial basins and indicates that z20 does not react to the same mechanisms as the thermocline. This could have important consequences in the assessment of air-sea coupling in current general circulation models and bias reduction strategies.