A Shallow Thermocline Bias in the Southern Tropical Pacific in CMIP5/6 Models Linked to Double-ITCZ Bias

A basin-wide strong shallow bias in the thermocline of the southern tropical Pacific is found in an ensemble of models from the coupled model intercomparison project phases 5 and 6 (CMIP5/6). The bias is present in over 90% of models examined and is equally represented in CMIP5 and CMIP6 models. In contrast to observations, where the southern thermocline is far deeper than its northern counterpart, models have a hemispherically symmetric tropical thermocline. The shallow thermocline bias is closely linked to the well known double intertropical convergence zone (ITCZ) bias; more so for the physical thermocline (i.e., depth of maximal vertical thermal gradient) than the commonly used (Formula presented.) C isotherm thermocline proxy. A shallow thermocline bias is further found to be associated with climatic conditions including wider separation of double ITCZ peaks, a pronounced cold tongue, and a spurious south equatorial counter current.