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

Abstract

AbstractA 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 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.