An Intermodel Approach to Identify the Source of Excessive Equatorial Pacific Cold Tongue in CMIP5 Models and Uncertainty in Observational Datasets

Abstract

An excessive cold tongue error in the equatorial Pacific has prevailed in several generations of climate models. However, the causes of this problem remain a mystery, partly owing to uncertainty and/or a lack of observational datasets. Based on the multimodel ensemble from phase 5 of the Coupled Model Intercomparison Project (CMIP5), this study introduces a novel intermodel approach to identify the bias source by going beyond comparison with observational datasets. Intermodel statistics show that the excessive cold tongue bias could be traced back to a too strong oceanic dynamic cooling linked to a too shallow thermocline along the equatorial Pacific. A heat budget analysis suggests that the excessive oceanic dynamic cooling is balanced by the surface latent heat flux (LHF) adjustment. This is consistent with a variety of oceanic and atmospheric observations but at odds with the popular objectively analyzed air-sea heat fluxes (OAFlux) products. Further analyses suggest an alarming overestimation of OAFlux net surface heat flux (Q net ) into the tropical Pacific, mainly ascribed to observational uncertainly in air specific humidity. Implications for intermodel statistics in assessing model processes, validating observational data, and regulating future climate projections are discussed.

Characterizing CMIP5 model spread in simulated rainfall in the Pacific Intertropical Convergence and South Pacific Convergence Zones

“…For example, the MEM is too wet in the SH eastern Pacific, reflecting the model simulation of a spurious SH ITCZ there, while the dipole appearing along the poleward portion of the SPCZ is consistent with its too zonal orientation in the models. Along the equator, especially to the west of the International Dateline, the MEM considerably underestimates rainfall relative to GPCP, consistent with too cool SSTs and excessive westward extension of the cold tongue [ Li et al , , ]. Farther to the west near the Maritime continent, the CMIP5 models are excessively wet, as they also are over the western coast of South America.…”

Double ITCZ in Coupled Ocean-Atmosphere Models: From CMIP3 to CMIP5

“…More elaborate metrics may be designed such as the Taylor diagram [ Taylor , ] or empirical orthogonal function modes as by Li et al . []. It is adopted here because of its simplicity and intuitive interpretation.…”

Tropical Indian Ocean response to the decay phase of El Niño in a coupled model and associated changes in south and east-Asian summer monsoon circulation and rainfall

“…The observed coupling between the SSH anomalies and Ekman pumping is not evident in the model. The underestimated SSH anomalies and coupling may be due to a too deep SWIO thermocline dome in model (e.g., Chowdary et al 2015) as in many other coupled models (e.g., Li et al 2015a;Zheng et al 2015). This would largely limit the TIO basin-wide warming response to El Niño.…”