The Double-ICTZ Problem in IPCC AR4 Coupled GCMs: Ocean-Atmosphere Feedback Analysis

This study examines the double-ITCZ (Intertropical Convergence Zone) problem in the coupled general circulation models (CGCMs) participating in the Inter-governmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The 20th century climate simulations of 22 IPCC AR4 CGCMs are analyzed, together with the available AMIP runs from 12 of them. To understand the physical mechanisms for the double-ITCZ problem, the main ocean-atmosphere feedbacks, including the zonal sea surface temperature (SST) gradient?trade wind feedback (or Bjerknes feedback), the SST?surface latent heat flux (LHF) feedback, and the SST?surface shortwave flux (SWF) feedback, are studied in detail.

The results show that most of the current state-of-the-art CGCMs have some degree of the double-ITCZ problem, which is characterized by excessive precipitation over much of the tropics (e.g. northern hemisphere ITCZ, South Pacific Convergence Zone, maritime continent, and equatorial Indian Ocean), and often associated with insufficient precipitation over equatorial Pacific. The excessive precipitation over much of the tropics usually causes overly strong trade winds, excessive LHF and insufficient SWF, leading to significant cold SST bias in much of the tropical oceans. Most of the models also simulate insufficient latitudinal asymmetry in precipitation and SST over eastern Pacific and Atlantic Oceans.

The AMIP runs also produce excessive precipitation over much of the tropics including equatorial Pacific, which also leads to overly strong trade winds, excessive LHF and insufficient SWF. This suggests that the excessive tropical precipitation is an intrinsic error of the atmospheric models, and that the insufficient equatorial Pacific precipitation in the coupled runs of many models comes from ocean-atmosphere feedback. Feedback analysis demonstrates that the insufficient equatorial Pacific precipitation in different models is associated with one or more of the following three biases in ocean-atmosphere feedback over equatorial Pacific: (1) excessive Bjerknes feedback, which is caused by excessive sensitivity of precipitation to SST and overly strong time-mean surface wind speed; (2) overly positive SST-LHF feedback, which is caused by excessive sensitivity of surface air humidity to SST; and (3) insufficient SST-SWF feedback, which is caused by insufficient sensitivity of cloud amount to precipitation. Off the equator over the eastern Pacific stratus region, most of the models produce insufficient stratus-SST feedback associated with insufficient sensitivity of stratus cloud amount to SST, which may contribute to the insufficient latitudinal asymmetry of SST in their coupled runs. These results suggest that the double-ITCZ problem in CGCMs may be alleviated by reducing the excessive tropical precipitation and the above feedback-relevant errors in the atmospheric models.