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Abstract:
Over the east Pacific during February-April the Intertropical Convergence Zone (ITCZ) varies spatially on daily to weekly time scales. Climate models continue to struggle to mitigate long standing biases in the ITCZ over this region. While atmosphere-only models have relatively weak-biases, coupled climate models exacerbate the biases through air-sea feedback mechanisms. Our recent results using observational (TRMM) and reanalysis (ERA5) data showed that the distribution of sea surface temperature (SST) anomalies and latent heat flux anomalies follow the classic wind-evaporation-SST (WES) positive feedback mechanism during daily to weekly spatial variability of the ITCZ. However, a mechanism embracing the effect of SST anomalies on vertical stratification and momentum mixing acts in parallel, giving rise to a negative wind-evaporation-SST (WES) feedback.In this study, we compare daily precipitation from NCAR Community Atmosphere Model 6 (CAM6) to TRMM and IMERG. During January through May, CAM6 produces less than the observed precipitation in the northern hemisphere, while producing similar or slightly more than the observed precipitation in the southern hemisphere. We run a series of atmosphere-only simulations to test both the positive and negative WES feedback mechanisms using monthly and daily SSTs during these months. We hypothesize that the atmospheric and oceanic variables associated with the positive WES feedback mechanism are too strong or those associated with the negative WES feedback mechanism are too weak when there is an ITCZ in the southern hemisphere or both of these WES feedbacks are happening simultaneously, which contributes to the climate models producing a double ITCZ bias.
