Improved Simulations of Tropical Pacific Annual‐Mean Climate in the GFDL FLOR and HiFLOR Coupled GCMs

Abstract

The National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory has recently developed two global coupled general circulation models, the Forecast‐oriented Low Ocean Resolution (FLOR) model and the High atmospheric resolution Forecast‐oriented Low Ocean Resolution (HiFLOR) model, which are now being utilized for climate research and seasonal predictions. Compared to their predecessor Coupled Model version 2.1 (CM2.1), the new versions have improved ocean/atmosphere physics and numerics and refinement of the atmospheric horizontal grid from 220 km (CM2.1) to 55 km (FLOR) and 26 km (HiFLOR). Both FLOR and HiFLOR demonstrate greatly improved simulations of the tropical Pacific annual‐mean climatology, with FLOR practically eliminating any equatorial cold bias in sea surface temperature. An additional model experiment (Low Ocean Atmosphere Resolution version 1) using FLOR's ocean/atmosphere physics, but with the atmospheric grid coarsened toward that of CM2.1, is used to further isolate the impacts of the refined atmospheric grid versus the improved physics and numerics. The improved ocean/atmosphere formulations are found to produce more realistic tropical Pacific patterns of sea surface temperature and rainfall, surface heat fluxes, ocean mixed layer depths, surface currents, and tropical instability wave activity; enhance the near‐surface equatorial upwelling; and reduce the intercentennial warm drift of the tropical Pacific upper ocean. The atmospheric grid refinement further improves these features and also improves the tropical Pacific surface wind stress, implied Ekman and Sverdrup transports, subsurface temperature and salinity structure, and heat advection in the equatorial upper ocean. The results highlight the importance of nonlocal air‐sea interactions in the tropical Pacific climate system, including the influence of off‐equatorial surface fluxes on the equatorial annual‐mean state. Implications are discussed for improving future simulations, observations, and predictions of tropical Pacific climate.

The GFDL Earth System Model Version 4.1 (GFDL‐ESM 4.1): Overall Coupled Model Description and Simulation Characteristics

“…Compared to CM4.0, ESM4.1 also has somewhat larger tropical climatological rainfall biases (poleward‐shifted ITCZ and double ITCZ), weaker climatological zonal surface wind stress off‐equator, and more cyclonic wind stress curl over the tropical Pacific. This enhanced cyclonic curl acts to shoal the equatorial Pacific thermocline via poleward Sverdrup transport (Wittenberg et al, 2018), exacerbating the equatorial Pacific subsurface cold bias in ESM4.1 relative to CM4.0.…”

The GFDL Global Ocean and Sea Ice Model OM4.0: Model Description and Simulation Features

“…As in Wittenberg et al. (), the TAO/TRITON data are processed using daily means of measurements from both fixed depth mechanical current meters and acoustic Doppler current profilers, with a simple average of the two where both are available. The merged daily means from the moorings are vertically interpolated and then averaged onto a monthly grid.…”

On the Angola Low Interannual Variability and Its Role in Modulating ENSO Effects in Southern Africa

“…Qualitatively similar results and conclusions are obtained with and without flux adjustment. However, in FLOR El Niño events are too strong and frequent (Vecchi et al 2014;Wittenberg et al 2018), thus exaggerating interannual variability of SA summer rainfall. Furthermore, the ENSO pattern over SA is abnormally shifted westward as in most of CMIP5 models (Dieppois et al 2015), an error that is largely removed in FLOR-FA by reducing SST biases.…”