Simulation of monsoon intra‐seasonal oscillations in Geophysical Fluid Dynamics Laboratory models from Atmospheric Model Intercomparison Project integrations of Coupled Model Intercomparison Project phase 5

Abstract

AbstractThe simulation of the intra‐seasonal variability (ISV), especially the fidelity of monsoon intra‐seasonal oscillations (MISO) over the South Asian summer monsoon (SASM), has been evaluated in Atmospheric Model Intercomparison Project run of three Geophysical Fluid Dynamics Laboratory (GFDL) atmospheric general circulation models (AGCM), participating in Coupled Model Intercomparison Project phase 5. Two of the models, namely, “GFDL‐HIRAM‐C180” and “GFDL‐HIRAM‐C360” are global high‐resolution atmospheric models (HIRAMs), which are same but at different horizontal resolutions. Third model “GFDL‐CM3” is at moderate horizontal resolution, whose atmospheric component differs from HIRAMs. Results have led to the conclusion that simulation of ISV over SASM remains poor in GFDL AGCMs at higher horizontal resolution, except northwards propagation of 30–60‐day mode of MISO by HIRAMs. Our diagnostics revealed resemblance in key characteristics of ISV simulated in two HIRAMs, except spectral peak at lower frequencies (30–60 day), which is captured only by “GFDL‐HIRAM‐C180” over equatorial Indian Ocean. Based on these results, it is conjectured that although high horizontal resolution is crucial for AGCMs to reproduce observed northwards propagation of 30–60‐day mode of MISO, the simulation of nature of ISV over SASM is not influenced by the difference of horizontal resolution in two HIRAMs. Analysis implicates that a reasonable representation of meridional migration of horizontal moisture advection, the meridional gradient of low‐level specific humidity over Indian longitudes and precipitation–SST relationship over north Indian Ocean, the northwestern tropical Pacific Ocean and tropical Indian Ocean assures the northwards propagation of 30–60‐day mode of MISO in HIRAMs.