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

The 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.

Monsoons, plumes, and blooms: intraseasonal variability of subsurface primary productivity in the Bay of Bengal

“…Surface PAR fluctuates on timescales of weeks to months over the entire BoB during the southwest monsoon (Figure 5a-b) 4,5,6 . Both the DBASIS and BGC-Argo observations indicate the intraseasonal variability in surface light then projects onto the SCM primary productivity, where SCM productivity is typically light-limited.…”

“…Current generation coupled atmosphere-ocean numerical models lack fidelity on 10-90 day timescales in the tropics and subtropics 4,5,6 . This so-called intraseasonal variability is a major component of global weather and is fundamental to the prediction of important climate phenomena like the South Asian monsoon 6 .…”

“…Since coupled ocean-atmosphere models struggle to accurately represent the magnitude or the timing of intraseasonal fluctuations 4,5,6 , these fluctuations must also be inaccurately represented in coupled BGC models. Intraseasonal oscillations like the Madden-Jullian Oscillation or the Monsoon Intraseasonal Oscillation impact huge swaths of the subtropical ocean, so the potential impact on BGC processes is significant.…”

Dynamic Downscaling the South Asian Summer Monsoon From a Global Reanalysis Using a Regional Coupled Ocean‐Atmosphere Model

“…The role of the local scales of the cumulonimbus clouds to the view of the SASM as a shift of the ITCZ from the equatorial Indian Ocean to the continental region with quasi‐divergent planetary circulations make SASM an complex mix of many spatio‐temporal scales (Chen et al., 2019, 2021; Goswami, 2005; Meehl, 1987; Webster et al., 1998). Second, the observed anharmonic sub‐seasonal variations of the SASM precipitation have been a significant shortcoming feature of many climate models across generations of model development (Ajayamohan et al., 2011; Kang et al., 2002; Konda & Vissa, 2022; Mandke et al., 2020; Rajendran & Kitoh, 2006; Slingo et al., 1996). More recently attempts at simulating the SASM and more importantly its intraseasonal variations at comparatively higher resolutions than global models have been made from regional climate models with limited success (e.g., Bhaskaran et al., 1998; Bhate et al., 2012; Maharana & Dimri, 2016; Samala et al., 2013).…”

Impact of horizontal resolution on monsoon precipitation for CORDEX-South Asia: A regional earth system model assessment

“…Such a coarse resolution model is not capable of resolve the orography in complex regions such as the steep topography of the Himalayas and the Western Ghats and various important climate processes such as atmospheric convection and mesoscale boundary currents and eddies and subgrid-scale processes (Tiedtke, 1996;Jakob and Klein, 1999;Stephens et al, 2004;Kumar et al, 2006Barker et al, 2016;Mishra and Dwivedi, 2019) and has to be parameterized, which may compromise dynamic processes and their interactions (Collins et al, 2018). Increased resolution correctly represents the orography of the region and the local features, such as the propagation of mesoscale systems (Vellinga et al, 2016;Mandke et al, 2020).…”