Representation of Ocean-Atmosphere Processes Associated with Extended Monsoon Episodes over South Asia in CFSv2

T S Mohan,Larry Marx,Bohua Huang,H Annamalai,James Kinter

doi:10.3389/feart.2018.00009

Abstract

In the present study, we analyze 30-years output from free run solutions of CFSv2 coupled model to assess the model’s representation of extended (>7 days) active and break monsoon episodes over south Asia. Process based diagnostics is applied to the individual and composite events to identify precursor signals in both ocean and atmospheric variables. Our examination suggests that CFSv2, like most coupled models, depict systematic biases in variables important for ocean-atmosphere interactions. Nevertheless, model solutions capture many aspects of monsoon extended break and active episodes realistically, encouraging us to apply process-based diagnostics. Diagnostics reveal that sea surface temperature (SST) variations over the northern Bay of Bengal where the climatological mixed-layer is thin, lead the in-situ precipitation anomalies by about 8 (10) days during extended active (break) episodes, and the precipitation anomalies over central India by 10-14 days. Mixed-layer heat budget analysis indicates for a close correspondence between SST tendency and net surface heat flux (Q_net). MSE budgets indicate that horizontal moisture advection to be a coherent precursor signal (~10 days) during both extended break (dry advection) and active (moist advection) events. The lead timings in these precursor signals in CFSv2 solutions will be of potential use to monitor and predict extended monsoon episodes. Diagnostics, however, also indicate that for about 1/3 of the identified extended break and active episodes, inconsistencies in budget terms suggest precursor signals could lead to false alarms. Apart from false alarms, compared to observations, CFSv2 systematically simulates a greater number of extended monsoon active episodes.

Highlights

Rainfall during the Indian summer monsoon (ISM) season (June–September) fluctuates between active and break spells
A similar investigation on Climate Forecast System version 2 (CFSv2) solutions suggests that anomalous winds advecting climatological moisture gradient (–V′ ·∇qo) with origins over northern Arabian continent is the dominant source of dry air prior to extended breaks over NCEN, and that the climatological winds acting on anomalous moisture gradient (Vo·∇q′ ) with origin over western Arabian Sea is the source of moist air prior to extended active episodes over NCEN
Since CFSv2 is employed for real-time prediction of monsoon over South Asia, for individual and composite events we apply process-based diagnostics such as vertically integrated moist static energy (MSE) budget and mixed layer (ML) heat budget to identify precursor signals in both ocean and atmospheric variables, and to identify any false alarms

Summary

Introduction

Rainfall during the Indian summer monsoon (ISM) season (June–September) fluctuates between active and break spells. With that goal in mind, here we diagnose the representation of extended active and break monsoon episodes in the Climate Forecast System version 2 (CFSv2) integrations, and assess whether the leading thermodynamical processes are faithfully represented in the solutions. Diagnosing CFSv2 integrations several studies have identified similar dry bias in seasonal mean precipitation climatology over continental India and cold SST bias over the tropical Indian Ocean The climatological biases in CFSv2 are similar to those of many present-day coupled climate models We recognize that such systematic errors in a time-mean sense arise due to errors in both ocean and atmospheric components of the coupled model (Annamalai et al, 2017), and could impact the simulated variance across all time scales

Objectives

Discussion

Conclusion