Abstract
The variability of predicted variables at daily to seasonal scales in coupled models is primarily governed by surface boundary conditions between the ocean and atmosphere, namely, sea surface temperature (SST), turbulent heat, and momentum fluxes. Although efforts have been made to achieve good accuracy in surface fluxes and SST in observation and reanalysis products, less attention has been paid toward achieving improved accuracy in coupled model simulations. Improper diurnal phase and amplitude in intra-daily SST and precipitation are well-known problems in most global coupled general circulation models, including the Climate Forecast System v2 (CFSv2) model. The present study attempts to improve the representation of ocean-atmosphere surface boundary conditions in CFSv2, primarily used for India's operational forecasts at different temporal/spatial scales. In this direction, the diurnal warm layer and cool skin temperature correction scheme are implemented along with the surface flux parameterization scheme following Coupled Ocean-Atmosphere Response Experiment (COARE) v 3.0. The coupled model re-forecasts with a revised flux scheme showed better characteristics in various ocean-atmosphere parameters and processes at diurnal and seasonal time scales. At the diurnal scale, the phase and amplitude of intra-daily SST and mixed layer depth variabilities are improved over most tropical oceans. Improved diurnal SSTs helped in enhancing the diurnal range of precipitation by triggering stronger intra-daily convection. The corrected diurnal ocean-atmospheric boundary state translated into a reduction in seasonal mean dry bias over Indian landmass and the wet bias over tropical oceans. Better simulation of non-linearity associated with El Niño–Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), ENSO-Indian Summer Monsoon Rainfall (ISMR), and IOD-ISMR relation is among the most critical improvements achieved by revising the turbulent flux parameterization. The revised flux scheme showed enhanced prediction skills for tropical SST indices and ISMR.
Highlights
Tropical oceans, being the largest storage of heat energy, drive atmospheric circulations that carry energy to other regions of the globe
The second set of hindcasts is by implementing COARE3.0, where turbulent surface flux, diurnal cool skin, and warm layer temperature corrections are computed by following Fairall et al (1996a,b, 2003)
Implementation of cool skin and warm layer corrections (Fairall et al, 1996a) to the bulk ocean temperature as a part of the COARE3.0 algorithm helped in improving the diurnal warming/cooling of the surface ocean, thereby increasing the diurnal range of ocean temperature
Summary
Tropical oceans, being the largest storage of heat energy, drive atmospheric circulations that carry energy to other regions of the globe. Observational and modeling studies (Fairall et al, 1996b; Slingo et al, 2003; Shinoda, 2005; Bernie et al, 2007, 2008; Woolnough et al, 2007; Mujumdar et al, 2011; Yan et al, 2021) have reported possible impacts of diurnal skin temperature variability on daily, intra-seasonal, and seasonal mean and variability by modulating ocean-atmospheric parameters such as SST, mixed layer depth, air pressure, humidity, low-level cloud, and convection. The study documents the impact of the implementation of skin temperature parameterization along with a revised flux scheme on the diurnal and seasonal scale mean biases, variability, and prediction skill of various ocean-atmosphere parameters with the help of coupled model simulations. The second set of hindcasts is by implementing COARE3.0, where turbulent surface flux, diurnal cool skin, and warm layer temperature corrections are computed by following Fairall et al (1996a,b, 2003). Descriptions about the various datasets used in the present study are summarized in Supplementary Table 1
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