Impact of sub-daily wind forcing on mixed-layer depth and sea surface temperature: study using observations and a one-dimensional upper ocean model

Abstract

The impact of sub-daily wind sampling on the diurnal cycle of oceanic mixed-layer depth (MLD) and sea surface temperature (SST) is investigated using a one-dimensional upper ocean model and observations at two locations: the Central Arabian Sea (CAS) and Eastern Equatorial Indian Ocean (EEIO). Motivation to carry out this study is twofold: first, it will help in understanding the possible error in model-simulated MLD and SST due to the non-inclusion of high-temporal wind sampling; and second, it will also emphasize the requirements of temporal sampling from space-based measurements of surface winds. Temporal decorrelation analysis suggests that over a 24-hour period, auto-correlation falls rapidly in the EEIO region, whereas the fall is less even at a lag of 24 hours in CAS. Time series analysis with different sub-daily sampling rates suggests that the optimum sampling rate is three hours for MLD and SST. A suite of one-dimensional model simulations performed at the CAS and EEIO locations with sub-daily wind suggests that once-daily synoptic measurements of wind, which is the most likely scenario with one scatterometer, results in small biases but large standard deviations in MLD. In the case of SST, there is a small positive bias in the order of 0.1°C at the CAS buoy location while at the EEIO location, no such bias is observed. With two scatterometers in a constellation resulting in two observations per day, one can obtain a small standard deviation in MLD, but the bias is greater in this case. For SST, except for a small bias (about 0.1°C) at the CAS location, the distribution is mostly well-behaved Gaussian in all cases. The present study suggests the advisability of acquiring more frequent wind measurements from space-borne scatterometers. A well-coordinated satellite scatterometer constellation will help in resolving the diurnal variability and associated feedback mechanism of air–sea exchange processes, enhancing the understanding of large-scale phenomena such as the Indian summer monsoon, El Niño-southern oscillations, and the Madden–Julian oscillation.

Acknowledgements

The authors would like to thank the Director, Space Applications Centre (SAC); the Deputy Director, Earth, Ocean, Atmosphere, Planetary Sciences and Applications Area, SAC; Group Director, Atmospheric and Oceanic Sciences Group, SAC; and Former Head, Oceanic Sciences Division, SAC, for constant encouragement. The authors are also grateful to R.A. Weller for providing Central Arabian Sea buoy data from the surface mooring. Hourly measurements of wind and other parameters in the Eastern Equatorial Indian Ocean were obtained from http://pmel.noaa.gov and are part of the TAO/TRITON-RAMA buoy network programme. We are grateful to Dr P.K. Thapliyal, Dr (Ms) Bipasha Paul Shukla, and Ms Smitha Ratheesh for their help in using statistical tools. Useful comments and suggestions made by anonymous reviewers helped the authors to improve the manuscript.