Mixed Layer Depth and its Variability in the Eastern Equatorial Indian Ocean as Revealed by Observations and Model Simulations

Abstract

Mixed layer depth (MLD) variability in the Eastern Equatorial Indian Ocean (EEIO) from a hindcast run of an Ocean General Circulation Model (OGCM) forced by daily winds and radiative fluxes from NCEP-NCAR reanalysis from 2004 to 2006 is investigated. Model MLD compares well with the ∼20,000 observations from Argo floats and a TRITON buoy (1.5°S and 90°E) in the Indian Ocean. Tests with a one-dimensional upper ocean model were conducted to assess the impact on the MLD simulations that would result from the lack of the diurnal cycle in the forcing applied to the OGCM. The error was of the order of ∼12 m. MLD at the TRITON buoy location shows a bimodal pattern with deep MLD during May–June and December–January. MLD pattern during fall 2006 was significantly different from the climatology and was rather shallow during December–January both in the model and observation. An examination of mixed layer heat and salt budget suggested salinity freshening caused by the advective and vertical diffusive mixing to be the cause of shallow MLD.

Keywords:

• ARGO
• Equatorial Indian Ocean
• mixed layer depth
• OGCM
• TRITON

Acknowledgements

The authors thank the director of the Space Applications Centre (SAC); the deputy director of the Remote Sensing Application Area, SAC; group director, Meteorology and Oceanography. Group (MOG); and group head, MOG, SAC, for encouragement. We are extremely grateful to the two reviewers whose valuable comments were helpful in enhancing the quality of the present work. MOM-3 code has been acquired from Geophysical Fluid Dynamics Laboratory (NOAA). Argo data were downloaded from ftp://ifremer.fr. Daily surface and subsurface data from TRITON buoy have been obtained from http://www.jamstec.go.jp/jamstec/TRITON.