Argo observations of barrier layer in the tropical Indian Ocean Neeraj Agarwal a , Rashmi Sharma b , Anant Parekh c , Sujit Basu b,⇑ , Abhijit Sarkar b , Vijay K. Agarwal b a Max Planck Institute for Meteorology, Hamburg, Germany b Atmospheric and Oceanic Sciences Group, Space Applications Centre, Ahmedabad, India c Indian Institute of Tropical Meteorology, Pune, India Received 24 March 2011; received in revised form 18 May 2012; accepted 22 May 2012 Available online 1 June 2012 Abstract All the available Argo float observations of temperature and salinity for the period January 2002 through December 2008 have been analyzed to evaluate the seasonal characteristics of barrier layer (BL) in the entire tropical Indian Ocean (TIO). The annual average bar- rier layer thickness (BLT) in the TIO varies from 10 to 50 m, with larger values in the warm pool regions viz., eastern TIO, Bay of Bengal (BOB) and southeastern Arabian Sea (SEAS). Standard deviation is also high (10–35 m) indicating strong seasonal variations. The annual average mixed layer depth is low (<50 m), whereas isothermal layer depth (ILD) is high (>50 m) in these regions. Annual average BLT pattern largely coincides with the average precipitation pattern. The relative importance of atmospheric forcings (wind and precip- itation) obtained from satellites and ocean dynamics (once again using satellite-derived currents) has been investigated. Also, net pro- duction of energy has been computed to find out the reason for the formation, sustenance and destruction of barrier layer. Expectedly, regions of deep barrier layer have been found to coincide with those of high net energy. A significant interannual variation in BLT has been found in the SEAS and eastern equatorial Indian Ocean (EEIO). Ó 2012 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Barrier layer; Tropical Indian Ocean; Argo observations 1. Introduction The barrier layer (BL) is defined as the intermediate layer that separates the base of the mixed layer from the top of the thermocline (Vinayachandran et al., 2002; Shenoi et al., 2004; Qu and Meyers, 2005). BL is an impor- tant regulator of air-sea interaction since it is known to sig- nificantly affect the sea surface temperature (SST) as shown by Vialard and Delecluse (1998) in a model of the tropical Pacific. Moreover, it leads to an enhanced oceanic response to imposed wind forcing, since in the presence of BL, the kinetic energy transferred from the wind to the ocean is trapped in a thinner Ekman layer (Masson et al., 2002), causing acceleration of the flow in the mixed layer. Because of the importance of BL in the air–sea interaction, many researchers have studied the mechanism of BL formation in different basins of World Ocean. An early study by Sprintall and Tomczak (1992) focused on BL formation in the tropical region of World Ocean using Levitus climatology. During Tropical Ocean-Global Atmosphere Coupled Ocean Atmosphere Response Exper- iment (TOGA-COARE), many studies were devoted to BL in the warm pool region of the Pacific Ocean (Tomczak, 1995; Ando and McPhaden, 1997; Cronin and McPhaden, 1998; Vialard and Delecluse, 1998). There have been rela- tively fewer studies related to BL in the TIO. Shetye et al. (1996) observed large and robust BL in the BOB and underlined the important role of salinity in the vertical temperature profile of that region. Using a reduced gravity model, Murtugudde and Busalacchi (1999) were the first to 0273-1177/$36.00 Ó 2012 COSPAR. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.asr.2012.05.021 ⇑ Corresponding author. E-mail addresses: neeraj.agarwal@zmaw.de (N. Agarwal), rashmi@ sac.isro.gov.in (R. Sharma), anant@tropmet.res.in (A. Parekh), skbasu@ sac.isro.gov.in (S. Basu), sarkar.abhi@gmail.com (A. Sarkar), vkagarwal6 @yahoo.com (V.K Agarwal). www.elsevier.com/locate/asr Available online at www.sciencedirect.com Advances in Space Research 50 (2012) 642–654