Seasonal and interannual variabilities of the barrier layer thickness in the tropical Indian Ocean

Abstract

Abstract. The seasonal and interannual variations of the barrier layer thickness (BLT) in the tropical Indian Ocean (TIO) is investigated in this study using the Simple Ocean Data Assimilation version 3 (SODA v3) ocean reanalysis dataset. Analysis of this study suggests energetic but divergent seasonal variabilities of BLT in the western TIO (5∘ N–12∘ S, 55–75∘ E) and the eastern TIO (5∘ N–12∘ S, 85–100∘ E). For instance, the thicker barrier layer (BL) is observed in the western TIO during boreal winter as a result of decreasing sea surface salinity (SSS) and deeper thermocline, which are associated with the intrusion of freshwater flux and the weakened upwelling, respectively. On the contrary, the variation of BLT in the eastern TIO mainly corresponds to the variation in thermocline depth in all seasons. The interannual variability of BLT with the Indian Ocean Dipole (IOD) and El Niño–Southern Oscillation (ENSO) is explored. During the mature phase of positive IOD events, a thinner BL in the eastern TIO is attributed to the shallower thermocline, while a thicker BL appears in the western TIO due to deeper thermocline and fresher surface water. During negative IOD events, the thicker BL only occurs in the eastern TIO, corresponding to the deeper thermocline. During ENSO events, prominent BLT patterns are observed in the western TIO corresponding to two different physical processes during the developing and decaying phase of El Niño events. During the developing phase of El Niño events, the thicker BL in the western TIO is associated with deepening thermocline induced by the westward Rossby wave. During the decaying phase of El Niño events, the thermocline is weakly deepening, while the BLT reaches its maxima induced by the decreasing SSS.

Numerical Investigation of Tropical Indian Ocean Barrier Layer Variability

“…The far western region of the tropical Indian Ocean shows positive BLT anomalies, a feature in the EOF-1 of BLT in Figure 4. It appears that the salinity advection and basin-scale dynamics cause such positive polarity in the far western part of tropical Indian Ocean BLT during a positive IOD, consistent with conclusions arrived by Yuan et al (2020). The PC-1 of both CTRL and SENS-3 are correlated at 0.74, significant above 95% confidence.…”

“…The MLD heat-budget analysis based on moored buoy observations from the eastern equatorial Indian Ocean during two recent IOD years (2018-19 and 2019-20) shows that the thinning of BLT exacerbates SST cooling by giving way to the entrainment of momentum and heat fluxes in extreme IOD years (Aparna & Girishkumar, 2022). Prominent BLT variability is observed in the western tropical Indian Ocean during the developing and decaying phase of El Niño events corresponding to two different physical processes (Yuan et al, 2020). The IOD is responsible for most variations of MLD in the 10°N-10°S band, with positive phases associated with a shallow MLD in the equatorial and southeastern Indian Ocean and a deepening in the south-central Indian Ocean (Keerthi et al, 2013).…”

“…A fair amount of understanding about the interannual variability of BLT of the eastern tropical Indian Ocean exists from the past literature and can be summarized for a positive IOD year as follows. (a) IOD-related upwelling Kelvin wave and surface Ekman drift in response to the southeasterly anomalies uplift the thermocline in the eastern tropical Indian Ocean (Yuan et al, 2020). (b) The isothermal layer shoals relatively more than MLD due to the thermocline uplift and shoals the BLT (Aparna & Girishkumar, 2022).…”

“…Tropical Indian Ocean barrier layer thickness (BLT) has been synthesized using ship-based observations and autonomous profiles in past studies (Qiu et al, 2012;Thadathil et al, 2007Thadathil et al, , 2008Thadathil et al, , 2016Yuan et al, 2020). The annual average BLT in the tropical Indian Ocean varies from 10 to 50 m, with larger values in the warm pool regions, viz., eastern tropical Indian Ocean, the Bay of Bengal and the southeastern Arabian Sea, with strong seasonal variations of 10-35 m (Agarwal et al, 2012).…”

“…The interannual variability of the tropical Indian Ocean BLT is closely related to the Indian Ocean Dipole (IOD), with the IOD leading the BLT variability by 1 month (Qiu et al, 2012;Yuan et al, 2020). During the positive IOD season, equatorial easterly-induced upwelling Kelvin waves raise the isothermal layer off Sumatra; a salinity-stratified mixed layer shoals due to a reduced eastward salty water transport by a weaker Wyrtki Jet, despite an offset by a reduced freshwater flux (Qiu et al, 2012).…”