Subseasonal variations in salinity and barrier‐layer thickness in the eastern equatorial Indian Ocean

Abstract

The barrier layer, the layer between the bottom of the density-defined mixed layer and the isothermal layer in the upper ocean, may play a role in air-sea dynamics. In the present study, data from Argo profiling floats in the tropical Indian Ocean and a mooring at 90 E, 0 N are used to examine subseasonal variations in upper ocean salinity and barrier-layer thickness (BLT) during boreal winter. In the eastern equatorial Indian Ocean, subseasonal variations in BLT are energetic. However, composites used to isolate the Madden-Julian Oscillation (MJO) component of the subseasonal signal reveal that, on average, the MJO anomaly in BLT is negligible despite large swings in both the mixed-layer depth and the isothermal-layer depth. This discrepancy is likely due to (a) noise from other subseasonal processes; and (b) the diversity of individual MJO events: the thickness of the mixed layer and the isothermal layer are sensitive to wind and rain forcing, so even subtle differences in the phasing and strength of MJO-related atmospheric anomalies can produce a very different effect on upper ocean stratification and hence on the thickness of the barrier layer. The effect of the barrier layer on the upper ocean response to MJO forcing is also evaluated. When the barrier layer is thick, entrainment cooling during the MJO is reduced, so the MJO drives a weaker sea surface temperature anomaly. This suggests that modulation of BLT can have significant consequences for the response of the upper ocean to the MJO, and hence, potentially, for feedbacks of the ocean onto the atmosphere on MJO time scales.

Drastic thickening of the barrier layer off the western coast of Sumatra due to the Madden-Julian oscillation passage during the Pre-Years of the Maritime Continent campaign

“…On 15-16 December, the BLT thinned to 45 m because of the ML deepening associated with the second peak of the surface wind speed exceeding 8 m/s. On 16-17 December, however, the MLD shallowed again, resulting in the BL thickening up to 85 m. This value is very large compared with those found in previous statistical studies (Drushka et al 2014;Sato et al 2004Sato et al , 2006Vissa et al 2013a).…”

“…The formation mechanisms and seasonal variations of the BLs differ from region to region (Sato et al 2004(Sato et al , 2006, and they are deeply involved in the Indian Ocean Dipole (IOD; Saji et al 1999) (Masson et al 2004;Qiu et al 2012) and ENSO (Ando and McPhaden 1997;Maes et al 2002Maes et al , 2005Maes et al 2006). Previous studies have noted that the BL shows various responses to atmospheric forcing associated with westerly wind bursts and the continuous precipitation of the Madden-Julian oscillation (MJO; Madden and Julian 1971) (Cronin and McPhaden 2002;DeMott et al 2015;Drushka et al 2014).…”

Extreme Marine Warming Across Tropical Australia During Austral Summer 2015–2016

“…The reduced BLT may be due to increased salinity associated with El Niño events (Zhang et al, ). A reduced MLD is likely associated with weakened wind speeds (Figures c–f), increased solar radiation (Drushka et al, ) or increased precipitation over evaporation (Zhang et al, ). For February 2016, in the different regions, the negative MLD anomalies correlated well with linear relationships for the positive total heat flux anomalies, and positive latent heat flux anomalies caused by weakened winds (Figures b–d).…”

Bay of Bengal salinity stratification and Indian summer monsoon intraseasonal oscillation: 1. Intraseasonal variability and causes

“…Both MLD and barrier layer thickness (BLT) are important factors in determining the amplitude of intraseasonal SST variability [e.g., Sengupta and Ravichandran , ; Schiller and Godfrey , ; Drushka et al ., ; Li et al ., ]. Waliser et al .…”