Mechanisms of Barrier Layer Formation and Erosion from In Situ Observations in the Bay of Bengal

Abstract

During the Bay of Bengal (BoB) Boundary Layer Experiment (BoBBLE) in the southern BoB, time series of microstructure measurements were obtained at 8 • N, 89 • E from 4-14 July, 2016. These observations captured events of barrier layer (BL) erosion and re-formation. Initially, a three-layer structure was observed: a fresh surface mixed layer (ML) of thickness 10-20 m; a BL below of 30-40 m thickness with similar temperature but higher salinity; a high salinity core layer, associated with Summer Monsoon Current. Each of 93 reason for this is the lack of direct turbulence and mixing observations, particularly in 94 the BoB. In the BoB, measurements of vertical mixing have been made in the north (Lu-95 cas et al. 2016; Mahadevan et al. 2016) and near Sri Lanka (Jinadasa et al. 2016). Here 96 we present micro-structure measurements that captured the erosion of the barrier layer 97 and its re-formation during a 10-day time series in the southern BoB during the summer 98 monsoon of 2016. The data have been used to understand the characteristics of mixing 99 5in the barrier layer, and the mechanism of barrier layer formation and erosion. Our data 100 suggest that the advection of high salinity surface waters by the SMC to the southern 101 BoB has an important role in the barrier layer erosion. 102The paper is organized as follows: The measurements and methodologies are de-103 scribed in Section 2. Observations of barrier layer formation and erosion are presented 104 in Section 3. Formation mechanisms of the barrier layer and its turbulent characteristics 105 are addressed in Section 4. Section 5 details the mechanism of barrier layer erosion. 106 A 1D model analysis is presented in Section 6. The summary and conclusions of the 107 present study are given in Section 7. 108 2. Methods and field campaign 109

Freshwater Lens Fronts Propagating as Buoyant Gravity Currents in the Equatorial Indian Ocean

“…Salinity stratification induced by local precipitation is known to significantly impact upper ocean structure and dynamics, for example, through barrier layer formation and isolation of the surface layer from the thermocline. Subsurface barrier layers of relatively higher salinity in an otherwise thermally homogeneous layer have been well documented (George et al., 2019; Lukas & Lindstrom, 1991; Sprintall, 2019) though the details of how these features are related to well‐defined FWLs has yet to be ascertained. This uncertainty is largely set by the understanding of FWL evolution.…”

“…thermocline. Subsurface barrier layers of relatively higher salinity in an otherwise thermally homogeneous layer have been well documented (George et al, 2019;Lukas & Lindstrom, 1991;Sprintall, 2019) though the details of how these features are related to well-defined FWLs has yet to be ascertained. This uncertainty is largely set by the understanding of FWL evolution.…”

Enhanced Intraseasonal Variability of the Upper Layers in the Southern Bay of Bengal During the Summer 2016

“…An extreme negative IOD event occurred in the summer and fall of 2016, which induces the westerly anomaly and has impact on the upper Ocean of the equatorial eastern Indian Ocean and BOB (Lu et al., 2018). Recent studies show observed strong SMC during June–July 2016 (Vinayachandran et al., 2018) may be related to westerly burst caused by the negative IOD (George et al., 2019; Webber et al., 2018). These studies mainly focus on the intensity and salinity transport of the SMC related to the equatorial waves.…”

Progress in understanding of Indian Ocean circulation, variability, air-sea exchange and impacts on biogeochemistry

“…Here, the advection of freshwater by the mesoscale stirring also plays an important role in determining SST over the northern BoB (Buckley et al 2020), as these waters are typically associated with relatively shallow mixed layers. In the southern Bay, measurements have suggested the competing influences of mixing and advection of salty Arabian Sea water in the erosion and reformation of the barrier layer during the southwest monsoon (George et al, 2019;. In particular, George et al (2019) show that maintenance of the barrier layer and the associated maximum depth of mixing was critically dependent on horizontal advection through its impact on stratification.…”

“…In the southern Bay, measurements have suggested the competing influences of mixing and advection of salty Arabian Sea water in the erosion and reformation of the barrier layer during the southwest monsoon (George et al, 2019;. In particular, George et al (2019) show that maintenance of the barrier layer and the associated maximum depth of mixing was critically dependent on horizontal advection through its impact on stratification. Surface freshwater input also has an impact on barrier layer evolution; several freshening events were captured at various stages of their seasonal evolution in the southern Bay of Bengal in recent observations .…”