Subsurface mixing and ventilation of oxygen minimum zone waters in the southern Bay of Bengal during the summer monsoon

doi:10.21203/rs.3.rs-3790094/v1

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Subsurface mixing and ventilation of oxygen minimum zone waters in the southern Bay of Bengal during the summer monsoon

https://doi.org/10.21203/rs.3.rs-3790094/v1

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During the summer monsoon, the local wind forcings around Sri Lanka causes the formation of a cold dome called the Sri Lanka Dome (SLD), which upwells subsurface waters. To the east of SLD, the summer monsoon current (SMC) flows into the Bay of Bengal (BoB), transporting high-salinity water from the Arabian Sea. We show that the SMC and the upwelled waters of the SLD are ventilated episodically during summer monsoon in the southern BoB, leading to a net exchange of low oxygen subsurface waters with saturated mixed layers. We observed presence of hypoxic boundary < 63 µmol kg^− 1 very close to the surface. Within the SLD, it shoaled between 35 to 40 m, with oxygen values reaching as low as 6 µmol kg^− 1 at the bottom of the thermocline. Negative fluxes showing the ingassing rates ranged between − 0.33 and − 9.43 µmol m^− 2 sec^− 1 within the SLD and SMC. We propose that the episodic ventilation seen during this investigation may lead to disequilibrium between mixed layer and below thereby contributing to mid-depth oxygen enrichment. This study possibly illustrates a pathway through which the oxygen minimum zone in BoB may be gaining oxygen, thereby preventing from becoming denitrifying.

Earth and environmental sciences/Biogeochemistry

Earth and environmental sciences/Climate sciences

Earth and environmental sciences/Environmental sciences

Earth and environmental sciences/Ocean sciences

The Bay of Bengal (BoB) is a semienclosed basin in the North Indian Ocean characterized by strong surface layer stratification ^1,2. The strongest stratification covaries with the onset of the summer monsoon (May to September) in the northern BoB, where heavy rainfall and river influx result in a low-salinity surface layer ^{3, 4, 5}. In contrast to the northern BoB, the southern BoB receives less rainfall; therefore, the surface salinity is greater ^{6, 7, 8}. The existence of a strong perennial OMZ (oxygen minimum zone) with moderate depth oxygen depletion has been reported ^{9, 10}. Despite this, no denitrification has been observed, which is generally characterized by the presence of secondary nitrite maxima typically associated with Winkler oxygen (O₂) close to ~ 4 µmol kg^− 1 in the Arabian Sea ^{9, 11, 12, 13, 14, 15}. Within the OMZ core, O₂ concentrations can range between < 63 and < 4 µmol kg^− 1, and such low O₂ levels are particularly harmful to marine ¹⁴.

The low oxygen content at mid-depth in the Bay of Bengal can be attributed to primary production in surface waters along with the rapid sinking of organic matter and moderate bacterial respiration rates ^{12, 16, 17, 18}. Recent observations highlighted very low levels of O₂ in the BoB and OMZ, and a further decrease in O2 may lead to an anammox process ¹⁹. These authors reported low but significant nitrogen loss in the Bay of Bengal. However, this hypothesis was recently contested by who reported episodic O₂ injection into the BoB OMZ due to the presence of eddies ²⁰. These authors showed that even though the western boundary upwelling system in the BoB is weak anticyclonic eddies that form in the east supply O₂-rich waters to the OMZ as it moves toward the western BoB ²⁰. Based on the mean lifetime of the anticyclonic eddies, ventilation rates at 100–300 m were estimated to be 0.06 ± 0.019 µmol kg^− 1 day^− 1, which is 3 to 4 times greater than the bacterial respiration rate (0.019 µmol kg^− 1 day^− 1) ²⁰. In addition, an exchange of waters with O₂ values between 5 and 10 µmol kg^− 1 with the core of the BoB OMZ is recently reported ²¹. These authors highlighted that such processes prevent the BoB OMZ from becoming denitrifying. More recently, it has been shown that Persian Gulf Water brings O₂-rich waters to the BoB and possibly plays an important role in keeping O₂ levels higher below which ecological functioning would be significantly affected ^{22, 23}. Thus, new mechanisms have been identified recently that keep BoB O₂ levels just above the denitrification threshold; these mechanisms are not well understood and are not well represented in biogeochemical models ²⁴.

During the summer monsoon, due to local wind forcings, the formation of a cold dome, the Sri Lankan dome (SLD), occurs in the southern BoB. The upward Ekman pumping induced by this cyclonic curl brings cooler water to the near-surface layers ²⁵. The upwelling accompanying the SLD influences the local environment by modulating water column properties by cooling the sea surface temperature and enhancing biological production and air-sea interactions ^{26, 27}. Further to the east of the SLD, the intrusion of the summer monsoon current (SMC) occurs during the same time, and the eastward flow in the North Indian Ocean during the summer monsoon is called the summer monsoon current ^{3, 8}. The direction of the SMC is eastward to the south of India, and the SMC turns around Sri Lanka and enters the BoB, carrying high-salinity water from the Arabian Sea along its path ^{4, 28, 29}. When lighter water from lower-salinity BoB water is encountered, the Arabian Sea water subducts beneath the latter ⁶. High-salinity (35–35.6 psu) water intruded below the mixed layer to a maximum depth of approximately 200 m ^{25, 30}. The nutrient-rich water carried by the SMC promotes an increase in surface chlorophyll throughout its path ^{26, 31}. The biogeochemical manifestations of the upwelled waters associated with the SLD and its interaction with the SMC in the vicinity of the southern BoB are poorly understood. Taken together, these findings underscore that understanding the O₂ biogeochemical cycles in the BoB is particularly important, especially during the summer monsoon, as upwelling around the Sri Lankan Dome (SLD) coincides with the flow of Arabian Sea high-salinity water mass (ASHSW) in this region during which both are characterized by low O₂ content ^{4, 5}.

The data presented here were collected during the Bay of Bengal Boundary Layer Experiment (BoBBLE) field program in the southern BoB ⁵ (Fig. 1) from 24 June to 24 July 2016, coinciding with the summer monsoon. The present study investigated the oxygen distribution associated with the upwelled waters of the SLD and SMC and discussed the impact of episodic ventilation of subsurface waters observed during the curise. We hypothesize that the episodic ventilation observed during this investigation may have led to O₂ disequilibrium/anomalies within the surface mixed layers and contributed to mid-depth oxygen enrichment. Our study illustrates a possible mechanism by which the BoB oxygen minimum zone may gain oxygen, which may counteract OMZ expansion, as predicted, and possibly maintain the oxygen levels of the southern BoB above the denitrifying threshold.

Thermohaline structure and O ₂ within SLD

At the beginning of our observations at TSW and Z1 (28 to 30 June; Fig. 2), the surface mixed − 29. The water column distribution of O₂ up to a depth of 1050 m is illustrated in Fig. 3, and the temporal variability is shown in Fig. 4. The surface water was less saline, which created an oxygenated freshwater layer on the top. It is apparent that this layer remained well oxygenated with concentrations > 190 µmol kg^− 1. Within the SLD, oxygen concentrations as low as < = 50 µmol kg^− 1 were observed at a shallow depth of 50 m and remained close to 30 m on the 30th June, after which a freshening event was recorded on the 2nd June. Below a depth of 100 m, O₂ concentrations varied between 6 and 10 µmol kg^− 1 within the SLD. This value is just above the denitrifying threshold and remains constant until a depth of 200 m. Below this depth, it showed a minor increase and ranged between 16 µmol kg^− 1 and 24 µmol kg^− 1 until a depth of 500 m. The observed nitrite values are presented in Fig. 5.

Variability in O₂ within the SMC

While TSW and Z1 were within the SLD in the western part of the study area, Z2 to Z3 were located within the core of the SMC (Fig. 1). Our hydrographic observations at Z2, Z3, and the TSE suggest the presence of high-salinity water in the Arabian Sea (Fig. 3). The oxygen concentrations within the top 100 m at Z2 were marginally greater than those observed within SLD, and a gradual increase was noted within Z3 and TSE. Within Z2, the hypoxic boundary (~ 63 µmol kg^− 1) was located much deeper (108 m) than what was observed within SLD (~ 40–45 m) and thereafter moved close to 158 m (Fig. 3) within Z3; after this, there was strong depletion in O₂ concentrations, attaining a minimum of 13 µmol kg^− 1 at a depth of 220 m (Fig. 3). However, at the TSE, upward propagation was noted, and the hypoxic boundary shoaled between 65 and 88 m (Fig. 3).

BL erosion and ventilation of O ₂ at the Time Series Station (TSE)

The formation of BL and associated erosion were pronounced at this location (TSE; Fig. 2). The temporal variability of the thermohaline structure shows two freshening events (4 to 5th July and 10 to 15 July 2016) (Fig. 2 and Supplementary Fig. S2). During these events, the MLD was confined to the base of the low-salinity BoB surface waters, and the formation of the BL took place. Briefly, a three-layer structure was observed initially at TSE. At first, there was the presence of a fresh surface mixed layer with a thickness of 10 to 20 m, below which there was the evolution of the BL which had the same temperature but higher salinity, after this there was intrusion of high salinity core layer associated with the SMC. The barrier layer formed rapidly after the first frenshing event. The sea surface salinity on 4 July decreased by 0.3 in 2 h, decreasing from 34.3 psu at 0530 UTC to 33.9 psu by 0730 UTC. The mixed layer depth (MLD) decreased from 70 to 18 m, leading to the formation of a barrier layer that was approximately 50 m thick and had a temperature of 29°C. The upper layer warmed by approximately 0.3°C relative to the barrier layer below, and a diurnal cycle occurred. The second barrier layer formed gradually. Consequently, this new barrier layer formed at a thickness of approximately 40 m .

Oxygen anomalies and fluxes in the surface layer and during deep mixing events in the southern Bay of Bengal

The average O₂ anomalies and fluxes associated with the mixed layer are presented in Table 1. The anomalies were strongest within the SLD, where the presence of hypoxic waters was observed at much shallower depths. The O₂ anomalies ranged between − 3.2 and − 76.3 µmol kg^− 1 during this investigation. The derived mixed layer ventilation times for complete equilibration of upwelled waters were on the order of 3.7 days within the SLD and 7.2 within the SMC based on the observed gas transfer velocity and MLD. In general, negative fluxes representing the ingassing rates ranged between − 0.33 and − 9.43 µmol m^− 2 sec^− 1 within the SLD and SMC. The highest ingassing rate of 9.43 µmol m^− 2 sec^− 1 was associated with station Z1 (SLD), which had BoB OMZ waters very close to the surface.

Table 1

Estimates show O₂ anomalies Δ [O₂] = ([O₂]_m - [O₂]_sat]) and expected fluxes of O₂ under shallow mixed layer condition and deep mixing events observed at Z1 and TSE. Erosion of barrier layer at TSE resulted in mixing of low O₂ SMC waters during the BoBBLE campaign. Note strong under saturation was observed below surface layer at all stations. Positive [Δ O₂] indicates that the ocean is supersaturated and will tend to outgas to the atmosphere and negative means under saturation and ocean will gain O₂.
Stn ID	Lat [°N]	Long [°E]	MLD (m)	Average Δ [O₂] (within MLD in µmol kg^− 1 )	Outgassing/ Ingassing rates within MLD in (µmol m^− 2 sec^− 1)
TSW	7.999	85.3013	30	− 6.7	− 0.69
Z1	7.999	86.0183	22	− 76.3	− 9.43
Z2	8.013	87.0071	18	− 26.9	− 2.79
Z3	8.007	88.0018	24	− 3.2	− 0.33
TSE (During BL erosion)	8.007	89.0006	60	− 6.9	− 0.71
TSE (Without BL)	8.007	89.0006	18	+ 10.9	+ 1.13

The atmospheric conditions observed during the BoBBLE campaign are described elsewhere⁵. Briefly, there was influence of intraseasonal variability in June 2016. The sampling locations in the southern BoB were under the influence of a convectively active phase of the boreal summer intraseasonal oscillation (BSISO), which propagated northward and was replaced by a convectively suppressed phase of the BSISO during July 2016 ⁵. Toward the end of the campaign, conditions returned to the convectively active phase, with the incursion of the next cycle of the BSISO. Hence, the main BoBBLE deployment sampled the transition between the end of one active BSISO event, the subsequent suppressed phase and the initiation of the active phase in the following BSISO event. Therefore, our observations captured in detail the warming of the ocean mixed layer and preconditioning of the atmosphere to convection⁵ .

During BoBBLE upwelled waters from the SLD and SMC were sampled to determine the influence of upwelling waters on the oxygen distribution in the southern BoB. While stations TSW and Z1 were within the SLD in the western part, Z2 to Z3 were located within the core of the SMC and TSE on the outer edge of the SMC. We show that the presence of SMCs alongside the upwelling waters of the SLD was episodically ventilated during the summer monsoon in the southern BoB. The water masses within TSW and Z1 had signatures of upwelling, with negative sea level anomalies observed within the SLD (Supplementary Fig. 1). At TSW and Z1, the hypoxic boundary (< 63 µmol kg^− 1) shoaled just below the mixed layer (~ 35–40 m). The impingement of the oxygenated mixed layer during deepening within the SLD is shown in Fig. 6. An increase of ~ 38 µmol kg^− 1 was observed at 50 m after the first mixing event, with the hypoxic boundary shifting from 28 m to 50 m, thereby increasing the concentration of O₂ at the top of the thermocline. Based on the calculations the relative increase in oxygen concentration was found to be ~ 0.34 µmol kg^− 1 for one mixing event (assuming a mixing depth 50 m) within the SLD. This result is comparable to the eddy-induced oxygen enrichment observed within the northern BoB ^{32, 20}. Within the SLD no secondary nitrite maxima were observed in our dataset, suggesting the absence of denitrification (Fig. 5). This result is consistent with the O₂ concentrations observed within the OMZ (Fig. 3).

In general, the water columns at TSW and Z1 were less oxygenated than those at stations Z2, Z3 and TSE (Fig. 3),, which were under the influence of SMC. During the investigation period, the SMC was fully developed with near-surface speeds of 0.5^–1 ms^{− 1 29} (Fig. 1). The most common feature here was the presence of a high-salinity core (Fig. 2 & Supplementary Fig. 2). The subsurface high-salinity core is the expression of ASHSW transported by the subsurface branch of the SMC with a salinity greater than 35 psu. Our observations from the southern BoB show strong shoaling of hypoxic waters very close to the surface. For example, it was close to 40 m within the SLD and varied between 65 and 80 m within the SMC. This presumably interchanged with the surface mixed layer during deep mixing events (Fig. 6). As the spread of high-salinity water along the SMC weakens vertical stratification, which in turn causes BL erosion and subsequent ventilation ³⁰; therefore, the exchange of low-O₂ subsurface waters with O₂₋rich surface waters is expected. Moreover, we did not observe any secondary nitrite maxima at any of the stations investigated, which possibly supports the hypothesis that periodic oxygen enrichment in subsurface waters keeps the bay above the denitrifying threshold.

The mixed layer deepening and erosion of the barrier has been described in detail in George et al. (2019) ³⁰. After the first freshening event, the mixed layer deepened on 6 July in response to a salinization event that occurred between 6 and 9 July (Supplementary Fig. 2). During this event, the surface salinity increased from 33.84 to 34.35 psu. This led to mixed layer deepening and erosion of the barrier layer (Fig. 2 and Supplementary Figs. 2 and 3). In contrast, when a barrier layer (BL) was present, the surface mixed layer was shallow and less saline. These processes occurred episodically during our investigation, where O₂-depleted subsurface waters mixed with the surface layer, initiating a possible mixing process. The air-sea exchange anomalies are presented in Table 1. In general, the thickness of the BL layer ranged between 40 and 50 m and was formed due to the advection of low-salinity (33.35–33.8 psu) water at the TSE. Due to this BL, the surface mixed layer remained confined to the upper 10 to 20 m, whereas the isothermal layer penetrated until 60 m (see Fig. 2b). The salinity budget presented in George et al. (2019) ³⁰ showed that during BL erosion, advection brought high-salinity surface waters (~ 34.5 psu) with weaker stratification to the time series location and replaced the three-layer structure with a deep mixed layer (~ 60–70 m). The resulting weakened stratification at the time series location led to the mixing of low O₂ subsurface waters. The changes in O₂ concentrations with depth after BL erosion are shown in Fig. 6. Once the barrier layer was eroded, the O₂ concentration at 60 m increased to ~ 135 µmol kg^− 1 from earlier observations of ~ 63 µmol kg^− 1 (Fig. 3), suggesting O₂ intrusion from the upper layers. The hypoxic boundary shifted from 60 m to 125 m after BL erosion. Further we also calculated how much O₂ can be supplied to the low-oxygen waters below due to BL erosion, as observed in the TSE. Our calculations suggest that the relative increase in oxygen content due to BL erosion considering a 4-day mixing event within an SMC for a 1° × 1° grid can reach ~ 100 µmol kg^− 1. While the concentration of O₂ in surface waters is an end product of complex interactions of O₂ produced via primary production and lost via respiration, ventilation through atmospheric exchange, and advection by currents, a decrease in O₂ in subsurface waters is generally due to poor ventilation and associated heterotrophic processes ^{33, 34}. The deepening of the mixed layer between 2 and 4 July is consistent with our hypothesis, which suggests mixing of the low-O₂ subsurface waters with the oxygenated surface mixed layer.

Large-scale analysis of oxygen data from Arabian Sea OMZ waters revealed that isopycnal resupply of O₂ was the dominant process only between 300 and 500 m, while at 200 to 250 m, O2 was the most likely diapycnal process due to eddies ³⁵. Therefore, the exchange between the SLD and SMC at greater depths cannot be ignored; however, we focused here on mixed layer deepening, which was restricted to ~ 70 m, and calculated the relative increase in oxygen concentration that can occur if the parcel of water is redistributed over a given area as detailed in method section. Our observations at TSE capture an important mechanism wherein low O₂ ASHSW is observed to be occasionally ventilated. The depth profile of salinity vs. O₂ also suggested the occasional presence of ASHSW waters at depths of 40 to 50 m, with O₂ concentrations between 80 and 100 µmol kg^− 1 (Fig. 7). A potential temperature and salinity diagram also suggested the presence of Persian Gulf Water (PGW) and Red Sea Water (RSW), which were recently detected in the Bay of Bengal ^{36, 37}. The detailed physical processes involved and their implications for subsurface oxygen distribution are further discussed in Sheehan et al., 2020 ³⁷.

Several authors have reported that SLDs form during summer monsoons, and SMC flow occurs until September in the southern BoB ^{4, 38}. BoBBLE observations were carried out between 24th June and 23rd July 2016. Although we captured two freshing events and two BL erosion events within the SMC between 24th June and 25th July, more such events are expected to occur until the SMC remains active, coinciding with the active summer monsoon phase. Furthermore, the relative increase in the oxygen content because of mixing events needs detailed investigation. We believe that episodic exposure to low-O₂ waters may result in quick equilibration with the saturated surface mixed layer and represent a potential pathway of O₂ enrichment in this region. As the expansion of O₂ minimum zones is already predicted by climate models ^{39, 40}, understanding the complex interplay that maintains this delicate balance between O₂ supply and its subsequent demand in the BoB becomes important, as this may be crucial for deciding on future deoxygenation scenarios in this region.

Physical setting during BoBBLE field expedition

The sampling stations were along 8◦ N, extending from 85.3◦ E (hereafter referred to as TSW) to 89◦ E (hereafter referred to as TSE), with three additional stations in between, referred to as Z1, Z2, and Z3. The transects from TSW to Z3 were sampled between 24 July to 3 July, and the TSE was sampled on the 4th of July. Thereafter, time series observations were carried out at the TSE (8° N and 89° E) between the 4th and 15th of July 2016. The TSW station was located within the SLD (Fig. 1). Sea level anomalies (SLAs) from the region highlight the evolution of the SLD and are represented by negative SLAs (Supplementary Fig. 1). Z1 is on the outer edge of the SMC to the west, and station TSE is east of the SMC. Stations Z2 and Z3 were sampled within the core of the SMC (Fig. 1). The transect runs across the productive regions of the SLD and SMC and is further detailed in ³¹ Thushara and et al., 2019.

Biogeochemical analysis

A factory-calibrated SeaBird Electronics (SBE) 9/11 + Conductivity-Temperature-Depth profiler (CTD) was used to measure the vertical profiles and collected water samples at all the points marked in Fig. 1. Nominally, the casts were collected to a depth of 1000 m. A total of 138 CTD casts were taken, including from a time series station (TSE) ^{5, 30}. In addition, hydrographic samples were collected at discrete depths to measure various biogeochemical parameters and to compute air-sea O₂ fluxes⁴¹. For this purpose, the CTD was attached to a rosette frame with 12 Niskin sampler bottles (10 L each) and sampled at fixed depths. Water samples for chemical measurements were collected from the Niskin bottles using Tygon tubing connected to the spout. Analysis of dissolved O₂ (DO) at a precision of ± 0.03 µM was based on the Carpenter (1965) modification of the traditional Winkler titration. These O₂ data sets were used to calibrate the CTD O₂ with the Winkler DO and CTD O₂ data, which yielded a coefficient of determination of 0.99 (n = 306, p < 0.01). The mixed layer depth (MLD) is calculated as the depth at which the density is equal to the sea surface density plus an increase in density equivalent to 0.8°C ³⁰. The isothermal layer is defined as the depth where the temperature is 0.8°C less than the SST, and the barrier layer is the layer between the base of the isothermal layer and the base of the mixed layer (BL). Apparent oxygen utilization (AOU) was calculated using the calibrated CTD data and the solubility equations of ^{42, 43} Garcia and Gordon (1992) and coefficients of Benson and Krause (1984).

Calculation of the relative increase in dissolved O₂ that can occur within the BoB OMZ and ASHSW due to mixing

An attempt is made to understand the magnitude of O_2, which can be supplied to the BoB OMZ due to observed deep mixing events within the SLD and BL erosion within the SMC. The ventilation rate of the mixed layer/equibriation time can be calculated by dividing the mixed layer thickness by the gas transfer velocity, as further detailed in ⁴¹ Roy et al., 2021. For this purpose, we considered the water mass within the SLD as a parcel of water with a width (W) of 100 km that contains the BoB surface waters and OMZ waters below. Therefore, this mass of water can be written as

M_{parcel (1)} = W× H × U × Δt × ρ, (1)

where W (denotes the approximate width of the parcel within the SLD assumed here to be 100 km), H is the vertical extent of the deep mixing (in this case, 50 m as observed), U is the lateral speed of the parcel taken as ~ 0.5 m s^{− 1 15} within the SLD, Δt is the approximate time line of mixing, taken here as 1 day, and ρ is the density of the parcel of water (taken as 1022 kg m^− 3). This gives M_{parcel (1)} as = 2.20 × 10¹⁴ kg. The additional oxygen added from the surface mixed layer to the OMZ due to the mixing event can be written as the product of the excess oxygen observed (38 µmol kg^− 1) multiplied by M_{parcel (1),} which equates to 8.36 × 10¹⁵ µmol. However, the actual increase will depend on how much this parcel of water is redistributed. For a grid area between 7°N and 8°N and between 85°E and 86°E and a mixing depth of 50 m with a ρ of 1022 kg m^− 3, Mparcel ₍₂₎ equals 6.29 × 10¹⁴ kg. Our calculations suggest that the relative increase in the oxygen concentration will reach ~ 0.34 µmol for one mixing event until 50 m within the SLD.

Similarly, we calculated how much O₂ can be supplied to the low-oxygen waters below due to BL erosion within the SMC. For this purpose, we used an excess O₂ of 72 µmol kg^− 1, W (as 100 km), H (as 70 m), U (as 0.5 ms^− 1), Δt (as 4 days), and ρ (1022 kg m^− 3). This gives M_{parcel (3)} as = 1.23 × 1015 kg. However, the additional oxygen added due to BL erosion to OMZ waters below can be written as the product of the excess oxygen observed (72 µmol kg^− 1) multiplied by M_{parcel (1),} which equates to 88.56 × 10¹⁵ µmol. As above, the increase in oxygen concentrations will depend on the volume over which it is redistributed; if redistributed over a 7°N to 8°N and 87°E to 88°E and mixing depth of 70 m with a ρ of 1022 kg m^− 3, Mparcel₍₄₎ equals 8.81 × 10¹⁴ kg. Therefore, the actual increase in the oxygen content due to BL erosion considering a 4-day event within an SMC within a 1° × 1° grid was found to be ~ 100 µmol.

Fluxes of O ₂ due to upwelling/deep mixing events and erosion of the barrier layer (BL) during the BoBBLE campaign

The net air-sea flux of O₂ is directly proportional to the partial pressure difference across the air-sea interface or its corresponding concentration anomaly. Therefore, Δ O₂ = [O₂]m – [O₂]s, where [O₂]m represents the measured concentration of O₂ in µmol kg^− 1 and [O₂]s represents the saturated concentration of O₂; these values can be deduced as a product of the solubility coefficient (S_A) and partial pressure of O₂ in the atmosphere (p^A), where the units of the solubility coefficient are in mmol m^− 3 atm^− 1.

The solubility coefficients are calculated based on the in situ temperature and salinity at a given depth. Therefore, areal fluxes (µmol m^− 2 sec^− 1) across the water–atmosphere boundary can be calculated according to the expression:

F = Kw Δ O₂ (1)

where ‘Kw’ is the gas transfer velocity (cm hr^− 1) and Δ O₂ is the difference between the measured O₂ and its solubility. The gas transfer velocity ‘kw’ (cm hr^− 1) was calculated according to the equation given by Wanninkhof (1992), which depends on the wind speed measured at a height of 10 m (u₁₀₎ and Schmidt number (Sc) and is shown in Eq. 2 below.

Kw = 0.31.U². (Sc/600)^−0.5 (2)

Sc is the kinematic viscosity of water divided by the diffusion coefficient of O₂. The coefficients for calculating the Schmidt number (below Eq. 3), which is a function of temperature and salinity (35 psu) for O_2, were taken from ⁴⁴ Sarmiento and Gruber (2006), and the equation for gas transfer velocity from ⁴⁵ Wanninkhof (1992) was used to estimate the fluxes during barrier layer erosion (deep mixing events).

Sc = A - BT- CT² - DT³ (3)

where the in situ temperature (T) is in °C. The surface wind speed during the first half of the BoBBLE observation period was on the order of 8–12 ms^− 1. These wind speeds are typical for the southern BoB during the summer monsoon ⁵. A positive value of flux in (1) means outgassing O₂, whereas a negative flux denotes ingassing. Based on the wind speed, the estimated gas transfer velocity (K_W) ranged between 37.42 and 44.53 cm h^− 1 for oxygen.

Author Contribution

RR, AS, PNV, JG, CLP: Conceptualization, Methodology, Formal analysis and investigation, original draft preparation, RR & PNV: Conceptualization, review and editing, Funding acquisition and supervision, PNV: Review and editing.

Data availability

The datasets used during the current study is available from the corresponding author on reasonable request.

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Subsurface mixing and ventilation of oxygen minimum zone waters in the southern Bay of Bengal during the summer monsoon

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