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Abstract: Time series measurements (2010–2017) from the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA) moorings at 15°N, 90°E and 12°N, 90°E are used to investigate the effect of the seasonal barrier layer (BL) on the mixed-layer heat budget in the Bay of Bengal (BoB). The mixed-layer temperature tendency (∂T/∂t) is primarily controlled by the net surface heat flux that remains in the mixed layer (
${Q}^{{'}}$ ) from March to October, while both${Q}^{{'}}$ and the vertical heat flux at the base of the mixed layer ($ {Q}_{h} $ ), estimated as the residual of the mixed-layer heat budget, dominate during winter (November–February). An inverse relation is observed between the BL thickness and the mixed-layer temperature ($ \mathrm{M}\mathrm{L}\mathrm{T} $ ). Based on the estimations at the moorings, it is suggested that when the BL thickness is ≥25 m, it exerts a considerable influence on ∂T/∂t through the modulation of$ {Q}_{h} $ (warming) in the BoB. The cooling associated with$ {Q}_{h} $ is strongest when the BL thickness is ≤10 m with the$ \mathrm{M}\mathrm{L}\mathrm{T} $ exceeding 29°C, while the contribution from$ {Q}_{h} $ remains nearly zero when the BL thickness varies between 10 m and 25 m. Temperature inversion is evident in the BoB during winter when the BL thickness remains ≥25 m with an average MLT<28.5°C. Furthermore,$ {Q}_{h} $ follows the seasonal cycle of the BL at these RAMA mooring locations, with r>0.72 at the 95% significance level.-
Key words:
- barrier layer /
- vertical heat flux /
- temperature inversion /
- Bay of Bengal
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Figure 1. Seasonal climatology of mixed-layer depth (contours, m) (de Boyer Montegut et al., 2004) and sea surface temperature (SST) (colored shading) (Huang et al., 2017) in the Bay of Bengal (BoB) (a); seasonal climatology of barrier-layer thickness (contours, m) and top of thermocline depth (TTD) (colored shading) in the BoB (de Boyer Montegut et al., 2004) (b).
Figure 2. Location of the selected RAMA moorings in the Bay of Bengal (a) and the number of Argo profiles at 15°N, 90°E (area marked with red box) (b), 12°N, 90°E (area marked with blue box) (c) used in the present study.
Figure 3. Comparison of estimated mixed-layer depth (MLD) (a, b) and the barrier-layer thickness (BLT) (c, d) using observations at the RAMA moorings, Argo, HYCOM, and monthly climatology at 15°N, 90°E (a, c) and 12°N, 90°E (b, d). The numbers represent the correlation (r) values between the RAMA estimations with Argo (red), HYCOM (blue), and climatology (green). The standard deviations of the RAMA observations are: ±6.5 (a), ±8.6 (b), ±16 (c), and ±7.4 (d).
Figure 4. Comparison of estimated seasonal cycle of mixed-layer temperature (MLT) (a, b) and mixed-layer salinity (MLS) (c, d) using observations at the RAMA moorings, Argo, HYCOM, and OISST at 15°N, 90°E (a, c) and 12°N, 90°E (b, d). The standard deviations of the RAMA observations are: ±0.9 (a), ±0.7 (b), ±0.2 (c), and ±0.3 (d).
Figure 5. Seasonal cycles of turbulent and radiative heat fluxes (a, b) and mixed-layer heat budget terms (c, d) at 15°N, 90°E (a, c) and 12°N, 90°E (b, d) in the Bay of Bengal. The specific terms and their color notations are given in the legend. The standard deviations of the estimated heat budget terms are:
$\partial T/ \partial t = \pm 0.02,{Q}^{{'}} = \pm 0.06,\mathrm{H}\mathrm{A}\mathrm{d}\mathrm{v} = \pm 0.006,\;\mathrm{a}\mathrm{n}\mathrm{d}\;{Q}_{h} = \pm 0.04$ (c), and$\partial T/ \partial t = \pm 0.02,{Q}^{{'}} = \pm 0.03, $ $ \mathrm{H}\mathrm{A}\mathrm{d}\mathrm{v} = \pm 0.007,\;\mathrm{a}\mathrm{n}\mathrm{d}\;{Q}_{h} = \pm 0.02$ (d).
Figure 6. Seasonal variability of barrier-layer thickness (BLT) and mixed-layer temperature (MLT) (a), mixed-layer
$ \mathrm{H}\mathrm{A}\mathrm{d}\mathrm{v} $ (b), and$ {Q}_{h} $ (c) at 15°N, 90°E (solid lines) and 12°N, 90°E (dashed lines) in the Bay of Bengal. Shaded area represents the ±1σ.
Figure 7. Seasonal variability of barrier-layer thickness (BLT) and entrainment (a, c), and the seasonal variability of BLT and temperature inversion at 15°N, 90°E (a, b) and 12°N, 90°E (c, d) in the BoB. In the upper panels, the red lines represent entrainment estimated considering
${W}_{h} = \mathrm{T}\mathrm{T}\mathrm{D}$ (solid line) and${W}_{h} = \mathrm{D}23$ (dashed line). In the upper (lower) panels, the dahsed lines (black) represent the zero lines for entrainment (temperature inversion). The standard deviation of the estimated parameters are: ±0.02 (red solid) and ±0.01 (red dashed) (a), ±0.23 (green) (b), ±0.01 (red solid) and ±0.008 (red dahsed) (c), and ±0.14 (green) (d).
Figure 8. Seasonal variability of
$ {Q}_{h} $ and entrainment,${W}_{h} = \mathrm{T}\mathrm{T}\mathrm{D}$ and${W}_{h} = \mathrm{D}23$ at RAMA stations. Figures represent the average data for 8 years (2010–2017) that have been filtered using a 15-day running mean filter.Table 1. Correlations at the 95% significance level between the estimations from RAMA data and other data sources
Parameter Data source 15°N, 90°E 12°N, 90°E r RMSE r RMSE MLD RAMA-Argo 0.86 [0.58, 0.96] 3.70 0.82 [0.48, 0.94] 8.00 RAMA-HYCOM 0.42 [0.36, 0.52] 7.20 0.90 [0.67, 0.97] 4.00 RAMA-Climatology 0.77 [0.35, 0.93] 11.75 0.80 [0.42, 0.94] 11.80 BLT RAMA-Argo 0.95 [0.85, 0.98] 5.60 0.77 [0.36, 0.93] 4.90 RAMA-HYCOM 0.85 [0.55, 0.95] 12.50 0.95 [0.82, 0.98] 5.00 RAMA-Climatology 0.96 [0.86, 0.98] 5.70 0.88 [0.63, 0.97] 7.40 MLT RAMA-Argo 0.98 [0.95, 0.99] 0.19 0.88 [0.62, 0.97] 0.70 RAMA-HYCOM 0.98 [0.93, 0.99] 0.51 0.92 [0.73, 0.98] 0.62 RAMA-OISST 0.98 [0.97, 0.99] 0.18 0.97 [0.96, 0.98] 0.20 MLS RAMA-Argo 0.81 [0.45, 0.94] 0.23 0.89 [0.65, 0.97] 0.32 RAMA-HYCOM 0.62 [0.08, 0.88] 0.30 0.91 [0.71, 0.97] 0.20 Note: The confidence intervals are noted in square brackets. MLD: mixed-layer depth; BLT: barrier-layer thickness; MLT: mixed-layer temperature; MLS: mixed-layer salinity. 
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Table 2. Correlations at the 95% significance level between the estimated barrier-layer thickness (BLT) and several selected parameters at the RAMA moorings
15°N, 90°E 12°N, 90°E BLT-MLT −0.86 [−0.88, −0.83] −0.96 [−0.97, −0.95] BLT-HAdv −0.54 [−0.61, −0.47] −0.11 [−0.21, −0.01] BLT-Qh 0.72 [0.67, 0.77] 0.78 [0.74, 0.82] BLT-(Wh=TTD) 0.98 [0.97, 0.99] 0.88 [0.85, 0.90] BLT-(Wh=D23) 0.97 [0.96, 0.98] 0.79 [0.74, 0.83] BLT-(–ΔT) 0.94 [0.93, 0.95] 0.94 [0.92, 0.96] Note: The confidence intervals are noted in square brackets. MLT: mixed-layer temperature; TTD: top of thermocline depth.
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