The Dynamics of the Southwest Monsoon Current in 2016 from High-Resolution In Situ Observations and Models

The strong stratification of the Bay of Bengal (BoB) causes rapid variations in sea surface temperature (SST) that influence the development of monsoon rainfall systems. This stratification is driven by the salinity difference between the fresh surface waters of the northern bay and the supply of warm, salty water by the Southwest Monsoon Current (SMC). Despite the influence of the SMC on monsoon dynamics, observations of this current during the monsoon are sparse. Using data from high-resolution in situ measurements along an east–west section at 8°N in the southern BoB, we calculate that the northward transport during July 2016 was between 16.7 and 24.5 Sv (1 Sv ≡ 106 m3 s−1), although up to ⅔ of this transport is associated with persistent recirculating eddies, including the Sri Lanka Dome. Comparison with climatology suggests the SMC in early July was close to the average annual maximum strength. The NEMO 1/12° ocean model with data assimilation is found to faithfully represent the variability of the SMC and associated water masses. We show how the variability in SMC strength and position is driven by the complex interplay between local forcing (wind stress curl over the Sri Lanka Dome) and remote forcing (Kelvin and Rossby wave propagation). Thus, various modes of climatic variability will influence SMC strength and location on time scales from weeks to years. Idealized one-dimensional ocean model experiments show that subsurface water masses advected by the SMC significantly alter the evolution of SST and salinity, potentially impacting Indian monsoon rainfall.


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The monsoon depressions that originate over the Bay of Bengal (BoB) provide the majority of 45 the monsoon rain that falls over northern and eastern India (e.g., Gadgil 2003). The active-break  The vertical salinity structure is characterized by a fresh surface mixed layer, generally less than 222 34 g kg −1 , beneath which salinity increases sharply, with the 35 g kg −1 contour between 50 and 223 80 m (Fig. 2a). Throughout the BoB there is a broad subsurface salinity maximum between around   From the glider observations we calculate the time-mean total northward geostrophic transport 274 (i.e., ignoring the southward flow) between 85.3 • E and 88 • E to be 21.0 Sv between 5-15 July, 275 giving daily average values between 16.7 and 24.5 Sv during this period. There are two sources 276 of uncertainty in this estimate: sampling uncertainty due to the limited spatial coverage of the 277 observations, and measurement uncertainty due to errors in the temperature, salinity and dive-278 averaged current observations. We estimate the sampling uncertainty by subsampling the NEMO 279 model velocity at the glider locations and comparing the resultant transport with that calculated 280 from the model velocity at standard resolution. This comparison suggests that the glider sampling 281 underestimates the total transport by up to 5 Sv. However, this is partly compensated by the 282 overestimation of total transport by the geostrophic approximation, since the cyclonic curvature 283 of the SMC around the SLD means that the true velocity is less than the geostrophic velocity. As 284 a result, the mean bias of the geostrophic, subsampled transport relative to the total transport is 285 −0.6 Sv, with a root mean square error of 2.8 Sv. 286 We estimate the measurement uncertainty associated with temperature (O(0.001 • C)) and salin-  The current width is around 300 km at the surface, consistent with the 3 • width stated by Schott estimating their maximum transport of 8-16 Sv. It is clear that some of the northward transport is 296 associated with recirculating eddies, including the SLD to the west and the persistent anticyclonic 297 eddy centred on 88 • E (Fig. 1). We investigate the temporal variability of these features by exam-298 ining daily-mean velocity and salinity at 110 m from the NEMO model (Fig. 4). The SLD is at the 299 centre of a large cyclonic circulation extending over 82-86 • E, 5-15 • N, encompassing the SMC 300 and the EICC. Meanwhile, the anticyclonic recirculation to the east is centred on 88 • E, and ex-301 tends from 4 to 10 • N on July 1 (Fig. 4a). This feature subsequently splits into two quasi-stationary 302 eddies, and is clearly linked to the presence of the subsurface salinity maximum, since the core of 303 these eddies are associated with salinity maxima (Fig. 4d).

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If we defined the SMC as only the portion of the current that is continuous from the Arabian Sea 305 into the BoB, then the transport would be substantially less than the total northward transport esti-  Therefore, the total SMC transport from the Arabian Sea into the BoB may be between 1 3 and 1 2 of 314 the observed northward transport, or 7-10.5 Sv.

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To evaluate the influence of high frequency variability on our transport estimate, we calculate 316 the zonal mean velocity between 84 and 88 • E at the surface from AVISO altimetry data, and at 317 various levels from the NEMO data (Fig. 5). This analysis shows that the observational period was   the discrepancies between the glider and altimetry data may be due to the spatial and temporal 348 smoothing involved in the optimal interpolation of the AVISO altimetry data, for which the decor-

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Overall we conclude that the NEMO model faithfully represents the variability of the SMC. series; therefore it is likely that the large-scale SSH anomalies associated with the negative Indian SSH. The density from the NEMO ocean model (Fig. 14a,b) mirrors the SSH signal (Fig. 13a,b), 461 with high density associated with the upwelling in the SLD, and low density associated with the 462 downwelling propagating Rossby waves. This signal is also apparent in the conservative tempera-463 ture at 100 m depth (Fig. 14e,f), which shows a gradient of more than 10 • C across the BoB at 8  6. Impact of subsurface salinity advected by the SMC on SST 478 We have shown above that the SMC advects warm and saline ASHSW into the subsurface BoB, likely that the advection of this water mass also has a direct impact on SST by altering the vertical 481 structure of temperature and density, but this influence has not been previously quantified. To eval-482 uate the impact of subsurface temperature and salinity differences between the ASHSW advected 483 by the SMC and the colder, fresher water further west (Fig. 14), we conduct a set of idealised KPP 484 experiments with identical surface forcing but varying initial conditions below the surface mixed 485 layer. These experiments quantify the impact of the advected ASHSW in an idealised framework 486 without the influence of other processes such as horizontal advection or feedbacks on the surface 487 fluxes that would complicate the picture in a more complex model.

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The control initial conditions represent the mean vertical profiles of temperature and salinity 489 from SG579 for July 8-15 when SG579 was at 85.3 • E and the high salinity core was absent. increased temperature causes the perturbation density to be lower than the control below 40 m, 496 despite the generally higher salinity in the perturbation initial conditions (Fig. 15e).

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The surface forcing (Fig. 15a,b) for both KPP simulations is identical and represents June- In both the control and perturbation experiments the surface temperature (Fig. 15c) follows the 511 net heat flux as expected. Cooling is generally present throughout June, associated with the largely 512 negative net heat flux at this time in the convectively active phase of the BSISO (Fig. 15a). The 513 diurnal cycle is suppressed during this cooling phase until July when it becomes stronger due to 514 increased shortwave and net heat fluxes in the convectively active phase of the BSISO. As a result, 515 the surface warms to almost its initial temperature by the end of the simulation. Meanwhile, 516 the salinity (Fig. 15d) increases slightly during the simulation, but is punctuated by sharp drops 517 due to intermittent high precipitation. The overall increase in salinity is due to a combination 518 of evaporation and vertical mixing from persistently strong winds (Fig. 15b). The amplitude of 519 the variability over the active-break cycle associated with the BSISO is consistent with previous 520 observational estimates (Vecchi and Harrison 2002).

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The difference between control and perturbation experiments is shown in Fig. 15f. By the end 522 of the simulation, the perturbation surface temperature is around 0.08 • C warmer than the control, 523 and the salinity is 0.06 g kg −1 higher. The magnitude of the temperature difference between the 524 control and perturbation experiments is around 10% of the modelled variability over the lifetime 525 of the BSISO cycle, therefore, this represents a significant modulation of SST that will also affect 526 lateral SST gradients and thus atmospheric moisture convergence and convection. Most of the tem-527 perature difference accumulates between 10 June and 10 July 2016, associated with strong winds, 528 mixed layer deepening and entrainment. Since the perturbation initial conditions have warmer and 529 saltier water below the mixed layer, this accounts for the reduced cooling and increased salinity.

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The salinity changes during the simulation are around 20% of the variability over the entire run, 531 representing an important difference to the evolution of mixed-layer salinity and stratification. As 532 this water is advected into and around the BoB, this difference in mixed layer salinity may further 533 influence the stratification and air-sea interaction on longer time scales than accounted for here. In 534 summary, we find that the advection of subsurface ASHSW by the SMC has the potential to alter 535 SST and thus the development of monsoon rainfall systems over the BoB. in the NEMO model, and it is likely that the exact pathway of the SMC strongly influences the 574 strength of this feature. As the ASHSW is also relatively warm, the density of this water mass 575 is less than the density of the fresher but cooler water further west (Fig. 9), generating a density 576 gradient that strengthens the subsurface SMC to the west of the ASHSW core and generates the 577 anticylonic eddy with southward flow to the east. Therefore, there is a feedback from the advected 578 propagation of the SMC and the advected ASHSW, as observed (Fig. 14). 580 We use idealised 1-dimensional modelling experiments to test the hypothesis that the advection 581 of the subsurface warm and salty ASHSW will exert a significant influence on the evolution of 582 SST. Idealised KPP experiments confirm that initial conditions with subsurface ASHSW led to an 583 increase in SST of 0.08 • C relative to initial conditions from the SLD. Although some of this dif-584 ference is due to the relatively cold subsurface waters in the SLD, the ASHSW is typically warmer