Ocean Mixed Layer Response to Two Post‐Monsoon Cyclones in the Bay of Bengal in 2018

Abstract

The Bay of Bengal (BoB) is characterized by a shallow (∼10–20 m deep) fresh layer associated with 40–60 m deep warm near‐surface layer during the post‐monsoon season (October–November). We use hourly observations from a moored buoy at 15°N, 90°E along with satellite and ocean analysis data sets to understand the evolution of the near‐surface layer during the passage of two category‐3 cyclonic storms: Cyclone Titli (7–11 October 2018) and cyclone Gaja (10–15 November 2018). The mooring was ∼200 km away to the right of the two cyclone tracks. A 15 day (22 September–7 October) break in the Indian summer monsoon resulted in clear skies, calm winds, and sea surface temperature warming (SST) by 2.8°C before Titli. During Titli, near‐surface thermal stratification restricted vertical mixing to 20 m depth and limited SST cooling at the mooring. During passage of cyclone Gaja, fresh Irrawaddy River water associated with a subsurface warm layer was advected to the mooring by mesoscale eddy flow. Cyclone winds could not break through the near‐surface density stratification associated with this river water. Shear associated with near‐inertial currents led to delayed mixed layer deepening after the cyclone's passage but there was no cold wake along the track of Gaja due to mixing of subsurface warm water to the surface. We illustrate the relevant processes at work for the two cyclones based on mixed layer salt and heat balance at 15°N, 90°E. This study demonstrates the importance of ocean preconditioning and thermohaline stratification on SST evolution under the influence of post‐monsoon cyclones in BoB.

Tropical Cyclone-Induced Sea Surface Temperature Responses in the Northern Indian Ocean

“…TC-induced warming accounted for approximately 14% of the total locations, which aligns with findings reported by other researchers [44,60,61]. The study further identified that warming events typically occurred in the latitude range of 8 • -15 • N (Figure 6).…”

“…When TC-induced mixing occurs, the subsurface warm layer is mixed up, resulting in an increase in SST [20]. Previous studies using in situ measurement and numerical models have extensively investigated this feature and have shown that when mixing introduces subsurface warm water to the surface, there is no cold wake phenomenon [61]. The existence of the pre-storm subsurface warm layer has been confirmed through mooring observations during the passage of TC Phailin [63].…”

“…As a result, instead of breaking through the thermal inversion layer, these TCs lifted subsurface warm layers to the surface. This process triggered SST warming during the passage of the TCs [61]. It is worth noting that this phenomenon of abnormal warming is not confined to the Bay of Bengal alone but also in the Arabian Sea.…”

“…Additionally, the advected warm water replaces the previously lower SST (Figure 2a). Secondly, there is a subsurface warm layer at the depth of 20-60 m in the Bay of Bengal, primarily due to freshwater input from river discharge and monsoon rain [61]. The shallow topography of the Bay of Bengal facilitates the formation of subsurface warm water through coastal riverine discharge, which has higher density.…”

Climate, trends and variability of extreme winds along the southwest coast of India

“…In 2018, a low‐pressure system developed in the Arabian Sea, followed by monsoon depressions and caused heavy rainfalls and flash floods over Kerala. Further, a Very Severe Cyclonic Storm Gaja in November 2018 passed through the close vicinity of the study region (George et al, 2021; Jarugula & McPhaden, 2022). In 2019, similar conditions occurred when a Very Severe Cyclonic Storm Vayu in June 2019 and an Extremely Severe Cyclonic Storm Maha in October 2019 (Mishra et al, 2021) impacted the study region.…”