Jyothi Lingala

This study analyzed the upper ocean responses to tropical cyclone (TC) Ockhi (2017) from two regions where it underwent (a) rapid intensification (RI) and (b) rapid weakening (RW) using simulations from the high‐resolution HYbrid Coordinate Ocean Model. Pre‐existed oceanic conditions in the RI region were warmer and fresher than in the RW region. Though the translation speed was slower over the RI region, the storm‐induced sea surface temperature cooling was weaker (−0.8°C) while a significant cooling (−1.9°C) was observed at RW. The mixed layer heat budget analysis revealed that the dominant process driving the temperature tendency in the RI and RW regions were surface thermal forcing (−0.01°C h−1) and entrainment (−0.015°C h−1), respectively. The role of entrainment was restrained by a thick barrier layer in the RI region. The TC‐induced mixing over a weakly stratified ocean at RW resulted in a significant drop in dynamic temperature (Tdy) compared to the RI. The cooling differences were attributed to the contrasting salinity stratification at RI and RW regimes which modulated the ocean's negative feedback to TC intensity changes. Further, the difference between the depths of mixing length and 26°C isotherm at RI and RW regimes indicate that 26°C isotherm does not necessarily represent the mixing depth under the influence of TC. Hence, this study suggests Tdy may be a better metric for cyclone intensity changes than heat content above a fixed isotherm, particularly in the salinity stratified regions.
Plain Language Summary
Strong surface winds accompanying tropical cyclones usually cool the ocean surface by bringing cooler waters from below the surface. The magnitude of this cooling depends on the intensity of the cyclone, its travel speed, and the underlying ocean condition over which the storm moves. This study examined the ocean responses to cyclone Ockhi (2017), which occurred over the Arabian Sea during the November‐December month. In the early stage of Ockhi, where it had rapidly intensified, the storm moved slowly, and the ocean was forced by intense winds. These conditions, in general, lead to significant ocean surface cooling as it gets more time to interact with the TC. However, in the case of Ockhi, a thick barrier layer resulting from freshwater intrusion inhibited the TC‐induced cooling. On the contrary, there was prominent cooling during the rapidly weakening stage of Ockhi due to the absence of a barrier layer as the waters were more saline. The disparity in the cooling between RI and RW stages had provided distinct negative ocean feedback to the cyclone.
Key Points
Upper ocean responses to cyclone Ockhi are studied using a numerical ocean model (HYbrid Coordinate Ocean Model) in its rapidly intensified and weakened regions
Contrasting salinity structure at rapidly intensified (weakened) regions resulted in a negligible (significant) drop in dynamic temperature
Analysis of the mixed layer heat budget in the two regions highlighted the role of the barrier layer on entrainment‐based cooling

The upper oceanic thermal response induced by Tropical Cyclone Phailin (9–14 October 2013) under the influence of East India Coastal Current (EICC) and a cyclonic eddy is investigated and contrasted with the response from open ocean region using a high‐resolution HYbrid Coordinate Ocean Model simulation. There is significant cooling (7° C) inside the cold core eddy and negligible cooling (0.5° C) within the EICC region characterized by the shallow and deeper thermocline, respectively. Our analysis of mixed layer heat budget terms showed that the horizontal advection plays a significant role in determining the temperature tendency for the location within the EICC, in contrary to the general dominance of vertical processes as reported in previous studies during the cyclone period. The analysis for the locations inside eddy and open ocean concurs with the previous studies showing the dominance of vertical processes toward the temperature tendency. Further, near the coast, the surface cooling is minimal compared to the subsurface cooling, dominantly seen between 50‐ and 100‐m depth. This disparity indicates that the factors responsible for the surface temperature anomalies are different from those of subsurface. Our analysis of thermal signatures after the passage of cyclone showed that the EICC and cyclonic eddy contribute to the faster advection of cold wake and recovery of sea surface temperature to the prestorm state.
Key Points
Tropical Cyclone Phailin‐induced ocean thermal responses near to the coast are dominated by horizontal advection in contrary to the open ocean
Weak sea surface temperature cooling is observed near the coast despite the passage of an intense cyclone
Poststorm thermal recovery of ocean is modulated by coastal boundary current and associated cyclonic eddy