Tropical Cyclone Heat Potential Research Articles

Accelerated Increase in Tropical Cyclone Heat Potential in the Typhoon Rapidly Intensifying Zone during 1955-2020

Accelerated Increase in Tropical Cyclone Heat Potential in the Typhoon Rapidly Intensifying Zone during 1955-2020

Trends in Western North Pacific Tropical Cyclone Intensity Change Before Landfall

This study investigates the long-term trend in the average 24-h intensity change (ΔV24) of western North Pacific (WNP) tropical cyclones (TCs) before landfall during June-November for the period from 1970–2019. We find a significant increasing trend in basin-averaged ΔV24 during 1970–2019. The increase in ΔV24 is significant over the northern South China Sea (17.5°-25°N, 107.5°-120°E) and to the east of the Philippines (7.5°-15°N, 122.5°-132.5°E), implying a slower weakening rate before landfall for the South China Sea and an increased intensification rate before landfall for the region east of the Philippines. We find a significant linkage between changes in ΔV24 and several large-scale environmental conditions. The increased ΔV24 before landfall in the above two regions is induced by a warmer ocean (e.g., higher sea surface temperatures, maximum potential intensity and TC heat potential) and greater upper-level divergence, with a moister mid-level atmosphere also aiding the ΔV24 increase east of the Philippines. Our study highlights an increasing tendency of ΔV24 before landfall, consistent with trends in ΔV24 over water and over land as found in previous publications.

Tropical Cyclone Heat Potential (TCHP) from the NCMRWF NEMO based global ocean analysis and forecast system

In this study, the Nucleus European Modelling of Ocean (NEMO) based global ocean analysis and forecast system configured at National Centre for Medium Range Weather Forecasting (NCMWRF) is used to compute the upper ocean heat content up to 26° isotherm depth called as Tropical Cyclone Heat Potential (TCHP). NCMRWF has the real-time TCHP monitoring capability on daily basis. It is produced in real time using the global ocean forecast system up to 10 days using the ocean only model and up to 15 days using the coupled atmosphere-ocean model for monitoring the upper ocean and also for research purpose mainly for Tropical Cyclone (TC) study. Four pre-monsoon TCs with category above cyclonic storms in which two from Arabian Sea and two from Bay of Bengal are also examined. The Sea Surface Temperature drop is much more over the regions where the surface wind stress anomaly is strong. In both regions, various small scale warm and cold eddies are observed during TCs. TCHP anomaly is positive/negative over the region where Sea Surface Height (SSH) anomaly is positive/negative. From TCHP, it is manifest that the magnitude of TCHP anomaly is higher in BoB than AS region during TCs.

Decadal changes of cyclone tracks in the Bay of Bengal

Warming tropical sea surface temperatures may also change the environmental wind field, which could affect genesis, intensity and track of Tropical Cyclone (TC). In recent years under ever-increasing global warming, scientists have directed more attention to the variation in the track. In the Indian Ocean, most of the severe cyclones during pre-monsoon (April and May) and post-monsoon (October to December) seasons hit the east coast of India, causing loss of life and damage to property. The frequency of cyclones is 3 - 4 times higher over the Bay of Bengal (BoB) when compared with the Arabian Sea (AS). The cyclone tracks over BoB in October-December season, the upper tropospheric winds over the north Indian Ocean and Indian Summer Monsoon Rainfall exhibit large inter-annual variability. In this study an attempt has been made to examine the changes in TC tracks during October-December over BoB on decadal time scale. In addition to atmospheric parameters, oceanic parameters like SST, TCHP (tropical cyclone heat potential), UOHC (upper ocean heat content), thermocline depth, appear to be the best parameters to examine the variability for last three decades.

Simultaneous Occurrence of Tropical Cyclones in the Northern Indian Ocean: Differential Response and Triggering Mechanisms

The northern Indian Ocean, comprising of two marginal seas, the Arabian Sea (AS) and the Bay of Bengal (BoB), is known for the occurrence of tropical cyclones. The simultaneous occurrence of the cyclones Luban in the AS and Titli in the BoB is a rare phenomenon, and, in the present study, we examined their contrasting upper ocean responses and what led to their formation in October 2018. Being a category-2 cyclone, the maximum cooling of sea surface temperature associated with Titli was 1°C higher than that of Luban, a category-1 cyclone. The higher tropical cyclone heat potential in the BoB compared with the AS was one of the reasons why Titli was more intense than Luban. The enhancement of chlorophylla(Chl-a) and net primary productivity (NPP) by Luban was 2- and 3.7-fold, respectively, while that by Titli was 3- and 5-fold, respectively. Despite this, the magnitudes of both Chl-aand NPP were higher in the AS compared with the BoB. Consistent with physical and biological responses, the CO2outgassing flux associated with Titli was 12-fold higher in comparison to the pre-cyclone value, while that associated with Luban was 10-fold higher. Unlike the Chl-aand NPP, the magnitude of CO2flux in the BoB was higher than that in the AS. Although the cyclones Luban and Titli originated simultaneously, their generating mechanisms were quite different. What was common for the genesis of both cyclones was the pre-conditioning of the upper ocean in 2018 by the co-occurrence of El Niño and the positive phase of Indian Ocean dipole along with the cold phase of the Pacific decadal oscillation, all of which worked in tandem and warmed the AS and parts of the BoB. What triggered the genesis of Luban in the AS was the arrival of the Madden–Julian oscillation (MJO) and the mixed Rossby-gravity wave during the first week of October. The genesis of Titli in the BoB was triggered by the eastward propagation of the MJO and the associated enhanced convection from the AS into the region of origin of Titli along with the arrival of the downwelling oceanic Rossby wave.

Upper-Ocean Temperature Variability in the Gulf of Mexico with Implications for Hurricane Intensity

AbstractStrong winds in tropical cyclones (TCs) mix the ocean, causing cooler water from below the thermocline to be drawn upward, reducing sea surface temperature (SST). This decreases the air–sea temperature difference, limits available heat energy, and impacts TC intensity. Part of TC forecast accuracy therefore depends upon the ability to predict sea surface cooling; however, it is not well understood how underlying ocean conditions contribute to this cooling. Here, ~4400 Argo profiles in the Gulf of Mexico were used in a principal component analysis to identify the modes of variability in upper-ocean temperature, and a 1D mixed layer model was used to determine how the modes respond to surface forcing. It was found that the first two modes explain 75% of the variance in the data, with high mode-1 scores being broadly characterized as having warm SST and deep mixed layer and mode-2 scores being characterized as having high SST and a shallow mixed layer. Both modes have distinct seasonal and spatial variability. When subjected to the same model forcing, mode-1- and mode-2-characteristic waters with equal tropical cyclone heat potential (TCHP) respond very differently. Mode-2 SST cools faster than mode 1, with the difference being most pronounced at lower wind speeds and when comparing early-season storms with late-season storms. The results show that using TCHP as a marker for SST response during TC forcing is insufficient because it does not fully capture subsurface ocean thermal structure. This result underscores the need for continual subsurface monitoring so as to accurately initialize the upper ocean in coupled TC models.

Influence of El Niño decaying pace on low latitude tropical cyclogenesis over the western North Pacific

AbstractThe modulation of El Niño decaying pace on tropical cyclones (TCs) activities at low latitude (within 10°) over the western North Pacific (WNP) is investigated in this study. During rapidly decaying El Niño events, fewer TCs occur near the equator and the averaged genesis location is more westward. More TCs are found in the southeast of WNP during slowly decaying phase of El Niño events. The distinctive differences of TC genesis frequency and their respective meridional positions are related to the modulation of El Niño to large‐scale environmental flows. Both dynamical and thermodynamical conditions, such as low‐level vorticity, mid‐level relative humidity and vertical motion, are favourable for TC genesis in the southeast of WNP during slowly decaying El Niño events. The differences of sea surface temperature, vertical wind shear, and TC heat potential between the two El Niño events are similar to the spatial distribution of TC genesis at low latitude. Disturbances in southeastern WNP in slowly decaying El Niño events are more likely to develop into TCs due to the stronger zonal convergence and meridional shear of zonal wind. These findings may help to better understand the interaction between low‐latitude TCs and El Niño. The higher frequency of low‐latitude TC genesis and their more eastward TC genesis position in slowly decaying El Niño events may in turn slow down the transition speed of El Niño.

Variations of rapidly intensifying tropical cyclones and their landfalls in the Western North Pacific

ABSTRACT Rapid intensification (RI) plays a crucial role in intensifying tropical cyclones (TCs) within the context of climate change. This study investigates variations of RI TCs and their cyclonic energy upon landfall in the western North Pacific (WNP) during 1986–2017. Statistically, 31.2% of 946 historical TCs underwent RI events (RI TCs) and made 341 landfalls. The numbers of landfalling RI TCs show a significant increasing trend with strengthened high-intensity TCs. Both the lifetime maximum intensity (LMI) and landfalling cyclone energy of TCs that did not undergo RI exhibit decreasing trends. The mean genesis and on-set locations of RI TCs indicate northwestward migration. Cyclone energy from landfalling RI TCs has increasing trend along the coasts of southern China and Japan, while that along the Philippine coasts tend to exhibit an opposite trend. We attribute such trends to the poleward migration of the mean latitude of LMI. With increasing sea surface temperatures, relative high tropical cyclone heat potential provided by poleward-extended warmer mixed layer in warm seasons may play an important role in favoring RI events. Moreover, increases of 200–850 hPa vertical wind shear in the eastern WNP is likely to attribute to the westward migration of RI activities.

Effects of ocean eddies on the tropical storm Roanu intensity in the Bay of Bengal.

A tropical storm (TS) Roanu occurred in northern Sri Lanka in 2016, which transported northwards along the west coast of the Bay of Bengal (BoB). During the development of the TS, ocean eddies on its track had an important effect on the intensity of Roanu. The dynamic mechanism was investigated with multisource reanalysis and Argo float data in this study. The results show that ocean eddies were the main reason why Roanu first enhanced, weakened, and then enhanced again. Warm eddy W1 supports the initial development of the TS, cold eddy C1 weakens Roanu, and warm eddy W2 continues to support Roanu. On May 19, 2016, the maximum average latent heat flux over W1 was 260.85 w/m2, while that of C1 was only 200.71 w/m2. After the passage of Roanu, the tropical cyclone heat potential (TCHP) of eddies significantly decreased. The TCHP of W1, W2, C1 and C2 decreased by 20.95 kJ/cm2, 11.07 kJ/cm2, 29.82 kJ/cm2, 9.31 kJ/cm2, respectively. The mixed layer of warm eddies deepened much more than that of cold eddies, supporting Roanu development. In addition, changes in potential vorticity (PV) values caused by the disturbance of eddies may also reflect changes in the TS intensity. This study offers new insights on the influence of ocean eddies in regulating the development of tropical cyclone (TC) in the BoB.

On the interaction of mesoscale eddies and a tropical cyclone in the Bay of Bengal

Oceanic eddies span over a wide range of sizes and affect the thermodynamic properties of water column. By modifying the thermal structure of the upper ocean, these eddies influence the air-sea heat exchange and, therefore, the strength of a tropical cyclone (TC) that passes over the eddy zone. To understand the interaction of tropical cyclone with different types of eddies (cyclonic and anticyclonic) over the Bay of Bengal (BoB) region, three idealized numerical experiments are performed using an atmosphere–ocean–wave coupled model. In these experiments, the atmospheric and wave models are forced by the realistic surface forcing (i.e. wind, pressure, heat and momentum fluxes) of the Vardah cyclone period (10–15 December 2016) in the BoB. The ocean model is initialized with three idealized conditions, i.e. horizontally constant profile of temperature and salinity throughout the model domain, idealized three-dimensional (3-D) temperature and salinity profiles in the area of 2° radius for cyclonic eddy and anticyclonic eddy. The 3-D eddy profile is placed in the path of TC Vardah track. The strength and radius of both idealized cyclonic and anticyclonic eddies are kept same. The model-simulated results of these three experiments were intercompared and it is found that the tropical cyclone interaction with an anticyclonic eddy had provided an ancillary strength to the TC by raising the tropical cyclone heat potential (TCHP) in comparison with other experiments. The utilization of coupled ocean–atmosphere–wave model had also provided a better representation of ocean–atmosphere coupled feedback processes during TC interaction with different eddies. The analysis of feedback processes depicted that the TC interaction with an anticyclonic eddy has raised the magnitude of its existing moist-static-energy comparative to the cyclonic eddy interaction case which resulted into an increase in the strength of the TC.

Possible Influence of Tropical Indian Ocean Sea Surface Temperature on the Proportion of Rapidly Intensifying Western North Pacific Tropical Cyclones during the Extended Boreal Summer

AbstractThis study examines the possible impact of tropical Indian Ocean (TIO) sea surface temperature anomalies (SSTAs) on the proportion of rapidly intensifying tropical cyclones (PRITC) over the western North Pacific (WNP) during the extended boreal summer (July–November). There is a robust interannual association (r= 0.46) between TIO SSTAs and WNP PRITC during 1979–2018. Composite analyses between years with warm and cold TIO SSTAs confirm a significant impact of TIO SSTA on WNP PRITC, with PRITC over the WNP basin being 50% during years with warm TIO SSTAs and 37% during years with cold TIO SSTAs. Tropical cyclone heat potential appears to be one of the most important factors in modulating the interannual change of PRITC over the WNP with a secondary role from midlevel moisture changes. Interannual changes in these large-scale factors respond to SSTA differences characterized by a tropics-wide warming, implying a possible global warming amplification on WNP PRITC. The possible footprint of global warming amplification of the TIO is deduced from 1) a significant correlation between TIO SSTAs and global mean SST (GMSST) and a significant linear increasing trend of GMSST and TIO SSTAs, and 2) an accompanying small difference of PRITC (~8%) between years with detrended warm and cold TIO SSTAs compared to the difference of PRITC (~13%) between years with nondetrended warm and cold TIO SSTAs. Global warming may contribute to increased TCHP, which is favorable for rapid intensification, but increased vertical wind shear is unfavorable for TC genesis, thus amplifying WNP PRITC.

Genesis potential parameter using satellite derived daily tropical cyclone heat potential for North Indian ocean

ABSTRACT Tropical Cyclone Heat Potential (TCHP) is the heat content relative to 26°C isotherm, which is important in the development of cyclone. Cyclonic events last for few weeks and become intensified within few days or in some cases few hours. To study such low pressure systems, daily TCHP is required which is created in our study using Two Layer Gravity Model (TLGM) using monthly climatologies of temperature and salinity profiles, daily Sea Surface Height Anomaly (SSHA) and daily Sea Surface Temperature (SST). This satellite-based data of SSHA and SST are used to create daily TCHP fields which are validated using collocated ARGO in-situ data. ARGO derived TCHP and TLGM TCHP showed correlation coefficient , which indicates that TLGM can be used for near real time TCHP calculation for cyclone studies. Analysis done on 92 non-developing and 93 developing systems shows that, on the day of formation of depression, average TCHP is 89.3 kJ cm for developing systems while average TCHP is 78.3 kJ cm for non-developing systems in North Indian Ocean (NIO). Minimum TCHP was found to be 27.2 kJ cm and 28.6 kJ cm for developing and non developing systems out of 93 and 92 cases, respectively. Further, Genesis Potential Parameter (GPP) using low-level relative vorticity, middle troposphere relative humidity, middle troposphere temperature instability and vertical wind shear is created based on past similar studies, but with six hourly as well as daily resolution (1993 to 2017) and 0.125° 0.125° spatial resolution. Using daily TCHP, another index called Genesis Potential Index (GPI) is created and then both the GPP and GPI are analysed for 93 developing and 92 non-developing systems in north Indian ocean from the year 1993 to 2017. Our study shows that both are good predictors for the lead periods of less than 2 days, while for the lead periods of more than 2 days, the distinction becomes difficult between developing and non-developing systems. Abbreviations: TLGM - Two Layer Gravity Model; TCHP - Tropical Cyclone Heat Potential; - Depth of 26°C isotherm; - Depth of 20°C isotherm; NIO - North Indian Ocean; AS - Arabian Sea; India Meteorological Department - IMD; BOB - Bay of Bengal

Ocean heat content and its role in tropical cyclogenesis for the Bay of Bengal basin

Anthropogenic factors continue to warm the oceans by storing sufficient memory of past accumulated effects resulting in higher Sea surface temperature (SST) and Ocean heat content (OHC) invigorating tropical cyclogenesis and tropical cyclone intensity. Prior studies indicate that an increase in accumulated tropical cyclone heat potential, a measure of OHC is directly related to high intense storms of longer durations. The present study is an attempt to understand the role of OHC in tropical cyclogenesis for the Bay of Bengal (BoB). Unlike the other global ocean basins, the BoB is a semi-marginal sea in the North Indian Ocean (NIO) having differential water mass characteristics attributed due to tremendous influx from riverine discharges, seasonal reversing monsoonal wind system and complex ocean circulation features. This study provides a detailed assessment on SST and OHC distributions during the pre- and post-monsoon seasons covering a period of 21 years (1991–2016) over four identified sub-domains, the potential locations for tropical cyclogenesis. The daily ERA-Interim, ORAS4, ORAS5 and 3-h RAMA buoy datasets were used in the present study. The spatial distribution and variations of SST and OHC anomalies during pre- and post-monsoon seasons in the BoB were also examined. Differential nature of inter-annual SST variability was noticed in the study region. Highest variability in SST is evidenced during both pre- and post-monsoon seasons for the northern Bay region showing a warming bias of ∼ 1.7 °C in the past 2 decades. Contrasting variability is also noticed for the eastern Bay during pre-monsoon that is almost three times higher as compared to the post-monsoon season. Maximum OHC variability is found for the southern and central-eastern Bay regions. It is seen that OHC variability has a strong tele-connection with Oceanic Nino Index associated with El Nino/La Nina events in the Pacific basin with their amplitudes found to increase from northern to southern regions of the Bay.

Exceptional Tropical Cyclone Kenneth in the Far Northern Mozambique Channel and Ocean Eddy Influences

AbstractDuring summer 2018/2019, Mozambique was devastated by Idai and Kenneth, the first time two Intense Status tropical cyclones have ever made landfall here. The season broke numerous records for the Southwest Indian Ocean (the greatest deaths, the largest damage, and numbers of Intense Status cyclones and tropical storms). Kenneth, which is the strongest cyclone ever to make landfall in Mozambique, did so unusually late in the season, had an unusual due westward track and struck the coast far further north than previously, is examined. Changes in Kenneth's strength (21–25 April 2019) across the extreme northern Mozambique Channel are shown to be related to two cyclonic/anticyclonic eddy pairs in the ocean. These eddies led to changes in SST, enthalpy fluxes, tropical cyclone heat potential, and upper ocean heat content that influenced its rapid strengthening from moderate tropical storm to Very Intense tropical cyclone status and then back to Intense Status just before landfall.

Recent increase in extreme intensity of tropical cyclones making landfall in South China

This study examines the interdecadal variations in the frequency and intensity of tropical cyclones (TCs) making landfall in South China (SC) during the period 1975–2018. The annual frequency shows a decrease in 1997 but rises again since 2008 and the annual maximum landfall intensity (MLI) shows an increase since 2012. According to these variations, three subperiods, 1975–1996 (higher frequency but lower MLI), 1997–2011 (lower frequency and MLI) and 2012–2018 (higher frequency and MLI), are defined. The increase in MLI during 2012–2018 is related to the increases in the frequency of (1) TCs undergoing rapid intensification over the South China Sea (SCS) and landfalling in SC, with higher maximum intensity and location of maximum intensity closer to the coast of SC, and (2) intense typhoons (ITYs) over the western North Pacific (WNP), which maintain high intensity before landfall. These changes are closely related to the lower vertical wind shear and higher TC heat potential over the ocean east of the Philippines and the northern part of the SCS. Such an environment is more conducive for TC intensification, leading to the observed increases in the number of rapid-intensifying TCs over the SCS and ITYs over the WNP. Some of these latter TCs move across the SCS and tend to maintain high intensity during landfall in SC. The steering flow also changes, which allows more TCs to enter the SCS, resulting in an increase of ITYs making landfall in SC.

Influence of eddies and tropical cyclone heat potential on intensity changes of tropical cyclones in the North Indian Ocean

Eddies are known to play a crucial role in the sudden intensification of tropical cyclones. In this study, Sea Level Anomaly (SLA) data from satellite altimetry is utilized to investigate the role of eddies on tropical cyclones intensification in the North Indian Ocean (NIO) basin. SLA data obtained from Archiving Validation and Interpretation of Satellite Data in Oceanography (AVISO), Tropical Cyclone Heat Potential (TCHP) from National Remote Sensing Centre (NRSC) and cyclone intensity data from Indian Meteorological Department (IMD) have been utilized to analyse and understand the impact of TCHP and eddies on 60 tropical cyclones in the NIO spanning the years 2001–2018. Out of these 60 cyclones, 38 were formed in the Bay of Bengal (BoB) and 22 in the Arabian Sea (AS). From the analysis, it is found that nearly 40% cyclones are affected by Warm Core Eddy (WCE) generally intensifying them, 30% are influenced by Cold Core Eddy (CCE) that generally reduce cyclone intensities and the rest 30% are affected by TCHP variations (in the absence of eddies) in the NIO region during the study period. Cyclones which do not follow the established trend of intensifying due to WCEs or increasing TCHP; and diminishing owing to CCEs or TCHP decrease were found to be governed primarily by the Translational Speed of the cyclones. WCE are mostly present during the pre-monsoon season in the NIO (both BoB and AS). This analysis confirms that consideration of the underlying eddies in the cyclone track represented by SLA, TCHP variations, Translational Speed of cyclones and the ocean barrier layer are key factors that must be considered in forecasting cyclone intensity and track.

Effect of winter-to-summer El Niño transitions on tropical cyclone activity in the North Atlantic

El Nino-Southern Oscillation (ENSO) is known to significantly modulate Atlantic tropical cyclone (TC) activity. In this paper, we separate El Nino events into El Nino resurgence (ELR) and La Nina-transition-El Nino (LAT) events according to the warm or cold ENSO phase in the preceding winter. Different influences of the two types of El Nino events on North Atlantic TC activity are explored during the hurricane season of June–November. The average number of TCs is more, with shorter average duration and slightly larger intensity during ELR events than during LAT events. There are more TCs passing through and making landfall at the Gulf of Mexico, Caribbean Sea during ELR events, indicating that TCs may have a greater impact on coastal cities of Gulf of Mexico and Caribbean Sea during ELR events than LAT events. Composite analysis reveals that thermodynamic factors such as relative humidity (RH), sea surface temperature and tropical cyclone heat potential are mainly responsible for the TC activity difference between ELR and LAT events. Genesis potential index diagnosis shows that the RH and potential intensity are the dominant factors that influence TC genesis in the MDR during ELR events. During LAT events, the relative vorticity, vertical wind shear and RH over Cape Verde are favorable for TC genesis. In summary, the preceding winter ENSO phases associated with ELR and LAT events lead to opposite changes of environmental factors, and thus induce different TC activities in ELR and LAT events.

Interdecadal variation of frequencies of tropical cyclones, intense typhoons and their ratio over the western North Pacific

AbstractThis study explores whether a higher frequency of tropical cyclone (TC) occurrence would lead to more intense typhoons (ITYs) over the western North Pacific (WNP) during 1975–2016. The proportion of ITYs shows an upward trend, with a low‐ (1975–2001) and a high‐proportion period (2002–2016). There is an interdecadal variation in TC and ITY frequencies within the period 1975–2001, with three subperiods (1975–1986, 1987–1997 and 1998–2001) identified. Both the annual frequencies of TCs and ITYs are below normal during 1975–1986 and 1998–2001 but above normal during 1987–1997. A comparison between the periods 1975–1986 and 2002–2016 (similar TC frequency but different ITY frequency) shows a significant increase in ITY occurrence to the north and west of the major development region during 2002–2016, which is mainly due to the longer duration of intensification or higher intensification rate so that the TCs can develop into ITYs before making landfall or moving to higher latitudes. Comparing the periods 1987–1997 and 2002–2016 (similar ITY frequency but different TC frequency) shows that the decrease in TC frequency during 2002–2016 is mainly related to a reduction in TC genesis in the southeastern part of the WNP. The west‐northwestward shift in genesis positions during 2002–2016 leads to an increasing frequency of ITYs towards Japan, the Korean Peninsula and the east coast of China. The stronger vertical wind shear and lower moist static energy (MSE) (weaker vertical wind shear and higher MSE) in the southeastern and northwestern parts of the WNP are related to the lower (higher) annual frequencies of both TCs and ITYs during 1975–1986 (1987–1997). The higher tropical cyclone heat potential in the recent decade indicates an expansion of the area favourable for intensification, which is another factor responsible for the higher proportion of ITYs during 2002–2016.

Dynamic and thermodynamic upper-ocean response to the passage of Bay of Bengal cyclones 'Phailin' and 'Hudhud': a study using a coupled modelling system.

Understanding the upper-ocean response to tropical cyclones (TCs) in terms of sea surface temperature (SST) cooling is of prime importance in the prediction of TC intensity. However, the magnitude of cooling during the passage of TC varies depending on storm characteristics and pre-existing upper-ocean conditions such as the presence of ocean eddy and upper-ocean stratification. The present study investigates the upper-ocean response to two post-monsoon Bay of Bengal (BoB) cyclones, Phailin (October 2013) and Hudhud (October 2014), those followed almost a similar track, in association with pre-existing oceanic conditions using a fully coupled ocean-atmosphere modelling system. The spatial structure and temporal evolution of SST cooling induced by the two cyclones and the physical processes governing the cooling are examined. Analysis shows that the intensity of Phailin is significantly reduced when it encountered the regime of lower tropical cyclone heat potential (TCHP) associated with pre-existing cold core eddy (CCE). Intense upwelling with an average of 0.6 m/h is observed over CCE that resulted in strong temperature tendency of - 4.2 °C prior to landfall. Though average TCHP in the generation region of Hudhud was 50 kJ/cm2, the storm drew sufficient energy from the underlying ocean due to its slow translation speed. Presence of shallow thermocline over extended region and weaker upper-ocean stratification enhanced SST cooling over a larger region after passage of the TC Hudhud. Finally, the present study brings in clarity that the upper-ocean condition and the relative position of the mesoscale oceanic features to the storm track are responsible for the intensification of the TC and the recovery of the ocean surface.

The response of ocean parameters to tropical cyclones in the Bay of Bengal

AbstractThe Bay of Bengal (BoB) exhibits notable seasonal variations in tropical cyclone heat potential (TCHP), barrier layer thickness (BLT) and sea‐surface temperature (SST). These parameters also undergo profound changes in the presence of tropical cyclones (TCs). The composite structures of these ocean parameters as a function of the season of TC formation, intensity, and translation speed are unknown and are developed in the present study. Composite structures are examined based on 1,222 instantaneous samples from 83 TCs during 2003–2016 using INCOIS‐GODAS analyses. A BLT of 10–30 m and TCHP of 40–80 kJ/cm2 favours TC intensification in the central BoB. The multivariate regression of BLT and TCHP appears to be better for TC intensity up to 64 knots and is highly underestimated for the stronger TCs (>64 knots). The TC right‐rear sector experiences significant changes in TCHP anomaly (TCHPA) as the intensity increases. The TCHPA ranges ∼10–15, ∼20–25 and ∼25–30 kJ/cm2 when a TC is at Cyclone Storm (CS), Severe Cyclonic Storm (SCS) and Very SCS (VSCS) stages respectively. The maximum TCHPA is generally aligned along the TC track during the post‐monsoon season. Slow‐moving TCs produce maximum TCHPA cooling of ∼20 kJ/cm2 within 250 km storm radius in the rear sector, while it is less and away from the storm centre for normal and fast‐movers. The seasonal changes showed opposite relations between BLT and TCHP from pre‐ to post‐monsoon seasons during the TC intensification. TC‐induced SST cooling is maximum (∼0.5–1.2 °C) in the inner core for the strong (VSCS and above) and slow‐moving TCs. The cooling decreases with an increase in the translation speed and is more pronounced in the pre‐monsoon season. This study provides a baseline to verify and understand the limitations of the models, and also develop a climatological perspective of BoB TCs.