Comparing the upper-ocean response associated with two pre-monsoon tropical cyclones over the Arabian Sea

Recent warming in the tropical oceans has increased tropical cyclone (TC) activity globally. The Arabian Sea (AS) in the northern Indian Ocean is such a basin with rising TC activity. TCs have the potential to favor the short-term productivity of the ocean through extensive mixing of the upper ocean, bringing about temporary variations in the biogeochemistry. The current study investigates the upper ocean biogeochemical response of AS to a TC utilizing data sets from six Bio-Argos, multi-satellite observations, and biogeochemical models from Copernicus Marine Services. The extremely severe cyclonic storm “Biparjoy”, one of the long-duration TCs in the basin was chosen as a case study. It formed during the onset of the Indian summer monsoon in June 2023 and persisted for eleven days (6th to 16th June 2023). Profiles obtained from the Bio-Argos around the storm track showed extensive cooling (~4 °C) in the upper surface. The strong TC also resulted in significant vertical mixing which brought cold, nutrient-rich water to the upper surface. Dissolved oxygen concentration was observed to have decreased during the cyclone, compared to pre- and post-cyclone periods. Eventually, an increased chlorophyll concentration, persisting for more than a week, was observed along the track in the sub-surface and surface waters after the passage of the storm. Biogeochemical model data was used to analyze the intricate variations in the upper ocean biogeochemistry of AS during the storm. Further analysis for a better understanding of the upper ocean response of the AS to cyclonic storms in recent years is in progress.

The impacts of Persian Gulf water and ocean-atmosphere interactions on tropical cyclone intensification in the Arabian Sea

طوفان‌های حاره از پدیده‌های مهم پیرامون خط استوا هستند که در نیمه گرم سال در نیمکره شمالی یا جنوبی ایجاد می‌شوند. این چرخندها با گذر از اقیانوس و تکیه‌بر منبع عظیم انرژی گرمایی نهان تبخیر، قدرت قابل‌توجهی می‌یابند و در مدت کوتاهی به یکی از مخرب‌ترین مخاطرات طبیعی تبدیل می‌شوند. هدف این مطالعه، مقایسه و تحلیل ساختاری چرخندهای دریای عرب و عمان به‌منظور بررسی نقش پارامترهای جوی، اقیانوسی در تعیین مسیر حرکت آن‌هاست. بدین منظور با استفاده از آمار موجود در مرکز مشترک اخطار طوفان، اطلاعات مربوط به چرخندها تهیه شد. همچنین با استفاده از داده‌های باز تحلیل پایگاهECMWF متغیرهای فشار سطح دریا، ارتفاع ژئوپتانسیل سطح 850 هکتوپاسکال، دمای سطح 1000هکتوپاسکال و دمای سطح دریا در محدودۀ -5 تا 40 درجه عرض شمالی و 40 تا 80 درجه طول شرقی برای مدت‌زمان حیات چرخند استخراج گردید. تولید و تحلیل نقشه‌ها نیز در محیط GRADS و ArcGis با استفاده از تغییرات آزیموت، روابط همبستگی و قوانین کشش و رانش انجام شد. نتایج نشان داد که در لحظه تشکیل، جهت حرکت همه چرخندها به‌غیراز گونو شمال غرب بوده و همبستگی قوی منفی بین دما و فشار سطح دریا در زمان شروع وجود داشته است. اما به‌جز چرخند گونو در سایر چرخند‌ها زمان تغییر مسیر بازمان رسیدن آن به اوج، یکی نیست. تحلیل نقشه‌های فشار سطح دریا نیز نشان داد که مسیر حرکت چرخندهای موردمطالعه از قوانین کشش و رانش پیروی کرده و حاکمیت پرفشارها در فصل سرد باعث شده چرخندهای نیلوفر و چاپالا نسبت به سایر، به سمت عرض-های بالا گسترش پیدا نکنند.

Climate Change and Tropical Cyclone Activity.- A Climatology of Intense Tropical Cyclones in the North Indian Ocean Over the Past Three Decades (1980-2008).- Tropical Cyclones in a Hieararchy of Climate Models of Increasing Resolution.- Modeling Climate Change: Perspective and Applications in the Context of Bangladesh.- Changes in Tropical Cyclone Precipitation Over China.- Toward Improved Projection of the Future Tropical Cyclone Changes.- Global Warming and Tropical Cyclone Activity in the Western North Pacific.- Tropical Cyclones and Climate Change: An Indian Ocean Perspective.- Recent Trends in Tropical Cyclone Activity in the North Indian Ocean.- Progress on Tropical Cyclogenesis.- Generating Synthetic Tropical Cyclone Databases for Input to Modeling of Extreme Winds, Waves, and Storm Surges.- Numerical Simulation of the Genesis of Cyclone Nargis Using a Global Cloud-System Resolving Model, NICAM.- Simulation of the North Indian Ocean Tropical Cyclones Using the Regional Environment Simulator: Application to Cyclone Nargis in 2008.- Simulation of Track and Intensity of Gonu and Sidr with WRF-NMM Modeling System.- Operational Tropical Cyclone Forecasting & Warning Systems.- Monitoring and Prediction of Cyclonic Disturbances Over North Indian Ocean by Regional Specialised Meteorological Centre, New Delhi (India): Problems and Prospective.- Evaluation of the WRF and Quasi-Lagrangian Model (QLM) for Cyclone Track Prediction Over Bay of Bengal and Arabian Sea.- Simulation of Tropical Cyclones Over Indian Seas: Data Impact Study Using WRF-Var Assimilation System.- Impact of Rain-Affected SSM/I Data Assimilation on the Analyses and Forecasts of Tropical Cyclones, and Study of Flow-Dependent Ensemble Background Errors, Over the Southwest Indian Ocean.- Statistical Forecasting of Tropical Cyclones for Bangladesh.- THORPEX and Its Application for Nargis by Ensemble Prediction.- Cyclone Gonu: The Most Intense Tropical Cyclone on Record in the Arabian Sea.- Real-Time Prediction of Cyclone Over Bay of Bengal Using High-Resolution Mesoscale Models.- Performance Evaluation of DGMANs NWP Models During Gonu.- Capabilities of Using Remote Sensing and GIS for Tropical Cyclones Forecasting, Monitoring, and Damage Assessment.- Assessment of Risk and Vulnerability from Tropical Cyclones, Including Construction, Archival and Retrieval of Best-Track and Historic Data Sets.- On Developing a Tropical Cyclone Archive and Climatology for the South Indian and South Pacific Oceans.- Improving the Australian Tropical Cyclone Database: Extension of the GMS Satellite Digital Image Archive.- Coastal Vulnerability Assessment Based on Historic Tropical Cyclones in the Arabian Sea.- The International Best Track Archive for Climate Stewardship (IBTrACS) Project: Overview of Methods and Indian Ocean Statistics.- Remote Sensing Imagery Assessment of Areas Severely Affected by Cyclone Gonu in Muscat, Sultanate of Oman.- Urban Sprawl and City Vulnerability: Where Does Muscat Stand?.- Flood Studies in Oman and the Difficulties in Using Rainfall-Runoff Analysis.- Disaster Preparedness, Management and Reduction.- Cyclone Gonu Storm Surge in the Gulf of Oman.- How the National Forecasting Centre in Oman Dealt with Tropical Cyclone Gonu.- Cyclone Disaster Management: A Case Study of MODES Experience with Cyclone Gonu.- Recent High Impact Tropical Cyclone Events in the Indian Ocean: Nargis, SIDR, Gonu and Other Events.- The Impact of Cyclone Gonu on Selected Coral Rich Areas of the Gulf of Oman Including Indications of Recovery at the Daymanyiat Islands.- Cyclone Nargis Storm Surge Flooding in Myanmar's Ayeyarwady River Delta.- The First Ever Super Cyclonic Storm GONU over the Arabian Sea During 1-7 June 2007: A Case Study.- Characteristics of Very Severe Cyclonic Storm NARGIS over the Bay of Bengal During 27 April to 3 May 2008.- Characteristics of Very Severe Cyclonic Storm SIDR over the Bay of Bengal During 11-16 November 2007.- Influence of a Tropical Cyclone Gonu on Phytoplankton Biomass (Chlorophyll a) in the Arabian Sea.- Recent Outbreaks of Harmful Algal Blooms Along the Coast of Oman: Possible Response to Climate Change?.- Understanding the Tropical Cyclone Gonu.

The Arabian Sea (AS) are regularly in uenced by tropical cyclones (TCs). In this study, the TCs in the AS during the period 1980-2022 were examined and three TCs (Biparjoy, Shaheen, and Mekunu), that developed in the AS and followed various paths were simulated using the WRF model. The most TCs occurred in the Indian Ocean during the negative or neutral phase of El Niño-Southern Oscillation (ENSO) and neutral phase of the Indian Ocean Dipole (IOD). These storms typically formed in the pre-monsoon season in the eastern AS, moving north/northeastward and in the post-monsoon, in southern regions of the AS and moved westward. Satellite analysis revealed the presence of ice-phase cirrus clouds with signi cant thickness at higher altitudes near the storm's eye. The simulation showed that TCs moved towards areas with lower pressure and wind shear. In well-developed TCs, a relative humidity reduction in the eye region and moisture gradient between rain bands and subsiding cold air areas were observed. During the peak days of cyclonic activity, the helicity reached its maximum and tropopause folding were occured. The WRF model underestimated the speed of Shaheen and its intensity and precipitation extent. In the Biparjoy, the model indicated precipitation over a broader area around the storm center. The model suggested less precipitation in regions surrounding the storm center in Mekunu. The comparison of daily cumulative precipitation between the GPM data and the WRF output revealed the highest correlation and the lowest RMSE for the Biparjoy. Tropical cyclones (TCs), also known as hurricanes or typhoons in different regions, are powerful and organized low pressure systems that form over warm ocean waters near the equator (Spiridonov et al. 2021; Latos et al. 2023; Sekaranom et al. 2023). Damage caused by TCs can be extensive and multifaceted, affecting different aspects of the environment, infrastructure, and communities (Shultz 2005; Meyer et al. 2023) . Strong winds, heavy rainfall and ooding can damage crops and disrupt agricultural activities (Chikodzi et al. 2021; Igbal 2022) . The combination of wind, storm surge, and ooding can lead to loss of life, especially in areas with dense populations and inadequate infrastructure (Anderson et al. 2020; Bakkensen et al. 2019; Tennant and Gilmore 2020) . The TC formation process involves several key factors. They require warm ocean water as their primary energy source. Sea surface temperature (SST) must be at least 26.5°C or higher to provide the necessary warmth and moisture for TC development (Hibbert et al. 2023; Holliday 2019 ). An unstable atmosphere is essential for the development of TCs (Han et al. 2021; Kiehl et al. 2021) . The Coriolis effect is very important for TC formation (Al Mohit et al. 2022; de Mendonça et al. 2023 ). An initial disturbance, often in the form of a cluster of thunderstorms, plays an important role in TC development (Studholme et al. The Indian Ocean and the Arabian Sea (AS) are regularly affected by TCs (Ranji et al. 2022; Priya et al. 2022) . The occurrence of these storms has many socio-economic and environmental impacts on the countries along the coasts (Sattar 2022; Swain 2022). The number of TCs that develop or move into the

Using tropical cyclone data along with sea surface temperature data (SST) and atmospheric circulation reanalysis data during the period of 1980–2019, the seasonal variation of tropical cyclone genesis (TCG), and the related oceanic and atmospheric environments over the Arabian Sea (AS) and Bay of Bengal (BOB) are compared and analyzed in detail. The results show that TCG in both the BOB and AS present bimodal seasonal variations, with two peak periods in the pre-monsoon and post-monsoon season, respectively. The frequencies of TCG in the BOB and AS are comparatively similar in the pre-monsoon season but significantly different in the post-monsoon season. During the post-monsoon season of October–November, the TCG frequency in the BOB is approximately 2.3 times higher than that of the AS. The vertical wind shear and relative humidity in the low- and middle-level troposphere are the two major contributing factors for TCG, and the combination of these two factors determines the bimodal seasonal cycle of TCG in both the AS and BOB. In the pre-monsoon season, an increase in the positive contribution of vertical wind shear and a decrease in the negative contribution of relative humidity are collaboratively favorable for TCG in the AS and BOB. During the monsoon season, the relative humidity factor shows a significant and positive contribution to TCG, but its positive effect is offset by the strong negative effect of vertical wind shear and potential intensity, thus resulting in very low TCG in the AS and BOB. However, the specific relative contributions of each environmental factor to the TCG variations in the AS and BOB basins are quite different, especially in the post-monsoon season. In the post-monsoon season, the primary positive contributor to TCG in the AS basin is vertical wind shear, while the combined effect of vertical wind shear and relative humidity dominates in the BOB TCG. From the analysis of environmental factors, atmospheric circulations, and genesis potential index (GPI), the BOB is found to have more favorable TCG conditions than the AS, especially in the post-monsoon season.

Climatologically, tropical cyclone (TC) activity in the North Indian Ocean (NIO) is asymmetric between the Arabian Sea (AS) and Bay of Bengal (BoB) basin. For the 172 TCs formed over NIO during 1983–2015, only 56 formed over AS and the rest (116) over BoB. During the period, AS was very active in a few years (but with quiet BoB season), and the opposite occurred in some others. It is found that this contrast occurred mostly during the post‐monsoon season of October–December. The meteorological and climate factors that accounted for these contrasting AS and BoB TC seasons are analysed. While climate variability such as the El Niño Southern Oscillation and Indian Ocean Dipole have known influences to NIO TC activity, results reveal that no single climate mode can well explain the TC development concentrating on AS or BoB only. Instead, it is found that variability of the northeast monsoon is an important factor responsible for the difference between the two basins. Excess moisture is available over the AS due to anomalous low‐level flow from the equatorial IO in the years in which there are more TCs in that basin, and dryer condition is over the BoB. In these years, there is likely excess northeast monsoon rainfall. The relationship is opposite between post‐monsoon BoB TC activity and the northeast monsoon. Nevertheless, the anomalous flow during active AS TC seasons is similar to that occurs during positive Indian Ocean Dipole, and thus this climate variability may be responsible for redistributing the moisture content in the NIO.

This study investigates the upper ocean response and primary productivity over Arabian Sea due to tropical cyclone ‘Ockhi’, a rare tropical storm system that formed over southwest Bay of Bengal and moved westward into the Arabian Sea while rapidly intensifying into a severe storm. Multi-satellite data products comprising Sea Surface Temperature (SST), turbulent fluxes, surface wind, and Ekman pumping derived from European Reanalysis – 5 (ERA5) daily surface wind data is used to examine the evolution and life cycle of the storm. MODIS derived Chlorophyll-a (Chlor-a), daily averaged Photosynthetically Available Radiation (PAR) and Particulate Organic Carbon (POC) are utilized for detailed analysis of the impact of the storm. Evolution of SST starting from the week prior to Ockhi shows that Southeast Arabian Sea and South-west Bay of Bengal regions had high values ranging from 28 to more than 30ÂoC, temperatures favorable enough to fuel the intensification of Ockhi. As the storm system moved north-westward, a clear reduction in SST is observed over the west-central Arabian Sea, affirming the upper ocean cooling associated with strong upwelling in the storm centre.

A study of upper ocean characteristics in response to the three intense re-curving tropical cyclones from the Arabian Sea using satellite and in situ measurements

The current study looks at how the characteristics of Arabian Sea tropical cyclones (TCs) change over time. The results show that in the pre-monsoon (April–June) and the post-monsoon (October–December), the activity of TCs > 34 knots, including cyclonic storm (CS), severe cyclonic storm (SCS), very severe cyclonic storm (VSCS), extreme severe cyclonic storm (ESCS), and super cyclonic storm (Sup. CS), has significantly increased, while the tendency of TCs < 34 knots, depressions and deep depressions (Ds) over the Arabian Sea has only slightly increased. Most of the TC activity in the first two decades (1982–2001) over the Arabian Sea activated on the eastern side, while in the last two decades (2002–2021), there was an expansion toward the southwest region of the Arabian Sea, especially in the post-monsoon season. The composite analysis of environmental parameters over the Arabian Sea reveals that the negative anomalies of outgoing longwave radiation (OLR) and the positive anomalies of relative humidity at 500 hPa (RH–500 hPa) in the first decade (1982–1991) and the second decade (1992–2001) are more concentrated on the eastern side of the Arabian Sea, leading to increased activity for TCs. Decades three (2002–2011) and four (2012–2021) demonstrated a wide distribution of weak vertical wind shear (VWS) and strong convection (OLR and RH–500 hPa) over the Arabian Sea basin. This led to TCs occurring more frequently and stronger, especially in the post-monsoon season. SST over the Arabian Sea was sufficient for tropical storm activity (≥26.5 °C) for both typical seasons.

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.

In recent years, and particularly from 2000 onwards, the North Indian Ocean (NIO) has been acting as a major sink of ocean heat that is clearly visible in the sub-surface warming trend. Interestingly, a part of the NIO—the Arabian Sea (AS) sector—witnessed dramatic variations in recent sub-surface warming that has direct repercussion on intense Tropical Cyclone (TC) activity. This study investigated the possible causative factors and physical mechanisms towards the multi-decadal warming trends in surface and sub-surface waters over the AS region. Responsible factors towards warming are examined using altimetric observations and reanalysis products. This study used ORAS5 OHC (Ocean Heat Content), derived meridional and zonal heat transport, currents, temperature, salinity, Outgoing Longwave Radiation (OLR), and air-sea fluxes to quantify the OHC build-up and its variability at water depths of 700 m (D700) and 300 m (D300) during the past four decades. The highest variability in deeper and upper OHC is noticed for the western and southern regions of the Indian Ocean. The warming trend is significantly higher in the deeper regions of AS compared to the upper waters, and relatively higher compared to the Bay of Bengal (BoB). Increased OHC in AS show good correlation with decreased OLR in the past 20 years. An analysis of altimetric observations revealed strengthening of downwelling Kelvin wave propagation leading to warming in eastern AS, mainly attributed due to intrusion of low saline water from BoB leading to stratification. Rossby wave associated with deepening of thermocline warmed the southern AS during its propagation. Heat budget analysis reveals that surface heat fluxes play a dominant role in warming AS during the pre-monsoon season. Increasing (decreasing) trend of surface heat fluxes (vertical entrainment) during 2000–2018 played a significant role in warming the southeastern sector of AS.

Tropical cyclones (TCs) over the Arabian Sea pose significant threats to coastal populations and result in substantial economic losses, yet their variability in response to major climate modes remains insufficiently understood. This study examines the relationship between the El Niño–Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and the Indo-Pacific Warm Pool (IPWP) with TC activity over the Arabian Sea from 1982 to 2021. Utilizing the India Meteorological Department (IMD)’s best-track data, reanalysis datasets, and composite analysis, we find that ENSO and IOD phases affect TC activity differently across seasons. The pre-monsoon season shows a limited association between TC activity and both ENSO and IOD, with minimal variation in frequency, intensity, and energy metrics. However, during the post-monsoon season, El Niño enhances TC intensity, resulting in a higher frequency of intense storms, leading to increased accumulated cyclone energy (ACE) and power dissipation index (PDI) in a statistically significant way. In contrast, La Niña favors the development of weaker TC systems and an increased frequency of depressions. While negative IOD (nIOD) phases tend to suppress TC formation, positive IOD (pIOD) phases are associated with increased TC activity, characterized by longer durations and higher ACE and PDI (statistically significant). Genesis sites shift with ENSO: El Niño favors genesis in the eastern Arabian Sea, causing westward or northeastward tracks, while La Niña shifts genesis toward the central-western basin, promoting northwestward movement. Composite analysis indicates that higher sea surface temperatures (SSTs), reduced vertical wind shear (VWS), increased mid-tropospheric humidity, and lower sea level pressure (SLP) during El Niño and pIOD phases create favorable conditions for TC intensification. In contrast, La Niña and nIOD phases are marked by drier mid-level atmospheres and less favorable SST patterns. The Indo-Pacific Warm Pool (IPWP), particularly its westernmost edge in the southeastern Arabian Sea, provides a favorable thermodynamic environment for genesis and exhibits a moderate positive correlation with TC activity. Nevertheless, its influence on interannual variability over the basin is less significant than that of dominant large-scale climate patterns like ENSO and IOD. These findings highlight the critical role of SST-related teleconnections (ENSO, IOD, and IPWP) in regulating Arabian Sea TC activity, offering valuable insights for seasonal forecasting and risk mitigation in vulnerable areas.

Tropical cyclones (TCs) are among the most dangerous types of weather, originate over warm tropical oceans and can seriously harm people, infrastructure, ecosystems, and country economies. The Arabian Sea is an important area for the development of TCs, but not much research has been done on how cyclones behave there over time and in different seasons. This study looks at TC activity over the Arabian Sea from 1982 to 2021, focusing on TC tracks, energy metrics, including the accumulated cyclone energy (ACE) and power dissipation index (PDI), and TC duration. The results show a big change in TC tracks over time and between seasons. In the pre-monsoon, northwest and northeast tracks are the dominant tracks, whereas in the post-monsoon, northwest and westward tracks are the most common. There has been a big increase in the ACE, the PDI, and the lifespan of TCs, especially in the post-monsoon season, over the second half of the study. The study also looks at how large-scale synoptic characteristics, like sea surface temperature (SST), vertical wind shear (VWS), upper-level winds, sea level pressure (SLP), and relative humidity (RH) affect the behavior of TCs. The results show that higher SSTs and lower VWSs have made TCs stronger and last longer. Also, upper-level winds, SLP, and RH are significant for changing the paths of TCs. This study provides a comprehensive, seasonally resolved look at how TC activity has changed over the past four decades in the Arabian Sea. It also gives us new insights into how environmental factors have affected TC behavior over time.

Effects of vertical wind shear and storm motion on tropical cyclone rainfall asymmetries over the North Indian Ocean