Tropical Cyclone Genesis Research Articles

Strengthening the Early Detection and Tracking of Tropical Cyclones near Indonesian Waters

In early April 2021, the territory of Indonesia, around the province of East Nusa Tenggara in particular, was severely damaged due to being hit by tropical cyclone Seroja. The impact of tropical cyclone Seroja does not only occur in Nusa Tenggara but also in Australia. In fact, the impact that hit Australia exceeded the damage that occurred in East Nusa Tenggara. The impacts caused by tropical cyclone Seroja in East Nusa Tenggara included 181 deaths and 74,222 houses damaged. Tropical cyclones are extreme weather anomalies that hit many countries, especially in the middle latitudes associated with vast oceans, such as the area around the South China Sea, the Pacific Ocean and the Atlantic Ocean, such as the Philippines, Japan, America, Australia, Europe, etc. Early detection systems for the genesis of tropical cyclones are still being developed by international collaborations such as The Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) in the Indian Ocean, Tropical Atmosphere Ocean (TAO) in the Pacific Ocean, and Prediction and Research Moored, Array in the Tropical Atlantic (PIRATA). To find out the early sign of a tropical cyclone, it is characterized by sea surface temperatures > 26.5 C, the growth of very broad and thick convective clouds, and rotating wind speeds of > 63 km/hour. For this reason, continuous observations are needed in the area where the tropical cyclone first developed. Observation equipment required includes satellite observations, buoys, and weather radar. Unfortunately, in the territory of Indonesia, especially in the Indian and Pacific oceans around Indonesia, this equipment is not equipped with such equipment due to very expensive funding factors and vandalism constraints. For this reason, in the future, national and international cooperation will be needed to start building an early warning system for the emergence of tropical cyclones among research centers globally.

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.

Understanding differences in tropical cyclone activity over the Arabian Sea and Bay of Bengal

Within the North Indian Ocean basin, tropical cyclone (TC) activity over the Bay of Bengal (BoB) is substantially greater than over the Arabian Sea (AS). The authors attempt to quantify the roles of specific environmental factors in order to understand the reasons for this difference between the two basins. Environmental variables are considered in the basin as a whole and in the immediate times and places at which cyclognesis and storm intensification occur. The results for the two sub-basins are compared to determine which environmental variables differed significantly between the sub-basins. A tropical cyclone genesis index (TCGI) is also examined to determine whether the AS-BoB difference can be explained in terms of the environment using this index. Though the overall level of activity in both is under-predicted by the index by about a factor of two, the index shows a relative diffrence between the basins that is approximately consistent with that in observations. Based on that partial success, climatologies of the individual factors that comprise the index are examined to determine which ones are most important in the AS-BoB diffrence. Column relative humidity and vertical wind shear emerge as the most likely candidates. A closer examination suggests that column relative humidity is the more important factor and thus that the AS has fewer cyclones than the BoB primarily because it has a drier lower troposphere.

Evolution of IMD’s operational extended range forecast system of tropical cyclogenesis over North Indian Ocean during 2010-2020

The post monsoon seasons (October-December; OND) are known to produce tropical cyclones (TCs) of severe intensity over the North Indian Ocean (NIO) and particularly over the Bay of Bengal (BoB). The evolution of operational extended range forecast (ERF) of cyclogenesis probability during 2010 to 2020 based on dynamical models have been discussed. The ERF of cyclogenesis probability based on ECMWF and CFSv1 dynamical models had a modest beginning in 2010 with reasonable performance in case of severe cyclonic storm ‘Jal’ formed during the first week of November. The 2015 cyclone season with active Arabian Sea and inactive BoB was also very well captured in the real time ERF. IMD implemented CFSv2 coupled model for operational ERF in 2017 and based on it the Genesis Potential Parameter (GPP) is calculated for four weeks by using the dynamical variables like vorticity, divergence, vertical wind shear & mid-level relative humidity and was tested for the ‘Ockhi’ cyclone of 24-30 November, 2017. The GPP in case of ‘Ockhi’ cyclone was well predicted in the ERF, however, with a lead time of only one week. The Improved GPP (IGPP) is used since 2019, which can be applied both over the Ocean and the land region. In the case of IGPP the vorticity and middle tropospheric humidity terms of GPP have been retained but the thermodynamic term is modified as the scaled and averaged equivalent potential temperature between 1000 and 500 hPa. The vertical shear between 850 and 200 hPa is scaled and averaged over an annular region between 100 and 200 km radii for each grid point. In case of Super cyclone “Amphan” it indicated the genesis of the system in “Week 1” and “Week 2” forecast and also its re-curvature northeastward like the observed track. The cyclone “Nisarga” over the Arabian Sea and its track towards western coast of India was well captured in week 1 forecast based on Initial Condition of 27 May, 2020. The IGPP also showed reasonable skill in ERF in predicting the genesis of three intense cyclones viz., ‘Gati’ during 21-24 November, ‘Nivar’ during 22-26 November and ‘Burehi’ during 30 November to 5 December and the two depressions of October, 2020. Considering the significant role of Madden Julian Oscillation (MJO) in BoB TC genesis, IMD is making use of the operational IGPP along with other parameters like current and forecast MJO, Tropical Cyclone Heat Potential etc and the value-added cyclogenesis probability outlook is being issued for two weeks on every Thursday.

Absence of Tropical Cyclone Genesis Over the Western North Pacific in July 2020 and Its Prediction by CFSv2

There were no TCs generated in July 2020 over the western North Pacific (WNP), which was the first time this had happened during since 1980. This study attempts to understand the cause of there having been no TCs generated in July 2020, and evaluates the prediction skill for the large-scale environmental conditions associated with the TC genesis number (TCGN). Results show that the main causes were the abnormal warming of sea surface temperature (SST) in the North Indian Ocean (NIO) and North Atlantic in July and the abnormal decrease in SST from April and May in the Niño4 region. The NIO SST can affect the large-scale environmental conditions via the SST–precipitation–wind feedback mechanism. Through the interaction between the tropical North Atlantic and the NIO, the abnormally warm North Atlantic SST further strengthened the impact of the NIO SST on the environmental conditions. The monthly difference (MD) of the Niño4 index from April to May is significantly correlated with the TCGN in July. Not only can the Niño4 MD in May affect the environmental conditions by affecting the WNP anticyclone, but it can also affect the NIO SST and precipitation anomalies through a shift in the position of the Walker circulation. Besides, the activity of the MJO also had a certain impact on the absence of TC genesis in July 2020. Although CFSv2 can successfully predict the local feedback affecting the July TCGN, it fails to forecast the large-scale environmental conditions associated with the absence of TC genesis over the WNP in July 2020.

Hurricane annual cycle controlled by both seeds and genesis probability

Understanding tropical cyclone (TC) climatology is a problem of profound societal significance and deep scientific interest. The annual cycle is the biggest radiatively forced signal in TC variability, presenting a key test of our understanding and modeling of TC activity. TCs over the North Atlantic (NA) basin, which are usually called hurricanes, have a sharp peak in the annual cycle, with more than half concentrated in only 3 mo (August to October), yet existing theories of TC genesis often predict a much smoother cycle. Here we apply a framework originally developed to study TC response to climate change in which TC genesis is determined by both the number of pre-TC synoptic disturbances (TC "seeds") and the probability of TC genesis from the seeds. The combination of seeds and probability predicts a more consistent hurricane annual cycle, reproducing the compact season, as well as the abrupt increase from July to August in the NA across observations and climate models. The seeds-probability TC genesis framework also successfully captures TC annual cycles in different basins. The concise representation of the climate sensitivity of TCs from the annual cycle to climate change indicates that the framework captures the essential elements of the TC climate connection.

Dynamical Seasonal Prediction of Tropical Cyclone Activity Using the FGOALS-f2 Ensemble Prediction System

AbstractThere is a distinct gap between tropical cyclone (TC) prediction skill and the societal demand for accurate predictions, especially in the western Pacific (WP) and North Atlantic (NA) basins, where densely populated areas are frequently affected by intense TC events. In this study, seasonal prediction skill for TC activity in the WP and NA of the fully coupled FGOALS-f2 V1.0 dynamical prediction system is evaluated. In total, 36 years of monthly hindcasts from 1981 to 2016 were completed with 24 ensemble members. The FGOALS-f2 V1.0 system has been used for real-time predictions since June 2017 with 35 ensemble members, and has been operationally used in the two operational prediction centers of China. Our evaluation indicates that FGOALS-f2 V1.0 can reasonably reproduce the density of TC genesis locations and tracks in the WP and NA. The model shows significant skill in terms of the TC number correlation in the WP (0.60) and the NA (0.61) from 1981 to 2015; however, the model underestimates accumulated cyclone energy. When the number of ensemble members was increased from 2 to 24, the correlation coefficients clearly increased (from 0.21 to 0.60 in the WP, and from 0.18 to 0.61 in the NA). FGOALS-f2 V1.0 also successfully reproduces the genesis potential index pattern and the relationship between El Niño–Southern Oscillation and TC activity, which is one of the dominant contributors to TC seasonal prediction skill. However, the biases in large-scale factors are barriers to the improvement of the seasonal prediction skill, e.g., larger wind shear, higher relative humidity, and weaker potential intensity of TCs. For real-time predictions in the WP, FGOALS-f2 V1.0 demonstrates a skillful prediction for track density in terms of landfalling TCs, and the model successfully forecasts the correct sign of seasonal anomalies of landfalling TCs for various regions in China.

Subsidence Warming in the Tropical Cyclogenesis of Cindy (2017): CPEX Observations and Coupled Modeling

AbstractThe formation of tropical cyclones (TCs) in unfavorable large-scale environments remains a challenge for TC forecasting. Tropical Storm (TS) Cindy (2017) formed at 1800 UTC 20 June 2017 in the Gulf of Mexico despite strong vertical wind shear, low midtropospheric relative humidity, and poorly organized convection. A key to TC genesis is the initial development of a warm core within an emergent cyclonic vortex, a process that occurs on small spatial scales and is often difficult to observe. TS Cindy was observed during the Convective Processes Experiment (CPEX) field campaign in 2017 by the NASA DC-8 aircraft, equipped with a Doppler wind lidar, precipitation radar, and GPS dropsondes. This study combines CPEX observations and a cloud-resolving, fully coupled atmosphere–wave–ocean numerical simulation to investigate the formation of TS Cindy. Prior to TC genesis, a shallow cyclonic circulation was embedded in a deep layer of west-southwesterly flow associated with an upper-level trough. Within the disturbance, a warm and dry anomaly was observed by dropsondes near the center of the cyclonic circulation, with a maximum at about the 2.5-km level. In the coupled model simulation, the temperature perturbation reached 5°C along with a dewpoint temperature depression of 8°C. Backward trajectory analysis shows that subsidence is primarily associated with a thermally indirect circulation along the western flank of the storm. Air parcels descend more than 1000 m toward the lower troposphere while warming up by 9°–12°C. The subsidence-induced virtual temperature perturbation in the 1.5–3.5-km layer accounts for 50% of the sea level pressure depression. Subsidence warming, therefore, played a key role in the genesis of TS Cindy.

Modified tropical cyclone genesis potential index over the Bay of Bengal during southwest and post-monsoon seasons

Modified tropical cyclone genesis potential index over the Bay of Bengal during southwest and post-monsoon seasons

Tropical Cyclone Footprints in Long-Term Mean State and Multiscale Climate Variability in the Western North Pacific as Seen in the JRA-55 Reanalysis

AbstractThe monsoon trough and subtropical high have long been acknowledged to exert a substantial modulating effect on the genesis and development of tropical cyclones (TCs) in the western North Pacific (WNP). However, the potential upscaling effect of TCs on large-scale circulation remains poorly understood. This study revealed the considerable contributions of TCs to the climate mean state and variability in the WNP between 1958 and 2019, characterized by a strengthened monsoon trough and weakened subtropical anticyclonic circulation in the lower troposphere, enhanced anticyclonic circulation in the upper troposphere, and warming throughout the troposphere. TCs constituted distinct footprints in the long-term mean states of the WNP summer monsoon, and their contributions increased intraseasonal and interannual variance by 50%–70%. The interdecadal variations and long-term trends in intraseasonal variance were mainly due to the year-to-year fluctuations in TC activity. The size of TC footprints was positively correlated with the magnitude of TC activity. Our findings suggest that the full understanding of climate variability and changes cannot be achieved simply on the basis of low-frequency, large-scale circulations. Rather, TCs must be regarded as a crucial component in the climate system, and their interactions with large-scale circulations require thorough exploration. The long-term dataset created in this study provides an opportunity to study the interaction between TCs and TC-free large-scale circulations to advance our understanding of climate variability in the WNP. Our findings also indicate that realistic climate projections must involve the accurate simulations of TCs.

Monthly prediction of tropical cyclone activity over the South China Sea using the FGOALS-f2 ensemble prediction system

The monthly prediction skill for tropical cyclone (TC) activity in the South China Sea (SCS) during the typhoon season (July to November) was evaluated using the FGOALS-f2 ensemble prediction system. Specifically, the prediction skill of the system at a 10-day lead time for monthly TC activity is given based on 35-year (1981–2015) hindcasts with 24 ensemble members. The results show that FGOALS-f2 can capture the climatology of TC track densities in each month, but there is a delay in the monthly southward movement in the area of high track densities of TCs. The temporal correlation coefficient of TC frequency fluctuates across the different months, among which the highest appears in October (0.59) and the lowest in August (0.30). The rank correlation coefficients of TC track densities are relatively higher (R > 0.6) in July, September, and November, while those in August and October are relatively lower (R within 0.2 to 0.6). For real-time prediction of TCs in 2020 (July to November), FGOALS-f2 demonstrates a skillful probabilistic prediction of TC genesis and movement. Besides, the system successfully forecasts the correct sign of monthly anomalies of TC frequency and accumulated cyclone energy for 2020 (July to November) in the SCS.摘要本研究基于新一代FGOALS-f2动力集合预测系统35年 (1981–2015年) 的热带气旋历史回报试验对南海台风季 (7–11月) 热带气旋活动超前10天的月预测技巧进行评估, 并对2020年南海台风季热带气旋活动进行了实时月预测尝试. 结果表明: FGOALS-f2能较好地预测南海热带气旋路径密度演变特征, 预测的热带气旋生成个数与实际观测结果的相关系数在0.2–0.6之间. 2020年实时预测方面, FGOALS-f2较为准确地给出了南海台风季热带气旋生成位置与移动路径的概率预测, 且对南海热带气旋生成个数和强度异常的确定性预测结果与观测接近.

Radiation Feedback Accelerates the Formation of Typhoon Haiyan (2013): The Critical Role of Mid‐Level Circulation

AbstractThe prediction of tropical cyclones (TC) genesis remains a great challenge. This study shows that the radiation (mostly cloud‐radiation) feedback accelerates the formation of Typhoon Haiyan (2013) by first enhancing the mid‐level circulation. Simulations indicate that the response of the mid‐level circulation is about one day ahead of the low‐level circulation when radiation feedback is removed. The radiation‐heating anomaly in the disturbance region induces a secondary circulation transporting more water vapor upward at high levels. Thus, the stratiform precipitation is enhanced, manifested as the expansion of the stratiform coverage and the increase of the vertical gradient of latent heating. This leads to a stronger mid‐level potential vorticity production and enhancement of the mid‐level circulation, finally promoting TC genesis. Comparing with previous ideas on how radiation feedback accelerates TC genesis, our results emphasize the critical role of the mid‐level circulation and the cloud‐radiation‐induced thermodynamical and dynamic responses in the vertical.

An Observational Study of Aerosols and Tropical Cyclones over the Eastern Atlantic Ocean Basin for Recent Hurricane Seasons

The aerosol vertical distribution in the tropical cyclone (TC) main development region (MDR) during the recent active hurricane seasons (2015–2018) was investigated using observations from NASA’s Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) Satellite. The Total Attenuated Backscatter (TAB) at 532 nm was measured by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP Lidar) onboard CALIPSO which is a polar orbiting satellite that evaluates the role clouds and atmospheric aerosols play in Earth’s weather, climate and air quality. The TAB was used to illustrate the dispersion and magnitude of the aerosol vertical distribution in the TC-genesis region. A combination of extinction quality flag, cloud fraction, and cloud-aerosol discrimination (CAD) scores were used to filter out the impact of clouds. To better describe the qualitative and quantitative difference of aerosol along the paths of African Easterly Waves (AEWs), the MDR was further divided into two domains from 18° W to 30° W (Domain 1) and 30° W to 45° W (Domain 2), respectively. The distribution of average aerosol concentration from the time of active cyclogenesis was compared and quantified between each case. The resulting observations suggest that there are two distinct layers of aerosols in the vertical profile, a near surface layer from 0.5–1.75 km and an upper layer at 1.75–5 km in altitude. A quantification of the total aerosol concentration values indicate domain 2 cases were associated with higher aerosol concentrations than domain 1 cases. The environmental variables such as sea surface temperature (SST), vertical windshear (VWS), and relative humidity (RH) tended to be favorable for genesis to occur. Among all cases in this study, the results suggested tropical cyclone genesis and further development occurred under dust-loaded conditions while the environmental variables were favorable, indicating that dust aerosols may not play a significant role in inhibiting the genesis process of TCs.

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.

A forecasting approach of tropical cyclone genesis based on thresholds of multi-physical parameters and its verification using ECMWF model data

While considerable studies have proved that the track and intensity forecasts of tropical cyclone (TC) relied heavily on output from numerical weather prediction (NWP) model, few researches investigated how well NWP models forecast TC genesis in the western North Pacific (WNP) basin. In order to understand the characteristics of TC genesis forecast in WNP basin by NWP models, this study derives a set of criteria to identify the formation of TC using historical data and verifies it based on ECMWF model data between 2013 and 2015. The results show that the percentile values adopted as the criteria thresholds have a significant impact on the performance of algorithm based on the criteria. A reasonable adjustment of threshold in a specific interval can effectively improve the TC genesis prediction. For example, in the WNP basin the forecast results are most sensitive to small changes in the relative vorticity on the 850 hPa level. The results of forecast test of the optimal threshold combination scheme indicate that the turning point of performance lies between 24 and 48 hours with regard to the hit rate in the 12-72 hours prior to the formation of TC. For lead time less than 24 hours, the hit rate was basically maintained at a high level above 0.7 with a small decrease. After that, the performance drops sharply before stabilizing beyond 48 hours. In addition, the performance of the TC genesis prediction in ECMWF model varies significantly from year to year and also in different WNP regions. It performs better to the east of the Philippines than over the South China Sea (SCS). On the other hand, high false alarm (FA) rates are found in the central parts of the SCS up to the waters around the Philippines and the central part of the WNP. The significant discrepancy in ECMWF’s performance can also be observed between different basins. Within the 24 hours before the genesis of a TC, the forecasts for the WNP basin verify better than those for the Atlantic basin.

Central American mountains inhibit eastern North Pacific seasonal tropical cyclone activity

The eastern North Pacific (ENP) has the highest density of tropical cyclones (TCs) on earth, and yet the controls on TCs, from individual events to seasonal totals, remain poorly understood. One effect that has not been fully considered is the unique geography of the Central American mountains. Although observational studies suggest these mountains can readily fuel individual TCs through dynamical processes, here we show that these mountains indeed play the opposite role on the seasonal timescale, hindering seasonal ENP TC activity by up to 35%. We found that these mountains significantly interrupt the abundant moisture transport from the Caribbean Sea to the ENP, limiting deep convection over the open ocean area where TCs preferentially occur. This study advances our fundamental understanding of ENP TC genesis mechanisms across the weather-to-climate timescales, and also highlights the importance of topography representation in improving the ENP regional climate simulations, as well as TC seasonal predictions and future projections.

How does the onset time of El Niño events affect tropical cyclone genesis and intensity over the western North Pacific?

AbstractPrevious studies have shown that the equatorial Pacific Ocean warming can be separated into spring (April~June; SP) and summer (July~October; SU) modes based on the onset time of El Niño events. Different onset times are caused by different atmospheric and oceanic conditions, which, in turn, should influence tropical cyclone (TC) activity. While the location of TC formation in the western North Pacific (WNP) shifts significantly eastward and equatorward in the SP type relative to climatology (1951–2017), only an eastward shift occurs in the SU type. Only intense TCs (category 1 + 2 + 3 and 4 + 5) have an obvious eastward and equatorward shift in the SP type, with no such shift in any TC category in the SU type. The number of category 4 + 5 TCs in the SP type increases significantly, but fewer category 1 + 2 + 3 TCs in the SU type. In the SP type, TCs are generally stronger, with more in category 4 + 5, and longer life spans occurring across all categories. However, no obvious change in TC intensity can be identified except for category 1 + 2 + 3 in the SU type. The eastward and equatorward shift of the mean genesis position in the developing phases of SP can be attributed to a southeastward shift of favourable atmospheric and oceanic conditions, which include weaker zonal vertical wind shear, stronger relative vorticity, higher mid‐level relative humidity, and enhanced upper‐ocean temperature (5–100 m). All these conditions are mainly related to the onset time of the westerly anomalies near the central equatorial Pacific. These results suggest that the onset time of El Niño events could serve as a good forecasting indicator for TC genesis position and intensity over the WNP.

Seasonal to Interannual Variability of Vertical Wind Shear and Its Relationship with Tropical Cyclogenesis in the Mozambique Channel

This article explores the relationship between vertical wind shear (VWS) and tropical cyclone (TC) genesis in the Mozambique Channel (MC) for the period 1979–2019. Additionally, SST, low-level relative vorticity, 700 hPa relative humidity and upper-level divergence were also analyzed for the peak cyclogenesis months to explore their relative contributions. The analyses were done using NCEP/NCAR Reanalysis-1 for the atmospheric fields, monthly Optimum Interpolation SST V2, and for the cyclogenesis the TC best track data from the La Reunion–Regional Specialized Meteorological and Joint Typhoon Warning Centre. A total of 43 TCs generated in the MC were observed for the analysed period. The maximum frequency of cyclogenesis in the MC was observed during January and February and the spatial location of maximum TC genesis was coincident with the minimum values of the VWS. The VWS showed significant correlations with TC intensity, particularly when considering the upper atmosphere (200–500 hPa) or the bulk (200–850 hPa) VWS. The mean composites of the cyclogenesis months over the MC of SST, relative humidity at 700 hPa, divergence at upper atmosphere, showed significant values. However, linear correlations between these factors vs. TC genesis frequency and intensity were not significant. Analyses of interannual correlations between VWS and Niño-3.4 (subtropical southwest Indian Ocean dipole-SIOD) showed statistically significant positive (negative) correlations at different lags, suggesting that La Niña and the positive phase of SIOD conditions are favorable to weaker VWS and thus to intensification of TCs in the Mozambique Channel. Thirteen landfall cases were observed with seven over the Madagascar west coast and six over the Mozambique coast. The landfall over the Madagascar (Mozambique) coast was associated with strengthened (weakened) VWS.

Influence of Different ENSO Types on Tropical Cyclone Rapid Intensification Over the Western North Pacific

AbstractThis study investigates the influence of different types of El Niño‐Southern Oscillation (ENSO) on the properties of tropical cyclone (TC) rapid intensification (RI) over the western North Pacific (WNP). The RI numbers during all types of ENSO events have no significant difference from that of neutral years, except that eastern Pacific (EP) El Niño corresponds to fewer RI cases. The most robust response of RI properties is occurrence position. The mean RI occurrence position in La Niña events shifts northwards by about 2° in latitude during the peak TC season (July–September), which may be related to the northwestward shift of TC genesis positions. On the other hand, the mean RI occurrence positions during EP El Niño and La Niña events in the late TC season (October–December) shift westwards by about 10° and 14° in longitude, respectively. This westward shift in RI occurrence positions is mainly caused by the favorable conditions over the western part of the WNP in the late TC season. During October–December, EP El Niño shifts to a La Niña phase and warms the upper ocean of the western part of the WNP. La Niña events result in similar upper ocean conditions because they maintain negative phases during October–December. The warmed upper ocean is also accompanied by a moistened mid‐troposphere and weakened vertical wind shear, which are favorable for RI occurrence.

The influence of upper level surrounding winds on tropical cyclone motion: a case study of the Bay of Bengal region

The study is an attempt to classify the observed tropical cyclone (TC) track patterns over the Bay of Bengal (BoB) region. It is hypothesized that winds at 200 hPa level can determine the direction of TC motion. We found that the mean TC genesis locations were shifted southward from October to December with the shifting of the regional wind circulation patterns. The wind circulation pattern during the straight moving TCs were very similar to the recurving TCs. Straight moving TCs were embedded in a deep easterly flow whereas the recurving TCs significantly moved further northward compared to the straight tracks which brought them under the influence of mid-latitude westerlies and changed their paths towards northeast. A TC track prediction scheme is presented to evaluate the effectiveness of 200 hPa level surrounding winds in TC motion prediction. Sea level pressure fall and relative vorticity advection also have been used as supportive factors. We found results comparable with other TC track prediction models which did not consider the upper level surrounding winds as a TC track predictor. Hence, the use of 200 hPa level winds might be beneficial to improve the existing TC track prediction models over the BoB region.