The tropical cyclone genesis number (TCGN) in July–October (JASO) over the western North Pacific (WNP) exhibits a robust interannual variation. It shows a longitudinally tri-pole pattern with a high in the eastern WNP and South China Sea (SCS) and a low in the western WNP, which explain 42.2 and 23.4 % of total TCGN variance in the eastern WNP and SCS, respectively. The high–low–high pattern is similar to that derived from a TC genesis potential index (GPI). To understand the cause of the longitudinal distribution of the dominant interannual mode, we examine the contributions of environmental parameters associated with GPI. It is found that relative humidity and relative vorticity are important factors responsible for TC variability in the SCS, while vertical shear and relative vorticity are crucial in determining TC activity in eastern WNP. A simultaneous correlation analysis shows that the WNP TCGN in JASO is significantly negatively correlated (with a correlation coefficient of −0.5) with sea surface temperature anomalies (SSTA) in the tropical North Atlantic (TNA). The longitudinal distribution of TC genesis frequency regressed onto TNA SSTA resembles that regressed upon the WNP TCGN series. The spatial patterns of regressed environmental variables onto the SSTA over the TNA also resemble those onto TCGN in the WNP, that is, an increase of relative humidity in the SCS and a weakening of vertical shear in the eastern WNP are all associated with cold SSTA in the TNA. Further analyses show that the cold SSTA in the TNA induce a negative heating in situ. In response to this negative heating, a low (upper)-level anomalous aniti-cyclonic (cyclonic) flows appear over the subtropical North Atlantic and eastern North Pacific, and to east of the cold SSTA, anomalous low-level westerlies appear in the tropical Indian Ocean. Given pronounced mean westerlies in northern Indian Ocean in boreal summer, the anomalous westerly flows increase local surface wind speed and surface evaporation and cool the SST in situ. Cold SSTA in northern Indian Ocean further suppress local convection, inducing anomalous westerlies to its east, leading to enhanced cyclonic vorticity and low surface pressure over the WNP monsoon trough region. Idealized numerical experiments further confirm this Indian Ocean relaying effect, through which cold SSTA in the tropical Atlantic exert a remote impact to circulation in the WNP.

Effects of tropical North Atlantic SST on tropical cyclone genesis in the western North Pacific

The Bay of Bengal (BoB) basin in the North Indian Ocean reports the highest number of tropical cyclones, and the frequency is much higher almost five times compared with the Arabian Sea (AS). Semi-enclosed nature of this basin in conjunction with its funnel shape steers the cyclone pathway striking the land. Historical reports signify that most deadly cyclones with highest catastrophe and death tolls occurred in this basin. The four maritime states located in the east coast of India are highly vulnerable to tropical cyclones. Hence, there is a need to understand the risk factors attributed from extreme winds, storm surge, and associated coastal inundation. Historical data signify higher frequency of post-monsoon cyclones compared with pre-monsoon cyclones in the BoB. This study aims to develop a synthetic track or the most probable cyclone track for each state located along the east coast of India. The synthetic track is a general guidance for numerical models, with utility to evaluate and assess the risk factors along coastal belts. This study also deals with aspects on frequency and cyclogenesis locations in the BoB based on analysis from historical data. Results for decadal scale variability signify that in the present decade, the eastern BoB is quite conducive for cyclogenesis. In this region, oceanic thermal field exhibits a warming trend, and found to extend up to water depth of 600 m. In addition, this study also evaluates the energy metrics such as Power Dissipation Index (PDI), and Accumulated Cyclone Energy (ACE) for tropical cyclones that occurred during past four decades in the BoB. The results signify that PDI for tropical cyclones in the present decade is six times higher when compared with the past. The study has practical applications for mapping coastal vulnerability in a changing climate. Tropical cyclones are associated with devastation resulting in enormous loss of life and property during its landfall. In a broad perspective, lack of disaster management pre- paredness would contribute to high vulnerability. The gen- esis of a cyclone usually referred as 'cyclogenesis' is the development stage or strengthening of a cyclonic circula- tion in the atmosphere during the initial stage of its forma- tion. Tropical cyclones form over warm ocean surface, and favourable conditions are sea-surface temperature (SST) greater than 26 ∘ C; low magnitude of vertical wind shear; larger low-level vorticity; and higher mid-troposphere rel- ative humidity. In global oceans, the western part of the North Pacific reported the maximum number of tropical cyclones (average of 26 per year) followed by eastern North Pacific (average of 17 per year). The South Indian Ocean and North Atlantic has an average of 10 per year, while regions in the North Indian Ocean (NIO) have an average of five cyclones per year (Niyas et al., 2009). Notably, the NIO alone accounts for about 7% of global tropical cyclone activity, with a frequency about five

Assessment on historical cyclone tracks in the Bay of Bengal, east coast of India

Tropical cyclones devastate large areas, take numerous lives and damage extensive property in Bangladesh. Research on landfalling tropical cyclones affecting Bangladesh has primarily focused on events occurring since AD 1960 with limited work examining earlier historical records. We rectify this gap by developing a new tropical cyclone catalogue that maximizes the use of available sources. The catalogue consists of 304 tropical cyclones that occurred between AD 1000 and AD 2009 and made landfall along the coasts of Bangladesh, eastern India and Myanmar. One hundred and ninety-three events directly struck Bangladesh between AD 1484 and AD 2009, although the precise landfall location of six events is unknown. Of the remaining 187 events, Cox's Bazar, Chittagong, Noakhali, Barisal and Khulna were struck by 30, 46, 19, 41 and 51 tropical cyclones, respectively. There is a paucity of data about tropical cyclones before AD 1900 and this increases the further back in time we go. Inconsistencies in reported storm surge height, wind speed and exaggerations in the reporting of deaths are identified and discussed. Some 20 72 509 human deaths in Bangladesh are associated with 71 tropical cyclones that occurred between AD 1484 and AD 2009. Between AD 1923 and AD 2009, 11 tropical cyclones caused 94 35 000 people to become homeless and between AD 1961 and AD 2009, 10 tropical cyclones resulted in economic damage of over US$ 4.6 billion. Analysis of the deaths and damage associated with tropical cyclones in AD 1970, AD 1991 and AD 2007 indicates that while the number of deaths decreased between events, economic damage and the number of people made homeless increased. There are positive and significant correlations between increasing storm surge height and increasing human fatalities (r = 0.60, p < 0.01) and increasing human injuries and greater wind speed (r = 0.45, p < 0.01). Despite our best efforts, the catalogue is incomplete. As such, we suggest further ‘deep’ archival research coupled with regional geological studies of palaeostorm events to gain a more sophisticated understanding of the hazard. Our results have implications for both risk assessment and disaster risk reduction.

A new catalogue of tropical cyclones of the northern Bay of Bengal and the distribution and effects of selected landfalling events in Bangladesh

The role of the El Nino–Southern Oscillation (ENSO) on the modulation of tropical cyclone activity over the Bay of Bengal (BoB) for the 1979–2011 period is examined. It is shown that Nino-3.4 sea surface temperature (SST) anomalies are negatively correlated with the BoB tropical cyclone activity to a statistically significant percentage by a lead time of 5 months. Composites of 10-m zonal winds exhibit greater variance during La Nina events, favoring the development of low-level cyclonic vorticity. Low vertical wind shear over the central and northern BoB also aids in the development of tropical cyclones during La Nina events. Increased relative humidity is the result of enhanced moisture transport and higher precipitable water under La Nina conditions. Furthermore, storm-relative composites of relative humidity show stronger moisture pulses over the BoB during La Nina. The enhanced moisture associated with tropical cyclogenesis likely aids in the development and strengthening of the systems. ENSO...

/pdf/enso-modulated-cyclogenesis-over-the-bay-of-bengal-28wudnc1ld.pdf

ENSO-Modulated Cyclogenesis over the Bay of Bengal*

This study unravels the physical link between the weakening of the monsoon circulation and the decreasing trend in the frequency of monsoon depressions over the Bay of Bengal. Based on the analysis of the terms of Genesis Potential Index, an empirical index to quantify the relative contribution of large scale environmental variables responsible for the modulation of storms, it is shown here that the reduction in the mid-tropospheric relative humidity is the most important reason for the decrease in the number of monsoon depressions. The net reduction of relative humidity over the Bay of Bengal is primarily due to the decrease in the moisture flux convergence, which is attributed to the weakening of the low level jet, a characteristic feature of monsoon circulation. Further, the anomalous moisture convergence over the western equatorial Indian Ocean associated with the rapid warming of the sea surface, reduces the moisture advection into the Bay of Bengal and hence adversely affect the genesis/intensification of monsoon depressions. Hence, the reduction in the number of monsoon depression over the Bay of Bengal could be one of the manifestations of the differential rates in the observed warming trend of the Indian Ocean basin

https://iopscience.iop.org/article/10.1088/1748-9326/11/1/014011/pdf

On the decreasing trend of the number of monsoon depressions in the Bay of Bengal

The annual cycle of tropical cyclone (TC) frequency over the Bay of Bengal (BoB) exhibits a notable bimodal character, different from a single peak in other basins. The causes of this peculiar feature were investigated through the diagnosis of a genesis potential index (GPI) with the use of the NCEP Reanalysis I dataset during the period 1981–2009. A methodology was developed to quantitatively assess the relative contributions of four environmental parameters. Different from a conventional view that the seasonal change of vertical shear causes the bimodal feature, it was found that the strengthened vertical shear alone from boreal spring to summer cannot overcome the relative humidity effect. It is the combined effect of vertical shear, vorticity, and SST that leads to the GPI minimum in boreal summer. It is noted that TC frequency in October–November is higher than that in April–May, which is primarily attributed to the difference of mean relative humidity between the two periods. In contrast, mo...

/pdf/bimodal-character-of-cyclone-climatology-in-the-bay-of-vh81l1y7xm.pdf

Bimodal Character of Cyclone Climatology in the Bay of Bengal Modulated by Monsoon Seasonal Cycle

Monsoon onset vortex (OV) in the form of tropical cyclone is often observed in the pre-monsoon period and contributes to the subsequent abrupt establishment of summer monsoon over the Bay of Bengal (BoB). It is identified here that all historical OVs occurred during the convection-enhanced phase of the first northward-propagating intra-seasonal oscillation (FNISO). The individual contributions from the four large scale environmental fields associated with the intra-seasonal variations to the cyclone genesis are diagnosed with the aid of the genesis potential index. The significant moistening of mid-level atmosphere, which is embedded in the FNISO convection-enhanced phase, is shown to be the primary factor leading to the cyclone genesis. The water vapor budget analysis is further done to understand the governing process for the mid-level humidity increase. It is clearly seen that the vertical advection process, dominated by the anomalous vertical advection of the mean vertical water vapor gradient, plays the critical role. Hence the OVs are shown to be strongly modulated by FNISOs, both of which are important elements of the complex story of the BoB monsoon onset.

/pdf/strong-modulations-on-the-bay-of-bengal-monsoon-onset-vortex-119tbdxltr.pdf

Strong modulations on the Bay of Bengal monsoon onset vortex by the first northward-propagating intra-seasonal oscillation

https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/asl.636

What controls the interannual variation of tropical cyclone genesis frequency over Bay of Bengal in the post-monsoon peak season?

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018GL078109

Impacts of ENSO on the Bay of Bengal Summer Monsoon Onset via Modulating the Intraseasonal Oscillation

Sea surface temperature (SST) reaches its annual maximum just before the summer monsoon onset and collapses soon after in the central areas of the Bay of Bengal (BoB). Here, the impact of the peak in the pre-monsoon SST on triggering the earliest monsoon onset in the BoB is investigated, with a focus on the role they play in driving the first-branch northward-propagating intra-seasonal oscillations (FNISOs) over the equatorial Eastern Indian Ocean (EIO). During the calm pre-monsoon period, sea surface warming in the BoB could increase the surface equivalent potential temperature (θe) in several ways. Firstly, warming of the sea surface heats the surface air through sensible heating, which forces the air temperature to follow the SST. The elevated air surface temperature accounts for 30 % of the surface θe growth. Furthermore, the elevated air temperature raises the water vapor capacity of the surface air to accommodate more water vapor. Constrained by the observation that the surface relative humidity is maintained nearly constant during the monsoon transition period, the surface specific humidity exhibits a significant increase, according to the Clausius–Clapeyron relationship. Budget analysis indicates that the additional moisture is primarily obtained from sea surface evaporation, which also exhibits a weak increasing trend due to the sea surface warming. In this way, it contributes about 70 % to the surface θe growth. The rapid SST increase during the pre-monsoon period preconditions the summer monsoon onset over the BoB through its contributions to significantly increase the surface θe, which eventually establishes the meridional asymmetry of the atmospheric convective instability in the EIO. The pre-established greater convective instability leads to the FNISO convections, and the summer monsoon is triggered in the BoB region.

/pdf/possible-role-of-pre-monsoon-sea-surface-warming-in-driving-2qbvwwohl3.pdf

Zhi Li

Papers

Bimodal Character of Cyclone Climatology in the Bay of Bengal Modulated by Monsoon Seasonal Cycle

Strong modulations on the Bay of Bengal monsoon onset vortex by the first northward-propagating intra-seasonal oscillation

What controls the interannual variation of tropical cyclone genesis frequency over Bay of Bengal in the post-monsoon peak season?

Impacts of ENSO on the Bay of Bengal Summer Monsoon Onset via Modulating the Intraseasonal Oscillation

Possible role of pre-monsoon sea surface warming in driving the summer monsoon onset over the Bay of Bengal