Cyclone Season Research Articles

The Uncharacteristic Occurrence of the June 2013 Biomass-Burning Haze Event in Southeast Asia: Effects of the Madden-Julian Oscillation and Tropical Cyclone Activity

One of the worst haze events to ever hit Peninsular Malaysia occurred in June 2013 due to smoke from Riau, Central Sumatra. While biomass-burning in the region is common, the early occurrence of a haze episode of this magnitude was uncharacteristic of the seasonality of extreme fire events, which usually occur between August and October in the Maritime Continent (MC). This study aims to investigate the phenomenology of the June 2013 haze event and its underlying meteorological forcing agents. The aerosol and meteorological environment during the event is examined using the Moderate Resolution Imaging Spectroradiometer (MODIS) active fire hotspot detections and aerosol optical thickness retrievals, satellite-based precipitation retrievals, and meteorological indices. These datasets are then supported by a WRF-Chem simulation to provide a comprehensive picture of the event’s meteorology and aerosol transport phenomenology. While extreme fire events are more characteristic of El Nino years, the MODIS fire count over the MC in June for the years 2001–2015 was highest in 2013 when neutral El Nino/Southern Oscillation (ENSO) conditions prevailed. Although, the mean daily precipitation for June 2013 was below average for June 2003–2015. An early active tropical cyclone (TC) season occurred in 2013, and results show that the combined induced subsidence and flow enhancement due to TC Bebinca and the dry phases of a strong Madden–Julian Oscillation (MJO) event contributed to the event intensification. Results also show that Bebinca induced a decrease in surface relative humidity of at least 10% over Riau, where fire hotspots were concentrated.

Impact of the tropical cyclone Ockhi on ecological and geomorphological structures of the small low-lying Islands in the Central Indian Ocean

Ockhi, was an intense tropical cyclone that devastated parts of Sri Lanka and India in 2017 and was the most intense tropical cyclone in the Arabian Sea, Central Indian Ocean, since Cyclone Megh in 2015. The ninth depression, and the third and strongest named storm during 2017, North Indian Ocean cyclone season, Ockhi, originated from an area of low pressure that formed in the eastern Andaman Sea on November 21, 2017. Here we describe the impacts of Ockhi on benthic coral reef assemblages of an atoll island directly in the path of the cyclone. We assessed the effect of the cyclone in the reefs around Kavaratti Island, by comparing the data from recent pre-Ockhi documentation with its present status. The study provided a unique opportunity to identify and describe the impacts of an intense tropical cyclone on reefs of a small low lying island in the Central Indian Ocean, which is often assumed to be vulnerable to physical disturbance due to their poorly lithified framework. For the present study, the entire Kavaratti atoll was demarcated into three zones (lagoon, western outer reef, and eastern outer reef), and data was gathered from 29 pre-fixed stations. Observed impacts of Ockhi were site-specific and spatially highly heterogeneous, but appeared to have been strongly influenced by the level of exposure, the site received, and the biological make-up of individual reefs sites. Live coral cover on western reefs had a considerably lower impact due to the shelter provided by the land and the trends in the substrate composition before and after Ockhi is almost similar. However, drastic devastation was observed in the eastern reef system, after the Ockhi event. The combination of repeated, intense cyclonic activities and mass bleaching events will act together to significantly hamper the rapid recovery of these reefs and also adversely affect the livelihood of the community.

Activity features of tropical cyclone in Truong Sa archipelago region, Khanh Hoa province

This paper presents the study results of occurrence feature of tropical cyclone in Truong Sa archipelago region in the period from 1884 to 2018. Data on tropical cyclone such as storm track, intensity... were taken from the website www.weather.unisys.com/hurricane (National Weather Service, USA). The study results show that the study region is the most active area of tropical cyclone activity with about 1.61 times/year. The occurrence frequency of tropical cyclone was increasing from 1988 to 2018. The period from 1884 to 1944 the occurrence frequency of tropical cyclone was about 1 time/year, from 1945 to 2018 the occurrence frequency of tropical cyclone was higher at about 2.4 times/year. Especially, in the years of 2013 and 2017, tropical cyclone occurred 6 times/year. In the years of 2008, 1998, 1993, 1983, 1962, it occurred 5 times/year. The study region underwent the effect of two tropical cyclone seasons, the main season was from September to January of the next year with the occurrence frequency of tropical cyclone being about 4 times higher than that of the season from March to August. The period of highest occurrence frequency of tropical cyclone was from October to December whereas the lowest occurrence frequency of tropical cyclone was in July. The main season of tropical cyclone activity coincided with strong Northeast monsoon period.

On the Mechanisms of the Active 2018 Tropical Cyclone Season in the North Pacific

AbstractThe 2018 tropical cyclone (TC) season in the North Pacific was very active, with 39 tropical storms including eight typhoons/hurricanes. This activity was successfully predicted up to 5 months in advance by the Geophysical Fluid Dynamics Laboratory Forecast‐Oriented Low Ocean Resolution (FLOR) global coupled model. In this work, a suite of idealized experiments with three dynamical global models (FLOR, Nonhydrostatic Icosahedral Atmospheric Model, and Meteorological Research Institute Atmospheric General Circulation Model) was used to show that the active 2018 TC season was primarily caused by warming in the subtropical Pacific and secondarily by warming in the tropical Pacific. Furthermore, the potential effect of anthropogenic forcing on the active 2018 TC season was investigated using two of the global models (FLOR and Meteorological Research Institute Atmospheric General Circulation Model). The models projected opposite signs for the changes in TC frequency in the North Pacific by an increase in anthropogenic forcing, thereby highlighting the substantial uncertainty and model dependence in the possible impact of anthropogenic forcing on Pacific TC activity.

Bias Correction of Tropical Cyclone Parameters in the ECMWF Ensemble Prediction System in Australia

Abstract Global ensemble prediction systems have considerable ability to predict tropical cyclone (TC) formation and subsequent evolution. However, because of their relatively coarse resolution, their predictions of intensity and structure are biased. The biases arise mainly from underestimated intensities and enlarged radii, in particular the radius of maximum winds. This paper describes a method to reduce this limitation by bias correcting TCs in the ECMWF Ensemble Prediction System (ECMWF-EPS) for a region northwest of Australia. A bias-corrected TC system will provide more accurate forecasts of TC-generated wind and waves to the oil and gas industry, which operates a large number of offshore facilities in the region. It will also enable improvements in response decisions for weather sensitive operations that affect downtime and safety risks. The bias-correction technique uses a multivariate linear regression method to bias correct storm intensity and structure. Special strategies are used to maintain ensemble spread after bias correction and to predict the radius of maximum winds using a climatological relationship based on wind intensity and storm latitude. The system was trained on the Australian best track TC data and the ECMWF-EPS TC data from two cyclone seasons. The system inserts corrected vortices into the original surface wind and pressure fields, which are then used to estimate wind exceedance probabilities, and to drive a wave model. The bias-corrected system has shown an overall skill improvement over the uncorrected ECMWF-EPS for all TC intensity and structure parameters with the most significant gains for the maximum wind speed prediction. The system has been operational at the Australian Bureau of Meteorology since November 2016.

Statistical Projection of the North Atlantic Storm Tracks

AbstractA method for empirical–statistical downscaling was adapted to project seasonal cyclone density over the North Atlantic Ocean. To this aim, the seasonal mean cyclone density was derived from instantaneous values of the 6-h mean sea level pressure (SLP) reanalysis fields. The cyclone density was then combined with seasonal mean reanalysis and global climate model projections of SLP or 500-hPa geopotential height to obtain future projections of the North Atlantic storm tracks. The empirical–statistical approach is computationally efficient because it makes use of seasonally aggregated cyclone statistics and allows the future cyclone density to be estimated from the full ensemble of available CMIP5 models rather than from a smaller subset. However, the projected cyclone density in the future differs considerably depending on the choice of predictor, SLP, or 500-hPa geopotential height. This discrepancy suggests that the relationship between the cyclone density and SLP, 500-hPa geopotential height, or both is nonstationary; that is, that the statistical model depends on the calibration period. A stationarity test based on 6-hourly HadGEM2-ES data indicated that the 500-hPa geopotential height was not a robust predictor of cyclone density.

Comparison between the active tropical cyclone seasons over the Arabian Sea and Bay of Bengal

AbstractClimatologically, 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.

Sub‐seasonal predictability of the 2017–2018 Southern Hemisphere tropical cyclone season

Multi‐week forecasts of tropical cyclone genesis are evaluated for the 2017–2018 Southern Hemisphere cyclone season. These forecasts were created with the Australian Bureau of Meteorology's new seasonal forecast system, ACCESS‐S. Forecast performance over the season showed positive Brier Skill Scores with good forecast reliability and improved skill over climatology out to week 3. ACCESS‐S provided guidance of the formation of major cyclone events during the season at more than 2 weeks lead time, including Cyclone Gita in the South Pacific which later devastated Tonga, and the landfall of Cyclone Hilda in western Australia.

Two ‘faces’ of ENSO-induced surface waves during the tropical cyclone season

The response of the wave climate to tropical cyclones (TCs) is investigated using the significant wave height (SWH) observed from satellites and the WAVEWATCH III (WW3) model. Tropical cyclone wind generates local waves (wind seas) under the TC and longer-period waves (swells) that propagate long distances. The genesis location, intensity, and frequency of tropical cyclones over the global ocean are strongly affected by the phases of the El Niño Southern Oscillation (ENSO). It is shown that the interannual variation of global ocean surface waves in the subtropics during summer is dominated by ENSO-related TC activity. In particular, in the subtropical western North Pacific, the wind power is stronger in the TC season before an El Niño and weaker before a La Niña. These ENSO-related TC variations are shown, through composite and empirical orthogonal function analyses, as well as modeling, to dominate the spatial distribution and temporal variation of the SWH over the western North Pacific. The model confirms that longer-period waves (swells) are driven into northern South China Sea, toward Japan in mid latitudes and toward the central Pacific along the equator due to ENSO-related TC activities. The wind power and SWH over the subtropical western North Pacific lead and regress well with the ENSO index, suggesting that they may potentially serve as useful ENSO predictors.

The Curious Association between Hadley Circulation Intensity and the Meridional Distribution of Tropical Cyclones in the Eastern North Pacific

It has recently been shown that an interesting inverse relationship exists between the strength of the overturning in the regional boreal Hadley circulation and tropical cyclone genesis and lifetime maximum intensity latitudes in the eastern North Pacific. One of the particularly curious aspects of this association is its uniqueness to this ocean basin. We show here that this result can be understood as the outcome of an equatorward shift in the local Intertropical Convergence Zone (ITCZ) and a moderate reduction of tropical vertical shear. These two factors are not necessarily concomitant. The magnitude of the vertical shear change is low enough to suggest that the primary physical mechanism behind this inverse relationship is the equatorward shift in the ITCZ. Since a significant proportion of tropical cyclone genesis results from aggregated convective cells forming into a coherent convective vortex and being shed from the ITCZ, this shift is apparently sufficient to explain the observed association. The equatorward shift in the ITCZ is potentially the result of a relative warming of the Southern Hemisphere in the boreal tropical cyclone season. This more equatorward ITCZ, located over warmer surface waters, presumably explains the more intense Hadley circulation, despite a reduction of the large-scale boreal meridional temperature gradient. Although, these links require further research.

Consistent Late Onset of the Western North Pacific Tropical Cyclone Season Following major El Niño Events

Consistent Late Onset of the Western North Pacific Tropical Cyclone Season Following major El Niño Events

Environmental Energetics aspects of anomalous Cyclogenesis over Indian Ocean during 2013

An attempt has been made to understand the dynamics behind anomalous cyclogenesis over Indian Ocean during 2013 cyclone seasons, from an atmospheric energetics point of view. Salient features of the results of this study are: Maximum intensification of all the intense cyclonic vortices was associated with an enhancement in both conversion from eddy available potential energy to eddy kinetic energy (AE to KE ) and zonal kinetic energy to eddy kinetic energy (KZ to KE). During intensification of the storms both AE and AZ have shown to be dissipated, suggesting the intensification of storms at the cost of AE and AZ .  For all of these systems baroclinic eddy kinetic energy conversions [conversion from AE to KE ] dominates over barotropic eddy kinetic energy conversions [conversion from KZ to KE]. Anomalous cyclogenesis in 2013, at least partly may be attributed to a positive anomaly in Moist Static Energy (MSE) of the environment along with positive anomaly in baroclinic and barotropic eddy kinetic energy conversion during cyclone seasons over the region under study. Release of convective instability in the atmosphere may also be attributed, at least partly, for anomalous cyclogenesis in 2013.

Cloud Detection from Radio Occultation Measurements in Tropical Cyclones

Tropical cyclones (TC) are one of the main producers of clouds in the tropics and subtropics. Hence, most of the clouds in TCs are dense, with large water and ice content, and provide conditions conducive to investigate clouds’ impact on Radio Occultation (RO) measurements. Although the RO technique is considered insensitive to clouds, recent studies show a refractivity positive bias in cloudy conditions. In this study, we analyzed the RO bending angle sensitivity to cloud content during tropical cyclone seasons between 2007 and 2010. Thermodynamic parameters were obtained from the ERA-Interim reanalysis, whereas the water and ice cloud contents were retrieved from the CloudSat profiles. Our experiments confirm the positive mean RO refractivity bias in cloudy conditions that reach up to more than 0.5% at the geometric height of around 7 km. A similar bias but larger and shifted up is visible in bending angle anomaly (1.6%). Our results reveal that the influence of clouds is significant and can exceed the RO bending angle standard deviation for 21 out of 50 (42%) investigated profiles. Mean clouds’ impact is detectable between 9.0 and 10.5 km, while, in the case of single events, clouds in most of the observations are significant between 8 and 14 km. Almost 15% of the detectable clouds reach 16 km height, while the influence of the clouds below 5 km is insignificant. For more than half of the significant cases, the detection range is less than 3 km but for one observation this range spreads to 7–8 km.

Remote subsurface ocean temperature as a predictor of Atlantic hurricane activity

Predicting North Atlantic hurricane activity months in advance is of great potential societal significance. The ocean temperature, both in terms of North Atlantic/tropical averages and upper ocean heat content, is demonstrated to be a significant predictor. To investigate the relationship between the thermal state of the Atlantic Ocean and the tropical cyclone (TC) activity in terms of accumulated cyclone energy (ACE), we use observed 1980-2015 TC records and a 1/4° resolution global ocean reanalysis. This paper highlights the nonlocal effect associated with eastern Atlantic Ocean temperature, via a reduction of wind shear, and provides additional predictive skill of TC activity, when considering subsurface temperature instead of sea surface temperature (SST) only. The most active TC seasons occur for lower than normal wind shear conditions over the main development region, which is also driven by reduced trade wind strength. A significant step toward operationally reliable TC activity predictions is gained after including upper ocean mean temperatures over the eastern Atlantic domain. Remote effects are found to provide potential skill of ACE up to 3 months in advance. These results indicate that consideration of the upper 40-m ocean average temperature improves upon a prediction of September Atlantic hurricane activity using only SST.

Wave ensemble forecast system for tropical cyclones in the Australian region

Forecasting of waves under extreme conditions such as tropical cyclones is vitally important for many offshore industries, but there remain many challenges. For Northwest Western Australia (NW WA), wave forecasts issued by the Australian Bureau of Meteorology have previously been limited to products from deterministic operational wave models forced by deterministic atmospheric models. The wave models are run over global (resolution 1/4∘) and regional (resolution 1/10∘) domains with forecast ranges of + 7 and + 3 day respectively. Because of this relatively coarse resolution (both in the wave models and in the forcing fields), the accuracy of these products is limited under tropical cyclone conditions. Given this limited accuracy, a new ensemble-based wave forecasting system for the NW WA region has been developed. To achieve this, a new dedicated 8-km resolution grid was nested in the global wave model. Over this grid, the wave model is forced with winds from a bias-corrected European Centre for Medium Range Weather Forecast atmospheric ensemble that comprises 51 ensemble members to take into account the uncertainties in location, intensity and structure of a tropical cyclone system. A unique technique is used to select restart files for each wave ensemble member. The system is designed to operate in real time during the cyclone season providing + 10-day forecasts. This paper will describe the wave forecast components of this system and present the verification metrics and skill for specific events.

Landslide Hazards in Fiji, managing the risk and not the disaster, a literature review

A literature review for a proposed study has yielded that Fiji has both a perception and a risk problem when it comes to landslide hazards. The islands experience high amounts of rainfall and violent wind-speeds combined with steep slopes, lead to erosion and mass movements. In 1980 seventy four landslides were reported in one 230 hectare area during Fiji’s cyclone season. Roads in the area suffered vast damaged leading to closures and eventual re-routing which cost $F1.6 million. This was only one such event, over 20 other events have been recorded since 1931. Landslides are identified as one of the major hazards in Fiji, causing around twenty five percent of annual cumulative deaths. They have a direct impact to farm lands as well as dwellings. The lack of understanding of slope stability has led to a mixed perception of landslide risk in the community. Financially it is more effective to mitigate against a hazard then to react to it. In other communities similar to those in Fiji, where landslides are a risk, one part of the solution has been to educate the affected community of the risks. This “bottom up” approach has worked in studies carried out in Guatemala and Eastern Caribbean. Each study differed in their approach on how they empowered the local community to understand their landslide risk, and ways to increase their resilience to that risk. Coupling the “bottom up” approach with technical trainings on emerging technologies like UAVs and 3D Printing, we teach children and adults how to create their own resources. Combining UAVs with a digital camera can allow a community to get a Birdseye view of their landscape to define the hazard and the risk. Presenting an image in 2D can show the landscape, but a 3D printed model can demonstrate how exactly their house would fare within a rain event. It provides a better visual, a tangible resource to better explain the potential hazards. Changing the perception of the hazard will aid in better planning. Understanding how to address both perception and risk may be the difference between managing the risk to and not the disaster.

Salient Differences in Tropical Cyclone Activity over the Western North Pacific between 1998 and 2016

Previous studies have suggested that tropical cyclone (TC) seasons over the western North Pacific (WNP) in the decaying years of El Niño events are generally less active than normal. The two strongest El Niño events on record were 1997/98 and 2015/16, but TC activities over the WNP displayed a sharp contrast between the decaying years of the two events. In 1998, consistent with previous studies, the WNP witnessed an extremely quiet season with no TC genesis in the preseason (January–June) and with only 10 named TCs observed in the typhoon season (July–October), making 1998 the most inactive season in the basin on record. In 2016, no TC formed in the preseason, similar to 1998; however, the basin became remarkably active in the typhoon season with above-normal named TCs observed. Further analyses indicate that the absence of TCs in the preseason in both 1998 and 2016 and the less active typhoon season in 1998 were attributed to the strong western Pacific anomalous anticyclone associated with the super El Niño events. However, the pattern of sea surface temperature anomalies (SSTAs) in the Pacific in 2016 showed features distinct from that in 1998. During July–August, the extremely positive phase of the Pacific meridional mode (PMM) triggered an anomalous cyclonic circulation and negative vertical wind shear over the WNP, favorable for TC geneses, while during September–October, the combined effect of the equatorial western Pacific warming and the weak La Niña event enhanced TC geneses over the WNP.

Bone histology sheds new light on the ecology of the dodo (Raphus cucullatus, Aves, Columbiformes)

The dodo, Raphus cucullatus, a flightless pigeon endemic to Mauritius, became extinct during the 17th century due to anthropogenic activities. Although it was contemporaneous with humans for almost a century, little was recorded about its ecology. Here we present new aspects of the life history of the dodo based on our analysis of its bone histology. We propose that the dodo bred around August and that the rapid growth of the chicks enabled them to reach a robust size before the austral summer or cyclone season. Histological evidence of molting suggests that after summer had passed, molt began in the adults that had just bred; the timing of molt derived from bone histology is also corroborated by historical descriptions of the dodo by mariners. This research represents the only bone histology analysis of the dodo and provides an unprecedented insight into the life history of this iconic bird.

Variation of the Tropical Cyclone Season Start in the Western North Pacific

The variation of the tropical cyclone (TC) season in the western North Pacific (WNP) was analyzed based on the percentiles of annual TC formation dates. The results show that the length of the TC season is highly modulated by the TC season’s start rather than its end. The start of the TC season in the WNP has large interannual variation that is closely associated with the variation of the sea surface temperature (SST) in the Indian Ocean (IO) and the central-eastern Pacific (CEP). When the SSTs of the IO and CEP are warm (cold) in the preceding winter, anomalous high (low) pressure and anticyclonic (cyclonic) circulation are induced around the WNP TC basin the following spring, resulting in a late (early) start of the TC season. These results suggest that a strong El Niño in the preceding winter significantly delays the TC season start in the following year.

A climatological model of North Indian Ocean tropical cyclone genesis, tracks and landfall

Extensive damage and loss of life can be caused by tropical cyclones (TCs) that make landfall. Modelling of TC landfall probability is beneficial to insurance/re-insurance companies, decision makers, government policy and planning, and residents in coastal areas. In this study, we develop a climatological model of tropical cyclone genesis, tracks and landfall for North Indian Ocean (NIO) rim countries based on kernel density estimation, a generalised additive model (GAM) including an Euler integration step, and landfall detection using a country mask approach. Using a 35-year record (1979–2013) of tropical cyclone track observations from the Joint Typhoon Warning Centre (part of the International Best Track Archive Climate Stewardship Version 6), the GAM is fitted to the observed cyclone track velocities as a smooth function of location in each season. The distribution of cyclone genesis points is approximated by kernel density estimation. The model simulated TCs are randomly selected from the fitted kernel (TC genesis), and the cyclone paths (TC tracks), represented by the GAM together with the application of stochastic innovations at each step, are simulated to generate a suite of NIO rim landfall statistics. Three hindcast validation methods are applied to evaluate the integrity of the model. First, leave-one-out cross validation is applied whereby the country of landfall is determined by the majority vote (considering the location by only highest percentage of landfall) from the simulated tracks. Second, the probability distribution of simulated landfall is evaluated against the observed landfall. Third, the distances between the point of observed landfall and simulated landfall are compared and quantified. Overall, the model shows very good cross-validated hindcast skill of modelled landfalling cyclones against observations in each of the NIO tropical cyclone seasons and for most NIO rim countries, with only a relatively small difference in the percentage of predicted landfall locations compared with observations.