Changes In Tropical Cyclone Tracks Research Articles

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.

Interannual Variability of the Basinwide Translation Speed of Tropical Cyclones in the Western North Pacific

AbstractUnderstanding variations in tropical cyclone (TC) translation speed (TCS) is of great importance for islands and coastal regions since it is an important factor in determining TC-induced local damages. Investigating the long-term change in TCS was usually subject to substantial limitations in the quality of historical TC records, but here we investigated the interannual variability in TCS over the western North Pacific (WNP) Ocean by using reliable satellite TC records. It was found that both temporal changes in large-scale steering flow and TC track greatly contributed to interannual variability in the WNP TCS. In the peak season (July–September), TCS changes were closely related to temporal variations in large-scale steering flow, which was linked to the intensity of the western North Pacific subtropical high. However, for the late season (October–December), changes in TC track played a vital role in interannual variability in TCS while the impacts of temporal variations in large-scale steering were weak. The changes in TC track were mainly contributed by the El Niño–Southern Oscillation (ENSO)-induced zonal migrations in TC genesis locations, which make more or fewer TCs move to the subtropical WNP, thus leading to notable changes in the basinwide TCS because of the much greater large-scale steering in the subtropical WNP. The increased influence of TC track change on TCS in the late season was linked to the greater contrast between the subtropical and the tropical large-scale steering in the late season. These results have important implications for understanding current and future variations in TCS.

Influence of varying time scale flows on tropical cyclone track changes

AbstractIt is often difficult to predict tropical cyclone (TC) tracks for the case of binary or even multiple TCs. To investigate this issue, two TCs over the western North Pacific basin in September 2016, Meranti and Malakas, were explored in this study. The two TCs formed close to each other, and their tracks were similar at an early stage but then deflected northward over different regions. The results indicate that the Meranti track change may be due to the asymmetric high‐frequency flows induced by topography, while the Malakas track change is attributed to the formation and strengthening of low‐frequency southerlies. It was found that the formation and evolution of low‐frequency monsoon circulation is the main cause for the emergence of the low‐frequency flows, and the process is accompanied by the enhancement and movement of low‐frequency positive vorticity. Therefore, based on the diagnostic results using the low‐frequency vorticity budget equation, it is shown that the low‐frequency vorticity advection and convergence induced by the low‐frequency flows play varying roles in the development of low‐frequency positive vorticity. More noteworthy is that the high‐frequency positive vorticity of TCs can trigger the enhancement of low‐frequency positive vorticity through nonlinear interactions, which have rarely been discussed in previous studies. It can be concluded that the interactions between high‐ and low‐frequency flows and vorticity may affect the development and evolution of the TCs at different time scales, finally leading to TC track deflection.

Tropical Cyclones and Climate Change Assessment: Part I: Detection and Attribution

AbstractAn assessment was made of whether detectable changes in tropical cyclone (TC) activity are identifiable in observations and whether any changes can be attributed to anthropogenic climate change. Overall, historical data suggest detectable TC activity changes in some regions associated with TC track changes, while data quality and quantity issues create greater challenges for analyses based on TC intensity and frequency. A number of specific published conclusions (case studies) about possible detectable anthropogenic influence on TCs were assessed using the conventional approach of preferentially avoiding type I errors (i.e., overstating anthropogenic influence or detection). We conclude there is at least low to medium confidence that the observed poleward migration of the latitude of maximum intensity in the western North Pacific is detectable, or highly unusual compared to expected natural variability. Opinion on the author team was divided on whether any observed TC changes demonstrate discernible anthropogenic influence, or whether any other observed changes represent detectable changes. The issue was then reframed by assessing evidence for detectable anthropogenic influence while seeking to reduce the chance of type II errors (i.e., missing or understating anthropogenic influence or detection). For this purpose, we used a much weaker “balance of evidence” criterion for assessment. This leads to a number of more speculative TC detection and/or attribution statements, which we recognize have substantial potential for being false alarms (i.e., overstating anthropogenic influence or detection) but which may be useful for risk assessment. Several examples of these alternative statements, derived using this approach, are presented in the report.

Changes of tropical cyclone tracks in the western North Pacific over 1979−2016

The tropical cyclone (TC) trajectory model has been widely used to investigate the mechanism of the climate change of TC tracks. In this study, the Gaussian weight interpolation method is used to calculate the beta drift in the TC track or trajectory model. By simulating historical TC tracks, the new calculation can better simulate the spatial distribution of the frequency of TC occurrence. The improved track model is further used to understand the TC track change during the period of 1979–2016. In 1979–2016, the TC activity has been suppressed in the South China Sea (SCS) while the TC influence along the southeast coastal regions of China has been increased. The sensitivity experiments indicate that the change of the formation location plays a dominant role in decreasing TC influence in the SCS while the change of the environmental steering flows increases the TC influence in the eastern coastal region of China. This study suggests that the ongoing climate change can affect the TC activity along the coast of China.

Projected changes in tropical cyclones over the South West Indian Ocean under different extents of global warming

The Paris Agreement achieved in December 2015 established that the signatory countries should pursue to hold the increase in global average temperature to below 2 °C relative to the preindustrial period and to strive to limit the temperature increase to 1.5 °C below the preindustrial period. The potential changes in tropical cyclones over the basin making landfall over southern Africa under the key global temperature goals have not been thoroughly investigated. Using the Coordinated Regional Downscaling Experiment-Africa regional climate models, we downscale six global climate models of the Coupled Model Inter-comparison Project Phase 5 to high resolution. This serves towards studying changes in tropical cyclone tracks over the South West Indian Ocean under different extents of global warming (1.5 °C, 2 °C and 3 °C of warming with respect to pre-industrial conditions). It is projected that the number of tropical cyclones making landfalls over southern Africa under global warming will decrease, with 2 °C being a critical threshold, after which the rate of cyclone frequency with further temperature increases no longer has a diminishing effect. Fewer cyclones may bring benefits and reduce damage to the southern African region. Although a decrease in damages associated with flood events is desirable, general decreases in tropical cyclone and tropical lows may also be associated with decreased rainfall over the Limpopo River basin and southern, central and northern Mozambique (with negative impacts on dryland agriculture).

Impact of initial storm intensity and size on the simulation of tropical cyclone track and western Pacific subtropical high extent

Typhoon Megi, the 13th typhoon of the 2010 typhoon season, was selected for case study by utilizing the Weather Research and Forecasting (WRF) model. Twelve sensitivity experiments with various initial tropical cyclone (TC) intensities and sizes were conducted to investigate their impacts on the simulation of typhoon track. Interaction between TC and the western Pacific subtropical high (WPSH) was also analyzed to explore the mechanism for the impact on TC track of the initial TC intensity and size. Numerical results indicate that the simulated TC size and TC track are sensitive to initial TC intensity and size. Stronger initial TC intensity and larger initial TC size often lead to larger simulated TC size and make TC turn northward earlier. Further analysis suggests that, with the increase of initial TC intensity and size, more air mass enters into the TC region, which subsequently reduces the extent of WPSH. As a result, the steering flow changes significantly and eventually causes the TC to turn northward earlier. The present study confirms that the initial TC intensity and size have certain influences on the TC track simulation, which demonstrates the importance of accurate initial condition for successful simulation of the TC intensity and TC track. Moreover, it also deepens our understanding of the interaction between TC and WPSH, provides helpful clues for the TC track change study, and discusses the future directions for improvement of TC track forecast.

Impact of tropical cyclone track change on regional air quality

There has been an increase in tropical cyclones (TCs) in the western North Pacific (WNP) that traverse with a northward recurving track towards East Asia and a decrease in TC tracks entering the South China Sea (SCS) in the past few decades. To investigate the potential impact of the prevailing TC track change on Hong Kong air quality, an analysis has been carried out based on historical data (1991 to 2010) of TC tracks and air quality. Compared to TCs in other regions, TCs in the vicinity of Taiwan (Region 2, R2) have the greatest impact on Hong Kong air quality due to regional transport of air pollutants from the highly industrialized Pearl River Delta (PRD). In the last twenty years, the number of days with TCs in R2 (May to October) has increased by 45% from 111days in the period 1991–2000 to 161days in 2001–2010, during which there was an increase in yearly TC-related pollution episodes of approximately 3 episodes per year in Hong Kong. The enhancement of mean O3 concentration due to TCs in R2 is reported as 82% (~50.8μg/m3 at a rural station) and 58% (~16.8μg/m3 at an urban station) higher than the summer averages. A similar enhancement is also observed for PM10 (called RSP) and SO2 with an average of 70% (i.e., 22.2μg/m3) and 100% (i.e., 15.2μg/m3) increases, respectively. Overall, the 20years of historical data show that the O3 concentrations on the TC-affected days are increasing at the estimated rates of 0.5μg/m3 and 2.6μg/m3 per year, respectively, in the urban and remote areas, which are significantly higher than the increase of 0.3μg/m3 and 0.4μg/m3 per year in the average summer concentrations.

Impact of ocean warming on tropical cyclone track over the western north pacific: A numerical investigation based on two case studies

AbstractThe impact of ocean warming on tropical cyclone (TC) track over the western North Pacific is an open issue. Relatively little is known about possible changes in TC tracks under ocean warming conditions due to both inhomogeneous observation networks and large natural variability over a relatively short observational period. A suite of semiidealized numerical experiments on two TC cases is conducted to investigate the response of TC tracks to increases in sea surface temperature (SST). It is found that the simulated TC track is highly sensitive to underlying SST. Specifically, through its influence on the radial distribution of sea surface enthalpy, ocean warming can lead to changes in the tangential wind profile and thus increase the TC size in terms of the radius of gale force wind, which is attributed to the increase in maximum wind speed, the expansion of the radius of maximum wind, and the additional increase in outer winds. The increased TC size, as suggested by previous studies, further leads to the eastward withdrawal of the western Pacific subtropical high (WPSH) and thus a northward turning of the TC. Results of climate simulations in the present study provide further evidence for the aforementioned impact of ocean warming on TC size and thus the WPSH and TC track. Results of the present study also imply that the threat of storms to countries in East Asia may be reduced due to possible changes in TC tracks if ocean warming continues in the future.

Influence of future tropical cyclone track changes on their basin-wide intensity over the western North Pacific: Downscaled CMIP5 projections

The possible changes of tropical cyclone (TC) tracks and their influence on the future basin-wide intensity of TCs over the western North Pacific (WNP) are examined based on the projected large-scale environments derived from a selection of CMIP5 (Coupled Model Intercomparison Project Phase 5) models. Specific attention is paid to the performance of the CMIP5 climate models in simulating the large-scale environment for TC development over the WNP. A downscaling system including individual models for simulating the TC track and intensity is used to select the CMIP5 models and to simulate the TC activity in the future.

The Impact of Natural and Anthropogenic Climate Change on Western North Pacific Tropical Cyclone Tracks*

Abstract The impact of natural and anthropogenic climate change on tropical cyclone (TC) tracks in the western North Pacific (WNP) is examined using a beta and advection model (BAM) to isolate the influence of changes in the large-scale steering flow from changes in genesis location. The BAM captures many of the observed changes in TC tracks due to El Niño–Southern Oscillation (ENSO), while little change is noted for the Pacific decadal oscillation and all-India monsoon rainfall in either observations or BAM simulations. Analysis with the BAM suggests that the observed shifts in the average track between the phases of ENSO are primarily due to changes in the large-scale steering flow, with changes in genesis location playing a secondary role. Potential changes in TC tracks over the WNP due to anthropogenic climate change are also assessed. Ensemble mean projections are downscaled from 17 CMIP3 models and 26 CMIP5 models. Statistically significant decreases [~(4%–6%)] in westward moving TCs and increases [~(5%–7%)] in recurving ocean TCs are found. These correspond to projected decreases of 3–5 TCs per decade over the Philippines and increases of 1–3 TCs per decade over the central WNP. The projected changes are primarily caused by a reduction in the easterlies. This slows the storm movement, allowing more time for the beta drift to carry the storm northward and recurve. A previous study found similar results in the North Atlantic. Taken together, these results suggest that a weakening of the mean atmospheric circulation in response to anthropogenic warming will lead to fewer landfalling storms over the North Atlantic and WNP.

A downscaling technique to simulate changes in western North Pacific tropical cyclone activity between two types of El Niño events

Changes in tropical cyclone (TC) activity over the western North Pacific (WNP) basin between the Modoki and canonical El Nino years have been simulated and examined in this study based on a downscaling technique. The downscaling technique is used for generating synthetic TCs based on TC formation, TC track, and TC intensity models. Results suggest that the downscaling technique can well simulate the spatial distribution of TC activity during the two types of El Nino years and their differences. It is found that the observed changes in TC tracks during the two types of El Nino years are mainly due to the combined effects of changes in TC formation locations and large-scale steering flows. Further examinations have shown that changes in large-scale steering flows play a more important role than changes in TC formation locations. These results are in accordance with the cyclonic circulation anomaly found during the Modoki El Nino years compared to that during the canonical El Nino years. Numerical simulations further suggest that changes in TC tracks between the two types of El Nino years appear to be the most important factor affecting the TC intensity change. Compared to that during the Modoki El Nino years, TC formation is enhanced in the south quadrant of the WNP basin and more TCs take a northwestward track during the canonical El Nino years, leading to a longer TC lifespan and greater TC intensity.

Sudden Tropical Cyclone Track Changes over the Western North Pacific: A Composite Study

Abstract Tropical cyclones (TCs) over the western North Pacific (WNP) are usually embedded in the multitime-scale summer monsoon circulation and occasionally experience sudden track changes, which are currently a challenge in TC forecasting. A composite analysis of 15 sudden north-turning cases and 14 west-turning cases that occurred during the period 2000–10 was conducted with a focus on influences of low-frequency monsoon circulations. It is found that TCs in the two specific categories of track changes are embedded in a monsoon gyre of about 2500 km in diameter on the quasi-biweekly oscillation (QBW) time scale, which is also embedded in a larger-scale cyclonic gyre or monsoon trough on the Madden–Julian oscillation (MJO) time scale. The two types of track changes are closely associated with interaction between low-frequency and synoptic flows. Two different types of asymmetric flow patterns are identified on the synoptic time scale in the vicinity of these TCs. In the north-turning case, enhanced winds lie mainly on the southeast side of TCs due to strong ridging associated with interactions between low-frequency and synoptic flows. In the west-turning case, the westward extension of the subtropical high leads to ridging on the northwest side of TCs and the enhanced winds can largely offset the steering of enhanced southwesterly winds on the synoptic time scale. Thus the north-turning (west turning) sudden track changes are affected primarily by the synoptic-scale (low frequency) steering. This may be one of the reasons for the larger forecasting errors in the north-turning case than in the west-turning case.

Detection of tropical cyclone track changes from the ECMWF ensemble prediction system

A cluster analysis of the 51 member European Center for Medium‐range Weather Forecasts ensemble forecast tracks of existing tropical storms from the THORPEX Interactive Grand Global Ensemble archive is utilized to detect multiple track solutions, and weighted‐mean vector motion (WMVM) tracks are calculated for the ensemble tracks and for the individual track clusters. When three track clusters were present, larger errors occurred in the corresponding deterministic model forecast. Reliability of the European Center for Medium‐range Weather Forecasts ensemble forecast tracks when two distinct clusters exist is examined in terms of the hit rates when the cluster with the larger number of members is selected. A larger hit rate is achieved if track clusters with at least 70%, 80%, or 90% of the members are selected. In these situations, the forecaster can select the WMVM tracks for those clusters and have confidence that a more accurate track than the overall WMVM track will generally be predicted.

Vortex axial-symmetrization characterized by fractal dimension

The vortex axial-symmetrization is an important dynamical process affecting tropical cyclone track change, and it is identified by previous work, up to now, in terms of vorticity fields rather subjectively. Two-dimensional simulations whose integration time is 48 h using a quesi-geostrophic non-divergence barotropic vorticity equation model are performed. The evolutional processes from an initial non-axial-symmetric vortex to the final axial-symmetric vortex are produced. Fractal dimension DB of the perimeter of the vortex is calculated hourly based on the model output data. The results show that fractal dimension can be used to characterize the vortex axial-symmetrization objectively. Based on DB calculated, the correction coefficients CRD and CVD are calculated to be CRD=-0.70 and CVD=-0.75. They each have a significant correction. Here CRD denotes the correction coefficient between DB and Rm being TC scale parameter, and CVD denotes the correction coefficient between DB and Vmax being tropical cyclone intensity parameter. The relation between the vortex axial-symmetrization and the tropical cyclone parameters may be characterized. The results have potential applications in tropical cyclone forecasting.

Observational Analysis of Sudden Tropical Cyclone Track Changes in the Vicinity of the East China Sea

Abstract An observational analysis of observed sudden typhoon track changes is conducted with a focus on the underlying mechanism and the possible role of slowly varying low-frequency flows. Four typhoons that took a generally northwestward track prior to sharply turning northeastward in the vicinity of the East China Sea are investigated. It is found that the sudden track changes occurred near the center of the Madden–Julian oscillation (MJO)-scale cyclonic circulation or at the bifurcation point of the steering flows at 700 hPa, and they were all associated with a well-developed quasi-biweekly oscillation (QBW)-scale gyre. Calculation of vorticity advection suggests that the peripheral ridging resulting from the interaction between the typhoons and the flows on the MJO and QBW scales can compress the typhoon circulation, leading to an area of high winds to the east or south of the typhoon center. The enhanced synoptic-scale winds shifted the typhoons northward and placed them in a northeastward orbit under the steering of the flows associated with the Pacific subtropical high. The sudden track change can be likened to the maneuvering of satellite orbit change in that the enhanced synoptic-scale winds act as a booster rocket to shift the typhoons northward to the southwesterly steering flows.

Monsoonal Influence on Typhoon Morakot (2009). Part II: Numerical Study

Abstract In the second part of this study, numerical experiments are conducted to investigate the influences of multi-time-scale monsoonal flows on the track change of Typhoon Morakot (2009). While the control simulation captures the slowing and northward deflections in the vicinity of Taiwan Island, the highly asymmetric rainfall structure, and the associated rainfall pattern, sensitivity experiments suggest that the westward movement prior to the landfall on Taiwan and the subsequent northward shifts in the vicinity of Taiwan were closely associated with the interaction between Morakot and multi-time-scale monsoonal flows. Prior to the landfall on Taiwan, Morakot moved westward directly toward Taiwan because of a synoptic wave train–like pattern, which consisted of Goni over mainland China, Morakot, and a cyclone over the western North Pacific with an anticyclone to the west of Morakot. Numerical simulation suggests that strong northerly winds between Morakot and the anticyclone reduced the northward steering component associated with the low-frequency flow prior to the landfall. Numerical experiments confirm that the northward track shifts that occurred in the vicinity of Taiwan Island were a result of the coalescences of Morakot with a quasi-biweekly oscillation (QBW)-scale gyre prior to the landfall on Taiwan and a Madden–Julian oscillation (MJO)-scale gyre in the Taiwan Strait. The simulation of Morakot and the associated sensitivity experiments agree with the previous barotropic study that the interaction between tropical cyclones and low-frequency monsoon gyres can cause sudden changes in tropical cyclone tracks.

Typhoon Track Changes Associated with Global Warming

Abstract Increasing tropical cyclone (TC) influence in the subtropical East Asia and decreasing TC activity in the South China Sea over the past few decades have been researched in previous studies. The singular value decomposition (SVD) of observational data and the Intergovernmental Panel on Climate Change (IPCC) climate change simulations in the Fourth Assessment Report (AR4) shows that the observed TC track changes are linked to the leading SVD mode of global sea surface temperature (SST) warming and the associated changes in large-scale steering flows. The selected five IPCC models can generally simulate the leading mode in their ensemble control run and prediction, suggesting the possible persistence of the reported track changes by 2040.

Multiple-Scale Interactions Affecting Tropical Cyclone Track Changes

Tropical Cyclone (TC) track changes associated with Rossby wave energy dispersion are simulated in a shallow water primitive equation model with an initial field where a TC is located south of a subtropical high. An anticyclone east of the TC appears because of Rossby wave energy dispersion. The connection of the anticyclone with the subtropical high leads to a poleward TC track deflection. The TC eventually moves across the axis of the subtropical ridge. The formation of the track may be attributed to the nonlinear interaction between the subtropical high and the TC. This work validates the conceptual model proposed by previous observational research. The scenario of the nonlinear interaction between the TC and the subtropical high may also be modified through the influence of mesoscale vortices. The main modifications are (1) the anticyclone induced by energy dispersion of the TC weakens, (2) the connection between the anticyclone and the subtropical high is delayed, and (3) the TC shifts more westward and does not move across the ridge axis. We propose that some of the mesoscale vortices are axisymmetrized by the TC and results in an increase in TC size which modifies the properties of the energy dispersion. The phase and group speeds decrease and produce a simulated track deflection to the left compared to the simulation without mesoscale vortices. Our numerical results demonstrate that multiple scale nonlinear interactions have an essential role in influencing TC track changes.

Decreasing Trend in Rainfall over Indochina during the Late Summer Monsoon: Impact of Tropical Cyclones

We examined the decreasing trend in rainfall during the late summer monsoon season (September) in Thailand from 1951 to 2000 and associated changes in tropical cyclone (TC) activity. Thailand receives significant rainfall from May to October and experiences two rainy peaks in late May to early June and in September. A previous study reported a decreasing trend in September rainfall in Thailand and, based on a regional climate model, suggested that the trend was associated with local deforestation. However, the long-term trend may also be affected by changes in large-scale circulation. Thus, the purpose of this study was to investigate changes in large-scale circulation associated with the decreasing rainfall trend.Westward-propagating TCs from the South China Sea and the western North Pacific brought most of the rainfall over Thailand in September. TCs include tropical depressions, tropical storms, severe tropical storms, typhoons, and residual lows. 70% of the rainfall amount in September was estimated to be associated with TCs.The 50-year time-series of September rainfall over Thailand showed a significant decreasing trend. TC activity defined by 700-hPa relative vorticity, showed a weakening trend over the Indochina Peninsula. TC tracks also suggested the weakening of TC activity over this area. The long-term trend in rainfall during the late summer monsoon season was closely associated with changes in TC activity over the Indochina Peninsula; these changes were likely caused by changes in the major course of TCs. Concurrent with the changes in TC tracks, there was a change in the TC steering current around the Philippines archipelago and Taiwan. This led to the TC activity over the Indochina Peninsula being inactive, probably resulting in the long-term decrease in rainfall over Thailand.