Steering Flow Research Articles

The impact of weak environmental steering flow on tropical cyclone track predictability

AbstractTyphoonsHaiyan(2013) andHagupit(2014) are examples of two tropical cyclones (TCs) for which, despite similarities in the track and intensity, the predictabilities differed greatly. Both TCs made landfall over the Philippines having followed a similar track across the Pacific and both reached intensities in excess of 60 ms−1. Operational global ensemble forecasts showed large uncertainty in the track of , whereas the ensemble spread for was considerably less. Using the Met Office's Unified Model, 5‐day global ensemble forecasts were produced for both storms. Consistent with the operational forecasts produced at the time of the storms, the spread of tracks is greater in the forecasts produced for than . The latter was located on the southern periphery of the subtropical high and embedded in a strong easterly flow. In contrast, the position of between two anticyclones earlier in the forecast is key to the subsequent motion of the storm in determining whether would make landfall over the Philippines or turn to the north. Upper‐level winds contributed the most to the depth‐averaged steering flow. Statistically significant differences in the strength of the upper‐level anticyclone to the east of the storm, the strength and position of the upper‐level ridge downstream of the storm and the location of a detached potential vorticity (PV) streamer appear between two groups of ensemble members – those which turn to the north and those which make landfall. Positional differences of the TC in different ensemble members earlier in the forecasts, particularly in the east to west direction, are correlated to larger northeast to southwest position differences later in the forecast. Ensemble sensitivity analysis suggests that this initial east–west positional variance is linked to the upper‐level geopotential height directly south of the storm. Accurately representing both the steering flow and the position of is vital for an accurate forecast.

A long-term perspective of hydroclimatological impacts of tropical cyclones on regional heavy precipitation over eastern monsoon China

Combining a multitemporal clustering algorithm and an objective synoptic analysis technique, we develop an objective identification approach to identify tropical cyclone (TC)-induced regional heavy precipitation events (RHPEs) during 1961–2018 over eastern monsoon China (EMC) based on 0.25° gridded precipitation dataset. Over the past 58 years, there are around 500 TC-induced RHPEs over the EMC, and the features (i.e. duration, impact area, precipitation depth, and total precipitation volume) of these events show obviously skewed distribution and strong seasonality. Seasonal variations in the features of TC-induced RHPEs show inverted “V” shapes and peak values in the inverted “V” shapes occur in July. More than 40% of RHPEs in coastal areas of southern China are triggered by TCs and this fractional contribution is also observed in northeastern China for RHPEs exceeding 50 mm/grid/day. Both frequency and duration of TC-induced RHPEs are significantly increasing in southeastern and northeastern China. Additionally, the EMC is experiencing more frequent, more widespread, and greater TC-induced RHPEs. There is also a signal of interdecadal variability in the features of TC-induced RHPEs. Anomalies of prevalent easterlies, westward steering flow, weakening vertical wind shear, and enhanced integrated vapor transport occur in years with severe TC-induced RHPEs. Meanwhile, the exceedance probability of extreme TC-induced RHPEs is increasing in recent decades, and this increase is more pronounced for destructive events (i.e. TC-induced RHPEs with long return intervals). This probability of extreme TC-induced RHPEs depends on TC activity characteristics, such as intensity and translation speed. Given that TCs tend to get stronger, decay slower, and linger longer in a warming climate, it is of great challenge for society to address the increasing risk of catastrophic disasters associated with extreme TC-induced RHPEs.

Recurvature and movement processes of tropical cyclones over the Bay of Bengal

AbstractActive bimodal tropical cyclones (TCs) are embedded in nonuniform flows over the Bay of Bengal (BoB). More than half of the TCs that occurred from 1990 to 2019 recurved from their original path due to the influence of 500–300 hPa steering flows. Premonsoon TCs have a tendency to turn right and make landfall on the right‐side coastal areas of the BoB. On the other hand, most of the TCs that occurred in the postmonsoon season crossed the left coast or east coast of India, despite being left or right from their path. The simulation shows that Cyclone Roanu (2016) was driven by a warm, moist, unstable southwesterly that was blocked and diverted by the hot, dry stable air masses of northerlies and northwesterlies. As a result, the TC moved along the dry–moist boundary, keeping a constant distance of ∼400 km between the eye and dryline. Conversely, the combined steering of moist southeasterly and southward‐moving upper‐level westerly jets controlled the track of postmonsoon Cyclone Madi (2013), encompassing a left turn of ∼150°. In addition to the primary steering mechanism, TC vortex propagation can be explained by the strong vertical shear (>30 m·s−1) offered by the dryline during the premonsoon period and near the monsoon trough in the postmonsoon period. This vertical shear causes the upper‐level anticyclone to tilt downshear, resulting in the forward motion of the TC vortex. The deep convection and associated mesovortices, especially those that are more prominent along the dryline, additionally influence asymmetric diabatic heating, contributing to the advection of TC potential vorticity.

Monsoon Surges Enhance Extreme Rainfall by Maintaining the Circulation of Landfalling Tropical Cyclones and Slowing Down Their Movement

Extreme rainfall induced by landfalling tropical cyclones (ERLTCs) in China can cause flash floods and other disastrous impacts, so investigating their genesis and mechanism of enhancement has been attracting considerable attention. This study demonstrates that the extreme rainfall of landfalling tropical cyclones (LTCs) possesses two key properties—namely, maintenance of the LTC circulation and a lagging (slowing down or looping) of its movement, and the monsoon surge can provide a positive contribution to these properties. Specifically, diagnostics show that the low-level cyclonic vorticity and upper-level divergence of ERLTCs are significantly stronger than those of NERLTCs (non-extreme-rainfall-producing LTCs). The continuous intensification of the cyclonic rotation in the lower troposphere before the occurrence of extreme rainfall is a significant feature that distinguishes ERLTCs from NERLTCs. Vorticity budget analysis further shows that the relative vorticity advection term contributes the most to the local increase and maintenance of vorticity in the middle and lower troposphere of ERLTCs under the influence of the southwest monsoonal surge, thus demonstrating that the monsoonal surge favors the maintenance of LTC circulation. On the other hand, the activity of the southwest monsoonal surge is mainly manifested in the zonal wind anomaly, and the corresponding strong westerly transport can significantly reduce the zonal component of the steering flow. As a result, the total steering flow can be weakened, which decreases the northwestward translation speed of ERLTCs, and thus the monsoonal surge favors a lagging (slowing down or looping) of LTC movement. These results reveal the mechanism of influence through which the monsoonal surge affects ERLTCs via its direct impacts on the maintenance of their circulation and lagging of their movement—two distinct evolutionary characteristics.

Estimation of maximum seasonal tropical cyclone damage in the Atlantic using climate models

There are several different estimates of the observed cyclone damage potential of tropical cyclones based on observations of size, intensity and track. For the analysis of climate model data, previous work identified an index, the cyclone damage potential climate index (CDPClimate), based on relative sea surface temperature (SST) and tropical cyclone steering flow to estimate the damage potential in climate models. Using millennia-long climate models, CDPClimate is estimated for the North Atlantic basin and compared against values from reanalyses and the observed damage potential. The peak in SSTs in the cyclone main development region with respect to the tropical mean SSTs is smaller in these models than reanalyses, resulting in smaller variations in CDPClimate. Although the year 1995 had the highest observed cyclone damage potential, the year 2010 is a maximum for CDPClimate in the reanalysis data. The models exceed this 2010 value in less than 1% of model years. Using a model with 100 ensemble members, the variability in CDPClimate is examined further. The interannual variability of the ensemble mean results has a very high correlation (R = 0.95) with reanalyses. The high decadal variability is evident and interannual variability is found to have increased during the 30 years after 1981 relative to those prior. The 2010 ensemble mean value is exceeded in other years by individual ensemble members 1.1% of the time. The results from this study suggest that although it is possible to exceed the observed CDP, this is rare in the current climate. However, this study does not consider changes as we move to future climates.

Influence of water vapor distribution on the simulated track of Typhoon Hato (2017)

Predicting tropical cyclone (TCs) tracks is a primary concern in TC forecasting. Some TCs appear to move in a direction favorable for their development, beyond the influence of the steering flow. Thus, we hypothesize that TCs move toward regions with high water-vapor content in the lower atmosphere. In this study, four numerical experiments, including a control experiment and three sensitivity experiments, were performed using the Weather Research and Forecasting Model, to analyze the relationship between water vapor distribution and the track of Severe Typhoon Hato (2017). Observations validated the features reproduced in the control experiment. The sensitivity experiments were conducted to explore variations in the TC track under different water vapor environments. Results indicate that the horizontal distribution of water-vapor content exerted a greater impact on the TC track than the steering flow when both factors were significant. Further analysis revealed that the TC’s movement vector was between the direction of the steering flow and the direction toward the peak of vorticity increasing area. The peaks of vorticity increasing area were close to the peaks of water vapor increasing area, which also proved the effect of water vapor distribution on the TC track. These results are expected to improve TC track analysis and forecasting.

이례적으로 영향 태풍 수가 많았던 2019년 한반도 주변 종관장 특성 분석

Tropical cyclones (TCs) over the western North Pacific (WNP) mainly occur during June-October, and result in significant casualties and damages to property in East Asian countries (e.g., Korea, Japan, Taiwan, and China, etc.). Although the total number of TCs that occurred over WNP was similar to normal years, the numer of TCs that affected Korea in August and September 2019 was 3 times higher than with the same number of TCs in July. Therefore, this study examined why more TCs migrated into Korea in 2019 through analyzing four environmental conditions: steering flow, geopotential height at 500 hPa, vertical wind shear (VWS), and sea surface temperature (SST). Results showed that the tracks of TCs were significantly associated with steering flows from July to September. Furthermore, weaker VWS and warmer SST were distributed near the tracks of TCs during August and September, whereas strong VWS and lower SST were dominant in July. The environmental conditions in August and September were favorable for maintaining and developing TCs, explaining why more typhoons have affected Korea during August and September in 2019.

Impact of Assimilating FY-3D MWTS-2 Upper Air Sounding Data on Forecasting Typhoon Lekima (2019)

In this study, the Fengyun-3D (FY-3D) clear-sky microwave temperature sounder-2 (MWTS-2) radiances were directly assimilated in the regional mesoscale Weather Research and Forecasting (WRF) model using the Gridpoint Statistical Interpolation (GSI) data assimilation system. The assimilation experiments were conducted to compare the track errors of typhoon Lekima from uses of the Advanced Microwave Sounding Unit-A (AMSU-A) radiances (EXP_AD) with those from FY-3D MWTS-2 upper-air sounding data at channels 5–7 (EXP_AMD). The clear-sky mean bias-corrected observation-minus-background (O-B) values of FY-3D MWTS-2 channels 5, 6, and 7 are 0.27, 0.10 and 0.57 K, respectively, which are smaller than those without bias corrections. Compared with the control experiment, which was the forecast of the WRF model without use of satellite data, the assimilation of satellite radiances can improve the forecast performance and reduce the mean track error by 8.7% (~18.4 km) and 30% (~58.6 km) beyond 36 h through the EXP_AD and EXP_AMD, respectively. The direction of simulated steering flow changed from southwest in the EXP_AD to southeast in the EXP_AMD, which can be pivotal to forecasting the landfall of typhoon Lekima (2019) three days in advance. Assimilation of MWTS-2 upper-troposphere channels 5–7 has great potential to improve the track forecasts for typhoon Lekima.

Historical Variability and Lifecycles of North Atlantic Midlatitude Cyclones Originating in the Tropics

AbstractNorth Atlantic tropical and post‐tropical cyclones impact midlatitude regions, but the inhomogeneous observational record of the latter stages of tropical cyclones precludes many climatological analyses. The frequency of tropical‐origin storms basin‐wide is projected to increase under anthropogenic climate change, so establishing confidence in our knowledge of their historical variability and lifecycles—against which climate model simulations may be evaluated—is important. We used a Lagrangian feature‐tracking algorithm to identify tropical cyclones that impacted Northeast North America and Europe in seven global reanalysis data sets, distinguishing systems that retained warm‐core structures or underwent warm seclusion from those that underwent extratropical transition, acquiring cold‐core, frontal structures. Over the last four decades, ∼25% and ∼10% of tropical‐origin cyclones made landfall across Northeast North America and Europe, respectively, as warm‐core systems, with, on average, higher wind speeds than cold‐core systems. Historical warm‐core and cold‐core landfalls also exhibit distinct tracks, likely responding to differing steering flow and midlatitude conditions.

Understanding the Impacts of Upper-Tropospheric Cold Low on Typhoon Jongdari (2018) Using Piecewise Potential Vorticity Inversion

AbstractTyphoon Jongdari (2018) had an unusual looping path before making landfall in Japan, which posed a forecasting challenge for operational numerical models. The impacts of an upper-tropospheric cold low (UTCL) on the track and intensity of Jongdari are investigated using numerical simulations. The storm track and intensity are well simulated in the control experiment using the GFS analysis as the initial and boundary conditions. In the sensitivity experiment (RCL), the UTCL is removed from the initial-condition fields using the piecewise potential vorticity inversion (PPVI), and both the track and intensity of Jongdari change substantially. The diagnosis of potential vorticity tendency suggests that horizontal advection is the primary contributor for storm motion. Flow decomposition using the PPVI further demonstrates that the steering flow is strongly affected by the UTCL, and the looping path of Jongdari results from the Fujiwhara interaction between the typhoon and UTCL. Jongdari first intensifies and then weakens in the control experiment, consistent with the observation. In contrast, it undergoes a gradual intensification in the RCL experiment. The UTCL contributes to the intensification of Jongdari at the early stage by enhancing the eddy flux convergence of angular momentum and reducing inertial stability, and it contributes to the storm weakening via enhanced vertical wind shear at the later stage when moving closer to Jongdari. Different sea surface temperatures and other environmental conditions along the different storm tracks also contribute to the intensity differences between the control and the RCL experiments, indicating the indirect impacts of the UTCL on the typhoon intensity.

The slowdown tends to be greater for stronger tropical cyclones

AbstractUnderstanding the impact of climate change on tropical cyclones (TCs) has become a hot topic. The slowdown of TC translation speed contributes greatly to the locally accumulated TC damage. While the recent observational evidence shows that TC translation speed has decreased globally by 10% since the mid-twentieth century, the robustness of the trend is questioned by other studies as effects of changes in observational capability can strongly affect the global trend. Moreover, none of the published studies considered dependence of TC slowdown on TC intensity. This is the caveat of these analyses as the effect of TC slowdown is closely related to TC intensity. Here, we investigate the relationship between TC translation speed trend and TC intensity, and reveal possible reasons for the trend. We show that the global slowing trend without weak TC moments (≤ 17 m s-1) is about double of that with weak TC moments in a recent study. This is because the slowing trend is dominated by strong TCs’ trend. Stronger (weaker) TCs tend to be controlled more by upper-level (lower-level) steering flow, and the calculated trend of upper-level steering flow is much larger than that of lower-level steering flow. This may be an important reason for the large difference between the slowing trend without weak TC moments and that with weak TC moments. Furthermore, the changes of TC tracks (including inter-basin trend and latitudinal shift), which are partly attributed to data inhomogeneity, make a much larger contribution to the slowing trend, compared with the weakening of tropical circulation, which is related to anthropogenic warming.

Impact of the Monsoonal Surge on Extreme Rainfall of Landfalling Tropical Cyclones

A comparative analysis and quantitative diagnosis has been conducted of extreme rainfall associated with landfalling tropical cyclones (ERLTC) and non-extreme rainfall (NERLTC) using the dynamic composite analysis method. Reanalysis data and the tropical cyclone precipitation dataset derived from the objective synoptic analysis technique were used. Results show that the vertically integrated water vapor transport (Qvt) during the ERLTC is significantly higher than that during the NERLTC. The Qvt reaches a peak 1–2 days before the occurrence of the ERLTC and then decreases rapidly. There is a stronger convergence for both the Qvt and the horizontal wind field during the ERLTC. The Qvt convergence and the wind field convergence are mainly confined to the lower troposphere. The water vapor budget on the four boundaries of the tropical cyclone indicates that water vapor is input through all four boundaries before the occurrence of the ERLTC, whereas water vapor is output continuously from the northern boundary before the occurrence of the NERLTC. The water vapor inflow on both the western and southern boundaries of the ERLTC exceeds that during the NERLTC, mainly as a result of the different intensities of the southwest monsoonal surge in the surrounding environmental field. Within the background of the East Asian summer monsoon, the low-level jet accompanying the southwest monsoonal surge can increase the inflow of water vapor at both the western and southern boundaries during the ERLTC and therefore could enhance the convergence of the horizontal wind field and the water vapor flux, thereby resulting in the ERLTC. On the other hand, the southwest monsoonal surge decreases the zonal mean steering flow, which leads to a slower translation speed for the tropical cyclone associated with the ERLTC. Furthermore, a dynamic monsoon surge index (DMSI) defined here can be simply linked with the ERLTC and could be used as a new predictor for future operational forecasting of ERLTC.

Synoptic Climatology of Lake-Effect Snow Events off the Western Great Lakes

As the mesoscale dynamics of lake-effect snow (LES) are becoming better understood, recent and ongoing research is beginning to focus on the large-scale environments conducive to LES. Synoptic-scale composites are constructed for Lake Michigan and Lake Superior LES events by employing an LES case repository for these regions within the U.S. North American Regional Reanalysis (NARR) data for each LES event were used to construct synoptic maps of dominant LES patterns for each lake. These maps were formulated using a previously implemented composite technique that blends principal component analysis with a k-means cluster analysis. A sample case from each resulting cluster was also selected and simulated using the Advanced Weather Research and Forecast model to obtain an example mesoscale depiction of the LES environment. The study revealed four synoptic setups for Lake Michigan and three for Lake Superior whose primary differences were discrepancies in a surface pressure dipole structure previously linked with Great Lakes LES. These subtle synoptic-scale differences suggested that while overall LES impacts were driven more by the mesoscale conditions for these lakes, synoptic-scale conditions still provided important insight into the character of LES forcing mechanisms, primarily the steering flow and air–lake thermodynamics.

Binary interaction of typhoons Soulik and Cimaron in 2018 – Part Ⅰ: Observational characteristics and forecast error

To understand structural changes and forecast error, a case study of binary typhoons in the western North Pacific (WNP) of 2018 was investigated using best track and reanalysis data. Soulik was generated on August 16 and Cimaron was generated on August 18, respectively. The 19th typhoon Soulik and 20th typhoon Cimaron co-existed from August 18 to 24 and approached each other. Soulik was located on the western side and Cimaron was located on the eastern side of the WNP. They were located approximately 1300 km from each other at 00 UTC August 22. The Soulik structure began changing around August 22 and became weak and slow, while Cimaron maintained its intensity, size, and moving speed. This observational evidence is likely caused by the binary interaction between two typhoons within a certain distance and environmental steering flow, such as the location of the North Pacific high and strong jet stream of the northern flank of the North Pacific high.Soulik was initially forecasted to make landfall and reach Seoul; however, its track changed from northward to northeastward from August 21 to 23 according to both official guidance and unified model (UM). Four global numerical weather prediction models forecasted different tracks of Soulik. UM and JGSM forecasted a northward track whereas ECMWF and GFS showed a northeastward track for 12 UTC August 21 through 12 UTC August 24. The latter models were similar to the best track. The track forecast error and spread of Soulik were larger than those of Cimaron. The mean absolute error of the maximum wind speed of Soulik was similar to the average of total typhoons in 2018.

Future Changes in Tropical Cyclone Intensity and Frequency over the Western North Pacific Based on 20-km HiRAM and MRI Models

AbstractThe future changes in tropical cyclone (TC) intensity and frequency over the western North Pacific (WNP) under global warming remain uncertain. In this study, we investigated such changes using 20-km resolution HiRAM and Meteorological Research Institute (MRI) models, which can realistically simulate the TC activity in the present climate. We found that the mean intensity of TCs in the future (2075–99) would increase by approximately 15%, along with an eastward shift of TC genesis location in response to the El Niño–like warming. However, the lifetime of future TCs would be shortened because the TCs tend to have more poleward genesis locations and move faster due to a stronger steering flow related to the strengthened WNP subtropical high in a warmer climate. In other words, the enhancement of TC intensity in the future is not attributable to the duration of TC lifetime. To understand the processes responsible for the change in TC intensity in a warmer climate, we applied the budget equation of synoptic-scale eddy kinetic energy along the TC tracks in model simulations. The diagnostic results suggested that both the upper-level baroclinic energy conversion (CE) and lower-level barotropic energy conversion (CK) contribute to the intensified TCs under global warming. The increased CE results from the enhancement of TC-related perturbations of temperature and vertical velocity over the subtropical WNP, whereas the increased CK mainly comes from synoptic-scale eddies interacting with enhanced zonal-wind convergence associated with seasonal-mean and intraseasonal flows over Southeast China and the northwestern sector of WNP.

Heavy Rain-producing Terrestrial Low-Pressure Systems Over East Asian Summer Monsoon Region: Evolution, Energetics, and Trend

AbstractThe synoptic low-pressure systems (LPSs) formed over the downwind side of the Tibetan Plateau explain a substantial portion of summer rainfall extremes along their paths. Recent studies have found that the total extreme rainfall trend over the East Asian landmass, which features the “south flood-north drought” pattern, can be understood to a great extent by the changes in terrestrial LPSs. Yet, the energy sources fueling these storms and the environmental drivers of their long-term trends remain unclear. Utilizing a probabilistic clustering method, three clusters of terrestrial LPS tracks for the period 1979-2018 are identified. Besides the differences in trajectories that distinguish the clusters into northeastward-migrating and quasi-stationary types, prominent inter-cluster differences are found in the LPS evolution, energetics, and trends. The Lorenz energetics suggest that while condensational heating is indispensable for all three clusters, the migratory type, which has greater intensity and faster development, is more closely tied to baroclinicity. Nonetheless, the summer baroclinicity alone is not enough to sustain these LPSs as these storms dissipate quickly after propagating out of the humid monsoon region and into the drier extratropics. Over time, the occurrences of migratory LPSs decreases, and that of quasi-stationary LPSs increases. Using a Poisson model that links the LPS genesis to local environmental conditions, the decreasing occurrence of migratory LPSs is shown to result from the weakened baroclinicity, whereas the increasing occurrence of quasi-stationary LPSs is primarily driven by enhanced relative humidity and reduced steering flow in the mid-to-lower troposphere over East Asia.

The Role of Cyclones in Moisture Transport into the Arctic

AbstractIn this study, ERA5 reanalysis is used to quantify cyclone‐induced moisture transport into the Arctic on a seasonal basis (1998–2018). Moist airstreams are spatially identified and temporally linked to individual cyclones (50–90°N). Results show that cyclones account for 74% of the annual poleward flux across 70°N. For these cyclone cases, which primarily emerge over the central Arctic in summer or otherwise originate from the North Atlantic, the amount of moisture transport is found to closely depend on track orientation and poleward propagation. Cyclone tracks oriented in the poleward direction along with coupling to an upper‐level vortex which influences the steering flow are the most important characteristics that determine the moisture flux into the Arctic. Back trajectories highlight low‐level moisture source regions over the Atlantic Gulf Stream throughout the year and over the high‐latitude continental land surface during the warm season.

A study of track deflection associated with the landfall of Tropical Cyclone Sidr (2007) over the Bay of Bengal and Bangladesh

In this study, the dynamics of track deflection associated with Tropical Cyclone (TC) Sidr (2007) are explored using a numerical weather prediction model. It is found that (a) the simulated track of Sidr is sensitive to flow, orographic, and model vertical structure that change the environmental steering flow leading to the track deflection. In particular, the track of TC Sidr is deflected northwestward for cases with lower domain height, horizontal domain covering only part of Himalaya mountains, and varying mountain heights; (b) the simulated track of TC Sidr, when compared with GFS reanalysis data, is mainly controlled by its deep-layer environmental steering flow as a point vortex; (c) the northwestward deflection with lower domain height is caused by an artificially larger high pressure at lower levels in the vicinity of the Himalayas, due to the upward propagation of wave energy being reflected by the upper domain boundary; (d) the significant northwestward deflection associated with the varying mountain height cases is due to the cyclone vortex being advected by the northeasterly monsoonal flow, which is blocked by the mountains in the corresponding cases with mountains; (e) the northeastward track deflection after the landfall of Sidr is explained by the addition of the frictional force.In summary, the model vertical domain height and the Himalaya mountain representation play key roles in influencing the accuracy of TC Sidr track simulation, compared with other factors, such as the vertical resolution, at least for TC Sidr.

Recent migration of tropical cyclones toward coasts.

Poleward migrations of tropical cyclones have been observed globally, but their impact on coastal areas remains unclear. We investigated the change in global tropical cyclone activity in coastal regions over the period 1982-2018. We found that the distance of tropical cyclone maximum intensity to land has decreased by about 30 kilometers per decade, and that the annual frequency of global tropical cyclones increases with proximity to land by about two additional cyclones per decade. Trend analysis reveals a robust migration of tropical cyclone activity toward coasts, concurrent with poleward migration of cyclone locations as well as a statistically significant westward shift. This zonal shift of tropical cyclone tracks may be mainly driven by global zonal changes in environmental steering flow.

Meridional oscillation of tropical cyclone activity in the western North Pacific during the past 110 years

A prominent meridional oscillation of tropical cyclone (TC) activity in the western North Pacific (WNP) during the past 110 years is identified in this paper. Based on the best-track TC data, the first three Empirical Orthogonal Functions of TC occurrence frequency generally show a northeast-southwest dipole, with the major loadings over the South China Sea (SCS) and the area near Japan. During the periods 1951–1963 and 1997–2019, TC frequency is generally lower over the SCS but higher near Japan. The opposite is true during 1969–1988. Inclusion of the historical TC data from 1910 gives two periods (1943–1963 and 1997–2019) during which the TC numbers near Japan (NJP) are higher but those over the SCS (NSCS) are lower, and two periods (1926–1942 and 1969–1988) with just the opposite pattern. The main factors responsible for this meridional oscillation include changes in the genesis positions of the TCs and their subsequent tracks associated with the interdecadal variation of the large-scale environmental flow patterns. During the periods in which the NSCS (NJP) is higher (lower), the large-scale steering flow is favorable for TCs moving to the SCS. At lower latitudes, vertical wind shear is weaker and low-level cyclonic flow is stronger, which lead to a southward shift of the genesis positions. The patterns are just the reverse during the periods in which the meridional oscillation is in the opposite phase. The oscillation is also found to be partly related to the interdecadal Pacific oscillation through changes in its low-level flow pattern.