Increasing tropical cyclone intensity in the western North Pacific partly driven by warming Tibetan Plateau.

The increase in intense tropical cyclone (TC) activity across the western North Pacific (WNP) has often been attributed to a warming ocean. However, it is essential to recognize that the tropical WNP region already boasts high temperatures, and a marginal increase in oceanic warmth due to global warming does not exert a significant impact on the potential for TCs to intensify. Here we report that the weakened vertical wind shear is the primary driver behind the escalating trend in TC intensity within the summer monsoon trough of the tropical WNP, while local ocean surface and subsurface thermodynamic factors play a minor role. Through observational diagnoses and numerical simulations, we establish that this weakening of the vertical wind shear is very likely due to the increase in temperature of the Tibetan Plateau. With further warming of the Tibetan Plateau under the Representative Concentration Pathway 4.5 scenario, the projected TCs will likely become stronger.

Increase in tropical cyclone rain rate with translation speed.

In general, tropical cyclone (TC) rainfall accumulation usually decreases with faster TC translation speed but increases with heavier rain rate. However, how the TC rain rate changes with translation speed is unclear. Here we show that, in all TC basins, the average TC rain rate significantly increases with translation speed. On average, the rain rate in a fast-moving TC is 24% higher than in a slow one. This difference increases with TC intensity, with category 3-5 TCs having a 42% increase while tropical depressions exhibit only a 9% increase. The increase in the average TC rain rate with translation speed is mainly caused by the TC net inflow in the lower troposphere, as well as vertical wind shear. These findings have important implications not only for a deeper understanding of rain rate changes in a translating TC but also for short-term forecasts of TC rainfall and disaster preparedness.

Historical and recent change in extreme climate over East Asia.

This is an extended editors' commentary on the topical collection "Historical and recent change in extreme climate over East Asia", which collects a total of 15 papers related to the change and variability of extreme climate events in East Asia over the last few hundreds years. The extreme climate events are broadly classified into three categories: temperature and extreme warmth/coldness, precipitation and floods/droughts and western North Pacific typhoons. This commentary briefly summarizes the main findings presented in each paper in this topical collection, and outlines the implications of these findings for monitoring, detecting and modeling of regional climate change and for studying climate change impacts and adaptability. It also assesses the uncertainties of these studies, as well as the remaining knowledge gaps that should be filled in the future. One solid conclusion we can draw from these studies is that there was a marked decadal to multi-decadal variability of extreme climate events in East Asia in recent history, and the extreme events as observed during the last decades of the instrumental era were still within the range of natural variability except for some of those related to temperature. More severe and enduring droughts occurred in the early 20 th century or the earlier periods of history, frequently leading to great famines in northern China. Uncertainties remain in reconstructing historical extreme climate events and analyzing the early instrumental records. Further research could focus on the improvement of methodology in proxy based reconstruction of multi-decadal variations of surface air temperature and precipitation/drought, the recovery, digitization, calibration and verification of the early instrumental records, and the mechanisms of the observed multi-decadal variability of extreme climate in the region.

A new approach for location-specific seasonal outlooks of typhoon and super typhoon frequency across the Western North Pacific region.

With an average of 26 tropical cyclones (TCs) per year, the western North Pacific (WNP) is the most active TC basin in the world. Considerable exposure lies in the coastal regions of the WNP, which extends from Japan in the north to the Philippines in the south, amplifying TC related impacts, including loss of life and damage to property, infrastructure and environment. This study presents a new location-specific typhoon (TY) and super typhoon (STY) outlook for the WNP basin and subregions, including China, Hong Kong, Japan, Korea, Philippines, Thailand, and Vietnam. Using multivariate Poisson regression and considering up to nine modes of ocean-atmospheric variability and teleconnection patterns that influence WNP TC behaviour, thousands of possible predictor model combinations are compared using an automated variable selection procedure. For each location, skillful TY and STY outlooks are generated up to 6 months before the start of the typhoon season, with rolling monthly updates enabling refinement of predicted TY and STY frequency. This unparalleled lead time allows end-users to make more informed decisions before and during the typhoon season.

Recent global decrease in the inner-core rain rate of tropical cyclones.

Heavy rainfall is one of the major aspects of tropical cyclones (TC) and can cause substantial damages. Here, we show, based on satellite observational rainfall data and numerical model results, that between 1999 and 2018, the rain rate in the outer region of TCs has been increasing, but it has decreased significantly in the inner-core. Globally, the TC rain rate has increased by 8 ± 4% during this period, which is mainly contributed by an increase in rain rate in the TC outer region due to increasing water vapor availability in the atmosphere with rising surface temperature. On the other hand, the rain rate in the inner-core of TCs has decreased by 24 ± 3% during the same period. The decreasing trend in the inner-core rain rate likely results mainly from an increase in atmospheric stability.

Spatial heterogeneities of current and future hurricane flood risk along the U.S. Atlantic and Gulf coasts.

We evaluate the spatial heterogeneities of hurricane flood risk along the United States (U.S.) Atlantic and Gulf coasts under two different climate scenarios (current and future). The flood hazard is presented as the hurricane surge flood level with 1% annual exceedance probability (100-year flood) under the two scenarios, where the future scenario considers the effect of hurricane climatology change and sea level rise towards late-21st-century under a high emission scenario (RCP8.5). This hazard information is combined with estimated vulnerability and disaster resilience of coastal communities to map the relative current and future risk employing different risk definitions. Several geographical techniques and spatial distributional models (e.g., spatial autocorrelation, spatial hotspot analysis, and spatial multivariate clustering analysis) are applied to systematically analyze the risk and identify statistically significant hotspots of the highest risk. Most of the high-risk hotspots are found in the Gulf coast region, particularly along the west coast of Florida. However, two out of three risk evaluation approaches also indicate New York City as a risk hotspot under the future climate-showing that the resultant risk is sensitive to the consideration of evaluation factors (i.e., hazard, vulnerability, and resilience). Additionally, we apply a machine-learning algorithm-based spatial multivariate approach to map the spatially distinct groups based on the values of risk, hazard, vulnerability, and resilience. The results show that the counties in the highest risk group (value >3rd quartile, 15% of total counties, including New York City) in the future lack specifically in the community capital and the social components of community resilience. This assessment of coastal risk to hurricane flood has important policy-relevant implications to provide a focus-for-action for risk reduction and resilience enhancement for the U.S., where 6.5 million households live in the hurricane flood-prone areas.

Global warming changes tropical cyclone translation speed.

Slow-moving tropical cyclones (TCs) can cause heavy rain because of their duration of influence. Combined with expected increase in rain rates associated with TCs in a warmer climate, there is growing interest in TC translation speed in the past and future. Here we present that a slowdown trend of the translation speed is not simulated for the period 1951-2011 based on historical model simulations. We also find that the annual-mean translation speed could increase under global warming. Although previous studies show large uncertainties in the future projections of TC characteristics, our model simulations show that the average TC translation speed at higher latitudes becomes smaller in a warmer climate, but the relative frequency of TCs at higher latitudes increases. Since the translation speed is much larger in the extratropics, the increase in the relative frequency of TCs at higher latitudes compensates the reduction of the translation speed there, leading to a global mean increase in TC translation speed.

Tropical cyclones act to intensify El Niño.

Tropical cyclones (TCs), some of the most influential weather events across the globe, are modulated by the El Niño-Southern Oscillation (ENSO). However, little is known about the feedback of TCs on ENSO. Here, observational and modelling evidence shows that TC activity in the southeastern western North Pacific can affect the Niño-3.4 index 3 months later. Increased TC activity in July-September can significantly contribute to the intensity of ENSO in October-December by weakening the Walker circulation and enhancing eastward-propagating oceanic Kelvin waves in the tropical Pacific. Thus, the greater the accumulated cyclone energy, the stronger (weaker) the El Niño (La Niña). A new physics-based empirical model for ENSO is constructed that significantly outperforms current models in predicting ENSO intensity from July to December and addressing the problem about the target period slippage of ENSO. Results suggest that TCs may provide significant cross-scale feedback to ENSO.

Risk assessment for the sustainability of coastal communities: A preliminary study.

With communities increasingly concentrated in coastal regions globally, governments and stakeholders call for cohesive risk assessments for future sustainability in the wake of natural hazards. This can only be achieved through risk-based decision-making and smart resources treatment. To provide a basis for such actions, we here propose a risk assessment (RA) framework focusing on the risk-resilience-sustainability nexus. In contrast to focusing on the traditional RA approach, we propose an integrative approach based on hazard, vulnerability, and resilience covering the full spectrum of RA for effective risk reduction. We further explain how the proposed framework can simultaneously provide useful input for resilience management in parallel to achieving certain Sustainable Development Goals (SDGs). We apply this framework for typhoon risk assessment (represented by a Typhoon Risk Index-TRI) of coastal counties of mainland China. Different TRIs e.g. total population, elderly population, non-adult population, and economic status are calculated for each coastal county to supplement multi-objective empirical measures for risk reduction. The RA results show a large spatial heterogeneity in typhoon risk with an increase in the risk from north to south along the coast of mainland China. The comparative results from this study are relevant to the prioritization of different regions for immediate or gradual actions, wise decision-making, and risk reduction through proper treatment of resources-related policy implications. The evaluation of the SDGs achievement status reveals that the overall performance of coastal provinces in mainland China is higher to achieve SDGs 3 and 15 followed by 13 and 8. The study shows that while Guangdong province is in the highest risk category, its achievement status for SDG-13 (climate actions, strengthening resilience) is the lowest relative to other provinces, which is critical. This study represents a major scientific contribution to mainland China's coastal risk management and calls for aligning risk-informed planning and sustainability frameworks.

Sensitivity of precipitation statistics to urban growth in a subtropical coastal megacity cluster.

This short paper presents an investigation on how human activities may or may not affect precipitation based on numerical simulations of precipitation in a benchmark case with modified lower boundary conditions, representing different stages of urban development in the model. The results indicate that certain degrees of urbanization affect the likelihood of heavy precipitation significantly, while less urbanized or smaller cities are much less prone to these effects. Such a result can be explained based on our previous work where the sensitivity of precipitation statistics to surface anthropogenic heat sources lies in the generation of buoyancy and turbulence in the planetary boundary layer and dissipation through triggering of convection. Thus only mega cities of sufficient size, and hence human-activity-related anthropogenic heat emission, can expect to experience such effects. In other words, as cities grow, their effects upon precipitation appear to grow as well.

Asymmetric response of tropical cyclone activity to global warming over the North Atlantic and western North Pacific from CMIP5 model projections.

Recent improvements in the theoretical understanding of the relationship between tropical cyclones (TCs) and their large-scale environments have resulted in significant improvements in the skill for forecasting TC activity at daily and seasonal time-scales. However, future changes in TC activity under a warmer climate remain uncertain, particularly in terms of TC genesis locations and subsequent pathways. Applying a track-pattern-based statistical model to 22 Coupled Model Intercomparison Project Phase 5 (CMIP5) model runs for the historical period and the future period corresponding to the Representative Concentration Pathway 8.5 emissions scenarios, this study shows that in future climate conditions, TC passage frequency will decrease over the North Atlantic, particularly in the Gulf of Mexico, but will increase over the western North Pacific, especially that hits Korea and Japan. Unlike previous studies based on fine-resolution models, an ensemble mean of CMIP5 models projects an increase in TC activity in the western North Pacific, which is owing to enhanced subtropical deep convection and favorable dynamic conditions therein in conjunction with the expansion of the tropics and vice versa for the North Atlantic. Our results suggest that North America will experience less TC landfalls, while northeast Asia will experience more TCs than in the present-day climate.

Recent decrease in typhoon destructive potential and global warming implications.

Typhoons (tropical cyclones) severely impact the half-billion population of the Asian Pacific. Intriguingly, during the recent decade, typhoon destructive potential (Power Dissipation Index, PDI) has decreased considerably (by ∼ 35%). This decrease, paradoxically, has occurred despite the increase in typhoon intensity and ocean warming. Using the method proposed by Emanuel (in 2007), we show that the stronger negative contributions from typhoon frequency and duration, decrease to cancel the positive contribution from the increasing intensity, controlling the PDI. Examining the typhoons' environmental conditions, we find that although the ocean condition became more favourable (warming) in the recent decade, the atmospheric condition 'worsened' at the same time. The 'worsened' atmospheric condition appears to effectively overpower the 'better' ocean conditions to suppress PDI. This stronger negative contribution from reduced typhoon frequency over the increased intensity is also present under the global warming scenario, based on analysis of the simulated typhoon data from high-resolution modelling.

On the mechanisms of the recurvature of super typhoon Megi.

Tropical cyclones (TC) are one of the most threatening natural hazards to human beings. Although significant improvements have been made in the track prediction of TCs during the past several decades, considerable uncertainties still exist, especially for recurving tracks. In this study, we explore the physical mechanisms that drove the large recurvature of super typhoon Megi through numerical sensitivity experiments using a regional atmospheric model. The results indicate that the cold air intrusion from the northwest to the southeast of China is the main cause of the sharp turning of Megi. This finding suggests that a cold air intrusion could be taken as an indicator for predicting the recurvature of a tropical cyclone in the future.

Does warmer China land attract more super typhoons?

Accurate prediction of where and when typhoons (or named hurricanes which form over the North Atlantic Ocean) will make landfall is critical to protecting human lives and properties. Although the traditional method of typhoon track prediction based on the steering flow theory has been proven to be an effective way in most situations, it slipped up in some cases. Our analysis of the long-term Chinese typhoon records reveals that typhoons, especially super typhoons (those with maximum sustained surface winds of greater than 51 ms(-1)), have a trend to make landfalls toward warmer land in China over the past 50 years (1960-2009). Numerical sensitivity experiments using an advanced atmospheric model further confirm this finding. Our finding suggests an alternative approach to predict the landfall tracks of the most devastating typhoons in the southeastern China.

Comment on "Changes in tropical cyclone number, duration, and intensity in a warming environment".

Analyses of tropical cyclone records from the western North Pacific reveal that the recent increase in occurrence of intense typhoons reported by Webster et al. (Reports, 16 Sep. 2005, p. 1844) is not a trend. Rather, it is likely a part of the large interdecadal variations in the number of intense typhoons related to similar temporal fluctuations in the atmospheric environment.