Accumulated Cyclone Energy Research Articles

Feedback of tropical cyclones on El Niño diversity. Part II: possible mechanism and prediction

Part I of this study has shown that the tropical cyclones (TCs) over the western North Pacific (WNP) can affect El Niño diversity. In this part, we further explore the possible mechanism of this phenomenon: Compared with the composite situation of all El Niño months, when the preceding (3 months earlier) accumulated cyclone energy (ACE) is strong, the Walker circulation is further weakened and the east–west thermocline gradient is reduced. The eastward transport of warm sea water over the western Pacific is enhanced, the center of the maximum positive sea surface temperature (SST) anomalies is located in the equatorial eastern Pacific, supporting for the development of the eastern-Pacific (EP) El Niño. In contrast, compared with the composite situation of all El Niño months, when the preceding ACE is weak, the Walker circulation is enhanced and the east–west thermocline gradient is strengthened. Thus, the center of the maximum positive SST anomalies is limited to the equatorial central Pacific, supporting for the development of the central-Pacific (CP) El Niño. The modulation of thermocline depth by the WNP TCs mainly results from Kelvin wave propagation and Ekman pumping. In addition, WNP TCs are verified to contribute to the prediction of both the phase-locking of the peak of EP and CP El Niño events and the rapid decrease in SST anomalies during the decaying period of two types of El Niño.

Feedback of tropical cyclones on El Niño diversity. Part I: Phenomenon

Understanding of the El Niño phenomenon is improving and several studies have considered the dynamics of El Niño diversity, however, the important role of tropical cyclones has rarely been reported. Here we show a clear feedback of tropical cyclones (TCs) over the western North Pacific (WNP) on the El Niño diversity. Strong (weak) accumulated cyclone energy helps to shift the center of strongest sea surface temperature anomalies 3 months later to the equatorial eastern (central) Pacific, and thus supporting the development of the eastern-Pacific (central-Pacific) El Niño. Local sea surface temperature anomalies in the Niño-3.4/Niño-3/Niño-4 region and tropical western Pacific, zonal wind anomalies over the tropical central-western Pacific and Madden‒Julian Oscillation play small roles in the process that WNP TCs affect El Niño diversity. Moreover, the greater number of central-Pacific El Niño events after 1999/2000 may be associated with weaker accumulated cyclone energy in this period.

Forest Structure and Composition Are Critical to Hurricane Mortality

Hurricanes can cause severe damage to tropical forests. To understand the nature of hurricane impacts, we analyze and compare immediate effects from category-4 hurricane María in 2017 and category-3 hurricane Hugo in 1989 at Bisley Experimental Watersheds (BEW) in the Luquillo Experimental Forest, Puerto Rico. We show that hurricane María caused lower mortality than hurricane Hugo, even though hurricane María was a stronger event with higher sustained wind. The lower mortality was due to the combination of lower accumulated cyclone energy at the site and more wind-resistant forest structure and composition at the time of disturbance. We compare our study site with a nearby location that has the same forest type, Luquillo Forest Dynamics Plot (LFDP), and describe the similarities and differences of mortality and impact factors between the two sites during the two events. During hurricane Hugo, LFDP experienced much lower mortality than BEW, even though the accumulated cyclone energy at LFDP was higher. The difference in mortality was due to contrasting forest structure and composition of the two sites. Our results demonstrate that forest structure and composition at the time of the disturbance were more critical to hurricane-induced mortality at the two sites than accumulated cyclone energy.

A Hyperactive End to the Atlantic Hurricane Season October–November 2020

Abstract The active 2020 Atlantic hurricane season produced 30 named storms, 14 hurricanes, and 7 major hurricanes (category 3+ on the Saffir–Simpson hurricane wind scale). Though the season was active overall, the final two months (October–November) raised 2020 into the upper echelon of Atlantic hurricane activity for integrated metrics such as accumulated cyclone energy (ACE). This study focuses on October–November 2020, when 7 named storms, 6 hurricanes, and 5 major hurricanes formed and produced ACE of 74 × 104 kt2 (1 kt ≈ 0.51 m s−1). Since 1950, October–November 2020 ranks tied for third for named storms, first for hurricanes and major hurricanes, and second for ACE. Six named storms also underwent rapid intensification (≥30 kt intensification in ≤24 h) in October–November 2020—the most on record. This manuscript includes a climatological analysis of October–November tropical cyclones (TCs) and their primary formation regions. In 2020, anomalously low wind shear in the western Caribbean and Gulf of Mexico, likely driven by a moderate-intensity La Niña event and anomalously high sea surface temperatures (SSTs) in the Caribbean, provided dynamic and thermodynamic conditions that were much more conducive than normal for late-season TC formation and rapid intensification. This study also highlights October–November 2020 landfalls, including Hurricanes Delta and Zeta in Louisiana and in Mexico and Hurricanes Eta and Iota in Nicaragua. The active late season in the Caribbean would have been anticipated by a statistical model using the July–September-averaged ENSO longitude index and Atlantic warm pool SSTs as predictors.

Possible linkage between asymmetry of atmospheric meridional circulation and tropical cyclones in the Central Pacific during El Niño years.

The El Niño–Southern Oscillation is one of the most important drivers of climate change on Earth, and is characterised by warmer (El Niño) or colder (La Niña) ocean surface temperatures in the equatorial Pacific. Tropical cyclones (TCs) and meridional circulation are the most influential weather events and climate phenomena, respectively. However, the link between TCs and meridional circulation anomalies (MCA) during El Niño years is unclear. Therefore, we calculated the accumulated cyclone energy index of TCs and the mass stream function of MCA from 1980 to 2018. Our results showed that TCs were closely related to the asymmetry of the MCA in the Central Pacific during El Niño years. An updraft anomaly in the North Pacific was found, which affected the response of MCA to El Niño from May to October during El Niño years. Therefore, the MCA intensity difference between the North and South Pacific increased, and the asymmetry was strengthened. This phenomenon may be strengthened by the combined effects of the equatorial westerly wind, relative vorticity, and warm ocean surfaces, which are controlled by El Niño. The equatorial westerly wind produces positive shear north of the equator, which increases the relative vorticity. The increase in relative vorticity is accompanied by a monsoon trough, leading to increased precipitation and updrafts. The background of the relative vorticity, updraft, and monsoon trough may be conducive to the generation and development of TCs. Our results prove that the possible link between TCs and the asymmetry of the MCA during El Niño years is derived from the combined effect of the equatorial westerly wind, relative vorticity, and warm ocean surfaces, thus providing a partial explanation for the link between TCs and the MCA.

Skill, Predictability, and Cluster Analysis of Atlantic Tropical Storms and Hurricanes in the ECMWF Monthly Forecasts

Abstract In this paper we analyze Atlantic Ocean hurricane activity in the European Centre for Medium-Range Weather Forecasts (ECMWF) monthly hindcasts for the period 1998–2017. The main climatological characteristics of Atlantic tropical cyclone (TC) activity are considered at different lead times and across the entire ECMWF ensemble using three diagnostic variables: the number of tropical cyclones, the number of hurricanes, and the accumulated cyclone energy. The impacts of changing horizontal resolution and stochastic parameterization are clear in these diagnostic variables. The model skill scores for the number of tropical cyclones and accumulated cyclone energy by lead time are also computed. Using cluster analysis, we compare the characteristics of the forecast TC tracks with observations. Although four of the ECMWF clusters have similar characteristics to observed ones, one of the ECMWF clusters does not have a corresponding one in observations. We consider the predictability of each of these clusters, as well the modulation of their frequency by climate modes, such as the El Niño–Southern Oscillation and the Madden–Julian oscillation, taking advantage of the very large sample size of TC datasets in these hindcasts.

Understanding the post‐monsoon tropical cyclone variability and trend over the Bay of Bengal during the satellite era

AbstractAn effort is made to investigate the tropical cyclone (TC) variability over the Bay of Bengal in the warming climate scenario during the satellite era (1979–2018). In this study, the authors investigate interannual variations in the overall capacity of the Bay of Bengal (BoB) basin in the post‐monsoon season to produce either intense or moderate TCs. Globally accepted and widely used metrics accumulated cyclone energy (ACE) and power dissipation index (PDI) are used to examine the energy and damage generated by TCs. A statistical change‐point analysis is conducted to detect the shift in the tropical cyclone frequency and ACE during the study period, indicating the year 1999 as a change point. The results show the increasing trend in ACE in the BoB during the post‐monsoon season with a statistical significance of 95%. On the contrary, the overall frequency is almost unchanged but has shown an increasing trend in the last decade of the study period. Additionally, atmospheric and ocean factors influencing the TC intensity are investigated by applying principal component analysis for three categories of TCs, viz. the very severe cyclonic storm, severe cyclonic storm, and cyclonic storm. Implementation of principal component analysis and correlations of maximum sustained wind speed (MSW2; intensity) with relative vorticity, minimum sea‐level pressure (MSLP), sea‐surface temperature (SST), specific rainwater content, and vertical wind shear revealed relative vorticity as the most dominating parameter amongst all governing factors which determine the final intensity of the BoB TCs. On the contrary, SST shows the different nature of the relationship with different TC categories.

Effect of horizontal resolution on the simulation of tropical cyclones in the Chinese Academy of Sciences FGOALS-f3 climate system model

Abstract. The effects of horizontal resolution on the simulation of tropical cyclones were studied using the Chinese Academy of Sciences Flexible Global Ocean–Atmosphere–Land System Finite-Volume version 3 (FGOALS-f3) climate system model from the High-Resolution Model Intercomparison Project (HighResMIP) for the Coupled Model Intercomparison Project phase 6 (CMIP6). Both the low-resolution (about 100 km resolution) FGOALS-f3 model (FGOALS-f3-L) and the high-resolution (about 25 km resolution) FGOALS-f3 (FGOALS-f3-H) models were used to achieve the standard Tier 1 experiment required by HighResMIP. FGOALS-f3-L and FGOALS-f3-H have the same model parameterizations with the exactly the same parameters. The only differences between the two models are the horizontal resolution and the time step. The performance of FGOALS-f3-H and FGOALS-f3-L in simulating tropical cyclones was evaluated using observations. FGOALS-f3-H (25 km resolution) simulated more realistic distributions of the formation, movement and intensity of the climatology of tropical cyclones than FGOALS-f3-L at 100 km resolution. Although the number of tropical cyclones increased by about 50 % at the higher resolution and better matched the observed values in the peak month, both FGOALS-f3-L and FGOALS-f3-H appear to replicate the timing of the seasonal cycle of tropical cyclones. The simulated average and interannual variabilities of the number of tropical cyclones and the accumulated cyclone energy were both significantly improved from FGOALS-f3-L to FGOALS-f3-H over most of the ocean basins. The characteristics of tropical cyclones (e.g., the average lifetime, the wind–pressure relationship and the horizontal structure) were more realistic in the simulation using the high-resolution model. The possible physical linkage between the performance of the tropical cyclone simulation and the horizontal resolution were revealed by further analyses. The improvement in the response between the El Niño–Southern Oscillation and the number of tropical cyclones and the accumulated cyclone energy in FGOALS-f3 contributed to the realistic simulation of tropical cyclones. The genesis potential index and the vorticity, relative humidity, maximum potential intensity and the wind shear terms were used to diagnose the effects of resolution. We discuss the current insufficiencies and future directions of improvement for the simulation of tropical cyclones and the potential applications of the FGOALS-f3-H model in the subseasonal to seasonal prediction of tropical cyclones.

Simulated Changes in Tropical Cyclone Size, Accumulated Cyclone Energy and Power Dissipation Index in a Warmer Climate

Detection, attribution and projection of changes in tropical cyclone intensity statistics are made difficult from the potentially decreasing overall storm frequency combined with increases in the peak winds of the most intense storms as the climate warms. Multi-decadal simulations of stabilized climate scenarios from a high-resolution tropical cyclone permitting atmospheric general circulation model are used to examine simulated global changes from warmer temperatures, if any, in estimates of tropical cyclone size, accumulated cyclonic energy and power dissipation index. Changes in these metrics are found to be complicated functions of storm categorization and global averages of them are unlikely to easily reveal the impact of climate change on future tropical cyclone intensity statistics.

Interannual variabilities of the summer and winter extreme daily precipitation in the Southeastern United States

This study focuses on investigating how climate variabilities exhibited in summer and winter precipitation in the Southern United States (SEUS) are possibly modulated by large scale atmospheric activities, including El Niño-Southern Oscillation (ENSO) and cyclonic activities measured in Accumulated Cyclone Energy (ACE) Index. A stringent criterion was applied to the Global Historical Climate Network (GHCN)-Daily dataset to identify a total of 99gauge stations in the SEUS where data length is at least 60 years. Seasonal precipitation is normalized by annual total precipitation amount to obtain the season ratio. Two approaches, including the block maximum and peaks-over-threshold methods, were used to develop time series of extreme precipitations in the summer (June-October) and winter (December-February) seasons. Generalized Extreme Value (GEV) distribution was used to fit a probability density function to the 1-day maximum precipitation time series. Daily cumulative anomaly curves were developed to investigate potential changes in the onset and demise of rainy seasons for the region to the south of 30-degree latitude. Analyses found that summer ratios are over 55% for most stations in Florida and in the range of 45%-55% for most stations along the east coast. Summer total precipitation is significantly correlated with ACE for only 17.2% of the stations, while winter total is significantly correlated with Nino 3.4 for 46.5% of the stations. Similarly, the effect of ACE on the frequency and amount of extreme daily precipitation over the 95th threshold in the summer season is less strong compared to the effect of ENSO on winter extreme precipitation. Summer extreme precipitation is found to be modulated by geographic terrain as well as by local atmospheric activities. Analyses of the onset and demise of rainy season in Florida found no trend in the onset or demise dates between the years 1932 and 2003, although substantial interannual variability was observed.

Dynamical Seasonal Prediction of Tropical Cyclone Activity Using the FGOALS-f2 Ensemble Prediction System

AbstractThere is a distinct gap between tropical cyclone (TC) prediction skill and the societal demand for accurate predictions, especially in the western Pacific (WP) and North Atlantic (NA) basins, where densely populated areas are frequently affected by intense TC events. In this study, seasonal prediction skill for TC activity in the WP and NA of the fully coupled FGOALS-f2 V1.0 dynamical prediction system is evaluated. In total, 36 years of monthly hindcasts from 1981 to 2016 were completed with 24 ensemble members. The FGOALS-f2 V1.0 system has been used for real-time predictions since June 2017 with 35 ensemble members, and has been operationally used in the two operational prediction centers of China. Our evaluation indicates that FGOALS-f2 V1.0 can reasonably reproduce the density of TC genesis locations and tracks in the WP and NA. The model shows significant skill in terms of the TC number correlation in the WP (0.60) and the NA (0.61) from 1981 to 2015; however, the model underestimates accumulated cyclone energy. When the number of ensemble members was increased from 2 to 24, the correlation coefficients clearly increased (from 0.21 to 0.60 in the WP, and from 0.18 to 0.61 in the NA). FGOALS-f2 V1.0 also successfully reproduces the genesis potential index pattern and the relationship between El Niño–Southern Oscillation and TC activity, which is one of the dominant contributors to TC seasonal prediction skill. However, the biases in large-scale factors are barriers to the improvement of the seasonal prediction skill, e.g., larger wind shear, higher relative humidity, and weaker potential intensity of TCs. For real-time predictions in the WP, FGOALS-f2 V1.0 demonstrates a skillful prediction for track density in terms of landfalling TCs, and the model successfully forecasts the correct sign of seasonal anomalies of landfalling TCs for various regions in China.

Monthly prediction of tropical cyclone activity over the South China Sea using the FGOALS-f2 ensemble prediction system

The monthly prediction skill for tropical cyclone (TC) activity in the South China Sea (SCS) during the typhoon season (July to November) was evaluated using the FGOALS-f2 ensemble prediction system. Specifically, the prediction skill of the system at a 10-day lead time for monthly TC activity is given based on 35-year (1981–2015) hindcasts with 24 ensemble members. The results show that FGOALS-f2 can capture the climatology of TC track densities in each month, but there is a delay in the monthly southward movement in the area of high track densities of TCs. The temporal correlation coefficient of TC frequency fluctuates across the different months, among which the highest appears in October (0.59) and the lowest in August (0.30). The rank correlation coefficients of TC track densities are relatively higher (R > 0.6) in July, September, and November, while those in August and October are relatively lower (R within 0.2 to 0.6). For real-time prediction of TCs in 2020 (July to November), FGOALS-f2 demonstrates a skillful probabilistic prediction of TC genesis and movement. Besides, the system successfully forecasts the correct sign of monthly anomalies of TC frequency and accumulated cyclone energy for 2020 (July to November) in the SCS.摘要本研究基于新一代FGOALS-f2动力集合预测系统35年 (1981–2015年) 的热带气旋历史回报试验对南海台风季 (7–11月) 热带气旋活动超前10天的月预测技巧进行评估, 并对2020年南海台风季热带气旋活动进行了实时月预测尝试. 结果表明: FGOALS-f2能较好地预测南海热带气旋路径密度演变特征, 预测的热带气旋生成个数与实际观测结果的相关系数在0.2–0.6之间. 2020年实时预测方面, FGOALS-f2较为准确地给出了南海台风季热带气旋生成位置与移动路径的概率预测, 且对南海热带气旋生成个数和强度异常的确定性预测结果与观测接近.

Nearshore Hurricane Intensity Change and Post‐Landfall Dissipation Along The United States Gulf and East Coasts

AbstractIntensification and dissipation of a hurricane before and after landfall, respectively, are crucial for coastal and inland risk potential. This study examines the relationship between intensity change prior to landfall and post‐landfall dissipation. The relative difference of 24 h accumulated cyclone energy generated before and after landfall is defined as the landfall dissipation rate (LFDR). This study focuses on the continental United States and shows that the 24 h hurricane LFDR is significantly negatively related to the 24 h intensity change before landfall. This implies hurricanes undergoing rapid intensification before landfall weaken at a slower rate after landfall. The decay rate is also positively correlated with landfall intensity but is less certain for Category 4–5 hurricanes (>112 kt). The relationship between near‐shore wind change and post‐landfall decay is not equally distributed along the U.S. coast, with pre‐landfall intensification more common along the Gulf Coast and a LFDR that varies.

Do Sudden Stratospheric Warmings Boost Convective Activity in the Tropics?

AbstractThe impact of sudden stratospheric warmings (SSWs) on the tropical troposphere is investigated using 5000‐years scale ensemble simulations with a 60‐km global atmospheric model to detect signals from large natural variability of convection. Stratospheric temperature and static stability decrease in the tropics, and convective activity peaks simultaneously with the tropical upwelling. Tropical cyclone intensity and accumulated cyclone energy (ACE) show a weak but statistically robust enhancement of 5% and 9%, respectively, relative to the December–March climatology. However, the relative probability of the largest 10% of ACE anomalies increases by about 30%. Extratropical waves toward the tropical troposphere, which can alter convective activity, do not show a strong systematic variation during SSWs, and no significant association with convective activity is found if the sample size is large enough.

TempestExtremes v2.1: a community framework for feature detection, tracking, and analysis in large datasets

Abstract. TempestExtremes (TE) is a multifaceted framework for feature detection, tracking, and scientific analysis of regional or global Earth system datasets on either rectilinear or unstructured/native grids. Version 2.1 of the TE framework now provides extensive support for examining both nodal (i.e., pointwise) and areal features, including tropical and extratropical cyclones, monsoonal lows and depressions, atmospheric rivers, atmospheric blocking, precipitation clusters, and heat waves. Available operations include nodal and areal thresholding, calculations of quantities related to nodal features such as accumulated cyclone energy and azimuthal wind profiles, filtering data based on the characteristics of nodal features, and stereographic compositing. This paper describes the core algorithms (kernels) that have been added to the TE framework since version 1.0, including algorithms for editing pointwise trajectory files, composition of fields around nodal features, generation of areal masks via thresholding and nodal features, and tracking of areal features in time. Several examples are provided of how these kernels can be combined to produce composite algorithms for evaluating and understanding common atmospheric features and their underlying processes. These examples include analyzing the fraction of precipitation from tropical cyclones, compositing meteorological fields around extratropical cyclones, calculating fractional contribution to poleward vapor transport from atmospheric rivers, and building a climatology of atmospheric blocks.

Linking AMOC Variations With the Multidecadal Seesaw in Tropical Cyclone Activity Between Eastern North Pacific and Atlantic

AbstractTropical cyclone (TC) over the North Atlantic is an important extreme weather event, causing great social and economic damages to the American coastal regions. In this study, we find that the North Atlantic TC activity (genesis frequency and accumulated cyclone energy) has significant multidecadal variability, which exhibits an out‐of‐phase relationship with that over the eastern North Pacific, referring to as the inter‐basin seesaw in TC activity. By using five indicators of the Atlantic Meridional Overturning Circulation (AMOC) strength, we link this TC activity seesaw pattern to the AMOC and the associated North Atlantic sea surface temperature anomalies. Both observation and Atlantic pacemaker experiment indicate that the AMOC induces the North Atlantic sea surface temperature warming and eastern North Pacific cooling, which lead to contrasting backgrounds (vertical wind shear, maximum potential intensity, and humidity) for the TC formation and development, resulting in the seesaw pattern in TC activity. Our results provide additional evidence for the reversed relationship in TC activity between the two ocean basins and attribute this seesaw to the variations in the AMOC. More importantly, the atmospheric‐NAO‐based AMOC indicator shows a leading role in depicting the multidecadal seesaw, which has great potential in improving the decadal prediction of North Atlantic/eastern North Pacific TCs.

Arctic Cyclones and Their Interactions With the Declining Sea Ice: A Recent Climatology

AbstractIn this study, we applied a cyclone tracking algorithm to multiple reanalyses and categorized the detected cyclones based on season, intensity, and sea ice state to provide an Arctic cyclone climatology with emphasis on cyclone‐sea ice interactions over the Arctic Ocean and adjacent waters. Our study period is from 1979 to 2015 and the reanalyses we are using are ERA‐Interim, ERA5, and CFSR. As for our intensity variable, we use Accumulated Cyclone Energy (ACE), which has commonly been used as an intensity metric in the tropics but has not been utilized for high latitudes before. ACE has the advantage that it relates the cyclone intensity to the kinetic energy of the cyclone and therefore better represents the cyclone impact/interaction with the surface. We performed comparisons with the other more commonly used intensity metrics to assess the use of the ACE metric in the Arctic. Our main findings include increased cyclone counts with higher trends toward the end of the study period. Cold season cyclone counts were also found to be related to decreased sea ice concentration throughout the year. We also show that there is a relationship between the cyclone intensity measured by ACE and the surface state. Less sea ice was shown to be related to higher cyclone intensities. Comparisons between the the three reanalysis data sets are also presented.

Climate change and temporal-spatial variation of tropical storm-related Natechs in the United States from 1990 to 2017: Is there a link?

Natural hazard triggered hazardous materials (hazmat)-release accidents, known as Natechs, can cause extensive damage and losses. Several studies found that tropical storms are becoming an important cause of Natechs. Understanding whether and how climate change affects the incidence of tropical storms-related Natechs becomes a crucial issue for industrial risk management and adaptation to climate change. This study analyzed the temporal-spatial variation of tropical storms-related Natech incidence on the eastern side of the United States (US) from 1990 to 2017 based on the analysis of hazmat-release accidents reported to the US National Response Center. The results show that the frequency and density of tropical storms-related Natechs are on the rise. In order to explore the cause of such changes, this study investigates the relationships between the temporal-spatial variation of the incidence of tropical storms-related Natechs, and the accumulated cyclone energy, the North Atlantic Oscillation (NAO) index and the Oceanic Niño Index (ONI), and other variables. The results suggest an indirect link between climate change and the temporal-spatial variation of the incidence of related Natechs due to its effect on tropical storm activity. The presented evidence suggests that, when developing Natech risk management plans, the potential effects of climate change should be considered. Not only facilities’ owners/operators, but government, first responders and other stakeholders should consider how climate change will affect the Natech risk landscape, and implement more effective regulations to manage Natech risk in wider areas.

How Does the Arctic Sea Ice Affect the Interannual Variability of Tropical Cyclone Activity Over the Western North Pacific?

Although many studies have revealed that Arctic sea ice may impose a great impact on the global climate system, including the tropical cyclone (TC) genesis frequency over the western North Pacific (WNP), it is unknown whether the Arctic sea ice could have any significant effects on other aspects of TCs; and if so, what are the involved physical mechanisms. This study investigates the impact of spring (April-May) sea ice concentration (SIC) in the Bering Sea on interannual variability of TC activity in terms of the accumulated cyclone energy (ACE) over the WNP in the TC season (June-September) during 1981–2018. A statistical analysis indicates that the spring SIC in the Bering Sea is negatively correlated with the TC season ACE over the WNP. Further analyses demonstrate that the reduction of the spring SIC can lead to the westward shift and intensification of the Aleutian low, which strengthens the southward cold-air intrusion, increases low clouds, and reduces surface shortwave radiation flux, leading to cold sea surface temperature (SST) anomaly in the Japan Sea and its adjacent regions. This local cloud-radiation-SST feedback induces the persistent increasing cooling in SST (and also the atmosphere above) in the Japan Sea through the TC season. This leads to a strengthening and southward shift of the subtropical westerly jet (SWJ) over the East Asia, followed by an anomalous upper-level anticyclone, low-level cyclonic circulation anomalies, increased convective available potential energy, and reduced vertical wind shear over the tropical WNP. These all are favorable for the increased ACE over the WNP. The opposite is true for the excessive spring SIC. The finding not only has an important implication for seasonal TC forecasts but also suggests a strengthened future TC activity potentially resulting from the rapid decline of Arctic sea ice.

Optimal Climate Normals for North Atlantic Hurricane Activity

AbstractMost climatologies use 30‐year epochs that are updated at the start of each decade. They will shift from 1981–2010 to 1991–2020 in 2021. North Atlantic hurricane activity has large interdecadal variability that may lead to biases in a 30‐year climatology. A previous inactive hurricane period included 1981–1990, while 2011–2020 is a part of the ongoing active era. As a result, the 1991–2020 normals are more active than the 1981–2010 normals, with the median accumulated cyclone energy increasing by ∼40%. A 50‐year epoch would be more likely to capture a full cycle of multidecadal variability, and this study demonstrates that 50‐year climatologies have historically been better predictors of the subsequent decade's hurricane activity. This paper argues that the 1971–2020 climatology should, therefore, be the baseline for hurricane activity for the next decade with a possible adjustment for the non‐climatic increase in observed short‐lived tropical cyclones.