Power Dissipation Index Research Articles

A comprehensive data set for tropical cyclone storm surge‐induced inundation for the east coast of India

ABSTRACTThe Indian Ocean region is one among the six major cyclone prone regions in the world with an annual average of about five to six cyclones. Prior studies conducted for this region are location‐specific and confined to storm surge characteristic and associated inundation. A detailed study to develop precomputed scenarios on storm surge and inundation maps for the entire Indian coast is still lacking for emergency preparedness. An effort in this direction can drastically reduce preparedness time in the eventuality of a real cyclone approaching its landfall location for effective coastal zone management measures. The present study is a motivation in this direction leading to the generation of a comprehensive data set on storm surges and associated coastal flooding for entire Odisha coast located in the east coast of India and that can be extended to other coastal regions as well. Historically, the Odisha State experiences highest number of cyclone strikes with the highest risk of exposure to storm surge and inundation. The novelty of this study is in the preparation of a comprehensive storm data set using synthetic tracks simulated with various combinations of landfall location, angle of attack, intensity, and forward translation speed. It also generates possible storm surge and inundation associated with intensified cyclones expected in a changing climate scenario. Further, the study also correlates power dissipation index with storm tide all along this coast. The distribution of surge height follows a third‐order polynomial to wind speed. An in‐depth analysis was also carried out to understand the characteristics of storm surge height and extent of coastal inundation with respect to coastal geomorphology, angle of attack, and varying forward speed of cyclones. The authors believe that information and analysis from this comprehensive data set will have immense potential in understanding the impacts from tropical cyclone climatology and disaster management applications.

Large‐scale control of the lower stratosphere on variability of tropical cyclone intensity

Recent studies of tropical cyclones (TC) have suggested intricate impacts of the lower stratosphere layer (LSL) on TC development at the high-intensity limit. This study examines the potential realization of the impacts of the LSL interannual variability on TC intensity. By minimizing the effects of sea surface temperature and outflow temperature based on the potential intensity framework, partial correlation analyses show a negative correlation between the power dissipation index (PDI) for intense TCs and the tropopause height in the northwestern Pacific basin, but a weaker relationship in the north Atlantic basin. Similar analyses for the LSL stratification also reveals signals of negative correlations between the LSL stratification and PDI in both basins, corroborating recent modeling studies about the impacts of the LSL on TC development. Due to the complexity of the relationships and data limitations, however, all correlations are statistically insignificant, thus rendering the impacts of LSL on TC intensity inconclusive at present.

Change in Destructiveness of Landfalling Tropical Cyclones over China in Recent Decades

This study investigates changes in the destructiveness of landfalling tropical cyclones (TCs) over China during 1975–2014. Using four different TC datasets, it is found that TCs making landfall over east China (TCEC) have tended to be more destructive in recent decades, with a significant increase in the power dissipation index (PDI) after landfall. Both time series analysis and diagnostic analysis reveal that such an increase in the PDI of TCECis associated with concomitant enhancement in landfall frequency as well as landfall intensity over east China. In contrast, changes in the PDI of TCs making landfall over south China (TCSC) are less apparent. Examination of different TC-related parameters shows no obvious changes in terms of landfall frequency, duration, and maximum intensity of TCSC.Diagnostic analysis further suggests that the reduction in TC occurrence over south China offsets considerably the positive effects of the intensity and the nonlinear term.Further examination of the environmental parameters reveals significant changes in the large-scale steering flow in recent decades, which is characterized by a prominent cyclonic circulation centered over southeast China. The southeasterly flows on the eastern flank of the cyclonic circulation tend to favor subsequent landfall of TCs over east China, resulting in an increase in landfall frequency, which contributes in part to the enhanced PDI of TCs over this region. Meanwhile, the slowing down of the mean translation speed of TCECand the weakening of vertical wind shear coupled with warmer SSTs in the WNP tend to favor the intensification of TCEC, leading to an increase in intensity and hence the PDI of TCs over east China.

Variability of upper‐ocean characteristics and tropical cyclones in the South West Indian Ocean

AbstractTrack and intensity are key aspects of tropical cyclone behavior. Intensity may be impacted by the upper‐ocean heat content relevant for TC intensification (known as Tdy) and barrier layer thickness (BLT). Here the variability of Tdy and BLT in the South West Indian Ocean and their relationships with tropical cyclones are investigated. It is shown that rapid cyclone intensification is influenced by large Tdy values, thick barrier layers and the presence of anticyclonic eddies. For TC generation in the South West Indian Ocean, the parameter Tdy was found to be important. Large BLT values overlay with large Tdy values during summer. Both fields are modulated by the westward propagation of Rossby waves, which are often associated with ENSO. For example, the 1997–1998 El Niño shows a strong signal in Tdy, SST, and BLT over the South West Indian Ocean. After this event, an increasing trend in Tdy occurred over most of the basin which may be associated with changes in atmospheric circulation. Increasing SST, Power Dissipation Index and frequency of Category 5 tropical cyclones also occurred from 1980 to 2010. To further examine the links between tropical cyclones, Tdy, and BLT, the ocean response to Category 5 Tropical Cyclone Bansi that developed near Madagascar during January 2015 was analyzed. Its unusual track was found to be linked with the strengthening of the monsoonal north westerlies while its rapid intensification from Category 2 to Category 4 was linked to a high‐Tdy region, associated with a warm core eddy and large BLT.

Tropical Cyclone Interaction with the Ocean: The Role of High-Frequency (Subdaily) Coupled Processes

Through tropical cyclone (TC) activity the ocean and the atmosphere exchange a large amount of energy. In this work possible improvements introduced by a higher coupling frequency are tested between the two components of a climate model in the representation of TC intensity and TC–ocean feedbacks. The analysis is based on the new Centro Euro-Mediterraneo per I Cambiamenti Climatici Climate Model (CMCC-CM2-VHR), capable of representing realistic TCs up to category-5 storms. A significant role of the negative sea surface temperature (SST) feedback, leading to a weakening of the cyclone intensity, is made apparent by the improved representation of high-frequency coupled processes. The first part of this study demonstrates that a more realistic representation of strong TC count is obtained by coupling atmosphere and ocean components at hourly instead of daily frequency. Coherently, the positive bias of the annually averaged power dissipation index associated with TCs is reduced by one order of magnitude when coupling at the hourly frequency, compared to both forced mode and daily coupling frequency results. The second part of this work shows a case study (a modeled category-5 typhoon) analysis to verify the impact of a more realistic representation of the high-frequency coupling in representing the TC effect on the ocean; the theoretical subsurface warming induced by TCs is well represented when coupling the two components at the higher frequency. This work demonstrates that an increased horizontal resolution of model components is not sufficient to ensure a realistic representation of intense and fast-moving systems, such as tropical and extratropical cyclones, but a concurrent increase in coupling frequency is required.

Multi-model ensemble forecasting of North Atlantic tropical cyclone activity

North Atlantic tropical cyclones (TCs) and hurricanes are responsible for a large number of fatalities and economic damage. Skillful seasonal predictions of the North Atlantic TC activity can provide basic information critical to our improved preparedness. This study focuses on the development of statistical–dynamical seasonal forecasting systems for different quantities related to the frequency and intensity of North Atlantic TCs. These models use only tropical Atlantic and tropical mean sea surface temperatures (SSTs) to describe the variability exhibited by the observational records because they reflect the importance of both local and non-local effects on the genesis and development of TCs in the North Atlantic basin. A set of retrospective forecasts of SSTs by six experimental seasonal-to-interannual prediction systems from the North American Multi-Model Ensemble are used as covariates. The retrospective forecasts are performed over the period 1982–2015. The skill of these statistical–dynamical models is quantified for different quantities (basin-wide number of tropical storms and hurricanes, power dissipation index and accumulated cyclone energy) for forecasts initialized as early as November of the year prior to the season to forecast. The results of this work show that it is possible to obtain skillful retrospective forecasts of North Atlantic TC activity with a long lead time. Moreover, probabilistic forecasts of North Atlantic TC activity for the 2016 season are provided.

Variations in the power dissipation index in the East Asia region

This study examines the variability of the power dissipation index (PDI) for different regions of the East Asia region during the period 1960–2013. The annual PDI (APDI) in the region is calculated as the sum of the PDI, defined as the cube of the maximum sustained wind speed at landfall of each tropical cyclone (TC) making landfall at that region. Upward and downward trends in APDI are found in the northern and southern parts of East Asia respectively, suggesting a possible northward shift in TC landfall locations. Interdecadal variations of the APDI can also be found in some regions. The APDI in various regions show a close relation with the PDI distribution over the western North Pacific (WNP) with three characteristic patterns. The ENSO and basin-wide mode represents the PDI patterns associated with ENSO events and the overall PDI over the WNP. The east–west dipole mode and the north–south dipole mode denote the east–west and north–south shifts of PDI respectively. Based on the steering flow (average winds within the 850–300 hPa layer) near the East Asian coast, a three-cell model for TC landfall in East Asia is proposed, which corresponds to three major modes of the atmospheric circulation in the WNP. Each of these modes shows an anomalous circulation located east of Taiwan, east of Japan and the South China Sea, respectively and each of which has a significant impact on the APDI in some regions along the coast of East Asia. A northward shift in the APDI along the East Asian coast is identified in the period 1997–2013 as a result of the change in steering flow pattern, northward shift in TC genesis location and weaker vertical wind shear over the ocean near the coastal areas.

Towards Dependence of Tropical Cyclone Intensity on Sea Surface Temperature and Its Response in a Warming World

Tropical Cyclone (TC) systems affect global ocean heat transport due to mixing of the upper ocean and impact climate dynamics. A higher Sea Surface Temperature (SST), other influencing factors remaining supportive, fuels TC genesis and intensification. The atmospheric thermodynamic profile, especially the sea-air temperature contrast (SAT), also contributes due to heat transfer and affects TC’s maximum surface wind speed (Vmax) explained by enthalpy exchange processes. Studies have shown that SST can approximately be used as a proxy for SAT. As a part of an ongoing effort in this work, we simplistically explored the connection between SST and Vmax from a climatological perspective. Subsequently, estimated Vmax is applied to compute Power Dissipation Index (an upper limit on TC’s destructive potential). The model is developed using long-term observational SST reconstructions employed on three independent SST datasets and validated against an established model. This simple approach excluded physical parameters, such as mixing ratio and atmospheric profile, however, renders it generally suitable to compute potential intensity associated with TCs spatially and weakly temporally and performs well for stronger storms. A futuristic prediction by the HadCM3 climate model under doubled CO2 indicates stronger storm surface wind speeds and rising SST, especially in the Northern Hemisphere.

Interannual variation of tropical cyclone energy metrics over North Indian Ocean

There is decreasing trend in the tropical cyclone (TC) number over the North Indian Ocean (NIO) in recent years, though there is increasing trend in the sea surface temperature (SST) which is one of the main environmental parameters for the development and intensification of TCs. Hence, a study has been performed to understand whether any trend exists in other TC parameters such as velocity flux (VF), accumulated cyclone energy (ACE) and power dissipation index (PDI). The interseasonal and interannual variations of VF, ACE and PDI for the NIO as a whole and Bay of Bengal (BOB) and Arabian Sea (AS) are analysed based on the data of 1990–2013 (24 years). Role of large scale features like El Nino southern oscillation (ENSO) and Indian Ocean dipole (IOD) have also been analyzed. The mean ACE per year for TCs [maximum sustained wind of 34 knots (kt) or more] over the NIO is about 13.1 × 104 kt2 including 9.5 × 104 kt2 over the BOB and 3.6 × 104 kt2 over the AS. The mean PDI per year for TCs over the NIO is about 10 × 106 kt3 including 3 × 106 kt3 over the AS and 7 × 106 kt3 over the BOB. The VF, ACE and PDI of TCs are significantly less over BOB during post-monsoon season (Oct.–Dec.) of El Nino years than in La Nina and normal years. The VF for TCs over the BOB during post-monsoon season is significantly less (higher) during positive (negative) IOD years. There is significant decreasing trend at 95 % level of confidence in ACE and PDI of TCs over AS during post-monsoon season and PDI over the BOB and NIO during pre-monsoon season mainly due to similar trend in average intensity of TCs and not due to trends in SST over Nino regions or IOD index.

Comments on “Monitoring and Understanding Trends in Extreme Storms: State of Knowledge”

1175 JULY 2015 AMERICAN METEOROLOGICAL SOCIETY | DOI:10.1175/BAMS-D-13-00211.1 K unkel et al. (2013) reviewed the state of the science with regards to long-term changes and trends of various extreme storms that impact the continental United States. In particular, they addressed severe convective storms, precipitation, snowstorms, and—of interest to this comment— hurricanes. All of the analyses presented in Kunkel et al. (2013) (their Figs. 1–7 and Tables 1 and 2) were focused on observations taken over the continental United States, with the notable exception of hurr icanes. Instead, their F i g . 5 pr o v i d e d d a t a (updated from Kossin et al. 2007) for the entire North Atlantic and northwestern Pacific basins going back only to 1970 in the form of power dissipation index (PDI). They did reference one study on U.S. hurricane activity (Landsea 2 0 05) a nd ment ione d that it indicated no significant long-term trends. That paper, however, is now dated because of the passing of a decade of new observations as well as revisions being made to the first half of the twentieth century U.S. hurricane record through the Atlantic hurricane database project (Landsea et al. 2008, 2012, 2014; Hagen et al. 2012). Given that Kunkel et al.’s (2013) goal was to “present a clear record . . . about what is known and unknown and why about . . . extreme weather and climate types affecting the United States” (p. 499), this comment examines the most up-to-date record of U.S. hurricanes and the associated century timescale trends. Hurricanes striking the continental United States compose a sizable percentage (23%) of all Atlantic basin hurricanes since 1972, the first year for reliable all Atlantic basin hurricane frequency owing to the invention of Dvorak satellite intensity technique (Dvorak 1975) coupled with available satellite imagery for the basin. The rather lengthy coastline of the United States tends to experience more hurricane strikes in busy seasons, but not

Comments on “Monitoring and Understanding Trends in Extreme Storms: State of Knowledge”

1175 JULY 2015 AMERICAN METEOROLOGICAL SOCIETY | DOI:10.1175/BAMS-D-13-00211.1 K unkel et al. (2013) reviewed the state of the science with regards to long-term changes and trends of various extreme storms that impact the continental United States. In particular, they addressed severe convective storms, precipitation, snowstorms, and—of interest to this comment— hurricanes. All of the analyses presented in Kunkel et al. (2013) (their Figs. 1–7 and Tables 1 and 2) were focused on observations taken over the continental United States, with the notable exception of hurr icanes. Instead, their F i g . 5 pr o v i d e d d a t a (updated from Kossin et al. 2007) for the entire North Atlantic and northwestern Pacific basins going back only to 1970 in the form of power dissipation index (PDI). They did reference one study on U.S. hurricane activity (Landsea 2 0 05) a nd ment ione d that it indicated no significant long-term trends. That paper, however, is now dated because of the passing of a decade of new observations as well as revisions being made to the first half of the twentieth century U.S. hurricane record through the Atlantic hurricane database project (Landsea et al. 2008, 2012, 2014; Hagen et al. 2012). Given that Kunkel et al.’s (2013) goal was to “present a clear record . . . about what is known and unknown and why about . . . extreme weather and climate types affecting the United States” (p. 499), this comment examines the most up-to-date record of U.S. hurricanes and the associated century timescale trends. Hurricanes striking the continental United States compose a sizable percentage (23%) of all Atlantic basin hurricanes since 1972, the first year for reliable all Atlantic basin hurricane frequency owing to the invention of Dvorak satellite intensity technique (Dvorak 1975) coupled with available satellite imagery for the basin. The rather lengthy coastline of the United States tends to experience more hurricane strikes in busy seasons, but not

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.

Assessment on historical cyclone tracks in the Bay of Bengal, east coast of India

The Bay of Bengal (BoB) basin in the North Indian Ocean reports the highest number of tropical cyclones, and the frequency is much higher almost five times compared with the Arabian Sea (AS). Semi-enclosed nature of this basin in conjunction with its funnel shape steers the cyclone pathway striking the land. Historical reports signify that most deadly cyclones with highest catastrophe and death tolls occurred in this basin. The four maritime states located in the east coast of India are highly vulnerable to tropical cyclones. Hence, there is a need to understand the risk factors attributed from extreme winds, storm surge, and associated coastal inundation. Historical data signify higher frequency of post-monsoon cyclones compared with pre-monsoon cyclones in the BoB. This study aims to develop a synthetic track or the most probable cyclone track for each state located along the east coast of India. The synthetic track is a general guidance for numerical models, with utility to evaluate and assess the risk factors along coastal belts. This study also deals with aspects on frequency and cyclogenesis locations in the BoB based on analysis from historical data. Results for decadal scale variability signify that in the present decade, the eastern BoB is quite conducive for cyclogenesis. In this region, oceanic thermal field exhibits a warming trend, and found to extend up to water depth of 600 m. In addition, this study also evaluates the energy metrics such as Power Dissipation Index (PDI), and Accumulated Cyclone Energy (ACE) for tropical cyclones that occurred during past four decades in the BoB. The results signify that PDI for tropical cyclones in the present decade is six times higher when compared with the past. The study has practical applications for mapping coastal vulnerability in a changing climate. Tropical cyclones are associated with devastation resulting in enormous loss of life and property during its landfall. In a broad perspective, lack of disaster management pre- paredness would contribute to high vulnerability. The gen- esis of a cyclone usually referred as 'cyclogenesis' is the development stage or strengthening of a cyclonic circula- tion in the atmosphere during the initial stage of its forma- tion. Tropical cyclones form over warm ocean surface, and favourable conditions are sea-surface temperature (SST) greater than 26 ∘ C; low magnitude of vertical wind shear; larger low-level vorticity; and higher mid-troposphere rel- ative humidity. In global oceans, the western part of the North Pacific reported the maximum number of tropical cyclones (average of 26 per year) followed by eastern North Pacific (average of 17 per year). The South Indian Ocean and North Atlantic has an average of 10 per year, while regions in the North Indian Ocean (NIO) have an average of five cyclones per year (Niyas et al., 2009). Notably, the NIO alone accounts for about 7% of global tropical cyclone activity, with a frequency about five

The Influence of El Niño–Southern Oscillation on Tropical Cyclone Activity in the Eastern North Pacific Basin

Abstract The interannual variability of tropical cyclone (TC) activity due to El Niño–Southern Oscillation (ENSO) in the main development region of the eastern North Pacific basin has received scant attention. Herein the authors classify years of El Niño, La Niña, and neutral conditions using the multivariate ENSO index (MEI). Storm measurements of the net tropical cyclone activity index and power dissipation index are used to summarize the overall seasonal TC activity and TC intensity between 1971 and 2012. Both measures are found to be statistically dependent on the ENSO phases in the basin’s main development region. However, when the area is longitudinally divided, only the western portion of the development region experienced a significant difference (p < 0.05). Specifically, El Niño years are characterized by more frequent, more intense events compared to La Niña conditions for this subregion. Correlation analyses on the relationships between the MEI and both TC indices demonstrate correlations between ENSO and TC activity and intensity that are statistically significant (p < 0.05) only in the western region. These relationships have the potential to improve the short-term forecast of the local TC activity and intensity on a seasonal basis for public awareness and disaster preparation.

Cluster Analysis of Downscaled and Explicitly Simulated North Atlantic Tropical Cyclone Tracks

Abstract A realistic representation of the North Atlantic tropical cyclone tracks is crucial as it allows, for example, explaining potential changes in U.S. landfalling systems. Here, the authors present a tentative study that examines the ability of recent climate models to represent North Atlantic tropical cyclone tracks. Tracks from two types of climate models are evaluated: explicit tracks are obtained from tropical cyclones simulated in regional or global climate models with moderate to high horizontal resolution (1°–0.25°), and downscaled tracks are obtained using a downscaling technique with large-scale environmental fields from a subset of these models. For both configurations, tracks are objectively separated into four groups using a cluster technique, leading to a zonal and a meridional separation of the tracks. The meridional separation largely captures the separation between deep tropical and subtropical, hybrid or baroclinic cyclones, while the zonal separation segregates Gulf of Mexico and Cape Verde storms. The properties of the tracks’ seasonality, intensity, and power dissipation index in each cluster are documented for both configurations. The authors’ results show that, except for the seasonality, the downscaled tracks better capture the observed characteristics of the clusters. The authors also use three different idealized scenarios to examine the possible future changes of tropical cyclone tracks under 1) warming sea surface temperature, 2) increasing carbon dioxide, and 3) a combination of the two. The response to each scenario is highly variable depending on the simulation considered. Finally, the authors examine the role of each cluster in these future changes and find no preponderant contribution of any single cluster over the others.

Variation Tendency of TC Activity in the NWP

Based on the tropical cyclone dataset during 1945∼2013 by Joint Typhoon Warning Center (JTWC), this study has systematically analyzed the long-term variation of tropical cyclone (TC) in the Northwest Pacific (NWP). People recorded annual variations of Typhoon's maximal wind speed, power dissipation index (PDI) and frequency in this period. The results showed that these meteorology parameters display a rising trend, implying that the TC activity presents a feature of non-stationary stochastic processes. Geographically, we give spatial distribution of their historical maximal wind speed by combining database with parametric TC model. The results indicate that spatial distribution of TC intensity in the NWP is uneven and the sea area east to the Philippines is the most severely affected region by typhoon.

Future Changes in the Western North Pacific Tropical Cyclone Activity Projected by a Multidecadal Simulation with a 16-km Global Atmospheric GCM

Abstract How tropical cyclone (TC) activity in the northwestern Pacific might change in a future climate is assessed using multidecadal Atmospheric Model Intercomparison Project (AMIP)-style and time-slice simulations with the ECMWF Integrated Forecast System (IFS) at 16-km and 125-km global resolution. Both models reproduce many aspects of the present-day TC climatology and variability well, although the 16-km IFS is far more skillful in simulating the full intensity distribution and genesis locations, including their changes in response to El Niño–Southern Oscillation. Both IFS models project a small change in TC frequency at the end of the twenty-first century related to distinct shifts in genesis locations. In the 16-km IFS, this shift is southward and is likely driven by the southeastward penetration of the monsoon trough/subtropical high circulation system and the southward shift in activity of the synoptic-scale tropical disturbances in response to the strengthening of deep convective activity over the central equatorial Pacific in a future climate. The 16-km IFS also projects about a 50% increase in the power dissipation index, mainly due to significant increases in the frequency of the more intense storms, which is comparable to the natural variability in the model. Based on composite analysis of large samples of supertyphoons, both the development rate and the peak intensities of these storms increase in a future climate, which is consistent with their tendency to develop more to the south, within an environment that is thermodynamically more favorable for faster development and higher intensities. Coherent changes in the vertical structure of supertyphoon composites show system-scale amplification of the primary and secondary circulations with signs of contraction, a deeper warm core, and an upward shift in the outflow layer and the frequency of the most intense updrafts. Considering the large differences in the projections of TC intensity change between the 16-km and 125-km IFS, this study further emphasizes the need for high-resolution modeling in assessing potential changes in TC activity.

Contributing Factors to the Recent High Level of Accumulated Cyclone Energy (ACE) and Power Dissipation Index (PDI) in the North Atlantic*

Abstract In recent decades, tropical cyclone (TC) activity in the North Atlantic has shown a marked positive anomaly in genesis number, mean lifespan, number of intense hurricanes, and mean maximum intensity. The accumulated cyclone energy (ACE), which is defined as the sum of the square of the maximum surface wind velocity throughout the lifetime of a TC, is one of the measures that can be used to synthesize these factors. Similar to the ACE, the power dissipation index (PDI), which is defined as the integrated third power of maximum surface wind velocity, has also been used to describe TC activity. The basin-total ACE and PDI for the North Atlantic have also followed a large positive anomaly during the period 1995–2012; however, the relative importance of factors such as TC genesis number, TC track property (e.g., duration and lifespan), and TC intensity remains unclear in terms of their contribution to the positive anomalies in ACE and PDI. This study uses a new empirical statistical approach to analyze the TC data and finds that the increase in the TC genesis number is primarily responsible for the positive anomalies in ACE and PDI. Other factors, such as TC track property and TC intensity, appear to be minor influences.

Oceanic control of Northeast Pacific hurricane activity at interannual timescales

Sea surface temperature (SST) is not the only oceanic parameter that can play a key role in the interannual variability of Northeast Pacific hurricane activity. Using several observational data sets and the statistical technique of multiple linear regression analysis, we show that, along with SST, the thermocline depth (TD) plays an important role in hurricane activity at interannual timescales in this basin. Based on the parameter that dominates, the ocean basin can be divided into two sub-regions. In the Southern sub-region, which includes the hurricane main development area, interannual variability of the upper-ocean heat content (OHC) is primarily controlled by TD variations. Consequently, the interannual variability in the hurricane power dissipation index (PDI), which is a measure of the intensity of hurricane activity, is driven by that of the TD. On the other hand, in the Northern sub-region, SST exerts the major control over the OHC variability and, in turn, the PDI. Our study suggests that both SST and TD have a significant influence on the Northeast Pacific hurricane activity at interannual timescales and that their respective roles are more clearly delineated when sub-regions along an approximate north–south demarcation are considered rather than the basin as a whole.

Enhanced western North Pacific tropical cyclone activity in May in recent years

The tropical cyclone (TC) power dissipation index (PDI) in May over the western North Pacific (WNP) region shows a remarkable increase from the pre-1999 years (1979–1999) to the post-1999 years (2000–2011). Both increased TC numbers and enhanced TC intensity contributed to the change in the PDI. The averaged TC number in May increased from 1.05 per year in the pre-1999 years to 1.75 per year in the post-1999 years. In particular, the number of intense typhoon goes up from 0.14 per year to 0.83 per year, implying a sharp increase of TC intensity. Examination of the large scale background circulation in May shows that the epochal increase of TC number is caused by a significant increase of the genesis potential index (GPI), which has increased by about 33 % from the first (1979–1998) to the second (1999–2011) epoch over the TC genesis region (110°E–160°E, 5°N–20°N). The higher TC intensity is related to the increased maximum potential intensity and reduced TC ambient vertical wind shear in the second epoch. These decadal changes in background conditions over the WNP are the results of the enhanced summer monsoon in May over the both South Asia and South China Sea.