Tropical Cyclone Potential Intensity Research Articles

Asymmetric and Irreversible Response of Tropical Cyclone Potential Intensity to CO2 Removal

AbstractUnderstanding the behaviors of tropical cyclone (TC) intensity under the CO2 removal scenario is important for future climate adaptation and policy making. Based on the idealized CO2 ramp‐up (from 284.7 to 1,138.8 ppm) and symmetric ramp‐down experiments, our results suggest an asymmetric and irreversible response of TC potential intensity to CO2 reduction. Potential intensity shows an additional enhancement at the same CO2 level during the CO2 ramp‐down relative to the ramp‐up periods (though with regional differences), and does not completely return to the initial value even when CO2 recovers on multi‐decadal to centennial timescale. The enhanced potential intensity is dominated by the increased thermodynamic disequilibrium, which is mainly attributed to the weakened surface winds arising from the El Niño‐like warming pattern and inter‐hemispheric ocean temperature contrast. Our results highlight the potential risks of stronger storms on the socioeconomic development under the negative carbon emissions pathways.

A Westward Shift in Tropical Cyclone Potential Intensity and Genesis Regions in the North Atlantic During the Last Interglacial

AbstractGiven the longstanding debate on geomorphological features (i.e., megaboulders, chevron‐shaped sand ridges, and runup deposits) observed around the Bahamas, it is unclear whether Earth experienced superstorms over the North Atlantic during the Last Interglacial (LIG). Based on multimodels from the Paleoclimate Modeling Intercomparison Project, we illustrate that the tropical North Atlantic exhibits an overall warming (∼0.9°C) during the storm season of the LIG relative to preindustrial. Potential intensity shows a zonal dipole anomaly over the main development region during the LIG, with positive anomalies in the west, offering an increased probability of extremely strong storms. Favorable conditions for storm formation shift from the eastern to the western main development region during the LIG, with enhanced large‐scale steering flows at higher latitudes (∼15 – 30°N). These results suggest a westward migration of storm activity during the LIG, with potentially more frequent and intense storms over the western North Atlantic and increased coastal erosions.

PyPI (v1.3): Tropical Cyclone Potential Intensity Calculations in Python

Abstract. Potential intensity (PI) is the maximum speed limit of a tropical cyclone found by modeling the storm as a thermal heat engine. Because there are significant correlations between PI and actual storm wind speeds, PI is a useful diagnostic for evaluating or predicting tropical cyclone intensity climatology and variability. Previous studies have calculated PI given a set of atmospheric and oceanographic conditions, but although a PI algorithm – originally developed by Kerry Emanuel – is in widespread use, it remains under-documented. The Tropical Cyclone Potential Intensity Calculations in Python (pyPI, v1.3) package develops the PI algorithm in Python and for the first time details the full background and algorithm (line by line) used to compute tropical cyclone potential intensity constrained by thermodynamics. The pyPI package (1) provides a freely available, flexible, validated Python PI algorithm, (2) carefully documents the PI algorithm and its Python implementation, and (3) demonstrates and encourages the use of PI theory in tropical cyclone analyses. Validation shows pyPI output is nearly identical to the previous potential intensity computation but is an improvement on the algorithm's consistency and handling of missing data. Example calculations with reanalyses data demonstrate pyPI's usefulness in climatological and meteorological research. Planned future improvements will improve on pyPI's assumptions, flexibility, and range of applications and tropical cyclone thermodynamic calculations.

Increasing tropical cyclone intensity and potential intensity in the subtropical Atlantic around Bermuda from an ocean heat content perspective 1955–2019

We investigate tropical cyclone (TC) activity and intensity within a 100 km radius of Bermuda between 1955 and 2019. The results show a more easterly genesis over time and significant increasing trends in TC intensity (maximum wind speed (Vmax)) with a decadal Vmax median value increase of 30 kts from 33 to 63 kts (r = 0.94, p = 0.02), together with significant increasing August, September, October sea surface temperature (SST) of 1.1 °C (0.17 °C per decade) r = 0.4 (p < 0.01) and increasing average ocean temperature between 0.5 °C and 0.7 °C (0.08 °C–0.1 °C per decade) r = 0.3(p < 0.01) in the depth range 0–300 m. The strongest correlation is found between TC intensity and ocean temperature averaged through the top 50 m ocean layer () r = 0.37 (p < 0.01). We show how TC potential intensity (PI) estimates are closer to actual intensity by using as opposed to SST using the Hydrostation S time-series. We modify the widely used SST PI index by using to provide a closer estimate of the observed minimum sea level pressure (MSLP), and associated Vmax than by using SST, creating a PI (_PI) index. The average MSLP difference is reduced by 12 mb and proportional (r = 0.74, p < 0.01) to the SST/ temperature difference. We also suggest the index could be used over a wider area of the subtropical/tropical Atlantic where there is a shallow mixed layer depth.

An improved potential intensity estimate for Bay of Bengal tropical cyclones

The Bay of Bengal (BoB) is the most dangerous tropical cyclone (TC) region to humans in the world due to a combination of geographical and environmental factors. TC potential intensity (PI) estimates are a useful forecasting tool for evaluating favorable thermodynamic conditions for TC intensification; however, it assumes an idealized environment which omits several TC–environmental interactions. The present study compares the PI calculated using the SST (SST-PI) and the PI calculated taking into account ocean surface cooling and vertical wind shear (Environmental-PI) to the intensification rates of BoB TCs. A significant improvement is found in terms of explained variance when using Env-PI (13.5%) rather than SST-PI (3.5%). For slower moving TCs, the improvement is larger between Env-PI (25.5%) and SST-PI (4.9%). This study demonstrates that including an estimate of both oceanic and dynamical TC interactions in PI estimates lead to an improved estimate of future TC intensification in the BoB.

Superiority of Mega‐ENSO Index in the Seasonal Prediction of Tropical Cyclone Activity Over the Western North Pacific

AbstractIn this study, we compared the performance of two potential predicators, that is, El Niño–Southern Oscillation (ENSO) index (Niño‐3.4) and mega‐ENSO index, in the seasonal forecast of tropical cyclone (TC) activity and its spatial distribution over the western North Pacific (WNP) during the extended TC season, which is of public concern. Our results clearly show that, although both mega‐ENSO and Niño‐3.4 indices in the preceding May are important predictors for the seasonal predication, the relative‐sea surface temperature (SST)‐dependent mega‐ENSO exhibits a higher skill in the seasonal forecasting compared with the absolute‐SST‐dependent ENSO. Further results show that, despite of stronger destructiveness of TCs in high mega‐ENSO (El Niño) years than in low mega‐ENSO (La Niña) years, more attention should be paid to the TCs in low mega‐ENSO years, which are more likely to occur in coastal areas compared with the TCs in high mega‐ENSO years. Due to the responses of TC genesis, TC potential intensity, and large‐scale flow to the SST change in low mega‐ENSO years, the WNP TCs tend to originate in the northwestern quadrant and intensify at high latitudes and then turn northwestward over the TC prevailing region, which contributes to the northwestward migration of the WNP TC exposure in terms of track density and destructiveness density and thus imposes more risks in the coastal areas in low mega‐ENSO years. In addition, despite the significant predication skill in forecasting TC activity when using mega‐ENSO/Niño‐3.4 as a single predicator, it is still far to predict reliable WNP activity, especially its spatial distribution, without considering other predictors.

A Look at the Relationship between the Large-Scale Tropospheric Static Stability and the Tropical Cyclone Maximum Intensity

AbstractVarious modeling and observational studies have suggested that tropical cyclone (TC) intensity tends to increase in the future due to projected warmer sea surface temperature (SST). This study examines the effects of the tropospheric stratification that could potentially offset the direct increase of TC intensity associated with the warmer SST. Using reanalysis datasets and TC records in the northwestern Pacific and the North Atlantic basins, it is shown that there exists a consistently negative correlation between the annually averaged TC intensity and the basinwide average of the tropospheric static stability. This negative correlation is more robust in the northwestern Pacific basin when using the TC lifetime maximum intensity but is somewhat less significant in the North Atlantic basin. Further separation of the troposphere into a lower (1000–500 hPa) and an upper layer (500–200 hPa) reveals that it is the upper-tropospheric static stability that plays a more dominant role in governing the TC intensity variability. The negating effects of a stable troposphere on TC intensity as found in this study suggest a partial offset of the projected increase in the TC potential intensity due to the future warmer SST. Thus, the tropospheric static stability is one of the key large-scale factors that need to be properly taken into account in studies of long-term TC intensity change.

Aerosol versus Greenhouse Gas Effects on Tropical Cyclone Potential Intensity and the Hydrologic Cycle

AbstractAerosol cooling reduces tropical cyclone (TC) potential intensity (PI) more strongly, by about a factor of 2 per degree of sea surface temperature change, than greenhouse gas warming increases it. This study analyzes single-forcing and historical experiments from phase 5 of the Coupled Model Intercomparison Project, aiming to understand the physical mechanisms behind this difference. Calculations are done for the tropical oceans of each hemisphere during the relevant TC seasons, emphasizing multimodel means. PI theory is used to interpret the difference in the PI response to aerosol and greenhouse gas forcings in terms of three factors. The net surface turbulent heat flux (sum of the latent and sensible heat fluxes) explains half of the difference, thermodynamic efficiency explains at most a small fraction, and surface wind speed does not explain the remainder, perhaps because of the use of monthly mean data. Changes in turbulent surface heat fluxes are interpreted as responses to surface radiative flux changes in the context of the energy balance of the ocean mixed layer. Radiative kernels are used to estimate what fractions of the surface radiative flux changes are feedbacks due to temperature and water vapor changes. The greater effect of aerosol forcing occurs because shortwave forcing has a greater direct, temperature-independent component at the surface than does longwave forcing, for a forcing amplitude that provokes the same SST change. This conclusion recalls prior work on the response of precipitation to radiative forcing, and the similarities and differences between precipitation and potential intensity in this regard are discussed.

Tropical cyclone activity affected by volcanically induced ITCZ shifts

Volcanic eruptions can affect global climate through changes in atmospheric and ocean circulation, and therefore could impact tropical cyclone (TC) activity. Here, we use ensemble simulations performed with an Earth System Model to investigate the impact of strong volcanic eruptions occurring in the tropical Northern (NH) and Southern (SH) Hemisphere on the large-scale environmental factors that affect TCs. Such eruptions cause a strong asymmetrical hemispheric cooling, either in the NH or SH, which shifts the Intertropical Convergence Zone (ITCZ) southward or northward, respectively. The ITCZ shift and the associated surface temperature anomalies then cause changes to the genesis potential indices and TC potential intensity. The effect of the volcanic eruptions on the ITCZ and hence on TC activity lasts for at least 4 years. Finally, our analysis suggests that volcanic eruptions do not lead to an overall global reduction in TC activity but rather a redistribution following the ITCZ movement. On the other hand, the volcanically induced changes in El Niño-Southern Oscillation (ENSO) or sea-surface temperature do not seem to have a significant impact on TC activity as previously suggested.

The Indian Ocean Dipole: A Missing Link between El Niño Modokiand Tropical Cyclone Intensity in the North Indian Ocean

This study is set out to understand the impact of El Niño Modoki and the Tropical Cyclone Potential Intensity (TCPI) in the North Indian Ocean. We also hypothesized and tested if the Indian Ocean Dipole (IOD) reveals a likely connection between the two phenomena. An advanced mathematical tool namely the Empirical Mode Decomposition (EMD) is employed for the analysis. A major advantage of using EMD is its adaptability approach to deal with the non-linear and non-stationary signals which are similar to the signals used in this study and are also common in both atmospheric and oceanic sciences. This study has identified IOD as a likely missing link to explain the connection between El Niño Modoki and TCPI. This lays the groundwork for future research into this connection and its possible applications in meteorology.

A Review on the Long Term Variations of Tropical Cyclone Activity in the Typhoon Committee Region

ABSTRACT A review on the long term variations of tropical cyclone (TC) activity in the Typhoon Committee region was conducted based on the assessments of IPCC, IWTC and ESCAP/WMO Typhoon Committee and a number of recent publications. The results reveal strong inter-annual and inter-decadal variations in the TC activity over the western North Pacific (WNP). Analysis of available TC data since 1950s indicates that most of the TC datasets depict a decreasing trend, and some statistically significant, in the annual number of TCs and typhoons in the WNP. For TC intensity, differences in TC databases for the WNP do not allow a convincing detection of a long term trend in this basin. Climate model projections suggest a noticeable decrease in the frequency of the WNP TCs in the 21st century. Some of the model simulations also report an increase in the number of intense TCs and the TC potential intensity in the WNP in a warmer climate. Looking forward, data homogeneity in TC databases and uncertainties in model simulation are two main hurdles hindering the determination of the past and future trends of TC activity in this basin. Inter-agency co-operations are urgently required to improve the homogeneity and consistency of TC databases in the WNP. Continuous research would also be needed to further improve our understanding of the influence of natural variability and anthropogenic warming on the TC activity in the WNP.

Evaluating the Evidence of a Global Sea Surface Temperature Threshold for Tropical Cyclone Genesis

Recent studies have reaffirmed a global threshold sea surface temperature (SST) of 26°C for tropical cyclone (TC) genesis. However, it is well understood that other thermodynamic variables influence TC genesis and that high SST in isolation is not a sufficient criterion for genesis. Here, a basin-by-basin analysis of the SST distributions in the five most active ocean basins is performed, which shows that there is no global SST threshold for TC genesis. The distributions of genesis SST show substantial variations between basins. Furthermore, analysis of the conditional probability of genesis for a given TC season main development region SST suggests that the SST bounds for TC genesis are largely determined by the climatological bounds of the basin and that the SST values within this environmental range have similar probabilities of genesis. The distribution of relative SST (the difference between local and tropical mean) and tropical cyclone potential intensity at TC genesis are more distinct from those of the TC season environment, consistent with their utility in TC genesis indices.

On the Seasonal Cycles of Tropical Cyclone Potential Intensity

Recent studies have investigated trends and interannual variability in the potential intensity (PI) of tropical cyclones (TCs), but relatively few have examined TC PI seasonality or its controlling factors. Potential intensity is a function of environmental conditions that influence thermodynamic atmosphere–ocean disequilibrium and the TC thermodynamic efficiency—primarily sea surface temperatures and the TC outflow temperatures—and therefore varies spatially across ocean basins with different ambient conditions. This study analyzes the seasonal cycles of TC PI in each main development region using reanalysis data from 1980 to 2013. TC outflow in the western North Pacific (WNP) region is found above the tropopause throughout the seasonal cycle. Consequently, WNP TC PI is strongly influenced by the seasonal cycle of lower-stratospheric temperatures, which act to damp its seasonal variability and thereby permit powerful TCs any time during the year. In contrast, the other main development regions (such as the North Atlantic) exhibit outflow levels in the troposphere through much of the year, except during their peak seasons. Mathematical decomposition of the TC PI metric shows that outflow temperatures damp WNP TC PI seasonality through thermodynamic efficiency by a quarter to a third, whereas disequilibrium between SSTs and the troposphere drives 72%–85% of the seasonal amplitude in the other ocean basins. Strong linkages between disequilibrium and TC PI seasonality in these basins result in thermodynamic support for powerful TCs only during their peak seasons. Decomposition also shows that the stratospheric influence on outflow temperatures in the WNP delays the peak month of TC PI by a month.

A Reformulation of Tropical Cyclone Potential Intensity Theory Incorporating Energy Production along a Radial Trajectory

AbstractA modified formula for calculating tropical cyclone (TC) potential intensity (PI) from a balance between energy production and frictional dissipation in the TC surface layer is developed. This modified formula accounts for energy production and frictional dissipation at multiple radii (and therefore at multiple wind speeds) along the TC inflow trajectory. The PI maximum wind speed values VMAX are calculated using this expanded formula for four canonical radial profiles of wind speed. These results are compared to PI VMAX values calculated using the standard assumption that all energy production and frictional dissipation relevant to maximum intensity occurs at the radius of maximum winds (RMW).The new PI formulation developed here results in PI VMAX values substantially higher than the standard PI VMAX for all four of the radial wind speed profiles examined; the difference is explained by the increase in the outer radial limit of energy production. This increase holds true even if outflow temperature increases with increasing radius, although the VMAX increases with increasing outer radius are somewhat more modest in this case. The extended PI formula yields VMAX values 3–17 m s−1 higher than the standard PI VMAX value when calculated with outer energy production–dissipation limits of 2.0–2.5 RMW, although it yields potentially unrealistic values when calculated with larger outer limits (e.g., 6 RMW). These results are presented as a potential explanation for why individual TCs can exceed their standard PI VMAX values in terms of the storm thermodynamics.

Dynamical downscaling of tropical cyclones from CCSM4 simulations of the Last Glacial Maximum

AbstractDynamical downscaling of simulations of the Last Glacial Maximum (LGM) and late twentieth century (20C) were conducted using the Weather Research and Forecasting (WRF) model with the aim of (1) understanding how the downscaled kinematic and thermodynamic variables influence simulated tropical cyclone (TC) activity over the western North Pacific during the LGM and the 20C periods and (2) to test the relevance of TC genesis factors for the colder LGM climate. The results show that, despite the lower temperatures during the LGM, the downscaled TC climatology over the western North Pacific in the LGM simulation does not differ significantly from that in the 20C simulation. Among the TC environmental factors, the TC potential intensity, mid‐tropospheric entropy deficit, and vertical wind shear during the LGM were consistent with previous analyses of TC genesis factors in LGM global climate model simulations. Changes in TC genesis density between the LGM and the 20C simulations seem to be well represented by the ventilation index, a nondimensional measure of the combined effects of vertical wind shear, and thermodynamic properties, suggesting the potential applicability of those factors for TC activity evaluation during the LGM and possibly other climates.

西北太平洋10~12月台风活动的年代际突变

本文基于日本气象厅区域专业气象中心-东京台风中心(Regional Specialized Meteorological Center-Tokyo Typhoon Center, Japan Meteorological Agency)的热带气旋资料和欧洲中尺度天气预报中心(European Centre for Medium-Range Weather Forecasts)的再分析资料，在前人的研究基础上，对西北太平洋10~12月期间台风活动对大尺度环境因子年代际突变的响应进行了讨论，并且通过台风数量和相关大尺度环境因素的分析，对其影响机制进行了比较详细的阐述。分析西北太平洋1980~2013年10~12月生成的台风，与1980~1994年相比，10~12月的台风数量在1995~2013年期间显著减少。分析发现热带气旋最大潜在强度的年代际变化是导致1995~2013年西北太平洋热带气旋潜在生成指数(Genesis Potential Index, GPI)减少的最主要因素，1995年后西北太平洋SST异常场呈La Nina形态，导致热带气旋最大潜在强度(The Maximum Tropical Cyclone Potential Intensity, MPI)增大，但在热带气旋的发展过程中过早受到西边界的影响使得增强为台风的热带气旋减少。突变后垂直风切变增强造成西北太平洋东南部的动力条件不利于台风生成，这都是造成台风突变减少的原因。分析还发现垂直风切变主要对西北太平洋东南部的台风活动造成影响。 Based on the data sets of Regional Specialized Meteorological Center-Tokyo Typhoon Center, Japan Meteorological Agency and the reanalysis data of European Centre for Medium-Range Weather Forecasts, this paper demonstrated the decadal shift between October to December (OND) typhoon activity over the Western North Pacific (WNP) during 1980 to 2013, and discussed the response of the OND typhoon activity to decadal shift of the large-scale circulation. Meanwhile, by means of the analysis of typhoon activity and its relative large-scale environmental factors, it brought a detail exposition of its impact mechanisms. We analyzed that the WNP typhoon activity during October to December from 1980 to 2013, compared with that from 1980 to 1994, the typhoon activity during October to December among 1995 to 2013 decreased significantly. We discovered the main factor that led to the decrease of Tropical Cyclone Genesis Potential Index was the decadal shift of The Maximum Tropical Cyclone Potential Intensity. The anomalous SST patterns appear to be a La Nina pattern, leading to the increasing of MPI. However, it early affected by the western boundary during tropical cyclones development process so that fewer tropical cyclones enhanced to be typhoon. After 1995, the vertical wind shear and subtropical high to be strengthener, resulting in the southeastern Pacific Northwest dynamic conditions are not conducive to typhoon genesis; all these are the reasons for the decadal shift of typhoon activity. We also found the vertical wind shear mainly affects the eastern part of the Northwest Pacific typhoon activity.

On the factors affecting trends and variability in tropical cyclone potential intensity

On the factors affecting trends and variability in tropical cyclone potential intensity

The Effects of Imposed Stratospheric Cooling on the Maximum Intensity of Tropical Cyclones in Axisymmetric Radiative–Convective Equilibrium

AbstractThe effects of stratospheric cooling and sea surface warming on tropical cyclone (TC) potential intensity (PI) are explored using an axisymmetric cloud-resolving model run to radiative–convective equilibrium (RCE). Almost all observationally constrained datasets show that the tropical lower stratosphere has cooled over the past few decades. Such cooling may affect PI by modifying the storm's outflow temperature, which together with the sea surface temperature (SST) determines the thermal efficiency in PI theory. Results show that cooling near and above the model tropopause (∼90 hPa), with fixed SST, increases the PI at a rate of 1 m s−1 per degree of cooling. Most of this trend comes from a large increase in the thermal efficiency component of PI as the stratosphere cools. Sea surface warming (with fixed stratospheric temperature) increases the PI by roughly twice as much per degree, at a rate of about 2 m s−1 K−1. Under increasing SST, most of the PI trend comes from large changes in the air–sea thermodynamic disequilibrium. The predicted outflow temperature shows no trend in response to SST increase; however, the outflow height increases substantially. Under stratospheric cooling, the outflow temperature decreases and at the same rate as the imposed cooling. These results have considerable implications for global PI trends in response to climate change. Tropical oceans have warmed by about 0.15 K decade−1 since the 1970s, but the stratosphere has cooled anywhere from 0.3 to over 1 K decade−1, depending on the dataset. Therefore, global PI trends in recent decades appear to have been driven more by stratospheric cooling than by surface warming.

Influence of Tropical Tropopause Layer Cooling on Atlantic Hurricane Activity

AbstractVirtually all metrics of Atlantic tropical cyclone activity show substantial increases over the past two decades. It is argued here that cooling near the tropical tropopause and the associated decrease in tropical cyclone outflow temperature contributed to the observed increase in tropical cyclone potential intensity over this period. Quantitative uncertainties in the magnitude of the cooling are important, but a broad range of observations supports some cooling. Downscalings of the output of atmospheric general circulation models (AGCMs) that are driven by observed sea surface temperatures and sea ice cover produce little if any increase in Atlantic tropical cyclone metrics over the past two decades, even though observed variability before roughly 1970 is well simulated by some of the models. Part of this shortcoming is traced to the failure of the AGCMs examined to reproduce the observed cooling of the lower stratosphere and tropical tropopause layer (TTL) over the past few decades. The authors caution against using sea surface temperature or proxies based on it to make projections of tropical cyclone activity as there can be significant contributions from other variables such as the outflow temperature. The proposed mechanisms of TTL cooling (e.g., ozone depletion and stratospheric circulation changes) are reviewed, and the need for improved representations of these processes in global models in order to improve projections of future tropical cyclone activity is emphasized.

Influence of local and remote SST on North Atlantic tropical cyclone potential intensity

We examine the role of local and remote sea surface temperature (SST) on the tropical cyclone potential intensity in the North Atlantic using a suite of model simulations, while separating the impact of anthropogenic (external) forcing and the internal influence of Atlantic Multidecadal Variability. To enable the separation by SST region of influence we use an ensemble of global atmospheric climate model simulations forced with historical, 1856–2006 full global SSTs, and compare the results to two other simulations with historical SSTs confined to the tropical Atlantic and to the tropical Indian Ocean and Pacific. The effects of anthropogenic plus other external forcing and that of internal variability are separated by using a linear, “signal-to-noise” maximizing EOF analysis and by projecting the three model ensemble outputs onto the respective external forcing and internal variability time series. Consistent with previous results indicating a tampering influence of global tropical warming on the Atlantic hurricane potential intensity, our results show that non-local SST tends to reduce potential intensity associated with locally forced warming through changing the upper level atmospheric temperatures. Our results further indicate that the late twentieth Century increase in North Atlantic potential intensity, may not have been dominated by anthropogenic influence but rather by internal variability.