Cyclone Strength Research Articles

Spatial Changes of Driving Parameters Affecting Cyclonic Activity over the North Indian Ocean from 1960 to 2020

This study undertakes a thorough analysis of the elements that influence the variability of tropical cyclones (TC) in the North Indian Ocean (NIO) from 1960 to 2020, with a specific focus on the periods before and after the monsoon season. The study utilizes historical satellite data to investigate the factors that impact the formation, strength, and trajectories of cyclones. The primary method for evaluating cyclone strength is by calculating the Accumulated Cyclone Energy (ACE). The study observes a decreasing trend in ACE levels during 1991–2005, which started increasing just after from 2006 to 2020. The Bay of Bengal (BoB) has a more uniform distribution of ACE in comparison to the Arabian Sea (AS), with higher average values and more variability over the Main Development Region (MDR), which is the area where cyclone development occurs most frequently. Cyclones of greater intensity generally occur following the monsoon season. Examination of storm paths reveals that cyclones with greater intensity frequently hit the northeastern and southeastern coastal regions of India. The study emphasizes notable discrepancies in parameters within the MDR, which impact cyclone strength and ACE values throughout various periods.

Assessment of tangible coastal inundation damage related to critical infrastructure and buildings: The case of Mauritius Island

Storm tides, which combine sea level rise (SLR), astronomical tides, and storm surges generated by tropical cyclones, pose significant threats to coastal zones, leading to flooding and substantial damage to property and infrastructure.There is a clear upward trend in the frequency of storms reaching tropical cyclone strength. A notable example is Cyclone Belal, which struck Mauritius on January 2024, during high tide, causing extensive infrastructure damage. This underscores the importance of conducting risk assessments to identify vulnerable areas and develop risk reduction strategies. However, quantitative risk assessments of storm tides are often challenging due to the lack of long-term projections. To address this, we developed a GIS-based flood model for Mauritius to simulate inundation areas and quantify the assets exposed to flooding. Under current conditions, the estimated damage exposure from extreme coastal flood events with return periods of 50 to 500 years is significant, with 6.2% and 27.1% of the area inundated, respectively. By 2100, damage exposure associated with these events is projected to increase by a factor of 1.1, with minimal variation between sea-level rise scenarios (0.3m). However, by 2200 and 2300, damage exposure is expected to rise by factors of 3.1 and 6.6, respectively. In the worst-case scenario for 2500, Mauritius could experience maximum inundation of 66.3 km2, with buildings covering 5.02 km2 submerged. Additionally, this study presents a detemporalized inundation scenario to assess impacts from any coastal flood event. This approach enables the identification of critical thresholds ( 1.5 m and 4.5 m) and , beyond which significant increases in damage exposure are likely, and allows for evaluating adaptation strategies against user-defined levels of change, rather than relying solely on predefined scenarios. These findings highlight the urgent need for strategic sectoral interventions to address the widespread consequences of coastal inundation, especially in light of critical thresholds for remedial action.

Modes of Weather System-Induced Flows through an Arctic Lagoon

With the increasing warming of the Arctic, the summertime ice-free period in the coastal Arctic becomes longer and the water exchange between arctic lagoons and coastal Beaufort Sea becomes more important for land–ocean interaction. This study examined the dynamics of water exchange between the arctic lagoons and the Arctic Ocean under the influence of weather systems (the transient arctic cyclones and hovering Beaufort High pressure system). We implemented rare observations, numerical modeling with the Finite Volume Community Ocean Model (FVCOM), and a forcing-response Empirical Orthogonal Function (fr-EOF) analysis to determine the weather-driven flow patterns and characteristics in the micro-tidal arctic lagoon (Elson Lagoon) with little freshwater discharge. The results were validated for both tidal and subtidal currents with in situ data. The inlets of the lagoon were significantly impacted by wind associated with the weather systems and the flows through the inlets were highly correlated with each other. The fr-EOF analysis for the 1.5-month FVCOM output indicated three significant modes of wind-driven flow. In the deepest (~16 m) northwestern-most inlet, a counter-wind flow occurred more than 96% of the time due to setup and set down of water level inside the lagoon and the vorticity balance related to the wind stress and water depth. For about 60–80% of the time, the exchange flow was out of the lagoon through the northwestern-most and deepest inlet due to the strong easterly winds dictated by the Beaufort High; this dominant flow is mainly caused by the persistent easterly wind as a limb of the Beaufort High pressure system, modified by the transient arctic cyclones with a westerly wind and inward flows at the westernmost inlet of Elson Lagoon. This study shows that the alternating influence from the cyclone-anticyclone weather systems produces a meteorological tide in the subtidal spectrum which dominates the water exchange in the region through the multiple inlets. With the observed increase in cyclone strength and frequency under the warming trend, this may imply a greater contribution from the westerly wind because of the increased cyclonic activities. If this is the case, the inward flow might increase and have an effect on sediment, larval, and nutrient transports through this system.

Advanced hybrid CNN-Bi-LSTM model augmented with GA and FFO for enhanced cyclone intensity forecasting

Predicting cyclone intensity is an important aspect of weather forecasting since it influences disaster preparation and response. This framework addresses the pressing need for precise cyclone intensity prediction by presenting a unique predictive model based on a hybrid CNN and Bi-LSTM architecture optimized using a Genetic Algorithm (GA) enhanced Fruit Fly Optimizer (FFO). Existing methods have primarily relied on traditional machine learning models and meteorological data, demonstrating limitations in capturing the complex spatial-temporal patterns inherent in cyclone evolution. These drawbacks include insufficient feature extraction abilities, underutilization of convolutional neural networks (CNN), and poor model tuning. This unique method incorporates a hybrid CNN and Bi-LSTM architecture that is tuned by a Genetic Algorithm (GA) enhanced Fruit Fly Optimizer (FFO), resulting in higher cyclone intensity prediction accuracy. The experimental results are implemented in Python software, and they reveal that this method outperforms current models by an average of 21% when compared to existing methods such as VGG-16 achieved an accuracy of 78% and Ty 5-CNN (95.23%). The suggested CNN-Bi-LSTM model predicts cyclone strength with an excellent accuracy of 99.4%. This unique approach offers a possible avenue for increasing cyclone intensity prediction, hence improving disaster preparedness and risk mitigation efforts in sensitive locations.

Cloud and Precipitation in Low-Latitude Extratropical Cyclones Conditionally Sorted on CYGNSS Surface Latent and Sensible Heat Fluxes

Abstract Extratropical cyclones are the primary cause of midlatitude winds, precipitation, and clouds. Surface latent and sensible heat fluxes from the ocean impact cyclone intensity, but their role in moisture transport leading to cloud and precipitation is still under investigation. While numerical simulations can help establish such links, evaluating and constraining these simulations require observations. To this end, satellite-based observations of cloud and precipitation in low-latitude extratropical cyclones are examined for four distinct classifications based on the Cyclone Global Navigation Satellite System (CYGNSS) latent and sensible heat fluxes, averaged in the post-cold-frontal or warm-sector areas of the cyclones. Using a cyclone compositing approach, contrasts in cloud and precipitation in strong- versus weak-surface-flux conditions are examined. In the post-cold-frontal region, stronger latent or sensible heat fluxes are associated with lower precipitation rates and higher cloud opacity, indicating more vigorous shallow convection. However, larger sensible heat flux cases display larger cloud fraction, while larger latent heat flux cases exhibit lower cloud fraction, which could indicate differing cloud morphologies. In the comma region of the cyclones, clouds and precipitation depend on both cyclone strength and moisture availability. Consistent with this, larger cloud amount and precipitation are found for strong fluxes in the post-cold-frontal region, and weak or negative sensible heat fluxes, indicative of poleward warm advection, in the warm sector. The strong regional differences in the surface heat flux–cloud and precipitation relationships highlight the need for further investigation into moisture supply and transport in cyclones, while providing guidance for future work.

Atmospheric forcing of the Hatteras coastal ocean during 2017–2018: The PEACH program

The Hatteras coastal ocean is centrally located along the east coast of the 48 contiguous United States, offshore of Cape Hatteras in a complex land/ocean/atmosphere region where major ocean currents of differing temperatures and salinities meet and interact, where the atmosphere fluctuates on a wide range of time scales, and where atmosphere-ocean interactions vary both spatially and temporally. The Gulf Stream current typically leaves its contact with the continental margin here. Continental shelf currents from the north and from the south converge here, resulting in a net shelf-to-ocean transport of shelf waters that carry important water properties and constituents. The two major drivers of these shelf currents and exchanges are the atmosphere and the oceanic Gulf Stream. Atmospheric driving of the Hatteras coastal ocean is through surface wind stress and heat flux across the air-sea interface. The complexity and importance of this region motivated the NSF-sponsored PEACH research program during 2017–2018 (PEACH: Processes driving Exchange At Cape Hatteras). In this paper, we utilize the substantial number of observations available during PEACH to describe the atmospheric forcing of the ocean then. Atmospheric conditions are described in terms of two seasons: the warm season (May to mid-September), with predominantly mild northeastward winds punctuated by occasional tropical cyclones (TCs); and the cool season (mid-September through April), with a nearly continuous, northeastward progression of energetic extratropical cyclones (ETCs) through the region. Cool season ETCs force the region with strong wind stress and ocean-to-atmosphere heat flux episodes, each with a time-scale of several days. Wind stress fluctuation magnitudes typically exceed mean stress magnitudes in each season by a factor of 3–5. These stresses account for just over 40% of the total current variability in the region, showing the wind to be a major driver of the ocean here. Atmosphere-ocean heat flux is typically into the ocean throughout the warm season (~100 W m-2); it is essentially always out of the ocean during the cool season (~500 W m-2 or more). New results herein include: southward intraseasonal oscillations of the jet stream’s position drove the strongest ETCs (including one “bomb” cyclone); and during the 41 years leading up to and including PEACH, the season-averaged number and strength of atmospheric cyclones passing over the Hatteras coastal ocean have shown little long-term change. Looking ahead, the NSF Pioneer Array is scheduled to be relocated to the northern portion of the Hatteras coastal ocean in 2024, and the NASA SWOT satellite has begun its ocean topography mission, which has a ground-track cross-over here.

Tropical cyclone strength, precipitation, and environment in variable resolution CAM-MPAS simulations over Western North Pacific

Tropical cyclone strength, precipitation, and environment in variable resolution CAM-MPAS simulations over Western North Pacific

Influence of ASCAT soil moisture on prediction of track and intensity of landfall tropical cyclones

ABSTRACT The current study attempted to evaluate the impact of Soil Moisture (SM) initial conditions in a regional NWP system on the simulation of land falling cyclones over North Indian Ocean region. SM initial conditions used in this study are created using a simplified EKF based LDAS, in which satellite derived SM information is also used. Here, two sets of numerical experiments using regional NWP system are performed with two different sets of SM initial conditions. The CTL (assimilated only surface level observations in LDAS) and ASCAT (assimilated satellite derived SM plus observations used in CTL) experiments are performed to understand the impact of SM initial conditions in NCUM-R NWP system. The atmospheric initial conditions for both experiments are prepared through 4DVAR assimilation technique. This study shows that high resolution regional model initialized with improved SM initial conditions created using satellite observations could improve structure, intensity, track and evolution of the storms. The positive values of humidity Jacobians are noticed over the regions affected by storms mainly in ASCAT. The evolution of vorticity tendency and convergence profiles is well differentiated in ASCAT as the system moved from higher to lower intensity stage or vice versa, but that pattern is not observed in CTL. The value of static stability indices is varying with strength of cyclones, which are well represented in ASCAT and are also well matched with observations. The evolution of magnitude of diabatic heating in and around the storm core region is more realistic in ASCAT than CTL. The moisture budget terms are improved in ASCAT, the contribution of moisture convergence (evaporation) to the precipitation is more over the oceanic (land) region before and after landfall of storm. The study deduced that accurate SM initial conditions are essential for better prediction of structure and intensity of high impact weather events.

Climate Change Impact and Frequency of Cyclone Surge in Bangladesh Coast: Proper Plan to Address Sustainably

The Bay of Bengal (BoB) is a familiar ground for tropical cyclones. Consequently, cyclone surge and tidal inundation are very common in the Bangladesh coast which causes massive loss of human lives and properties. The present paper is an attempt to delineate the frequency of cyclones in the BoB, and loss of human lives and properties of Bangladesh coast on the basis of published reports, open source data and field checking, and finally to propose an effective way out to protect the area from cyclone surge. This study exposes that the frequency of cyclones in the area increased recently due to rise of global temperature. It further reveals that though the frequency of cyclones is increased, the loss of human lives is significantly decreased. Death toll in the last two decades decreased a lot for similar strength of cyclones comparing to the toll in last thirty or fifty years. Improvement in the early warning system, evacuation plan with proper training and construction of good number of cyclone shelter all over the coastal region are the main reason for this improvement. Though the loss of human lives is significantly reduced, the loss of economic resources in the coastal areas of the country is enormous as in earlier episodes. Therefore, adaptation for the Climate Change effect is needed to expedite with afforestation in the coastal region. Economic loss might also be reduced obstructing the cyclonic surge in the coastal region. Considering the cyclone path and tidal amplitude of cyclonic surge, upgradation or reconstruction of the coastal embankments all along the coast is essential. In this process, it is important to ensure that the tidal channels act as an inlet and outlets all over the coast to keep the natural processes active. An integrated approach including geoscientists, engineers, planners, policy makers, etc. is required to protect the resources as well as proper management of the coastal areas of the country.

Assessing the Forecast Impact of a Geostationary Microwave Sounder Using Regional and Global OSSEs

Abstract Forecast observing system simulation experiments (OSSEs) are conducted to assess the potential impact of geostationary microwave (GeoMW) sounder observations on numerical weather prediction forecasts. A regional OSSE is conducted using a tropical cyclone (TC) case that is very similar to Hurricane Harvey (2017), as hurricanes are among the most devastating of weather-related natural disasters, and hurricane intensity continues to pose a significant challenge for numerical weather prediction. A global OSSE is conducted to assess the potential impact of a single GeoMW sounder centered over the continental United States versus two sounders positioned at the current locations of the National Oceanic and Atmospheric Administration Geostationary Operational Environmental Satellites (GOES) East and West. It is found that assimilation of GeoMW soundings result in better characterization of the TC environment, especially before and during intensification, which leads to significant improvements in forecasts of TC track and intensity. TC vertical structure (warm core thermal perturbation and horizontal wind distribution) is also substantially improved, as are the surface wind and precipitation extremes. In the global OSSE, assimilation of GeoMW soundings leads to slight improvement globally and significant improvement regionally, with regional impact equal to or greater than nearly all other observation types. Significance Statement This work seeks to determine the impact of a new geostationary microwave (GeoMW) sounder on tropical cyclone forecasts in particular, and on weather forecasts in general. It does so by assimilating simulated GeoMW sounder data into two different forecast models: one global and one regional. The data have a small positive impact globally, and a significant positive impact over the region viewed by the GeoMW instrument. In particular, assimilation of GeoMW data has a significant and positive impact on forecasts of tropical cyclone track, strength, and structure.

On the relationship between CYGNSS surface heat fluxes and the lifecycle of low-latitude ocean extratropical cyclones

AbstractSurface latent and sensible heat fluxes are important for extratropical cyclone evolution and intensification. Because extratropical cyclone genesis often occurs at low-latitude, CYGNSS surface latent and sensible heat flux retrievals are composited to provide a mean picture of their spatial distribution in low-latitude oceanic extratropical cyclones. CYGNSS heat fluxes are not affected by heavy precipitation and offer observations of storms with frequent revisit times. Consistent with prior results obtained for cyclones in the Gulf Stream region, the fluxes are strongest in the wake of the cold fronts, and weakest to negative in the warm sector in advance of the cold fronts. As cyclone strength increases, or mean precipitable water decreases, the maximum in surface heat fluxes increases while the minimum decreases. This impacts the changes in fluxes during cyclone intensification: the post-cold frontal surface heat flux maximum increases due to the increase in near surface winds. During cyclone dissipation, the fluxes in this sector decrease, due to the decrease in winds and in temperature and humidity contrast. The warm sector minimum decreases throughout the entire cyclone lifetime and is mostly driven by sea-air temperature and humidity contrast changes. However, during cyclone dissipation, the surface heat fluxes increase along the cold front in a narrow band to the east, independently from changes in the cyclone characteristics. This suggests that, during cyclone dissipation, energy transfers from the ocean to the atmosphere are linked to frontal in addition to synoptic-scale processes.

Do cyclone impacts really matter for the long-term performance of an offshore wind turbine?

With the transition on planning and construction of offshore wind turbine (OWT) from North Europe to other regions like America and East Asia, challenges are proposed for the direct application of international OWT experience to these territories due to disparate natural condition like cyclones. This article is intended to evaluate the impact of cyclone on the long-term performance of an OWT to be installed in cyclone-prone regions. To have a comprehensive consideration on the aero-hydro-structural-soil interaction, an improved decoupled method is proposed and validated for an onshore wind turbine before its application to an OWT. Two cyclone models combined with two wave theories are considered in the fatigue evaluation of an OWT under different working status, and their implications on the final estimation of fatigue damage are compared and discussed. The results obtained from this study indicate that the fatigue life reduction which is caused by cyclone, for an OWT can be conspicuous for a reasonable cyclone strength and average recurrence interval. This implies that potential premature failure of an OWT tower and relevant economic losses can be encountered during its service life if the cyclone contribution to fatigue damage is ignored in the initial conceptual design.

Climate-Induced Mass Relocation in Fiji

Climate change is, undeniably, a global phenomenon, which requires timely and sincere global efforts and commitments to save the planet before it is too late. The island nations in the blue Pacific region are arguably experiencing the destructive nature of climate change more than any other nation in the world. Scientists warn that this slow-motion phenomenon is claiming entire nations, which will not exist on the face of the earth as early as next century. Sea-level rise is one of the biggest existential threats that the region is facing. Countries such as Tuvalu, Kiribati and Marshall Islands have already started sinking with their citizens looking for alternative countries. In Fiji, more than 200 low-lying villages are at risk of sinking and the government hopes to relocate these communities to higher ground, despite the pressure this would place on its weak economy. The relocatees will lose their most precious commodity, the land, which is their identity, status and source of survival. The other most precious commodity to which they attach a sense of belonging and will be lost for life are their ancestral homes, culture and traditional way of life. The relocation plan also creates distance between people and the sea, which is the source of their food. This article argues that despite being considered an effective adaptation mechanism to climate change, the relocation plan is facing multiple hurdles. The plan is far beyond the financial capacity and technical prowess of the Fijian government. The other possible alternative to mass relocation is strengthening the locally-made seawalls into strong durable structures, which can withstand the strength of cyclones and be an effective barrier to further shoreline erosion. The small island developing nations of the Pacific region will need financial and technical assistance from the industrialised nations to implement such a project successfully.

Planned Relocation

Climate change is undeniably a global phenomenon, which requires timely global efforts and sincere commitments to save the planet before it is too late. The blue Pacific region as a whole is experiencing the destructive effects of climate change, arguably more so than any other nation in the world. Scientists warn us that this slow-in-motion phenomenon is claiming entire nations and will continue to do so. Nations will cease to exist on the face of the earth as early next century. Sea level rise is one of the biggest existential threats that the region is facing. Countries such as Tuvalu, Kiribati and the Marshall Islands have already started sinking, with their citizens looking for alternate countries.
 In Fiji, more than 200 low-lying villages are at risk of sinking and the government with its meagre economy has decided to relocate these communities to higher ground. The relocates will lose their most precious commodity, the land, which is their identity, status and source of survival. The other most precious commodity to which they attach a sense of belonging which will be lost for life is their ancestral home, along with their culture and traditional way of life. The relocation plan also creates a large distance between people and the sea, which is the source of their food.
 This article argues that despite being considered as an effective adaptation mechanism to climate change, the relocation plan is facing multiple hurdles. The plan is far beyond the financial capacity and technical prowess of the Fijian government. The other possible alternative to mass relocation is strengthening the locally-made seawalls into strong durable structures, which can withstand the strength of cyclones and be an effective barrier to further shoreline erosion into the sea. The small island developing nations of the Pacific region will need the financial and technical assistance of industrialised nations to implement the project successfully.

A Closer Look at the Role of the Cyprus Low on Dust Events in the Negev Desert

The Negev Desert in Israel is susceptible to frequent atmospheric events of high dust loading which have been linked with negative human health outcomes, including cardiovascular and respiratory distress. Previous research suggests that the highest levels of dust over the region occur during an atmospheric pattern with a cyclone situated over the eastern Mediterranean. This Cyprus Low can bring unsettled weather and strong westerly winds over the Negev. However, while the overall pattern associated with dust events in the Negev Desert is generally well-understood, it remains unclear why days with seemingly similar weather patterns result in different levels of atmospheric dust. Thus, the goal of this study is to better differentiate the atmospheric patterns during dust events over the Negev. Using PM10 data collected in Be’er Sheva, Israel, from 2000 to 2015 in concert with 72-h HYSPLIT back trajectories at three different height levels (surface, 200 m, 500 m), we examine the source region, trajectory groups using a K-Means clustering procedure, and overall synoptic pattern during dust events. Further, we use sea-level pressure data across the region to determine how cyclone strength and location impact dust events in Be’er Sheva. We find that the highest levels of atmospheric dust in the Negev are associated with the Cyprus Low pattern, and air traversing Libya seems to play an especially important role, likely due to the country’s arid surface cover. Cyclone strength is also a critical factor, as lower sea-level pressure results in more severe dust events. A better understanding of the atmospheric features associated with dust events over the Negev Desert will hopefully aid in forecasting these occurrences across the region.

Study of cyclogenesis of developing and non-developing tropical systems of NIO using NCUM forecasting system

A study has been carried out to understand the similarities and differences in a pair of developing and a non-developing tropical disturbances using NCMRWF Unified Model (NCUM) and the concepts of marsupial theory of tropical cyclogenesis. We focused on two systems, tropical cyclone (TC) Ockhi that attained Very Severe Cyclonic Storm (VSCS) strength, and another system that attained deep depression state, however, subsequently weakened. As per the first hypothesis of Marsupial Paradigm, genesis of both the systems is found to be associated with a westward moving parent disturbance. The pouch region associated with the parent disturbance is identified as a region of enhanced moisture. The vertical profiles of vorticity and convergence show that the pathway of genesis is bottom-up for both the systems. Further analyses have been carried out to understand why the depression failed to attain tropical cyclone strength. It has been evidenced that the vertical wind shear is too high (> 50 ms− 1) in the premises of the depression core during the post-genesis evolution. At the same time, the center of the depression happened to be in the close proximity of a ridge present over central Indian land mass. Additionally, significant evidence of dry air intrusion into the core of the depression is seen in the Total Precipitable Water (TPW) analysis and HYSPLIT back trajectory analysis. Furthermore, the depression immediately interacts with the colder sea water as it moves northwestward. Though the marsupial paradigm of tropical cyclogenesis is applicable in the North Indian Ocean as well, factors such as SST, wind shear, closeness to the land mass may play a critical role in further development of the system.

Evaluation of Modeled Precipitation in Oceanic Extratropical Cyclones Using IMERG

ABSTRACTUsing a high-spatial- and high-temporal-resolution precipitation dataset, Integrated Multi-satellite Retrievals for GPM (IMERG), extratropical cyclone precipitation is evaluated in two reanalyses and two climate models. Based on cyclone-centered composites, all four models overestimate precipitation in the western subsiding and dry side of the cyclones, and underestimate the precipitation in the eastern ascending and moist side. By decomposing the composites into frequency of occurrence and intensity (mean precipitation rate when precipitating), the analysis reveals a tendency for all four models to overestimate frequency and underestimate intensity, with the former issue dominating in the western half and the latter in the eastern half of the cyclones. Differences in frequency are strongly dependent on cyclone environmental moisture, while the differences in intensity are strongly impacted by the strength of ascent within the cyclone. There are some uncertainties associated with the observations: IMERG might underreport frozen precipitation and possibly exaggerate rates in vigorously ascending regions. Nevertheless, the analysis suggests that all models produce extratropical cyclone precipitation too often and too lightly. These biases have consequences when evaluating the changes in precipitation characteristics with changes in cyclone properties: the models disagree on the magnitude of the change in precipitation intensity with a change in environmental moisture and in precipitation frequency with a change in cyclone strength. This complicates accurate predictions of precipitation changes in a changing climate.

Variations in the Intensity and Spatial Extent of Tropical Cyclone Precipitation

AbstractThe intensity and spatial extent of tropical cyclone precipitation (TCP) often shapes the risk posed by landfalling storms. Here we provide a comprehensive climatology of landfalling TCP characteristics as a function of tropical cyclone strength, using daily precipitation station data and Atlantic U.S. landfalling tropical cyclone tracks from 1900 to 2017. We analyze the intensity and spatial extent of ≥1 mm/day TCP (Z1) and ≥50 mm/day TCP (Z50) over land. We show that the highest median intensity and largest median spatial extent of Z1 and Z50 occur for major hurricanes that have weakened to tropical storms, indicating greater flood risk despite weaker wind speeds. We also find some signs of TCP change in recent decades. In particular, for major hurricanes that have weakened to tropical storms, Z50 intensity has significantly increased, indicating possible increases in flood risk to coastal communities in more recent years.

Critical behavior of tropical cyclones

Critical phenomena have previously been reported in rainfall patterns over the tropical oceans. Confirming and extending that discovery, we identify a critical temperature Tc, and a linear T-Tc power law dependence, typical of a continuous phase transition, in existing data and excellent fits for the power dissipation index (PDI) for Northern Hemisphere hurricanes vs sea surface temperature T. It is suggested that tropical cyclone formation can usefully be regarded as a phase transition of the warm ocean-atmosphere system, driven by disequilibrium in atmospheric water content. The related implication of this previously unrecognized, rapid temperature dependence of tropical cyclone strength is noted.

The role of cyclone activity in snow accumulation on Arctic sea ice

Identifying the mechanisms controlling the timing and magnitude of snow accumulation on sea ice is crucial for understanding snow’s net effect on the surface energy budget and sea-ice mass balance. Here, we analyze the role of cyclone activity on the seasonal buildup of snow on Arctic sea ice using model, satellite, and in situ data over 1979–2016. On average, 44% of the variability in monthly snow accumulation was controlled by cyclone snowfall and 29% by sea-ice freeze-up. However, there were strong spatio-temporal differences. Cyclone snowfall comprised ~50% of total snowfall in the Pacific compared to 83% in the Atlantic. While cyclones are stronger in the Atlantic, Pacific snow accumulation is more sensitive to cyclone strength. These findings highlight the heterogeneity in atmosphere-snow-ice interactions across the Arctic, and emphasize the need to scrutinize mechanisms governing cyclone activity to better understand their effects on the Arctic snow-ice system with anthropogenic warming.