Variability Of Tropical Cyclone Frequency Research Articles

Observed Changes in Extreme Precipitation Associated with U.S. Tropical Cyclones

Abstract Numerous recent tropical cyclones have caused extreme rainfall and flooding events in the CONUS. Climate change is contributing to heavier extreme rainfall around the world. Modeling studies have suggested that tropical cyclones may be particularly efficient engines for transferring the additional water vapor in the atmosphere into extreme rainfall. This paper develops a new indicator for climate change using the enhanced rainfall metric to evaluate how the frequency and/or intensity of extreme rainfall around tropical cyclones has changed. The enhanced rainfall metric relates the amount of rain from a storm over a given location to the 5-yr return period rainfall in that location to determine the severity of the event. The annual area exposed to tropical-cyclone-related 5-yr rainfall events is increasing, which makes it a compelling climate change indicator. Quantile regression illustrates that the distribution of tropical cyclone rainfall is also changing. For tropical storms, all quantiles are increasing. However, major hurricanes show large increases in their most extreme rainfall. This study does not attempt to make any detection claims (vs natural variability) or attribution of the observed trends to anthropogenic forcing. However, the sensitivity of the results to natural variability in tropical cyclone frequency was somewhat constrained by comparing 2 decades from the previous active era (1951–70) with two from the current era (2001–20). This comparison also shows that both the mean rainfall and the maximum rainfall associated with tropical cyclones are increasing over most areas of the eastern CONUS with the most significant increases from northern Alabama to the southern Appalachians. Significance Statement The purpose of this study is to analyze the changes in frequency and magnitude of extreme precipitation events associated with tropical cyclones with the goal of developing a new indicator for climate change. This is important because heavy rainfall and associated flooding is one of the primary causes of tropical cyclone destruction and fatalities, especially in inland locations away from where storms initially make landfall. Our results show that both the frequency and magnitude of extreme rainfall events from tropical cyclones have increased over the CONUS. The strongest storms (major hurricanes) also show more of an increase in extreme rainfall than storms of weaker intensities.

Variability of Tropical Cyclone Frequency Over the Western North Pacific in 2018–2020

In this study, we examine the tropical cyclone (TC) activity over the western North Pacific (WNP) in 2018–2020 and its relationship with planetary scale convection and circulation anomalies, which play an important role for TC genesis. To determine the sea surface temperature (SST)-forced atmospheric variability, atmospheric general circulation model (AGCM) ensemble simulations are executed along with the observed SST. For AGCM experiments, we use two different convection schemes to examine uncertainty in convective parameterization and robustness of simulated atmospheric response. The observed TC activity and genesis potential demonstrated consistent features. In our AGCM ensemble simulations, the updated convection scheme improves the simulation ability of observed genesis potential as well as planetary scale convection and circulation features, e.g., in September–October–November (SON), a considerable increase in the genesis potential index over the WNP in SON 2018, WNP in SON 2019, and South China Sea (SCS) in SON 2020, which were not captured in the Emanuel scheme, have been simulated in the updated convection scheme.

Internal variability of North Atlantic tropical cyclones

AbstractUsing a regional model initial condition ensemble, this study quantifies the magnitude of internal variability of North Atlantic tropical cyclone frequency for a case study year and identifies potential physical sources. For tropical cyclone formations from easterly waves, the simulated internal variability of tropical cyclone frequency for 1998 is approximately two fifths of the total (externally forced and internal) variability of observed tropical cyclone frequency. The simulated internal variability of tropical cyclone frequency is found to arise in approximately equal measure from variability of easterly wave occurrence and development and variability of the transition from incipient warm cores to tropical cyclones. Variable interaction between developing tropical cyclones and vertical wind shear associated with upper level cyclones is identified as a potentially important contributing factor to tropical cyclone internal variability.

Interdecadal variability in tropical cyclone frequency over the South China Sea and its association with the Indian Ocean sea surface temperature

During the period 1958–2001, the frequency of tropical cyclone (TC) genesis over the South China Sea (SCS) experienced an obvious interdecadal change around the mid‐1970s. Compared to the period from late 1950s to early 1970s, the number of TCs is significantly reduced during mid‐1970s through late 1990s. This interdecadal change in the TC frequency appears to relate to the increase in sea surface temperature (SST) of the tropical Indian Ocean. The difference of the circulation between the warm phase and the cold phase of the tropical Indian Ocean SST provides support for the influence of the tropical Indian Ocean warming on the variability of the TC frequency over the SCS. In the warm phase, lower‐level convergent and ascending flows over the tropical Indian Ocean are accompanied by upper‐level divergent flows, part of which go toward the SCS, leading to upper‐level convergence and descent there. Consequently, two lower‐level anticyclones develop and the TC genesis is suppressed over the SCS.

Future Change of North Atlantic Tropical Cyclone Tracks: Projection by a 20-km-Mesh Global Atmospheric Model*

Abstract Possible future change in tropical cyclone (TC) activity over the North Atlantic (NA) was investigated by comparison of 25-yr simulations of the present-day climate and future change under the A1B emission scenario using a 20-km-mesh Meteorological Research Institute (MRI) and Japan Meteorological Agency (JMA) atmospheric general circulation model. The present-day simulation reproduces many essential features of observed climatology and interannual variability in TC frequency of occurrence and tracks over the NA. For the future projection, the model is driven by the sea surface temperature (SST) that includes a trend projected by the most recent Intergovernmental Panel on Climate Change (IPCC) multimodel ensemble and a year-to-year variation derived from the present-day climate. A major finding is that the future change of total TC counts in the NA is statistically insignificant, but the frequency of TC occurrence will decrease in the tropical western NA (WNA) and increase in the tropical eastern NA (ENA) and northwestern NA (NWNA). The projected change in TC tracks suggests a reduced probability of TC landfall over the southeastern United States, and an increased influence of TCs on the northeastern United States. The track changes are not due to changes of large-scale steering flows; instead, they are due to changes in TC genesis locations. The increase in TC genesis in the ENA arises from increasing background ascending motion and convective available potential energy. In contrast, the reduced TC genesis in the WNA is attributed to decreases in midtropospheric relative humidity and ascending motion caused by remotely forced anomalous descent. This finding indicates that the impact of remote dynamical forcing is greater than that of local thermodynamical forcing in the WNA. The increased frequency of TC occurrence in the NWNA is attributed to reduced vertical wind shear and the pronounced local warming of the ocean surface. These TC changes appear to be most sensitive to future change in the spatial distribution of rising SST. Given that most IPCC models project a larger increase in SST in the ENA than in the WNA, the projected eastward shift in TC genesis is likely to be robust.

Interdecadal Variability of Tropical Cyclone Frequency over the Western North Pacific in a High-Resolution Atmosphere-Ocean Coupled GCM

The variability of tropical cyclone (TC) frequency over the western North Pacific (WNP) was investigated using a high-resolution atmosphere-ocean Coupled General Circulation Model (CGCM) of the National Research Institute for Earth Science and Disaster Prevention. A 50-year CGCM integration simulated highly frequent occurrence periods (HFOPs), comprising periods with a greater number of TCs than the long-term mean number, and lowly frequent occurrence periods (LFOPs), comprising periods with a lower number than the mean, as the interdecadal variability of TC frequency. The variability corresponded to the interdecadal variability of the actual TC frequency over the WNP during the past 50 years, 1951-2000. This CGCM integration also showed that the oceanic and atmospheric conditions (sea surface temperature, horizontal wind and relative vorticity at the 850 hPa level, equivalent potential temperature, and precipitation rate) favorable for TC formation during the HFOPs were the same as the observed oceanic and atmospheric conditions during the observed HFOPs. In the HFOPs, the sea surface temperatures over the tropical WNP are higher than in the LFOPs. The differences in atmospheric conditions between HFOPs and LFOPs are characterized by an anomalous westerly wind and positive relative vorticity at the 850 hPa level and an increase in the convective available potential energy. These differences in the environmental conditions simulated by the CGCM are consistent with those found in the observations. The CGCM was able to simulate the interdecadal variability of model TC frequency and the oceanic and atmospheric conditions related to the variability, which were similar to the observations. However, we need to improve the CGCM to more realistically simulate TC activity and oceanic and atmospheric conditions hereafter, because the CGCM has several deficiencies.