Atlantic tropical cyclones are getting stronger on average, with a 30-year trend that has been related to an increase in ocean temperatures over the Atlantic Ocean and elsewhere. Over the rest of the tropics, however, possible trends in tropical cyclone intensity are less obvious, owing to the unreliability and incompleteness of the observational record and to a restricted focus, in previous trend analyses, on changes in average intensity. Here we overcome these two limitations by examining trends in the upper quantiles of per-cyclone maximum wind speeds (that is, the maximum intensities that cyclones achieve during their lifetimes), estimated from homogeneous data derived from an archive of satellite records. We find significant upward trends for wind speed quantiles above the 70th percentile, with trends as high as 0.3 +/- 0.09 m s(-1) yr(-1) (s.e.) for the strongest cyclones. We note separate upward trends in the estimated lifetime-maximum wind speeds of the very strongest tropical cyclones (99th percentile) over each ocean basin, with the largest increase at this quantile occurring over the North Atlantic, although not all basins show statistically significant increases. Our results are qualitatively consistent with the hypothesis that as the seas warm, the ocean has more energy to convert to tropical cyclone wind.
Changes in the frequency of U.S. landfalling hurricanes with respect to the El Nino-Southern Oscillation (ENSO) cycle are assessed. Ninety-eight years (1900-97) of U.S. landfalling hurricanes are classified, using sea surface temperature anomaly data from the equatorial Pacific Ocean, as occurring during an El Nino (anomalously warm tropical Pacific waters), La Nina (anomalously cold tropical Pacific waters), or neither (neutral). The mean and variance of U.S. landfalling hurricanes are determined for each ENSO phase. Each grouping is then tested for Poisson distribution using a chi-squared test. Resampling using a "bootstrap" technique is then used to determine the 5% and 95% confidence limits of the results. Last, the frequency of major U.S. landfalling hurricanes (sus-tained winds of 96 kt or more) with respect to ENSO phase is assessed empirically. The results indicated that El Nino events show a reduction in the probability of a U.S. landfalling hurricane, while La Nina shows an increase in the chance of a U.S. hurricane strike. Quantitatively, the probability of two or more landfalling U.S. hurricanes during an El Nino is 28%, of two or more landfalls during neutral conditions is 48%, and of two or more landfalls during La Nina is 66%. The frequencies of landfalling major hurricanes show similar results. The probability of one or more major hurricane landfall during El Nino is 23% but is 58% during neutral conditions and 63% during La Nina.
Although a theory of the climatology of tropical cyclone formation remains elusive, high-resolution climate models can now simulate many aspects of tropical cyclone climate. T he effect of climate change on tropical cyclones has been a controversial scientific issue for a number of years. Advances in our theoretical understanding of the relationship between climate and tropical cyclones have been made, enabling us to understand better the links between the mean climate and the potential intensity (PI; the theoretical maximum intensity of a tropical cyclone for a given climate condition) of tropical cyclones. Improvements in the capabilities of climate models, the main tool used to predict future climate, have enabled them to achieve a considerably improved and more credible simulation of the present-day climatology of tropical cyclones. Finally, the increasing ability of such models to predict the interannual variability of tropical cyclone formation in various regions of the globe indicates that they are capturing some of the essential physical relationships governing the links between climate and tropical cyclones. HURRICANES AND CLIMATEPrevious climate model simulations, however, have suggested some ambiguity in projections of future numbers of tropical cyclones in a warmer world. While many models have projected fewer tropical cyclones globally (Sugi et al. 2002;Bengtsson et al. 2007b; Gualdi et al. 2008; Knutson et al. 2010), other climate models and related downscaling methods have suggested some increase in future numbers (e.g., Broccoli and Manabe 1990;Haarsma et al. 1993; Emanuel 2013a). When future projections for individual basins are made, the issue becomes more serious: for example, for the Atlantic basin there appears to be little consensus on the future number of tropical cyclones or on the relative importance of forcing factors such as aerosols or increases in carbon dioxide (CO 2 ) concentration. One reason could be statistical: annual numbers of tropical cyclones in the Atlantic are relatively small, making the identification of such storms sensitive to the detection method used.Further, there is substantial spread in projected responses of regional tropical cyclone (TC) frequency and intensity over the twenty-first century from downscaling studies (Knutson et al. 2007; Emanuel 2013a). Interpreting the sources of those differences is complicated by different projections of large-scale climate and by differences in the present-day reference period and sea surface temperature (SST) datasets used. A natural question is whether the diversity in responses to projected twenty-firstcentury climate of each of the studies is primarily | a reflection of uncertainty arising from different large-scale forcing (as has been suggested by, e.g., Villarini et al. 2011;Villarini and Vecchi 2012;Knutson et al. 2013) or whether this spread reflects principally different inherent sensitivities across the various downscaling techniques, even including different sensitivity of responses within the same model due to...
Modern typhoon data and historical documents from Guangdong Province, southern China, are analyzed and found to support the El Niño-Southern Oscillation (ENSO)-typhoon hypothesis. The hypothesis states that tropical cyclone formation during an El Niño event shifts eastward, with typhoons tending to recurve north, staying away from China. From the comprehensive but short modern record, typhoon tracks are grouped into 3 distinct clusters based on geographic position at maximum and terminal typhoon intensities. The majority of typhoons originate between 110 and 170°E longitude in the latitude belt between 8 and 25°N. In general, typhoons take 1 of 3 paths away from this genesis region -a westerly path between latitudes (straight moving), a west-northwesterly path (recurving), or a north-oriented path that keeps them out to sea. Straight-moving typhoons are a significant threat to the Philippines, southern China, and Vietnam, whereas recurving typhoons occasionally threaten Japan, Korea, and northern China. The number of straight-moving typhoons, when grouped by year, is found to be significantly positively correlated with the number of landfalls over China south of the Tropic of Cancer. Thus, the abundance of straight-moving typhoons is a good indicator of the typhoon threat to portions of southern China. Moreover, the number of straightmoving typhoons is correlated with the ENSO cycle. A long annual time-series (1600-1909) of typhoon landfall counts from Guangdong, extracted from historical documents together with treering proxy records of the ENSO cycle, provide data that independently support this relationship. KEY WORDS: Typhoons · Typhoon tracks · ENSO · Guangdong · Southern China · Cluster analysisResale or republication not permitted without written consent of the publisher
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