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Philip J. Klotzbach

Abstract

Caribbean basin tropical cyclone activity shows significant variability on interannual as well as multidecadal time scales. Comprehensive statistics for Caribbean hurricane activity are tabulated, and then large-scale climate features are examined for their impacts on this activity. The primary interannual driver of variability is found to be El Niño–Southern Oscillation, which alters levels of activity due to changes in levels of vertical wind shear as well as through column stability. Much more activity occurs in the Caribbean with La Niña conditions than with El Niño conditions. On the multidecadal time scale, the Atlantic multidecadal oscillation is shown to play a significant role in Caribbean hurricane activity, likely linked to its close relationship with multidecadal alterations in the size of the Atlantic warm pool and the phase of the Atlantic meridional mode. When El Niño–Southern Oscillation and the Atlantic multidecadal oscillation are examined in combination, even stronger relationships are found due to a combination of either favorable or unfavorable dynamic and thermodynamic factors. For example, 29 hurricanes tracked into the Caribbean in the 10 strongest La Niña years in a positive Atlantic multidecadal oscillation period compared with only two hurricanes tracking through the Caribbean in the 10 strongest El Niño years in a negative Atlantic multidecadal oscillation period.

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Philip J. Klotzbach

Abstract

The 30–60-day Madden–Julian oscillation (MJO) has been documented in previous research to impact tropical cyclone (TC) activity for various tropical cyclone basins around the globe. The MJO modulates large-scale convective activity throughout the tropics, and concomitantly modulates other fields known to impact tropical cyclone activity such as vertical wind shear, midlevel moisture, vertical motion, and sea level pressure. The Atlantic basin typically shows the smallest modulations in most large-scale fields of any tropical cyclone basins; however, it still experiences significant modulations in tropical cyclone activity. The convectively enhanced phases of the MJO and the phases immediately following them are typically associated with above-average tropical cyclone frequency for each of the global TC basins, while the convectively suppressed phases of the MJO are typically associated with below-average tropical cyclone frequency. The number of rapid intensification periods are also shown to increase when the convectively enhanced phase of the MJO is impacting a particular tropical cyclone basin.

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Philip J. Klotzbach

Abstract

El Niño–Southern Oscillation (ENSO) has been shown in many previous papers to impact seasonal levels of Atlantic basin tropical cyclone activity. This paper revisits this relationship by examining a longer period (1900–2009) than has been examined in earlier analyses. Alterations in large-scale climate parameters, especially vertical wind shear, are shown to be the primary reasons why tropical cyclone activity in the Atlantic is reduced in El Niño years. Climate signals are found to be somewhat stronger in the Caribbean than for the remainder of the tropical Atlantic. The focus of the paper then shifts to U.S. landfalls, confirming previous research that U.S. landfalls are reduced in El Niño years. The reduction in landfall frequency is greater along the Florida peninsula and East Coast than it is along the Gulf Coast, especially for major hurricanes. The probability of each state being impacted by a hurricane and major hurricane is given for El Niño, La Niña, and neutral years. The most dramatic probability differences between warm and cold ENSO events lie along the East Coast and, in particular, the state of North Carolina. The relationship between ENSO and the Atlantic multidecadal oscillation (AMO) is also examined. In general, the negative phase of the AMO is characterized by a stronger ENSO modulation signal than a positive phase of the AMO.

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Philip J. Klotzbach

Abstract

The large-scale equatorial circulation known as the Madden–Julian oscillation (MJO) has been shown to impact tropical cyclone activity in several basins around the globe. In this paper, the author utilizes an MJO index created by Wheeler and Hendon to examine its impacts on tropical genesis and intensification in the Atlantic. Large differences in frequency and intensity of tropical cyclone activity are seen, both in the tropical Atlantic as well as in the northwest Caribbean and Gulf of Mexico depending on the MJO phase. Coherent changes in upper- and lower-level winds and relative humidity are likely responsible for these differences. Since the MJO shows potential predictability out to about two weeks, the relationships discussed in this paper may be useful for short-term predictions of the probability of tropical cyclone activity in the Atlantic as a complement to the already available longer-term seasonal predictions.

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Philip J. Klotzbach

Abstract

The Tropical Meteorology Project at Colorado State University currently issues seasonal forecasts for Atlantic basin hurricane activity in early April, June, and August. This paper examines the potential for issuing an additional seasonal forecast on 1 July, using a two-predictor forecast model. The two predictors are selected from the ECMWF Interim Re-Analysis (ERA-Interim) and explain over 60% of the cross-validated variance in post–30 June accumulated cyclone energy over the hindcast period from 1979 to 2012. The two predictors selected are May–June-averaged 2-m temperatures in the eastern tropical and subtropical Atlantic along with May–June 200-mb zonal winds in the tropical Indian Ocean. The May–June-averaged 2-m temperatures are shown to strongly correlate with August–October 2-m temperatures in the main development region, while the 200-mb zonal wind flow over the tropical Indian Ocean is shown to strongly correlate with El Niño–Southern Oscillation. In addition, each predictor is shown to correlate significantly with accumulated cyclone energy, both during the hindcast period of 1979–2012 and with an independent period from 1948 to 1978.

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Philip J. Klotzbach

Abstract

Predictions of the remainder of the season’s Atlantic basin tropical cyclone activity from 1 August have been issued by Gray and his colleagues at the Tropical Meteorology Project at Colorado State University since 1984. The original 1 August prediction scheme utilized several predictors, including measures of the stratospheric quasi-biennial oscillation (QBO), West African rainfall, El Niño–Southern Oscillation, and the sea level pressure anomaly and upper-tropospheric zonal wind anomalies in the Caribbean basin. The recent failure of the West African rainfall and QBO relationships with Atlantic hurricanes has led to a general degradation of the original 1 August forecast scheme in recent years. It was decided to revise the scheme using only surface data. The development of the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis has provided a vast wealth of globally gridded meteorological and oceanic data from 1948 to the present. In addition, other datasets have been extended back even further (to 1900), which allows for a large independent dataset. These longer-period datasets allow for an extended period of testing of the new statistical forecast scheme. A new prediction scheme has been developed on data from 1949 to 1989 and then tested on two independent datasets. One of these datasets is the 16-yr period from 1990 to 2005, and the other dataset is from 1900 to 1948. This allows for an investigation of the statistical significance over various time periods. The statistical scheme shows remarkable stability over an entire century. The combination of these four predictors explains between 45% and 60% of the variance in net tropical cyclone activity over the following separate time periods: 1900–48, 1949–89, 1949–2005, and 1900–2005. The forecast scheme also shows considerable skill as a potential predictor for giving the probabilities of United States landfall. Large differences in U.S. major hurricane landfall are also observed between forecasts that call for active seasons compared with those that call for inactive seasons.

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Jinjie Song
and
Philip J. Klotzbach

Abstract

Symmetric and wavenumber-1 asymmetric characteristics of western North Pacific tropical cyclone (TC) outer wind structures are compared between best tracks from the Joint Typhoon Warning Center (JTWC) and the Japan Meteorological Agency (JMA) from 2004 to 2014 as well as the Multiplatform Tropical Cyclone Surface Wind Analysis (MTCSWA) product from 2007 to 2014. Significant linear relationships of averaged wind radii are obtained among datasets, in which both gale-force and storm-force wind radii are generally estimated slightly smaller (much larger) by JTWC (JMA) than by MTCSWA. These correlations are strongly related to TC intensity relationships discussed in earlier work. Moreover, JTWC (JMA) on average represents a smaller (greater) derived shape parameter than MTCSWA does, implying that JTWC (JMA) typically assesses a more compact (less compact) storm than MTCSWA. For the wavenumber-1 asymmetry, large differences among datasets are found regardless of the magnitude or the direction of the longest radius. JTWC estimates more asymmetric storms than JMA, and it provides greater wavenumber-1 asymmetry magnitudes on average. Asymmetric storms are most frequently oriented toward the east, northeast, and north in JTWC and MTCSWA, whereas they are most frequently oriented toward the southeast, east, and northeast in JMA. The direction of the longest gale-force (storm force) wind radius in JTWC is statistically rotated 18° (32°) clockwise to that in JMA. Although the wind radii in JTWC are of higher quality than those in JMA when using MTCSWA as a baseline, there remains a need to provide a consistent and reliable wind radii estimating process among operational centers.

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Philip J. Klotzbach
and
William M. Gray

Abstract

September is the most active month for Atlantic basin tropical cyclone activity with about 50% of all hurricane activity occurring during this month. Utilizing National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data, a prediction scheme for forecasting September tropical cyclone activity has been developed. Based on hindcasting results from 1950 to 2000, 30%–75% of the variance for most tropical cyclone parameters can be hindcast by the end of July. This hindcast skill improves to 45%–75% by the end of August. Similarly, cross-validated hindcast skill explains from 20% to 65% for most variables by the end of July, improving to 30%–65% by the end of August. Simple least squared linear regression was utilized to calculate hindcast skill, and variables were selected that explained the largest degree of variance when combined with the other predictors in the scheme. These predictors tend to be global in nature and include zonal and meridional wind at 200 and 1000 mb and sea level pressure measurements at various global locations. Many of the predictors are strongly correlated with global modes such as El Niño–Southern Oscillation (ENSO) or the Pacific decadal oscillation (PDO). Based on this September hurricane prediction scheme, U.S. tropical cyclone landfall probability forecasts can also be issued. In addition, the 1 August forecast of September activity can also be applied to improve the hindcast skill of Gray's 1 August seasonal statistical forecast.

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Philip J. Klotzbach
and
William M. Gray

Abstract

An updated statistical scheme for forecasting seasonal tropical cyclone activity in the Atlantic basin by 1 December of the previous year is presented. Previous research by Gray and colleagues at Colorado State University showed that a statistical forecast issued on 1 December of the previous year could explain up to about 50% of the jackknife hindcast variance for the 1950–90 time period. Predictors utilized in the original forecast scheme included a forward extrapolation of the quasi-biennial oscillation (QBO) and two measures of West African rainfall. This forecast has been issued since 1991 but has shown little skill because of the as yet unexplained failure of the West African rainfall predictors during the 1990s.

The updated scheme presented in this paper does not utilize West African rainfall predictors. It employs the new NCEP–NCAR reanalysis data and involves predictors that span the globe. Much experimentation has led to the choosing of conditions associated with the El Niño-Southern Oscillation (ENSO), the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), the Pacific–North American pattern (PNA), and the QBO. This new statistical scheme shows similar cross-validated (jackknifed) hindcast skill to the original scheme (explaining up to about 50% of the variance) but is developed over a decade longer time period (1950–2001). In addition, since all predictors are taken directly from the NCEP–NCAR reanalysis, one does not need to consider rainfall collection issues that have caused major problems with the West African rainfall data since 1995. Hypothetical physical linkages between the predictors and the following year's hurricane activity are also presented. Based on the net tropical cyclone (NTC) activity prediction and a weighted average of North Atlantic sea surface temperatures, forecasts of U.S. hurricane landfall probability that showed considerable hindcast skill over the 52-yr period of 1950–2001 can also be issued.

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Philip J. Klotzbach
and
Christopher W. Landsea

Abstract

Ten years ago, Webster et al. documented a large and significant increase in both the number as well as the percentage of category 4 and 5 hurricanes for all global basins from 1970 to 2004, and this manuscript examines whether those trends have continued when including 10 additional years of data. In contrast to that study, as shown here, the global frequency of category 4 and 5 hurricanes has shown a small, insignificant downward trend while the percentage of category 4 and 5 hurricanes has shown a small, insignificant upward trend between 1990 and 2014. Accumulated cyclone energy globally has experienced a large and significant downward trend during the same period. The primary reason for the increase in category 4 and 5 hurricanes noted in observational datasets from 1970 to 2004 by Webster et al. is concluded to be due to observational improvements at the various global tropical cyclone warning centers, primarily in the first two decades of that study.

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