S. BAO
Department of Marine, Earth, Atmospheric Sciences, North Carolina State University, North Carolina, USA
Cooperative Institute for Research in Environmental Science, University of Colorado at Boulder, Boulder Colorado, USA
L. J. PIETRAFESA
Burroughs & Chapin Center for Marine & Wetland Studies, Coastal Carolina University, Conway, USA
NORDEN E. HUANG
Research Center for Adaptive Data Analysis, National Central University, Zhongli, Taiwan 32001, Taiwan
Z. WU
Department of Earth, Ocean and Atmospheric Science, Florida State University, USA
Center for Ocean-Atmospheric Prediction Studies, Florida State University, USA
D. A. DICKEY
Department of Statistics North Carolina State University, North Carolina, USA
P. T. GAYES
Burroughs & Chapin Center for Marine & Wetland Studies, Coastal Carolina University, Conway, USA
T. YAN
Burroughs & Chapin Center for Marine & Wetland Studies, Coastal Carolina University, Conway, USA

The trends and intrinsic frequencies in the time series of the number of Tropical Cyclones (TCs), hurricanes and typhoons, and Categories 4 and 5 hurricanes and typhoons in the Atlantic and Pacific Ocean domains, and the yearly integral of hurricane wind energy, represented by the Power Density Index (PDI), in the Atlantic and Eastern North Pacific Ocean domains are studied. The results show that the Empirical Modal Decomposition (EMD) method [Huang et al. (1998)] successfully reveals that there are intrinsic modes of variations that are controlled by climate systems such as the Quasi-Biennial Oscillation (QBO), the El Nino Southern Oscillation (ENSO), and the Atlantic and Pacific Multi-Decadal Oscillations (AMO and PDO), along with the Meridional Overturning Circulation (MOC). It also reveals some oscillation modes whose controlling factors are not yet identified. In both the Atlantic and Pacific Ocean domains, the frequencies of TCs, hurricane/typhoon-strength TCs and the strongest (Saffir-Simpson Categories 4 and 5) TCs have slowly rising trends. In the Atlantic Ocean, our study indicates that since the mid-1970s, the observed rise in the number of the strongest (Cats. 4 and 5) TCs as discussed previously by Webster et al. [2005] and the rise in the measure of destructiveness, the Power Density Index (PDI), developed by Emanuel [2005], were not the cause of rising trends, but instead, they are the result of the combination of positive phases of several intrinsic frequency modes. In the Pacific Ocean, the rising trends have larger amplitudes than those in the Atlantic Ocean, but the higher frequency modes appear to play a more important role in deciding the year-to-year Pacific TC, hurricane/typhoon and Cats. 4 and 5 TC activity levels.

Tropical cyclone activity; Atlantic Ocean; Pacific Ocean