No abstract

No abstract

[1] Interdecadal variations in the early (May -June) summer monsoon rainfall over South China (SCMR) are found to be related to the ENSO (El Niño/Southern Oscillation) and the PDO (Pacific Decadal Oscillation). An interdecadal variation in SCMR can be identified, with more dry (wet) monsoon years during the periods of high (low) PDO index. Such variations are also related to ENSO in association with PDO. When ENSO and PDO are in phase, i.e. high PDO phase/El Niño events, or low PDO phase/La Niña events, the SCMR tends to be below or above normal respectively more often. But when the ENSO and PDO are out-of-phase, the SCMR has no wet or dry preference. Such relationships appear to be related to the intensity of the subtropical high determined by the superposition of the effects of ENSO and PDO.

In this paper, the application of partial wavelet coherence (PWC) and multiple wavelet coherence (MWC) to geophysics is demonstrated. PWC is a technique similar to partial correlation that helps identify the resulting wavelet coherence (WTC) between two time series after eliminating the influence of their common dependence. MWC, akin to multiple correlation, is, however, useful in seeking the resulting WTC of multiple independent variables on a dependent one. The possible El Niño–Southern Oscillation–related impact of the large-scale atmospheric factors on tropical cyclone activity over the western North Pacific is used as an example. A software package for PWC and MWC has been developed. It also includes modified software that rectified the bias in the wavelet power spectrum and wavelet cross-spectrum. The package is available online (see http://www.cityu.edu.hk/gcacic/wavelet).

Tropical cyclone (TC) activity over the western North Pacific (WNP) exhibits a significant interdecadal variation during 1960-2011, with two distinct active and inactive periods each. This study examines changes in TC activity and atmospheric conditions in the recent inactive period (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011). The overall TC activity shows a significant decrease, which is partly related to the decadal variation of TC genesis frequency in the southeastern part of the WNP and the downward trend of TC genesis frequency in the main development region.The investigation on the factors responsible for the low TC activity mainly focuses on the effect of vertical wind shear and subtropical high on multidecadal time scales. A vertical wind shear index, defined as the mean magnitude of the difference of the 200-and 850-hPa horizontal zonal winds (108-17.58N, 1508E-1808) averaged between June and October, is highly correlated with the annual TC number and shows a significant interdecadal variation. Positive anomalies of vertical wind shear are generally found in the eastern part of the tropical WNP during this inactive period. A subtropical high area index, calculated as the area enclosed by the 5880-gpm line of the June-October 500-hPa geopotential height (08-408N, 1008E-1808), shows a significant upward trend. A high correlation is also found between this index and the annual TC number, and a strongerthan-normal subtropical high is generally observed during this inactive period. The strong vertical wind shear and strong subtropical high observed during 1998-2011 together apparently lead to unfavorable atmospheric conditions for TC genesis and hence the low TC activity during the period.