Johnny C. L. Chan

How Strong ENSO Events Affect Tropical Storm Activity over the Western North Pacific

Abstract An analysis of 35-yr (1965–99) data reveals vital impacts of strong (but not moderate) El Nino and La Nina events on tropical storm (TS) activity over the western North Pacific (WNP). Although the total number of TSs formed in the entire WNP does not vary significantly from year to year, during El Nino summer and fall, the frequency of TS formation increases remarkably in the southeast quadrant (0°–17°N, 140°E–180°) and decreases in the northwest quadrant (17°–30°N, 120°–140°E). The July–September mean location of TS formation is 6° latitude lower, while that in October–December is 18° longitude eastward in the strong warm versus strong cold years. After the El Nino (La Nina), the early season (January–July) TS formation in the entire WNP is suppressed (enhanced). In strong warm (cold) years, the mean TS life span is about 7 (4) days, and the mean number of days of TS occurrence is 159 (84) days. During the fall of strong warm years, the number of TSs, which recurve northward across 35°N, is 2.5 ...

Tropical Cyclone Activity over the Western North Pacific Associated with El Niño and La Niña Events

Abstract This paper presents the results of an investigation on the variations of tropical cyclone (TC) activity over the western North Pacific (WNP) associated with both El Nino (EN) and La Nina (LN) events. The study is based on the monthly number of TCs that occurred during the period 1959–97. Anomalies within each 5° lat × 5° long box from the year before (EN−1 and LN−1) to the year after (EN+1 and LN+1) are examined. During an EN−1 year, more (less) TCs are found in September and October over the South China Sea (southeast of Japan). In an EN year, TC activity is below normal during these two months over the South China Sea (SCS) but above normal especially in the late season in the eastern part of the WNP. After the mature phase of the warm event (i.e., during an EN+1 year), TC activity over the entire ocean basin tends to be below normal. No significant anomalies are found during an LN−1 year. However, in an LN year, the SCS tends to have more TCs in September and October, but for the rest of the W...

Tropical Cyclone Activity in the Northwest Pacific in Relation to the El Niño/Southern Oscillation Phenomenon

Abstract The interannual variations in tropical cyclone activity in the northwest Pacific (NWPAC) and their relationships with the El Nino/Southern Oscillation (ENSO) phenomenon were studied using the method of spectral analyses. Time series of a Southern Oscillation Index (SOI, defined as the sea-level pressure difference between Easter Island and Darwin) and tropical cyclone activity in the entire (NWPAC) ocean basin as well as in different regions of the NWPAC were analyzed. Two spectral peaks are apparent in all these time series. One corresponds to the generally accepted Southern Oscillation with a period of ∼3 to 3.5 years and another at the quasi-biennial oscillation (QBO) frequency. Cross-spectral analyses between the SOI and tropical cyclone activity show significant coherence in these two spectral peaks. The dominant peak is at the Southern Oscillation frequency with the SOI leading typhoon activity by almost a year. At the QBO frequency, the two series are almost in phase. Cyclone activity in t...

Analytical and Numerical Studies of the Beta-Effect in Tropical Cyclone Motion. Part I: Zero Mean Flow

Abstract The β-effect on tropical cyclone motion is studied using an analytical as well as a numerical model in a nondivergent barotropic framework. The analytical model and the linear version of the numerical model give essentially the same result: the linear β-effect causes a westward stretching of the model vortex but no significant movement of the vortex center. An east-west asymmetry in the meridional wind field is also created. It is the inclusion of the nonlinear term that produces the northwestward movement of the vortex previously found by other investigators (e.g., Kitade, 1981). This northwestward movement increases with both the maximum wind speed and the radius of maximum wind in a constant-shape vortex. A wind maximum is also found to the northeast of the vortex, which appears to be consistent with the observational findings of Shea and Gray. This asymmetry plays an important role in the vortex motion.

Tropical cyclone movement and surrounding flow relationships.

Abstract This paper presents results of a comprehensive study of the relationship between the movement of tropical cyclones and the large-scale circulation which surrounds them. Cyclones have been stratified by direction and speed of movement, latitude, intensity change and size (as determined by the radius of the outermost closed surface isobar) in three ocean basins: the northwest Pacific, the west Atlantic and the Australian-South Pacific region. Twenty-one different stratifications are available in the northwest Pacific, 13 in the west Atlantic and 6 in the Australian-South Pacific area. Cyclone movement and surrounding flow relationships were studied at different pressure levels and a variety of radii. Pressure-weighted layer-averages were also analyzed in search of such relationships. Results show an important relationship between surrounding large-scale flow and tropical cyclone movement. For all stratifications, the winds in the mid-troposphere (500–700 mb) at 5–7° latitude radius from the cyclone...

Global Warming and Western North Pacific Typhoon Activity from an Observational Perspective

Abstract Based on results from climate model simulations, many researchers have suggested that because of global warming, the sea surface temperature (SST) will likely increase, which will then lead to an increase in the intensity of tropical cyclones (TCs). This paper reports results of a study of the relationship between SST and observed typhoon activity (which is used as a proxy for the intensity of TCs averaged over a season) over the western North Pacific (WNP) for the past 40 yr. The average typhoon activity over a season is found to have no significant relationship with SST in the WNP but increases when the SST over the equatorial eastern Pacific Ocean is above normal. The mean annual typhoon activity is generally higher (lower) during an El Nino (La Nina) year. Such interannual variations of typhoon activity appear to be largely constrained by the large-scale atmospheric factors that are closely related to the El Nino–Southern Oscillation (ENSO) phenomenon. These large-scale dynamic and thermodyna...

Synoptic-Scale Controls of Persistent Low Temperature and Icy Weather over Southern China in January 2008

Abstract In January 2008, central and southern China experienced persistent low temperatures, freezing rain, and snow. The large-scale conditions associated with the occurrence and development of these snowstorms are examined in order to identify the key synoptic controls leading to this event. Three main factors are identified: 1) the persistent blocking high over Siberia, which remained quasi-stationary around 65°E for 3 weeks, led to advection of dry and cold Siberian air down to central and southern China; 2) a strong persistent southwesterly flow associated with the western Pacific subtropical high led to enhanced moisture advection from the Bay of Bengal into central and southern China; and 3) the deep inversion layer in the lower troposphere associated with the extended snow cover over most of central and southern China. The combination of these three factors is likely responsible for the unusual severity of the event, and hence a long return period.

Intraseasonal Variability of the South China Sea Summer Monsoon

The objective of this study is to explore, based on the National Centers for Environmental Prediction– National Center for Atmospheric Research (NCEP–NCAR) reanalysis data, the intraseasonal variability of the South China Sea (SCS) summer monsoon (SM) in terms of its structure and propagation, as well as interannual variations. A possible mechanism that is responsible for the origin of the 10–20-day oscillation of the SCS SM is also proposed. The 30–60-day (hereafter the 3/6 mode) and 10–20-day (hereafter the 1/2 mode) oscillations are found to be the two intraseasonal modes that control the behavior of the SCSSM activities for most of the years. Both the 3/6 and 1/2 modes are distinct, but may not always exist simultaneously in a particular year, and their contributions to the overall variations differ among different years. Thus, the interannual variability in the intraseasonal oscillation activity of the SCS SM may be categorized as follows: the 3/6 category, in which the 3/6 mode is more significant (in terms of the percentage of variance explained) than the 1/2 mode; the 1/2 category, in which the 1/2 mode is dominant; and the dual category, in which both the 3/6 and 1/2 modes are pronounced. Composite analyses of the 3/6 category cases indicate that the 30–60-day oscillation of the SCS SM exhibits a trough–ridge seesaw in which the monsoon trough and subtropical ridge exist alternatively over the SCS, with anomalous cyclones (anticyclones), along with enhanced (suppressed) convection, migrating northward from the equator to the midlatitudes. The northward-migrating 3/6-mode monsoon trough–ridge in the lower troposphere is coupled with the eastward-propagating 3/6-mode divergence–convergence in the upper troposphere. It is also found that, for the years in the dual category, the SCS SM activities are basically controlled by the 3/6 mode, but modified by the 1/2 mode. Composite results of the 1/2-mode category cases show that the 10–20-day oscillation is manifest as an anticyclone–cyclone system over the western tropical Pacific, propagating northwestward into the SCS. A close coupling also exists between the upper-level convergence (divergence) and the low-level anticyclone (cyclone). It is found that the 1/2 mode of the SCS SM mainly originates from the equatorial central Pacific, although a disturbance from the northeast of the SCS also contributes to this mode. The flow patterns from an inactive to an active period resemble those associated with a mixed Rossby–gravity wave observed in previous studies.

Seasonal Forecasting of Tropical Cyclone Activity over theWestern North Pacific and the South China Sea

Abstract This paper presents the development of operational statistical forecasts of seasonal tropical cyclone (TC) activity over the western North Pacific (WNP) and the South China Sea (SCS) based on 30 yr of data (1965–94). Predictors include monthly values of indices representing (a) the El Nino–Southern Oscillation phenomenon, and (b) the environmental conditions over East Asia and the WNP for the months from April of the previous year to March of the current year. Trends and short-term oscillations of the TC activity are also incorporated. The prediction equations are derived from the predictors of individual parameters using the Projection Pursuit Regression technique, which is a statistical method that reduces high-dimensional data to a lower-dimensional subspace before the regression is performed. This technique is found to provide explanations of certain nonlinear variations of the predictands. The predictions from individual parameters are then tested using the jackknife technique. Those predict...

Decadal variations of intense typhoon occurrence in the western North Pacific

The causes of one of the two major oscillation periods, 16–32 years, identified through a wavelet analysis, of the time series of the frequency of intense typhoon (categories 4 and 5 in the Saffir–Simpson scale) occurrence for the period 1960–2005 in the western North Pacific (WNP) is studied in this paper. By dividing this period into sub-periods during which the frequency of intense typhoon occurrence was above or below normal on this time scale, various thermodynamic and dynamic factors in each sub-period are examined. During the above-normal periods, the sea surface temperature in the southeastern part of the WNP (5–20° N, 150–180° E) is found to be slightly higher. Within this area, the moist static energy (MSE) is also higher and the vertical gradient of saturated MSE in the lower troposphere is more negative. At the same time, the low-level streamfunction anomalies tend to have a negative maximum and the vertical wind shear between 200 and 850 hPa is also relatively small. Thus, both the thermodynamic and dynamic conditions within this area are more conducive to the development of tropical cyclones (TCs). As these cyclones move northwestward, the favourable dynamic conditions continue to be present so that they can intensify further. The steering flow is such that many of these typhoons will stay over water for an extended period of time through low-latitude recurvature. As a result, they can intensify to become category 4 or 5 typhoons. The conditions during the below-normal periods are generally opposite. A major conclusion from the results of this study is that the frequency of intense typhoon occurrence undergoes a strong multi-decadal (16–32 years) variation due to similar variations in the planetary scale oceanographic and atmospheric conditions that govern the formation, intensification and movement of TCs. These latter variations are largely contributed by the El Niño and the Pacific Decadal Oscillation on similar time scales.