Kin Sik Liu

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...

Interdecadal Variability of Western North Pacific Tropical Cyclone Tracks

Abstract This study examines the interdecadal variability of the tropical cyclone (TC) tracks over the western North Pacific (WNP) during the 1960–2005 period. An empirical orthogonal function analysis of the 10-yr Gaussian-filtered annual frequency of TC occurrence shows three leading modes of TC occurrence patterns. The first mode is related to the variation of TC activity in the areas near Japan and its east. The second mode is characterized by a northeast–southwest dipole of TC occurrence anomalies along the southeast coast of China and an east–west dipole near Japan and its east. The third mode is similar to the second mode, except for the absence of the east–west dipole. These patterns are shown to be related to the decadal changes in the prevailing TC tracks. Two characteristic flow patterns related to the first and third modes of TC occurrence pattern are identified. The first pattern is characterized by a north–south dipole of 500-hPa geopotential anomalies over the WNP. Such a pattern may affect...

Improvements in the Seasonal Forecasting of Tropical Cyclone Activity over the Western North Pacific

A recent scheme to predict tropical cyclone (TC) activity over the western North Pacific partially failed in 1997 and 1998, during which a warm and a cold event of the El Nino-Southern Oscillation (ENSO) occurred, respectively. This paper presents results of two approaches to improve on such predictions. The first is to include new predictors that are related to ENSO based on some recent research, and the second is to provide an updated prediction by incorporating monthly values of predictors in April and May of the current year. The results suggest that new predictors related to ENSO can indeed be identified, which include temporal changes in the Southern Oscillation index, strength of the Australian monsoon, and intensity of the subtropical high in the South Pacific. These predictors, together with those selected from the original prediction scheme, are combined to form a modified scheme that in general gives better forecasts of TC activity. The updated scheme that includes April and May predictors further improves the accuracy of the predictions. Real-time predictions from both schemes for the year 2000, which were made in April and June, are found to be largely accurate. Both schemes show better skill compared with the original one.

Climatological Characteristics and Seasonal Forecasting of Tropical Cyclones Making Landfall along the South China Coast

Abstract This paper presents the important climatological features of the tropical cyclones making landfall along the South China coast and proposes a statistical scheme for the prediction of the annual number of such tropical cyclones. This number is found to have a large variation, which is mainly due to the occurrence or nonoccurrence of the El Nino–Southern Oscillation (ENSO) phenomenon. A strong El Nino event is found to reduce the number of landfalling tropical cyclones whereas more tropical cyclones tend to make landfall in years associated with La Nina events. Such variations are more prominent in some seasons. The late season (October–November) activity is generally suppressed (enhanced) in El Nino (La Nina) years whereas the chance of a tropical cyclone striking the South China coast increases (decreases) significantly in the early season (May and June) after the mature phase of a La Nina (El Nino) event. These anomalous activities are apparently linked to the ENSO-induced anomalies in the low- ...

Inactive Period of Western North Pacific Tropical Cyclone Activity in 1998-2011

AbstractTropical 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 (1998–2011). 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 (10°–17.5°N, 150°E–180°) averaged between June and October, is highly correlated with the annual TC number and shows a significant interdecadal variation. Positive anomalies of ve...

Size of Tropical Cyclones as Inferred from ERS-1 and ERS-2 Data

Abstract The sizes of the tropical cyclones (TCs) occurring over the western North Pacific (WNP) and the North Atlantic between 1991 and 1996 are estimated to establish a database for the study of the climatology of TC size and the physical processes responsible for the size changes of TCs. Wind data from the scatterometer onboard the European Remote-Sensing Satellites 1 and 2 (ERS-1 and ERS-2) form the data source for defining the TC size. The size of a TC is defined as the mean radius at which the relative vorticity decreases to 1 × 10−5 s−1. The mean TC size is found to be 3.7° lat for WNP TCs and 3.0° lat for those in the North Atlantic. Such a difference in size between the two basins is statistically significant at the 95% confidence level. The mean TC size in the WNP is also found to vary seasonally, with a value larger in the late season (October and November) than in midsummer (July and August). These results generally agree with those from previous studies using other measures of size. The size ...

Synoptic Flow Patterns Associated with Small and Large Tropical Cyclones over the Western North Pacific

Abstract This study investigates the synoptic flow patterns associated with small and large tropical cyclones (TCs) that occurred over the western North Pacific between 1991 and 1996. The size of a TC is defined as the azimuthally averaged radius from the TC center at which the relative vorticity decreases to 1 × 10−5 s−1. Calculation of the relative vorticity is based on the satellite-derived surface winds of the European Remote Sensing Satellites 1 and 2 (ERS-1 and ERS-2). Operational analyses of the U.K. Met Office are employed to identify the synoptic patterns around the TCs. Characteristic synoptic patterns at 850 hPa can be identified with TCs of different sizes. The southwesterly surge and late-season patterns are related to large TCs while the dominant ridge and monsoon-gyre patterns are associated with the occurrence of a small TC. A case study of Typhoon Bart demonstrates the time evolution of the synoptic pattern and its relationship with the TC size change. Bart exhibited a distinct transition...