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Eric K. W. Ng
and
Johnny C. L. Chan

Abstract

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).

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Johnny C. L. Chan
,
Jiu-en Shi
, and
Cheuk-man Lam

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 Niño–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 predictions that have correlations (with the observed) significant at the 95% level or higher are retained. The values of the correlation coefficients are then used as weights in combining the predictions to form a single forecast of each predictand. The forecasts obtained this way are found to be superior to those from individual parameters.

The combined forecast equations are then used to predict the TC activity over the WNP and the SCS for 1997. The prediction is for a slightly above-normal activity for the entire WNP but slightly below normal for the SCS. The former is found to be correct and the latter has the right trend although the activity over the SCS was far below normal, probably as a result of the El Niño of 1997.

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Dongliang Wang
,
Xudong Liang
,
Yihong Duan
, and
Johnny C. L. Chan

Abstract

The fifth-generation Pennsylvania State University–National Center for Atmospheric Research nonhydrostatic Mesoscale Model is employed to evaluate the impact of the Geostationary Meteorological Satellite-5 water vapor and infrared atmospheric motion vectors (AMVs), incorporated with the four-dimensional variational (4DVAR) data assimilation technique, on tropical cyclone (TC) track predictions. Twenty-two cases from eight different TCs over the western North Pacific in 2002 have been examined. The 4DVAR assimilation of these satellite-derived wind observations leads to appreciable improvements in the track forecasts, with average reductions in track error of ∼5% at 12 h, 12% at 24 h, 10% at 36 h, and 7% at 48 h. Preliminary results suggest that the improvement depends on the quantity of the AMV data available for assimilation.

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Johnny C. L. Chan
,
Jiu-en Shi
, and
Kin Sik Liu

Abstract

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 Niño–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.

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James E. Peak
,
William E. Wilson
,
Russell L. Elsberry
, and
Johnny C-L. Chan

Abstract

Tropical wind fields from the U.S. Navy Global Band Analyses (GBA) are studied to depict the synoptic flow surrounding tropical cyclones. The composite fields of the zonal and meridional wind components on a grid centered on the tropical cyclone indicate physically realistic flow patterns.

Scalar empirical orthogonal function (EOF) analysis is used to represent the zonal and meridional GBA wind component fields. The representation of these components in terms of the first 35 out of a total of 527 EOF coefficients accounts for at least 80% of the total variance and eliminates much of the noise from the fields. This truncation requires only 7% of the storage needed for the original gridpoints. The eigenvectors can be interpreted as representing different synoptic flow patterns.

Statistical regression equations are derived to predict the future zonal and meridional translation of the tropical cyclone. The EOF coefficients are used as predictors to represent the synoptic information for the scheme. The track forecast errors are slightly smaller than those from the Joint Typhoon Warning Center. Further reduction in forecast errors results from stratification of the cases. Stratification by prior 12-h motion results in 72-h weighted mean forecast errors of only 481 km for the dependent sample. Stratification by synoptic situation based on the EOFs is also tested. From discriminant analysis, the second zonal eigenvector at 700 mb best relates to the 72-h zonal storm motion while the second meridional eigenvector at 250 mb has the best correlation with the meridional motion. Weighted mean forecast errors for regression equations derived within the synoptic subgroups are 484 km at 72 h. Thus, stratification by synoptic situations in terms of wind-based EOFs is as effective as stratification by past storm motion in improving track forecasts.

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Richard C. Y. Li
,
Wen Zhou
,
Johnny C. L. Chan
, and
P. Huang

Abstract

The present study investigates the modulation by the Madden–Julian oscillation (MJO) and the impact of the El Niño–Southern Oscillation (ENSO) on tropical cyclone (TC) genesis in the western North Pacific (WNP) during the period 1975–2010. Results reveal a stronger modulation of cyclogenesis by the MJO during El Niño years, while the modulations in neutral and La Niña years are comparable to each other.

The asymmetric background modification by ENSO is found to greatly affect the extent of MJO modulation under different ENSO conditions. First, MJO activity is intensified and extends farther eastward during El Niño years, instead of being confined west of 150°E as in neutral and La Niña periods. Thus, the influence of MJO is stronger and more zonally widespread in El Niño years, causing significant differences in cyclogenesis parameters in most parts of the WNP. In El Niño years, cyclogenesis is further enhanced in the active phase due to synchronization of MJO signals with favorable background ENSO conditions. While in the inactive phase, the dominance of the strong MJO signals leads to further suppression in TC formation. This leads to overall enhancement of the MJO–TC relationship during El Niño years. On the other hand, the MJO signals confined to the western region west of 150°E in neutral and La Niña years lead to changes in TC-related parameters mainly in the western region, which contribute to the comparatively weaker TC modulations. It can thus be concluded that the MJO has an asymmetric modulation on cyclogenesis in the WNP under different ENSO conditions.

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Wen Zhou
,
Johnny C. L. Chan
,
Wen Chen
,
Jian Ling
,
Joaquim G. Pinto
, and
Yaping Shao

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.

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Johnny C. L. Chan
,
Francis M. F. Ko
, and
Ying Man Lei

Abstract

This paper proposes a consistent conceptual framework to explain tropical cyclone (TC) motion based on the concept of potential vorticity tendency (PVT) and to verify this framework based on analyses of different observational datasets. The framework suggests that a TC is likely to move toward an area of maximum wavenumber-1 (WN1) PVT, which is mainly contributed by the corresponding WN1 components of potential vorticity (PV) advection and diabatic heating (DH). The PV advection process consists of advection of symmetric PV by the asymmetric flow [AASPV, which includes, but is not limited to, the environmental “steering flow” and the beta-induced circulation (the so-called ventilation flow)] and the advection of asymmetric PV by the symmetric flow (SAAPV). The asymmetric PV includes any asymmetry in the TC circulation, the beta gyres and contributions from asymmetric convective heating. The modification of PVT by the DH process depends on the vertical gradient of convective heating and the coupling between horizontal gradient of convective heating and vertical wind shear. In steady (i.e., without much change in direction or speed) TC motion, the PV advection processes are generally dominant while the contribution by DH is usually less significant. However, the latter process becomes important for irregular TC motion. Changes in TC motion are then not only caused by those in steering, but can also be induced by variations in the other processes.

Composites of the Met Office operational analyses associated with TCs that had similar and relatively steady motion are first made to verify the contribution by the advection terms. In all motion categories examined, while the magnitude of the AASPV term is found to be generally dominant, its maximum is not downstream of the TC motion. The SAAPV term also contributes to the overall PV advection. The sum of these two terms gives a maximum at a location that generally aligns with the direction of TC motion.

The contribution of the DH process to PVT, and hence TC motion, is then examined using satellite-derived temperatures from high-resolution geosynchronous satellite images for individual TCs. It is found that DH appears to be important especially for slow-moving TCs. Track oscillations as well as irregular track changes may be explained by changes in the convection pattern that lead to variations in the location of maximum WN1 DH.

The entire PVT concept is further investigated using analyses from the Tropical Cyclone Motion Experiment TCM-90 for individual TCs with different track types. The results are consistent with those from the composites (for straight-moving cases) as well as from the satellite image analyses (for the irregular-moving case). Further, in the recurving case, the locations of the maximum in the advection terms rotate ahead of the turning motion of the TC, which is consistent with previous studies of TC motion based on the concept of absolute vorticity conservation.

An integration of all these observational analyses generally verifies the validity of the proposed conceptual framework, which appears to explain most types of TC motion.

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Rui Jin
,
Hui Yu
,
Zhiwei Wu
,
Johnny C. L. Chan
,
Ming Ying
, and
Peng Zhang

Abstract

This study examines the East Asia and western North Pacific (WNP) monsoon circulation patterns for strong and weak WNP tropical cyclone (TC) numbers in summer. It suggested that years with more intense TCs are coupled with the tropical monsoon circulations, including the northward cross-equatorial airflow and the extending tropical monsoon trough toward the central-eastern WNP. However, a higher frequency of weak TCs can be largely attributed to the mutual interactions among the tropical monsoon trough west of 140°E, the westward South Asia high, and the high pressure anomaly in Northeast Asia (NEA). Then the potential influence of the NEA extratropical system is focused on. The resultant local negative potential vorticity (PV) anomaly is carried southeastward by the prevailing flow. It stimulates a descending flow around 30°N, which favors the westward retreat of the South Asian high and the decreased zonal vertical wind shear around 20°N. The associated lower-level outflow converges in the tropical WNP and reinforces the ascending motion around 10°–20°N. Meanwhile, the warm air column in NEA also contributes to anomalous easterlies in a band around 30°N, intensifying the lower-level cyclonic vorticity in the northwestern WNP. Consequently, the ascending motion, cyclonic vorticity, and the weakened zonal vertical wind shear in northwestern WNP promote the WTC formation. A set of physically based empirical models is developed using various physically based predictors to reconstruct the number of intense and weak TCs. Cross-validated hindcasts suggest that the NEA extratropical circulation can serve as an additional source of predictability for the weak TC variability.

Significance Statement

Tropical cyclones (TCs) are a highly destructive type of natural disaster that have garnered widespread attention. By comparison with intense TCs (ITCs), weak TCs (WTCs) are much more numerous and often form closer to the coastal regions of East Asia, whose mechanism has not been fully understood. In this study, we suggest that more ITCs are controlled by tropical monsoon circulations, while the WTC variability is closely coupled with both tropical and extratropical monsoon systems. In addition to the tropical monsoon trough west of 140°E and the westward South Asian high, the Northeast Asian circulation can regulate the WTC number by changing the lower-level vorticity, vertical motion, and vertical wind shear in the WTC genesis-prone region, which can be applied to improve the seasonal prediction skill of WTCs.

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Savin S. Chand
,
Kevin J. E. Walsh
, and
Johnny C. L. Chan

Abstract

This study presents seasonal prediction schemes for tropical cyclones (TCs) affecting the Fiji, Samoa, and Tonga (FST) region. Two separate Bayesian regression models are developed: (i) for cyclones forming within the FST region (FORM) and (ii) for cyclones entering the FST region (ENT). Predictors examined include various El Niño–Southern Oscillation (ENSO) indices and large-scale environmental parameters. Only those predictors that showed significant correlations with FORM and ENT are retained. Significant preseason correlations are found as early as May–July (approximately three months in advance). Therefore, May–July predictors are used to make initial predictions, and updated predictions are issued later using October–December early-cyclone-season predictors. A number of predictor combinations are evaluated through a cross-validation technique. Results suggest that a model based on relative vorticity and the Niño-4 index is optimal to predict the annual number of TCs associated with FORM, as it has the smallest RMSE associated with its hindcasts (RMSE = 1.63). Similarly, the all-parameter-combined model, which includes the Niño-4 index and some large-scale environmental fields over the East China Sea, appears appropriate to predict the annual number of TCs associated with ENT (RMSE = 0.98). While the all-parameter-combined ENT model appears to have good skill over all years, the May–July prediction of the annual number of TCs associated with FORM has two limitations. First, it underestimates (overestimates) the formation for years where the onset of El Niño (La Niña) events is after the May–July preseason or where a previous La Niña (El Niño) event continued through May–July during its decay phase. Second, its performance in neutral conditions is quite variable. Overall, no significant skill can be achieved for neutral conditions even after an October–December update. This is contrary to the performance during El Niño or La Niña events, where model performance is improved substantially after an October–December early-cyclone-season update.

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