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Johnny C. L. Chan

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Johnny C. L. Chan

Abstract

The interannual variations in tropical cyclone activity in the northwest Pacific (NWPAC) and their relationships with the El Niño/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 the eastern part of NWPAC, however, is ∼180° out of phase with the SOI series at the Southern Oscillation frequency.

It appears that fluctuations of cyclone activity at the dominant Southern Oscillation frequency may be explained in terms of the change in the horizontal and vertical circulations in the atmosphere during periods of low SOI. The establishment of an anomalous Walker Circulation shifts areas of enhanced or suppressed convection, leading to the observed variations in cyclone activity.

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Johnny C-L. Chan

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Supertyphoon Abby (1983), although not one of the most destructive on record, received a great deal of attention from the typhoon forecasters in Guam. For a large part of Abby's lifetime, nearly all objectively predicted tracks were almost 90° to the left of the actual track of the cyclone. This study is an attempt to understand the reasons for the failure of the forecast models.

The intensity and size (horizontal extent) of the supertyphoon are hypothesized to be the main factors contributing to such a forecast failure. After intensifying to a maximum wind speed of 75 m s−1 (145 kt), Abby continued to grow, with the radius of 15 m s−1 (30 kt) winds extending beyond 600 km. Abby's circulation, which can be readily identified on synoptic charts, apparently affected the performance of the dynamical models. The “steering flow” vector as estimated from the operational analyses is found to be almost normal to the motion vector of Abby, which might provide a partial explanation of the forecasts by the objective methods.

These results suggest the need to analyze the performance of forecast models under different synoptic as well as storm-related factors. They also suggest the importance of studying the interaction between the tropical cyclone circulation and its environment.

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Johnny C. L. Chan

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The flow patterns at various levels in the atmosphere around northwest Pacific tropical cyclones are studied using objectively-analyzed wind fields produced by the United States Navy. The results show large differences in the flow fields among groups of cyclones moving in different directions. Appreciable baroclinity is found in cyclones moving northward or northeastward. The results also demonstrate the impotence of stratifying cyclones by their characteristics and synoptic environment in the study and prediction of tropical cyclone motion

The relationships between tropical cyclone motion and the midtropospheric flow averaged around the 5–7° latitude radial band are also investigated using both the composite and individual cases. The composite results are generally consistent with those obtained from individual cases. In most cases, these relationships also agree with those derived in previous studies from rawinsonde composites and objectively-analyzed height fields. Since the objectively-analyzed wind fields used in this study are available for individual cases, the results suggest possible application of these fields to additional research studies of tropical cyclone motion as well as to development of short-term prediction techniques.

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Johnny C. L. Chan

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A simple method based on the cumulative number of tropical cyclones (TCs) up to a given month in the early season is proposed to update the seasonal prediction of the annual number of TCs in a given ocean basin. For the western North Pacific, if this number is below normal by July or August, it is very likely that the annual activity will also be below normal. The reverse (for relating above-normal number with above-normal annual activity) is also true although the probability is smaller than for the below-normal category. Similar results are found for TCs in the eastern North Pacific and the North Atlantic, with the latter having the smallest likelihood. These results change only slightly when the samples are separated into dependent and independent subsets.

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Johnny C. L. Chan

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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 Niño (EN) and La Niña (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 WNP, TC activity tends to be below normal from August to November. During the year after an LN event, the entire basin generally has more TCs. Such a situation is especially true over the SCS from May to July.

All these anomalous activities are apparently linked to anomalies in the large-scale flow patterns at 850 and 500 hPa. Because the 850-hPa flow is related to TC genesis and development, areas with anomalous cyclonic (anticyclonic) flow are generally found to be associated with above- (below-) normal TC activity. Anomalous 500-hPa flow is identified as responsible for steering TCs toward or away from a region, thus rendering the TC activity in that region above or below normal.

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Johnny C. L. Chan

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In 1991, Typhoon Nat over the western North Pacific made four directional reversals due to its interactions with two other tropical cyclones (TCs), Luke and Mireille. This paper analyzes the performance of three global and two regional models in predicting the movement of Nat to determine the extent to which each of the models was capable of correctly simulating such binary interactions. The global models include those of the European Centre for Medium-Range Weather Forecasts (ECMWF) and the U.K. Meteorological Office (UKMO) and the U.S. Navy Operational Global Atmospheric Prediction System (NOGAPS). The regional models studied are the Typhoon Model (TYM) of the Japan Meteorological Agency and the One-Way Tropical Cyclone Model (OTCM) of the U.S. Navy.

It was found that in general the global models made better predictions than the regional ones, especially when the large-scale flow was well defined. During the interaction periods, the UKMO model and the TYM were the best. The ECMWF model was also quite good in capturing the latter part of the Nat-Mireille interaction when Mireille had a large circulation. Although NOGAPS had a bogus vortex in the model, it did not predict the interactions very well. The OTCM was the worst of the models, possibly because of the steering flow imposed onto the model vortex.

The main conclusions from this study are that a bogus vortex representative of the actual TC appears to be necessary for properly simulating the interaction between TCs. An increase in resolution may also help in this respect. However, imposing a persistence vector into a model to simulate steering may prove detrimental in predicting binary interactions.

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Johnny C-L. Chan

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This paper presents an observational study of the physics of tropical cyclone motion. Analyses of the vorticity budget using both aircraft and rawinsonde composite data were performed. As expected, the results show a definite link between the local change in relative vorticity and tropical cyclone movement. The main contributor to this local change, at least in the middle troposphere, is the horizontal advection of absolute vorticity with the divergence term usually playing a secondary but not necessarily negligible role. The vertical advection and tilting terms are generally much smaller.

The contribution of the divergence term as an extra component in determining the movement of tropical cyclone is discussed. The mass to wind adjustment as a result of the increase in vorticity is viewed as a combination of the advection of temperature (or mass) and subsidence. Substantiating evidence of this viewpoint is presented for cyclones undergoing turning motion.

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M. C. Wu and Johnny C. L. Chan

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The upper-level features associated with the two kinds of winter monsoon surges over south China are studied: the easterly surge (ES) and the northerly surge (NS). The study is similar to that used by Wu and Chan, except that a broader region (0°–60°N, 70°–160°E) is considered.

The outbreak of an NS is associated with a breakdown of the Siberia–Mongolia high. The upper-level features suggest that the evolution of the Siberia–Mongolia high in an NS can be related to an eastward passage of a short-wave trough and the polar jet based on the quasigeostrophic theory. The intensification of the Siberia–Mongolia high appears likely to be caused mainly by the adiabatic cooling due to rising motion associated with the trough and the polar jet passages. After the passage of the trough and the jet, the Siberia–Mongolia high breaks down in response to the sinking motion upstream of the trough, causing a surge of the northerly winds over the south China coast.

For the ES, the passage of an upper ridge is observed. The zonal index increases in an ES but the subtropical jet weakens. The high pressure center responsible for the ES (the Dahingganling high) is found to be a split cell from the Siberia–Mongolia high, and the splitting is related to the ridge passage. A temperature inversion is only observed when the high is not far from the parent high (i.e., Siberia–Mongolia high). Unlike the NS, an ES is found not to be associated with a strong north–south thermal contrast. Significant differences are observed when comparing the features of the ES and NS. On the whole, the results from this study and those from Wu and Chan suggest that on the synoptic scale a clear distinction exists between the ES and NS on the synoptic scale both at upper levels and the surface.

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M. C. Wu and Johnny C. L. Chan

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The surface features associated with two kinds of winter monsoon surges over south China are studied: the easterly surge (ES) and the northerly surge (NS). Surface meteorological parameters over the region 15°–50°N, 90°–130°E for the surges that occurred in the three winters (October–March) from 1988 to 1991 are analyzed. For the northerly surge, the surface features found are 1) an abrupt temperature drop and wind direction turning from easterly to northerly, which can be related to the passage of a cold front; 2) an increase in the dewpoint depression; and 3) a large north–south pressure gradient. On the other hand, the easterly surge is found to be associated with strong easterly winds up to approximately 40 km h−1, little temperature or pressure change, and a southeastward motion of a high pressure center from Dahinggangling to the Yellow Sea together with a sharp pressure ridge along the east China coast. Furthermore, an ES and an NS are associated with different perturbations (anomalies) in pressure, wind, temperature, and dewpoint depression when compared with the wintertime normal condition. The results suggest a clear distinction between the two surges on the synoptic scale.

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