Search Results

You are looking at 11 - 20 of 72 items for

  • Author or Editor: Johnny C. L. Chan x
  • Refine by Access: All Content x
Clear All Modify Search
Johnny C. L. Chan

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 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.

Full access
Bin Guan
and
Johnny C. L. Chan

Abstract

The nonstationarity of the intraseasonal oscillations (ISOs) associated with the western North Pacific summer monsoon (WNPSM) is examined using a wavelet analysis of outgoing longwave radiation (OLR). Both the 10–20- and 30–60-day ISOs are found to display significant interannual modulations, and their relative strengths vary with time. The variation of OLR associated with a strong ISO, either 10–20- or 30–60-day, could be as large as 20 W m−2 in magnitude. Case studies showed that the mechanism for development of low OLR may differ in individual years, and that the 10–20-day ISO, the 30–60-day ISO, and the seasonal cycle may each become dominant in different years.

Full access
Jianjun Xu
and
Johnny C. L. Chan

Abstract

Based on the time of first occurrence of a significant sea surface temperature anomaly (SSTA) in the Niño-3.4 area (5°S–5°N, 170°–120°W), two types of El Niño episodes can be identified: the spring (SP) type in which the SSTA first increased to greater than 0.5°C in April or May, and the summer (SU) type in which this threshold is first reached in July or August. Composites of the SSTAs for these two types of events during the period 1950–97 show that the SP (SU) event is generally a stronger (weaker) warm episode in terms of the SSTA amplitude, and longer (shorter) in terms of the period during which the SSTA is greater than 0.5°C.

Before the occurrence of both types of El Niño episodes, the zonal wind anomalies over the western equatorial Pacific are always westerly. The east Asian winter monsoon is also strong. The difference between the two types is mainly in the timing of the occurrence of the westerly anomalies. For the SP (SU) events, these anomalies extend to the date line by January (May) of the El Niño year. A third component found in both types of El Niño episodes is anomalous southerlies over the northeastern coast of Australia during the El Niño year, which appear earlier in SP events. The difference between the two types of El Niño episodes is apparently phase locked to the annual variation in SST over the western equatorial Pacific.

A stronger east Asian winter monsoon and westerly anomalies in the previous summer are also found in some non–El Niño years. However, in these cases, no anomalous southerlies occur over the northeast of Australia. Therefore, it appears that only when anomalous northerlies from the east Asian winter monsoon converge with anomalous southerlies associated with the transition of Australian monsoon can sufficiently strong westerly anomalies form over the western equatorial Pacific to cause an El Niño event to occur. The presence of a strong south Asian summer monsoon in the previous year is also necessary. The timing of occurrence of southerlies over northeastern Australia apparently determines the onset time of an El Niño event.

Full access
Bin Wang
and
Johnny C. L. Chan

Abstract

An analysis of 35-yr (1965–99) data reveals vital impacts of strong (but not moderate) El Niño and La Niña 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 Niño 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 Niño (La Niña), 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 times more than during strong cold years. This implies that El Niño substantially enhances poleward transport of heat–moisture and impacts high latitudes through changing TS formation and tracks.

The enhanced TS formation in the SE quadrant is attributed to the increase of the low-level shear vorticity generated by El Niño–induced equatorial westerlies, while the suppressed TS generation over the NW quadrant is ascribed to upper-level convergence induced by the deepening of the east Asian trough and strengthening of the WNP subtropical high, both resulting from El Niño forcing. The WNP TS activities in July–December are noticeably predictable using preceding winter–spring Niño-3.4 SST anomalies, while the TS formation in March–July is exceedingly predictable using preceding October–December Niño-3.4 SST anomalies. The physical basis for the former is the phase lock of ENSO evolution to the annual cycle, while for the latter it is the persistence of Philippine Sea wind anomalies that are excited by ENSO forcing but maintained by local atmosphere–ocean interaction.

Full access
Johnny C-L. Chan

Abstract

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.

Full access
Johnny C. L. Chan

Abstract

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.

Full access
Johnny C. L. Chan

Abstract

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.

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

Abstract

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.

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

Abstract

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.

Full access
Johnny C. L. Chan
and
William M. Gray

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 center have the best correlation with cyclone movement. Tropical cyclones in the Northern Hemisphere move ∼10–20° to the left of their surrounding mid-tropospheric flow at 5–7° latitude radius, and those in the Southern Hemisphere move ∼10° to the right. It is also found that cyclones, in general, move ∼1 m s−1 faster than this flow. These general relationships appear to be modified by the vertical shear of the environmental wind, the zonal component of the cyclone velocity and other characteristics of the cyclone. The mean tropospheric flow (surface to 100 mb) at 5–7° latitude radius also correlates well with cyclone movement in most cases. For cyclones embedded in an environment with relatively small vertical wind shear, the mid-tropospheric flow is as good a descriptor of cyclone motion as the mean tropospheric flow. The average wind between the upper (200 mb) and lower (900 mb) troposphere also appears to correlate reasonably well with cyclone movement.

Full access