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Global Guide to Tropical Cyclone Forecasting:
CHAPTER 5: SEASONAL FORECASTING

5.3 TROPICAL CYCLONE MODULATION BY THE QBO

In addition to the ENSO, there is another important global-scale meteorological parameter whose slowly varying properties are related to seasonal trends in tropical cyclone frequency and intensity. This is the stratospheric Quasi-Biennial Oscillation (QBO), which appears to have an important modulating impact upon tropical cyclones within 20^o lat of the equator. During an easterly phase of the QBO (Fig. 5.7), the westward moving systems in these latitudes are inhibited from developing into tropical cyclones, and those that form tend to not become intense. By contrast, formation and intensity are typically enhanced when QBO winds are in the westerly phase.

5.3.1 North Atlantic Basin

Substantial variation of tropical cyclone frequency and lifetime is observed during west and east phases of the QBO in the North Atlantic (Table 5.3). There have been 44% more hurricanes and 74% more hurricane days during the west as opposed to east phase of the QBO. The association between the frequency of intense Atlantic hurricanes with maximum sustained winds greater than 50 ms^-1 and the September phase of the QBO is summarised in Table 5.4. Nearly twice as many intense hurricanes occur during the westerly compared to the easterly phase (Gray, 1988).

 Figure 5.7: Illustration of the two basic wind conditions for the stratospheric QBO, which occur over the tropics at 50 hPa (or 20 km altitude) during the summer seasons of both hemispheres. The left diagram shows conditions during the easterly phase when moderate easterly winds occur on the equator and strong easterly winds occur at 10-15^o lat. During the westerly phase (right diagram) stratospheric winds on the equator are from the west and weak easterlies occur at 10-15^o lat.

5.3.2 Western Pacific/Australian Region

Recent studies by Gray and Sheaffer (1989) and Collimore and Gray (1991) suggest that a modest reduction of the total frequency of western Pacific tropical cyclones may occur during the west phase of the QBO, especially in the Australian region (Table 5.5). These results are opposite to the North Atlantic observations in Table 5.3.

Although the Pacific relationship is only weak, the opposite association between tropical cyclone frequency and phase of the QBO between the North Atlantic and the western Pacific may be attributed to differences in upper tropospheric zonal wind regimes in the two oceans. Tropical cyclones in the North Atlantic typically form in a region of low-level easterlies and upper-level westerlies. Formation is thus: 1. inhibited by the increased upper-tropospheric westerly winds during the westerly QBO phase; and 2. enhanced by the weaker vertical shear in the easterly QBO phase. The opposite occurs in the western Pacific, where the 200 hPa zonal winds are typically stronger from the east during the cyclone season. Thus the QBO has less effect on the vertical shear, which is slightly reduced in the upper troposphere during easterly phases.

Contrary to the results in Table 5.5, intense typhoons over much of the western Pacific have a definite preference for the westerly phase of the QBO, when their frequency (Table 5.6) is nearly a factor of two greater than for the east phase. Because western North Pacific typhoons tend to be more intense than those in the Australian region (perhaps due to the lack of aircraft reconnaissance), it was necessary to lower the intense cyclone criteria as shown. This relationship breaks down in the western portion of the South Pacific Ocean for reasons that are as yet not well understood.

Willoughby et al., (1989) listed all aircraft reconnaissance measurements of western North Pacific supertyphoons with central pressure below 900 hPa during the period 1950-1987. The data cover the period of the 1950s through mid-1987. Thirty-six such supertyphoons were found in 11 seasons. Thirty of these (83%) occurred during a westerly QBO phase and 5 of the remaining six occurred in one easterly phase year (1979). Thus nearly all of the extreme intensity cyclones in the western North Pacific have occurred during QBO westerly years.

5.3.3 Summary of QBO Relationships

The long period quasi-periodic nature of the QBO allows for relatively reliable estimates of the lower stratospheric zonal winds well before the start of each regional tropical cyclone season. The aforementioned relationships with tropical cyclone frequency and intensity then provide a means of forecasting the seasonal activity, especially that associated with the relatively infrequent, but very damaging intense cyclones. Table 5.7 offers suggested qualitative forecast guidelines for tropical cyclone frequency and intensity for each region. Care needs to be taken to include the uncertainty involved in making such seasonal forecasts.

Contents Chapter 5.4