Before tropical cyclogenesis and development can occur, there are several precursor environmental conditions that must be in place (Gray 1968, 1979):
1. Warm ocean waters (of at least 26.5 C) throughout a sufficient depth (unknown how deep, but at least on the order of 50 m). Warm sea surface temperatures (SSTs) are necessary to fuel the heat engine of the tropical cyclone1.
2. An atmosphere which cools fast enough with height such that it is potentially unstable to moist convection. It is the precipitating convection typically in the form of thunderstorm complexes which allows the heat stored in the ocean waters to be liberated for tropical cyclone development.
3. Relatively moist layers near the mid-troposphere. Dry mid levels are not conducive for allowing the continued development of widespread thunderstorm activity because entrainment into the thunderstorms dries and cools the rising parcel, reducing buoyancy.
4. A minimum distance of around 500 km from the equator. For tropical cyclogenesis to occur, there is a requirement for non-negligible amounts of the Coriolis force to provide for near gradient wind balance to occur. Without a substantial Coriolis force, inflow into the low pressure is not deflected to the right (to the left in the Southern Hemisphere) and the partial vacuum of the low is quickly filled.
5. A pre-existing near-surface disturbance with sufficient vorticity and convergence. Tropical cyclones cannot be generated spontaneously. To develop, they require a weakly organized system with sizable spin and low level inflow.
6. Low values (less than about 10 ms-1) of vertical wind shear between the 850 and 200 mb levels. Vertical wind shear is the magnitude of wind change with height. Large values of vertical wind shear disrupt the incipient tropical cyclone and can prevent genesis, or, if a tropical cyclone has already formed, large vertical shear can weaken or destroy the tropical cyclone by interfering with the organization of deep convection around the cyclone center (DeMaria 1996).
Having these conditions met is necessary, but not sufficient, as many disturbances that appear to have favorable conditions do not develop. Recent work (Velasco and Fritsch 1987, Chen and Frank 1993, Emanuel 1993) has identified that large thunderstorm systems (called mesoscale convective complexes [MCCs]) often produce an inertially stable, warm core vortex in the trailing altostratus decks of the MCC. These mesovortices have a horizontal scale of approximately 100 to 200 km, are strongest in the mid-troposphere and have no appreciable signature at the surface. Zehr (1992) hypothesizes that genesis of the tropical cyclones occurs in two stages: stage one occurs when the MCC produces a mesoscale vortex and stage two occurs when a second blow up of convection at the mesoscale vortex initiates the intensification process of lowering central pressure and increasing swirling winds.
Variations in environmental conditions that affect tropical cyclone activity
Seasonal variations of tropical cyclone activity depend upon changes in one or more of the above parameters. Many studies have focused upon the variations in these values both before and during the tropical cyclone season. While the bulk of these studies has been centered upon the Atlantic basin, all of the global basins have been analyzed to some degree for interannual predictability.
Globally, tropical cyclones are affected dramatically by the El Niño-Southern Oscillation (ENSO). ENSO is a fluctuation on the scale of a few years in the ocean-atmospheric system involving large changes in the Walker and Hadley Cells throughout the tropical Pacific Ocean region (Philander 1989). The state of ENSO can be characterized, among other features, by the sea surface temperature (SST) anomalies in the eastern and central equatorial Pacific: warmings in this region are referred to as El Niño events and coolings are La Niña events. The Southern Oscillation Index (SOI), the standardized difference in sea level pressure between Tahiti and Darwin, Australia, also describes the state of ENSO with high (low) pressures at Darwin and low (high) pressure at Tahiti corresponding to El Niño (La Niña) events. ENSO greatly alters global atmospheric circulation patterns and it is able to affect tropical cyclone frequencies primarily by altering the lower tropospheric source of vorticity and by changing the vertical shear profile.
The various basins do not respond identically to ENSO. Some show changes in frequency of cyclogenesis, while others show shifts in the genesis locations. These variations are due to the time of year that the basin reaches its peak in activity versus the annual cycle of ENSO, the location of the basin with respect to the central equatorial Pacific and the background climatological flow features within the basin. Basins within the Pacific can be partially forced by direct alterations of the SSTs in the genesis regions, however, most basins experience remote forcing through alteration of the tropospheric flow features. It is the combination of spatial, temporal and climatological factors that determine how individual tropical cyclone basins will be altered by ENSO.
Nicholls (1979) first identified that the tropical cyclones In the vicinity of Australia (90E to 165E), are reduced in number during the warm phase (or El Niño) of ENSO. Revell and Goulter (1986) and Hastings (1990) demonstrated that the reduction of Australian region tropical cyclones is compensated by an increase in the South Pacific east of 165E (Fig. 1), because of a shift in the center of action in tropical cyclone genesis. There is also a smaller tendency to have the tropical cyclones originate a bit closer to the equator (Revell and Goulter 1986). The opposite is observed in La Niña events. This appears to be due to a weakening of the Australian monsoon trough (e.g. the boundary between the cross-equatorial near surface westerlies and the tradewind easterlies - see McBride 1995) in the western portion of the basin and an extension of this trough well to the east of its usual location during an El Niño event, thus changing the availability of lower tropospheric large scale cyclonic circulation and convergence for the storms to develop (Fig. 2 - Evans and Allen 1992). Evans and Allen (1992) also identified a regional change for the Northern Territory of Australia that is opposite to the general tendency for the entire basin. They found fewer tropical cyclones (and fewer landfalls) during La Niña than in El Niño years because of a stronger - though landlocked - monsoon trough. Such an overland positioning of the monsoon trough, while allowing for large rainfall production over northern Australia, is not conducive for tropical cyclone formation because genesis of tropical cyclones requires an oceanic moisture and heat source.
Likewise, the Northwest Pacific basin experiences a similar change in the location of tropical cyclone genesis without a total change in frequency. Pan (1981), Chan (1985), and Lander (1994) have detailed that west of 160E there are reduced numbers of tropical cyclones forming and from 160E to just east of the dateline an increase in the amount of genesis occurring during El Niño events (Fig. 3). The opposite occurs during La Niña events. Changes in the monsoon trough location and strength again appear to dictate the tropical cyclone variations, though there has been no documentation of this possible effect. Additionally, Lander (1994) uncovered a mid-season increase in tropical cyclones forming in subtropical latitudes (20 to 30N) during La Niña events, which he hypothesized to be tropical cyclogenesis forced by the Tropical Upper Tropospheric Trough (TUTT; a persistent, summer-autumn, "cold-core" trough with maximum amplitude at the tropopause that occurs primarily over the tropical and subtropical mid-oceans - see Fitzpatrick et al. 1995) within the tradewind belt.
The western portion of the Northeast Pacific basin near Hawaii (140W to the dateline) has been suggested to experience more tropical cyclone genesis during an El Niño year and more tropical cyclones tracking into the sub-region in the year following an El Niño (Schroeder and Yu 1995). The opposite effects of La Niña have yet to be analyzed and the mechanism for such changes is unclear at this time.
While the previously mentioned studies have focused upon the ability to change conditions locally in altering the tropical cyclogenesis frequencies, the Atlantic basin feels the effects of ENSO remotely through changes in the vertical shear wind profile. During El Niño events, the vertical shear increases primarily due to increases in the climatological westerly winds in the upper troposphere (Fig. 4) and reduced 200mb westerlies and shear during La Niña events (Gray 1984a, Shapiro 1987). The larger (smaller) vertical shear accompanying El Niño (La Niña) events lead directly toward decreased (increased) numbers of Atlantic hurricanes. Goldenberg and Shapiro (1996) identified that the area between 10 and 20N from North Africa to Central America (hereby known as the Atlantic "main development region") shows the largest sensitivity toward changes in the vertical shear, with weakly opposite conditions occurring in the subtropical latitudes of 20 to 35N (Fig. 5). This tendency for weaker (stronger) vertical shear in the subtropical latitudes during El Niño (La Niña) events may account for increasing (decreasing) the number of subtropical forming tropical cyclones, though these changes in the subtropical latitudes are weaker in magnitude to the changes occurring in the main development region. Additional impacts of ENSO on Atlantic climate can be found in Enfield and Mayer (1997) and in the Enfield and Mestas-Nuñez (1997) chapter in this book.
The remaining basins - the eastern portion of the Northeast Pacific (the North Pacific Ocean from 140 W to North America), the Southwest Indian and the North Indian - appear to have little ENSO-forced variations (i.e. Jury 1993, Dong and Holland 1994, McBride 1995), though there may be ENSO relationships produced in these areas that have not yet been identified.
Beside the El Niño-Southern Oscillation, there is another global factor that appears to force changes in tropical cyclones: the stratospheric Quasi-Biennial Oscillation (QBO), an east-west oscillation of stratospheric winds that encircle the globe near the equator (Wallace 1973). This oscillation has a distinct effect upon Atlantic (more activity in the west phase [Fig. 6] - Gray 1984a, Shapiro 1989), Southwest Indian (more activity in the east phase - Jury 1993) and Northwest Pacific (more activity in the west phase - Chan 1995) tropical cyclones. While the exact mechanism of the stratospheric QBO's influence on tropical cyclones is uncertain, it has been hypothesized that upper tropospheric to lower stratospheric vertical shear variations (Gray et al. 1992b) and/or upper tropospheric static stability changes (Knaff 1993) may be responsible.
In addition to the global effects of ENSO and QBO, there are also local effects that appear to directly impact tropical cyclone frequency within individual basins. These include variations of local sea level pressures, SSTs and tradewind and monsoon circulations.
Sea level pressures act to directly impact the strength of the vertical wind shear. For example in the Atlantic basin because of a relatively invariant sea level pressure field near the equator, above (below) normal SLP in the main development region from 10 to 20N between Africa and the Americas tightens (loosens) the local pressure gradient and strengthens (weakens) the easterly tradewinds by 1 to 3 m s-1, thereby contributing to increased (decreased) vertical shear (Gray et al. 1993, 1994). Additionally, Gray et al. (1993) have suggested that abnormally low SLP indicates a poleward shift and/or a strengthening of the Intertropical Convergence Zone (ITCZ). Both situations contribute to less subsidence and drying in the main development region through which easterly waves move. Knaff (1997) indicates that low SLP is accompanied by a deeper moist boundary layer and a weakened tradewind inversion. Moreover, an enhanced ITCZ provides more large-scale, low level cyclonic vorticity to incipient tropical cyclones, thereby creating an environment that is more favorable for tropical cyclogenesis (Gray 1968). In contrast, above normal SLP tends to be associated with opposite conditions which are unfavorable for tropical cyclogenesis. Ray (1935), Brennan (1935), Shapiro (1982), Gray (1984b) and Gray et al. (1993, 1994) have discussed the relationship between sea level pressure anomalies and Atlantic basin activity, while Nicholls (1984) has analyzed Australian tropical cyclones and local pressure values.
Sea surface temperatures in the genesis regions of tropical cyclone basins have a direct thermodynamic effect on tropical cyclones through their influence on moist static stability (Malkus and Riehl 1960). Pacific SSTs also indirectly influence the vertical shear through its strong inverse relationship with surface pressures in some regions (Shapiro 1982, Gray 1984b, Nicholls 1984). (These direct and indirect effects of local SST variations are considered separately from the remote forcings of the SST modulations directly due to ENSO.) In particular for the Atlantic basin, warmer than average waters are usually accompanied by lower than average surface pressures, and thus, weaker tradewinds and reduced shear. Cooler than average waters are usually accompanied by higher pressure, stronger tradewinds and increased shear. Somewhat surprisingly, interannual SST variations have relatively small or negligible contributions toward increasing the tropical cyclone frequency in most basins. Only the Atlantic, Southwest Indian and Australian regions have significant though small, positive associations in the months directly before the tropical cyclone seasons begin (Raper 1992, Shapiro and Goldenberg 1997). However, Saunders and Harris (1997) provide substantial evidence that both preceding and during the hurricane season that Atlantic SSTs in the main development region contribute a large percentage of the variance explained (over 30% during the height of the season) with the number of hurricanes generated in that area. Indeed they argue through a partial correlation analysis that these Atlantic SSTs are the dominant physical modulator of tropical Atlantic hurricanes. In addition to these studies, Ray (1935), Carlson (1971), Wendland (1977) and Shapiro (1982) have also examined the Atlantic basin, Jury (1993) has investigated the Southwest Indian, and Nicholls (1984) and Basher and Zheng (1995) have analyzed the Australian/Southwest Pacific for SST associations.
One aspect that has recently been uncovered is the association of a tropical cyclone basin with its generating (or nearby) monsoon trough. As previously discussed, Evans and Allen (1992) identified that variations in the Australian monsoonal flow can be associated with changes in tropical cyclone activity such that a strong (weak) monsoon circulation during a cold (warm) phase of ENSO is accompanied by many (few) tropical cyclones. Bate et al. (1989) also suggested that variations in the Australian monsoon could alter the tropical cyclone activity, independent of any pronounced ENSO events. Over the Atlantic basin, June through September monsoonal rainfall in Africa's Western Sahel has shown a very close association with intense hurricane activity (Fig. 7 - Reed 1988, Gray 1990, Landsea and Gray 1992, Landsea et al. 1992). Wet years in the Western Sahel (e.g. 1988 and 1989) are accompanied by dramatic increases in the incidence of intense hurricanes, while drought years (e.g. 1990 through 1993) are accompanied by a decrease in intense hurricane activity. Variations in tropospheric vertical shear and African easterly wave intensity have been hypothesized as the physical mechanisms that link the two phenomena (Gray 1990, Landsea and Gray 1992), although Goldenberg and Shapiro (1996) have demonstrated that changes in the vertical shear probably dominate. They note that wet (dry) years are associated with reduced (increased) wind shear, due to both weaker (stronger) than average lower tropospheric tradewinds and upper tropospheric westerlies throughout the main development region.
A final factor that has been considered for forcing interannual variations of tropical cyclone activity is changes in the "steering flow" in which the storms are embedded. (To a first approximation, tropical cyclones can be considered to be steered by the surrounding deep layer [the ocean surface to 100 mb] atmospheric flow features [Franklin et al. 1996].) Namias (1955) and Ballenzweig (1959) first suggested that interannual variations in the mid-tropospheric flow fields could help account for both variations in Atlantic basin tropical cyclogenesis and in the tracks of the storms once formed. While their ideas regarding genesis have not borne out, the hypothesis regarding changes in steering have held up. Shapiro (1982) confirmed that mid-tropospheric flow features can account for sub-regions within the Atlantic basin experiencing more or less activity in any particular year.