El Niño-Southern Oscillation and the seasonal predictabilityof tropical cyclones
Christopher W. Landsea
Landsea, C. W., 2000: El Niño-Southern Oscillation and the seasonal predictability of tropical cyclones. In press in El Niño : Impacts of Multiscale Variability on Natural Ecosystems and Society, edited by H. F. Diaz and V. Markgraf.
ABSTRACT
Perhaps the most dramatic effect that El Niño has upon the climate system is in changing tropical cyclone characteristics around the world. This chapter reviews how tropical cyclone frequency, intensity and areas of occurrence are altered in all of the cyclone basins by the phases of El Niño-Southern Oscillation (ENSO). In addition to ENSO, other global (such as the stratospheric Quasi-Biennial Oscillation) and local factors (such as sea surface temperature, monsoon intensity and rainfall, sea level pressures and tropospheric vertical shear) can also help modulate tropical cyclone variability. Understanding how these various factors relate to tropical cyclone activity can be challenging due to the fairly short (on the scale of only tens of years) record of reliable data. Despite this limitation, many of the factors that have been linked to tropical cyclones - the foremost of which being ENSO - have substantial lead relationships and can be utilized to provide seasonal forecasts of tropical cyclones. Details of methodologies that have been developed for the North Atlantic, Northwest Pacific, South Pacific and Australian basin tropical cyclones are presented as well as the real-time forecasting performance of Atlantic hurricanes as issued by Prof. William Gray.
Introduction
Tropical cyclones are the costliest and deadliest natural disasters around the world, as the approximate 300,000 death toll in the infamous Bangladesh Cyclone of 1970 and the $26.5 billion (U.S.) in damages due to Hurricane Andrew in the Southeast United States can attest (Holland 1993, Hebert et al. 1996). Pielke and Pielke (1997) show that hurricane property losses - exceeding that due to earthquakes by a factor of four - account for 40% of all insured losses in the United States for the period 1984 to 1993. Understanding and being able to predict how both tropical cyclone frequencies and intensities vary from year to year is obviously a topic of great interest to meteorologists, public and private decisionmakers and the general public alike. A review of multidecadal scale tropical cyclone variations and possible "greenhouse warming" effects has been covered in Landsea (1998). This chapter will explore the role that the El Niño-Southern Oscillation and other phenomena have upon tropical cyclones around the world and what progress has been made in utilizing such information to provide seasonal forecasting of these storms.
"Tropical cyclone" is the generic term for a non-frontal synoptic scale low-pressure system that develops over tropical or sub-tropical waters with organized convection and a well-defined cyclonic surface wind circulation. Its energy source is primarily derived from evaporation and sensible heat flux from the sea in the presence of high winds and lowered surface pressure. These energy sources are tapped through condensation and fusion in convective clouds concentrated near the cyclone's "warm-core" center (Holland 1993). Tropical cyclones with maximum sustained surface winds of less than 18 ms-1 are called "tropical depressions". Once the tropical cyclone reaches winds of at least 18 ms-1 they are typically called a "tropical storm" and assigned a name. Names are decided upon by representatives from countries in the basins affected at annual World Meteorological Organization regional meetings (Neumann 1993). If winds reach 33 ms-1, they are then called: a "hurricane" (the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E); a "typhoon" (the Northwest Pacific Ocean west of the dateline); a "severe tropical cyclone" (the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E); a "severe cyclonic storm" (the North Indian Ocean); and a "tropical cyclone" (the Southwest Indian Ocean) (Neumann 1993). Additionally, the category of "intense (or major) hurricane" has been utilized for the Atlantic basin for those tropical cyclones obtaining winds of at least 50 ms-1, which corresponds to a category 3, 4 or 5 on the Saffir-Simpson hurricane intensity scale (Simpson 1974, Hebert et al. 1996).
It should be pointed out that such definitions are quite arbitrary ones and that nearly all intensity wind values at the surface are an estimation (by satellite pictures) or an extrapolation (from aircraft reconnaissance downward to the surface). Thus by the nature of the tropical cyclone, by the limited data available and by the way that meteorologists have defined intensity thresholds, the strength of individual tropical cyclones can be difficult to pinpoint with certainty. Also, changes in observational platforms available to monitor tropical cyclones can produce as much or greater change in the cyclone record as can actual climate fluctuations. Studies of interannual (and especially interdecadal) changes of tropical cyclones must carefully consider both the relative arbitrariness of the intensity record of the storms and the dependency of intensity on the observations available.
Necessary (but not sufficient) environmental conditions
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.