9.5.3.6 Tropical Cyclones

Several recent events, including the active North Atlantic hurricane seasons of 2004 and 2005, the unusual development of a cyclonic system in the subtropical South Atlantic that hit the coast of southern Brazil in March 2004 (e.g., Pezza and Simmonds, 2005) and a hurricane close to the Iberian Peninsula in October 2005, have raised public and media interest in the possible effects of climate change on tropical cyclone activity. The TAR concluded that there was ‘no compelling evidence to indicate that the characteristics of tropical and extratropical storms have changed’, but that an increase in tropical peak wind intensities was likely to occur in some areas with an enhanced greenhouse effect (see also Box 3.5 and Trenberth, 2005). The spatial resolution of most climate models limits their ability to realistically simulate tropical cyclones (Section 8.5.3), therefore, most studies of projected changes in hurricanes have either used time slice experiments with high-resolution atmosphere models and prescribed SSTs, or embedded hurricane models in lower-resolution General Circulation Models (GCMs) (Section 10.3.6.3). While results vary somewhat, these studies generally indicate a reduced frequency of tropical cyclones in response to enhanced greenhouse gas forcing, but an increase in the intensity of the most intense cyclones (Section 10.3.6.3). It has been suggested that the simulated frequency reduction may result from a decrease in radiative cooling associated with increased CO2 concentration (Sugi and Yoshimura, 2004; Yoshimura and Sugi, 2005; Section 10.3.6.3; Box 3.5), while the enhanced atmospheric water vapour concentration under greenhouse warming increases available potential energy and thus cyclone intensity (Trenberth, 2005).

There continues to be little evidence of any trend in the observed total frequency of global tropical cyclones, at least up until the late 1990s (e.g., Solow and Moore, 2002; Elsner et al., 2004; Pielke et al., 2005; Webster et al., 2005). However, there is some evidence that tropical cyclone intensity may have increased. Globally, Webster et al. (2005) find a strong increase in the number and proportion of the most intense tropical cyclones over the past 35 years. Emanuel (2005) reports a marked increase since the mid-1970s in the Power Dissipation Index (PDI), an index of the destructiveness of tropical cyclones (essentially an integral, over the lifetime of the cyclone, of the cube of the maximum wind speed), in the western North Pacific and North Atlantic, reflecting the apparent increases in both the duration of cyclones and their peak intensity. Several studies have shown that tropical cyclone activity was also high in the 1950 to 1970 period in the North Atlantic (Landsea, 2005) and North Pacific (Chan, 2006), although recent values of the PDI may be higher than those recorded previously (Emanuel, 2005; Section 3.8.3). Emanuel (2005) and Elsner et al. (2006) report a strong correlation between the PDI and tropical Atlantic SSTs, although Chan and Liu (2004) find no analogous relationship in the western North Pacific. While changes in Atlantic SSTs have been linked in part to the AMO, the recent warming appears to be mainly associated with increasing global temperatures (Section 3.8.3.2; Mann and Emanuel, 2006; Trenberth and Shea, 2006). Tropical cyclone development is also strongly influenced by vertical wind shear and static stability (Box 3.5). While increasing greenhouse gas concentrations have likely contributed to a warming of SSTs, effects on static stability and wind shear may have partly opposed this influence on tropical cyclone formation (Box 3.5). Thus, detection and attribution of observed changes in hurricane intensity or frequency due to external influences remains difficult because of deficiencies in theoretical understanding of tropical cyclones, their modelling and their long-term monitoring (e.g., Emanuel, 2005; Landsea, 2005; Pielke, 2005). These deficiencies preclude a stronger conclusion than an assessment that anthropogenic factors more likely than not have contributed to an increase in tropical cyclone intensity.

The ACE of a season is the sum of the ACEs for each storm and takes into account the number, strength, and duration of all the tropical storms in the season.

As well as being squared ACE can also be cubed, and this version is known as the Power Dissipation Index (PDI).

During the last several decades the number of El Niño events increased, and the number of La Niña events decreased.[48] The question is whether this is a random fluctuation or a normal instance of variation for that phenomenon, or the result of global climate changes towards global warming.

The studies of historical data show that the recent El Niño variation is most likely linked to global warming. For example, one of the most recent results is that even after subtracting the positive influence of decadal variation, shown to be possibly present in the ENSO trend[49], the amplitude of the ENSO variability in the observed data still increases, by as much as 60% in the last 50 years[50].

It is not certain what exact changes will happen to ENSO in the future: different models make different predictions (cf.[51]) It may be that the observed phenomenon of more frequent and stronger El Niño events occurs only in the initial phase of the global warming, and then (e.g., after the lower layers of the ocean get warmer as well), El Niño will become weaker than it was[52]. It may also be that the stabilizing and destabilizing forces influencing the phenomenon will eventually compensate for each other[53]. More research is needed to provide a better answer to that question, but the current results do not completely exclude the possibility of dramatic changes.

[...]

El Niño is credited with suppressing hurricanes and made the 2009 hurricane season the least active in twelve years.[28] El Niño is also associated with increased wave-caused coastal erosion along the United States Pacific Coast.[citation needed]

There is some evidence that El Niño activity is correlated with incidence of red tides off the Pacific coast of California[citation needed].
[edit] Tropical cyclones

Most tropical cyclones form on the side of the subtropical ridge closer to the equator, then move poleward past the ridge axis before recurving into the main belt of the Westerlies.[29] When the subtropical ridge position shifts due to El Niño, so will the preferred tropical cyclone tracks. Areas west of Japan and Korea tend to experience much fewer September-November tropical cyclone impacts during El Niño and neutral years. During El Niño years, the break in the subtropical ridge tends to lie near 130°E, which would favor the Japanese archipelago.[30] During El Niño years, Guam's chance of a tropical cyclone impact is one-third of the long term average.[31] The tropical Atlantic ocean experiences depressed activity due to increased vertical wind shear across the region during El Niño years.[32]

NOAA’s updated 2010 Atlantic Hurricane Season Outlook calls for a 90% chance of an above normal season. There is only a 10% chance of a near-normal season, and no expectation the season will be below normal. Therefore, 2010 is expected to become the eleventh above-normal season since 1995. See NOAA definitions of above-, near-, and below-normal seasons. The Atlantic hurricane region (or basin) includes the North Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico.

[...]

We estimate a 70% probability for each of the following seasonal (June-November) ranges of activity during 2010. These ranges are consistent with NOAA’s May outlook, and reiterate a high likelihood of a very active season (i.e. hyperactive, defined by ACE > 175% of median), perhaps one of the more active on record.

* 14-20 Named Storms,
* 8-12 Hurricanes
* 4-6 Major Hurricanes
* An ACE range of 170%-260% of the median.

The activity is expected to fall within these ranges in about 70% of seasons with similar climate conditions and uncertainties to those expected this year. They do not represent the total possible ranges of activity seen in past similar years.