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Global climate change and hurricanes

Posted by Olive Heffernan on behalf of Kevin Trenberth

The 2007 hurricane season is about to get officially underway. Never mind that nature has already provided the first named storm in the North Atlantic: Andrea. Several forecasts suggest that the 2007 season in the North Atlantic will be well above average. Sea surface temperatures (SSTs) are above normal and atmospheric conditions look likely to be favorable for tropical storm activity.

In 2005, the record breaking year in the North Atlantic, record high SSTs in the critical region from 10 to 20 degrees N in the North Atlantic provided ample fuel for the 28 named storms (normal about 15, previous record 21 in 1933) and 15 hurricanes (normal about 6, previous record 12). Atmospheric conditions were favorable with weak wind shear (that otherwise tends to tear a vortex apart) and the absence of stable layers that prevent convection from developing. In contrast in 2006, SSTs were much lower and closer to the long term normal, and the atmospheric conditions were not favorable, as a developing El Niño in the Pacific created an atmospheric circulation that increased the wind shear in the Atlantic. This year, in 2007, there is no El Niño in the Pacific, and SSTs and the upper ocean heat content are more favorable for Atlantic storms.

There seems to be general agreement on these points, yet the whole issue of Atlantic hurricanes is mired in controversy over the role of global warming. It is not a disagreement that SSTs are higher but rather whether the warming is due to natural processes such as the Atlantic Multi-decadal Oscillation or global warming. To the public, the result is the same for now. To me this is obvious: global warming is “unequivocal” to quote the recent IPCC Working Group I report and global SSTs have increased about 0.6 degrees C. In the last half century this warming is associated with increased greenhouse gases in the atmosphere. It is not possible that the Atlantic has escaped from this warming.

The latest shot fired on this issue was by Chris Landsea in Eos on the long term Atlantic hurricane record. The Atlantic has the best record of tropical storms owing in part to aircraft surveillance since 1944. It is widely recognized that the older records are less reliable as the reports depend on shipping or landfall of the storms. Landsea makes this case quite well, but he goes further. He assumes that the percentage of landfalling storms should be constant over time and this provides a basis for adjusting the older records and increasing the numbers of storms by 2.2 per year from 1900 to 1965, when the satellite era began. The critical assumption is that this fraction should remain the same, yet it is abundantly clear that SSTs have increased and so other things have not remained the same. Given the dependence of hurricanes on high SSTs above about 26 degrees C, shouldn’t increased SSTs increase the scope for storms to develop farther to the east over what were colder waters and for the tracks to become longer? An example may be Vince in October 2005, which was the first tropical storm to make landfall in Spain and Portugal. Indeed shouldn’t one expect a decreasing fraction of landfalling storms in a warming climate?

The fact is that SSTs have increased and so there is every reason to believe that activity has increased also. Activity can be manifested in several ways:

1) increased number

2) increased intensity

3) increased size

4) increased duration.

What we expect from the dynamics is that the storms will experience increased intensity because there is more fuel for the storms (higher SSTs and more water vapor) but stronger storms take more energy out of the ocean and leave behind a colder wake, and so the expectation is that numbers could actually decrease. There is some evidence to suggest that size, and thus damage, may increase, but there are no statistics on size at all. A key issue for the Landsea paper is what about duration? As long as the storms keep moving to a new piece of ocean they do not get affected by the cold wake, and so in recent times the storms may be developing farther to the east and this may mean there are fewer storms missed than Landsea claims. Moreover, storms that were east of 55 degrees W that might not have been subject to aircraft surveillance tend to move west into the area where they are tracked, and so all that may be missing is a bit at the start of the storm?

These uncertainties argue that there should be a reanalysis of tropical storms in the satellite era since about 1970 or even 1966, to look not just at numbers but other statistics as well.

Kevin Trenberth

Head of the Climate Analysis Section

National Center for Atmospheric Research, USA


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    James Elsner said:

    One thing I find surprising about the debate on climate change and hurricanes is the lack of discussion on the North Atlantic oscillation (NAO). A weak preseason NAO phase tends to favor tracks that parallel lines of latitude. In contrast a strong NAO phase tends to favor tracks that cross latitudes (hurricanes that, in general, get steered away from the U.S. coast). We speculate the reason for this is related to the position and strength of the subtropical high pressure system during the season.

    Interestingly, the NAO was in a positive phase for much of the 1970s and 1980s with historic highs in the early 1990s and speculation about a link to global warming has been made. Osborn et al. (1999) show that the NAO from the 1960s to early 1990s is outside the range of earlier variability in the instrumental record and also outside the range of variability simulated using UK Hadley Centre’s numerical model. Thus with greater warmth and perhaps more Atlantic hurricanes it is possible that the threat to the United States as defined by the probability of a strike will remain relatively constant rather than increase.

    In fact there is some evidence for this in the historical record of U.S. hurricane counts which show no long term trend but a tendency for a smaller ratio of landfall counts to basin-wide counts. The differential influence of improvements in observing technologies on landfall and total counts tends to confound attempts to understand this tendency as noted in Elsner and Kara (1999). Moreover, conditional on the phase of the NAO, there are statistically significant positive relationships between Atlantic sea-surface temperature (SST) and both U.S. hurricane counts (Elsner and Jagger 2006) and insured losses (Jagger et al. 2007).

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    bubba said:

    I would just note as a point of order that Andrea is/was not a hurricane. She was an althogher different meteorological formation called a subtropical storm. And the subtropical storm (depression) season typically runs through the end of May.

    Thus, Andrea’s appearance in the first week of May is far from abnormal.

    Except that the National Hurricane Center only began borrowing names from the seasonal list of tropical cyclones, true hurricanes, and applying them to subtropical cyclones in 2002.

    Something to keep in mind when implying, at least it reads this way, that the Hurricane season has begun well ahead of it’s normal opening.

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    Harold Brooks said:

    Landsea “assumes the percentage of landfalling storms should be constant over time” because it’s what is observed in the satellite era. If you take five-year running fractions of storms making (or nearly making, as Landsea uses) landfall, the highest value in the satellite era is 65.3% (32 of 49 in 1970-1974). The range from the late 1970s through 2006 was from 56-65% for every five-year period. You could make an argument, based on the reported record, that there’s been a slight increase in the fraction of landfalling storms during the satellite era (63.5% in 2000-2006, 52.7% in 1965-1971), but that’s because of the low fraction in a few of those early years. After that, there’s no trend.

    Prior to 1970-1974, the last five-year period with a fraction as low as the 1970-1974 was 1901-1905. Until 1946-1950, it only got as low as 68.96% and most periods it was above 80%. From 1948-1964, it was ~68%.

    Overall, in 1903-1947, 81.7% were landfalling or near landfalling; 1948-1964 was 67.6& and 1965-2006 was 58.5%. The changes in the fraction in the mid-to-late 1940s and in the mid-1960s are relatively abrupt. It’s possible that meteorological changes occurred at those times, but it is an interesting coincidence that they occurred at the same time as significant changes in the observing practice.

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