April 14, 2008

The Lack of Recent Hurricane Activity?

Filed under: Climate Extremes, Hurricanes

This hurricane issue never goes away and new websites appear every day warning us of more hurricanes in the immediate future. We cover this issue over and over, and no fewer than three more articles on hurricane activity have appeared in the scientific literature recently of interest to us at World Climate Report:

1. A team of scientists from Florida State University of Louisiana State University examined sediment deposits from Lake Shelby in Alabama. The lake provides an opportunity to engage in a little paleotempestology, which if you don’t know, is the study of storms from geological evidence. Elsner et al. note “Coastal wetlands and lakes are episodically subjected to overwash processes during catastrophic hurricane strikes when barrier sand dunes are overtopped by storm surge. The frequency of overwash sand layers in lake and wetland cores provides an estimate of their return period.” Over the past 3,240 years, it seems that Lake Shelby has experienced 11 hurricanes sufficient to produce significant sediment layers in the lake. The date, in years before present (and the convention is to make 1950 AD “present”), are 770, 1,360, 2,190, 2,240, 2,450, 2,650, 2,960, 3,000, 3,110, 3,160, and 3,240. In other words, over the past three plus millennium, nine of the 11 large hurricanes occurred more that two thousand years ago, one occurred around 1,500 years ago, and one occurred over 700 years ago. None has occurred in the past 700 years – for whatever reason, Lake Shelby’s hurricane activity has been remarkably low for the past 1,000 years!

2. Our second feature paper was published in the prestigious Proceedings of the Royal Society by Johnny Chan of the University of Hong Kong. In discussing whether tropical cyclones (TCs) are becoming more frequent in recent years, Chan notes “Because all such studies generally focused on the data after 1970, others have pointed out that if the time series of intense TC occurrence is extended backwards to earlier years, such an upward trend in intense TC occurrence frequency is actually part of a multi-decadal oscillation in the frequency of TC occurrence.” To test his idea of oscillations rather than trends, Chan collected data on intense (category 4 and 5, aka NCat45) storms in the western North Pacific from 1960 – 2005. The solid line on Figure 1 shows the number of category 4 and 5 storms in the study area over that time period, and Chan shows that the time series contains a distinctive oscillation, but no trend. He concludes “A major conclusion from the results of this study is that the frequency of intense typhoon occurrence undergoes a strong multi-decadal (16–32 years) variation due to similar variations in the planetary scale oceanographic and atmospheric conditions that govern the formation, intensification and movement of TCs.” Furthermore, we learn “Based on the reconstructed 16–32 year series of NCat45, the period 1960–2005 is divided into three sub-periods according to whether the reconstructed values on this time scale are positive or negative, i.e. above or below normal: 1960–1970 (period A1, above normal), 1971–1986 (period B, below normal) and 1987–1997 (period A2, above normal). As the values from 1998 to 2000 are nearly zero, and the period from 2001 to 2005 is too short, these years are ignored in the subsequent analyses.” With respect to global warming and intense activity in the Western North Pacific (WNP), Chan firmly states “Thus, at least for the WNP, it is not possible to conclude that the variations in intense typhoon activity are attributable to the effect of global warming.”


Figure 1. The solid line shows the number of category 4 and 5 events in the Western North Pacific over the period 1960-2005 (from Chan, 2007).

3. Our third major article appears in the Bulletin of the American Meteorological Society by Kerry Emanuel and two of his associates at MIT; this is of course the same Kerry Emanuel who is a leader in forecasting more intense hurricane events in our future. The team describes the difficulty in predicting hurricanes from global climate model experiments in which the grid cells are too coarse to depict actual synoptic-scale events such as tropical cyclones. They basically take thermodynamic and kinematic statistics and relate them to hurricane activity in various regions. They then use the same thermodynamic and kinematic statistics from climate models and “predict” hurricane genesis, tracks, and intensity. They find that the frequency of hurricanes decreases globally into the future, and they note that “recent global model-based studies all show decreasing frequency of tropical cyclones globally, although some studies show regional increases.” However, they note that “the sign and magnitude of the changes vary a great deal from basin to basin and from model to model, reflecting large regional differences in the global model predictions as well as natural multidecadal variability in each model that cannot be averaged out over the 20-yr periods considered here.”

In other words, no one is all too sure what will happen to the frequency of hurricanes in the future, but based on these three recent articles, we see little support for any claim that hurricanes will become more frequent in the decades to come.

References:

Chan, J.C.L. 2007. Decadal variations of intense typhoon occurrence in the western North Pacific. Proceedings of the Royal Society, A., 464, 249-272.

Elsner, J.B., T.H. Jagger, and K.-B. Liu. 2008. Comparison of hurricane return levels using historical and geological records. Journal of Applied Meteorology and Climatology, 47, 368-374.

Emanuel, K, R. Sundararajan, and J. Williams. 2008. Hurricanes and global warming: Results from downscaling IPCC AR4 simulations. Bulletin of the American Meteorological Society, 89, 347-367.




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