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Climate Change

Cyclones move poleward as tropics expand

 Article

Kossin J. P., K. A. Emanuel and G. A. Vecchi, 2014: The poleward migration of the location of tropical cyclone maximum intensity. Nature. 509, 349-352. doi:10.1038/nature13278

Background

Perhaps you can recall when Hurricane Katrina surged through New Orleans in 2005, or when Hurricane Sandy flooded the streets of New Jersey and New York City in 2012, or even more recently, when Typhoon Haiyan leveled Tacloban City in the Philippines in November of last year. These storms are just a few examples of tropical cyclones that have destroyed coastal homes and have accounted for over 1.33 million deaths since the 20th century. This study seeks to answer the question of whether we can expect more intense tropical cyclones at higher latitudes in the future by tracking the distance from the equator where cyclones reach maximum intensity.

Figure 1. Types of tropical cyclones and their typical paths. Image by NASA.

Tropical cyclones are powerful storms that are almost exclusively oceanic, drawing tremendous amounts of energy from warm seawater and dissipating rapidly over land. They are characteristic of low-pressure centers with gusty winds spiraling counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Figure 2. The anatomy of a tropical cyclone. Red arrows show warm, moist air rising off the ocean. Blue arrows show cool, dry air sinking in towards the center. This cyclone has a counterclockwise rotation and is a Northern Hemisphere cyclone. Image from NASA.

As tropical cyclones develop over warm water, they eventually reach their lifetime maximum intensity (LMI) before dissipating. During formation, the intensification of a storm is influenced by gradients in vertical wind shear and levels of potential intensity. Vertical wind shear is the change in wind speed or direction over a relatively short distance, and potential intensity is the sea surface temperature dependent limit on the strength of a tropical cyclone. Greater vertical wind shear and decreased maximum potential intensity prevent the development of tropical cyclones. Both of these environmental factors control when and where the LMI is reached and how powerful a tropical cyclone is likely to become. This study shows that the LMI of tropical cyclones is moving further away from the equator at a rate of 1º of latitude per decade. This poleward trend may be caused by the overall expansion of the tropics.

Methods

One challenge of climatological studies is the inconsistency and uncertainty of historical data in both time and space. Kossin et al. limit their analysis to the 31-year period between 1982 and 2012, when geostationary weather satellites provided precise measurements on tropical cyclones. The authors find the location of each storm where the lifetime maximum intensity is reached. They use best-track data and the globally homogenized record (ADT-HURSAT) of cyclone intensity. When taking into account both Northern and Southern Hemispheres, both datasets were consistent in showing tropical cyclone shifting away from the equator. This article explores other environmental changes from reanalysis products to determine trends in vertical wind shear and maximum potential intensity.

Conclusion

This study finds overwhelming support that the LMI of globally averaged tropical cyclones is shifting towards the poles. This poleward migration is more pronounced in the North Pacific, South Pacific and South Indian Oceans, and there is no trend evident in the Atlantic and North Indian Oceans. One possible explanation for the lack of a trend in the Indian Ocean is due to the confines of land boundaries to the north. The best track dataset shows a global 115 km per decade trend of LMI away from the equator.

Figure 3. Global migration of LMI away from tropics derived from the best-track (red) and ADT-HURSAT (blue) data. Shaded regions show the two-sided 95% confidence intervals.

The authors show that decreasing wind shear in the subtropics and increasing wind shear in the deep tropics agree with their findings of tropical cyclones migrating closer towards the poles. Greater wind shear ultimately induces cyclolysis (the degeneration of a tropical cyclone). The authors also find that the potential intensity of cyclones increases towards the poles, again favoring the development of stronger cyclones away from the deep tropics.

Figure 4. This figure shows the distance north and south of the equator as a function of percent change in vertical wind shear (a, c, e) and potential intensity (b, d, f) from 1980-1994 to 1995-2010. Observations are consistent across three different reanalysis data products: MERRA (a, b), ERA-Interim (c, d), and NCEP/NCAR (e, f).

The poleward migration of tropical cyclone LMI may also be related to the overall expansion of the tropics. The expansion of the tropics is measured by the north-to-south extent of the tropical Hadley Cell. The Hadley Cell is a pattern of atmospheric circulation where low-lying warm air converges near the equator and rises to create a region of low pressure. This pattern helps to create storms throughout the tropics. Anthropogenic emissions of aerosols and the depletion of the ozone layer have contributed to the widening of the Hadley Cell in both hemispheres starting in the 1990’s. This has fueled the observed changes in vertical wind shear and potential intensity, which have now been shown to cause cyclones to intensify towards the poles. The movement of cyclones towards higher latitudes increases the risk of damaging storms further north and south of the equator.

Figure 5. Hadley Cell circulation in the tropics. Image from www.climatetheory.net.

Significance

If the observed trends in vertical wind shear and potential intensity continue, it is very likely that the poleward migration of tropical cyclones will continue. The consequences of these trends are significant and could lead to new storm tracks over coastal cities at higher latitudes. Although the poleward migration of cyclones is not consistent in all ocean basins, the trend is present in both Northern and Southern Hemispheres. Coastal cities like New York, Tokyo and Sydney, among others, may see larger and stronger cyclones as a result of decreased wind shear, increased potential intensity and the overall expansion of the Hadley Cells. This could increase the risk of flooding, property damage and fatalities in unprepared cities. Meanwhile the deep tropics may see a decrease in precipitation, as this region will become less favorable to water-bearing tropical cyclones.

 

Hillary Scannell
Hillary received her MS in oceanography from the University of Maine in 2014 and works in the Ecosystem Modeling Lab at the Gulf of Maine Research Institute in Portland, ME.

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