Tropical cyclones weaken after they reach land. But it emerges that for the North Atlantic basin, storms are weakening more slowly as regional sea surface temperatures increase.
Tropical cyclones can cause substantial damage and death when they reach land, as a result of wind, storm surges and rainfall. It is known that tropical-cyclone intensity (measured by maximum wind speed) typically decreases rapidly after the storm reaches land1. However, existing models do not take into account whether and how this rate of storm decay after landfall depends on climate1,2. Writing in Nature, Li and Chakraborty3 report that the rate at which tropical cyclones from the North Atlantic decay after landfall has changed since the 1960s — their intensity has been decreasing more slowly over time. This shift is mainly due to warming sea surface temperatures. The authors’ work adds weight to growing concerns4 that tropical cyclones might become more damaging in the future.
Li and Chakraborty analysed historical intensity data for storms that made landfall over North America between 1967 and 2018. They used the decrease in storm intensity over the 24 hours after landfall to define a timescale of decay for each storm. They then examined trends in this timescale.
The authors found a significant long-term shift towards slower decay (so storms maintain a higher intensity on land for longer). Furthermore, this trend aligned with long-term increases in regional mean sea surface temperature over the Gulf of Mexico and the western Caribbean, which are adjacent to land and supply the moisture for the storms before landfall. The changing timescales of decay also correlate well with year-to-year variations in mean sea surface temperature (Fig. 1).
Li and Chakraborty next asked whether other factors could also contribute to the change in the timescale of decay. They found that a portion of the long-term trend could be attributed to an eastward shift in landfall location. By contrast, other factors — including the speed of storm movement at landfall and intensity at the time of landfall — were not important.
The authors bolstered their empirical findings by performing hurricane-landfall experiments using a simple, state-of-the-art atmospheric model. For a range of temperatures, they allowed a mature tropical cyclone to form over a water surface that had a fixed temperature. When each storm reached a fixed maximum wind speed, they mimicked landfall by instantaneously changing the surface beneath the storm from wet to dry. Under this model, the timescale of decay again increases with temperature.
The researchers then sought a physical explanation for why warming causes slower decay. The primary energy source for a tropical cyclone is the evaporation of water from the surface beneath the eyewall5 (the band of cloud that surrounds the eye of the storm), which is rapidly cut off at landfall. But residual moisture in the storm provides a smaller, temporary, secondary source of energy6. The levels of this residual moisture are expected to increase with temperature on the basis of the laws of thermodynamics for moist air.
The authors tested the hypothesis that increased levels of residual moisture could cause slower decay using a second set of modelling experiments in which, in addition to drying the surface to mimic landfall, they removed all residual moisture in the atmosphere. These storms all showed identical timescales of decay, despite their different temperatures. Thus, it is the increased residual store of atmospheric moisture at warmer temperatures that slows the weakening of the storm.
A key outstanding question is the exact degree to which the decay rate depends on temperature. Although the empirical and modelling results are in qualitative agreement, temperature had a smaller effect on decay rate in the simulations than was estimated empirically. This difference might be due to the small size of the historical data set or to confounding factors in it. For example, there have been changes in the spatial distribution of landfall locations over time, and hence differences in the surface properties felt by the storms on land, such as surface moisture and roughness.
In addition, it is unclear whether the long-term trends seen in the historical data set might be affected by ongoing changes in the technologies with which researchers observe storms or in methods for estimating maximum storm wind speed over land. Information about these uncertainties is not readily available publicly, but an in-depth investigation of estimation practices would be worthwhile.
Analysis of historical data along coastal regions in other parts of the world, along with simulations over a broader range of temperatures and climates, could help to further test the robustness of the authors’ findings for predicting future changes in decay rates. The effects of residual storm moisture also warrant further investigation to clarify how this effect can slow decay after landfall.
Li and Chakraborty’s work highlights a key component of risk models that has been largely overlooked so far. Slower storm decay after landfall in the future would directly result in increases in total damage, and this would be exacerbated by increases in peak wind speed and total rainfall, both of which are expected to occur in a warming climate7. The extent of damage occurring inland depends on both the rate of storm decay and the speed of storm motion at landfall. Hence, a slower decay could also lead to increases in damage farther inland, although changes in the speed of motion remain a point of contention8,9. Longer-lived storms might also increase the chances of interaction with the jet stream, which can sometimes produce hazardous weather that can extend much farther inland10.
More generally, the current results indicate the need to broaden our thinking about how climate change affects tropical cyclones after landfall. We must take into account residual atmospheric effects from the adjacent ocean, landfall location and effects induced by the land surface itself6. Integrating this understanding into hurricane models should help to improve our predictions of the future risks posed by individual storms and over the long term.
Nature 587, 200-201 (2020)
Fuente/ Source: www.nature.com
Foto/ Photo: sgerendask
PAGINA 100 POPAYAN COLOMBIA