Midweek Wonk: What Does the Science Say about Cyclone Intensity?
November 20, 2013
Journal of Climate
Research on trends in western North Pacific tropical cyclone (TC) activity is limited by problems associated with different wind speed conversions used by the various meteorological agencies. This paper uses a quantile method to effectively overcome this conversion problem. Following the assumption that the intensity ranks of TCs are the same among agencies, quantiles at the same probability level in different data sources are regarded as having the same wind speed level. Tropical cyclone data from the Joint Typhoon Warning Center (JTWC) and Japan Meteorological Agency (JMA) are chosen for research and comparison. Trends are diagnosed for the upper 45% of the strongest TCs annually. The 27-yr period beginning with 1984, when the JMA began using the Dvorak (1982)technique, is determined to be the most reliable for achieving consensus among the two agencies regarding these trends. The start year is a compromise between including as many years in the data as possible, but not so many that the period includes observations that result in inconsistent trend estimates. The consensus of TC trends between the two agencies over the period is interpreted as fewer but stronger events since 1984, even with the lower power dissipation index (PDI) in the western North Pacific in recent years.
Geophysical Research Letters:
 The Main Development Region (MDR) for tropical cyclones (TCs) in the western North Pacific Ocean is the most active TC region in the world. Based on synergetic analyses of satellite altimetry and gravity observations, we found that the subsurface ocean conditions in the western North Pacific MDR has become even more favorable for the intensification of typhoons and supertyphoons. Compared to the early 1990s, a 10% increase in both the depth of the 26°C isotherm (D26) and Tropical Cyclone Heat Potential (TCHP) has occurred in the MDR. In addition, the areas of high TCHP (≥ 110 kJ cm−2) and large D26 (≥ 110 m) have 13% and 17% increases, respectively. Because these high TCHP and large D26 regions are often associated with intensification of the most intense TCs (i.e. supertyphoons), this recent warming requires close attention and monitoring.
Climate scientists are confident in three ways that climate change will make the impacts of hurricanes worse. First, global warming causes sea level rise, which amplifies storm surges and flooding associated with hurricanes. As a paper published in the Proceedings of the National Academy of Sciences by Aslak Grinsted and colleagues concluded,
“we have probably crossed the threshold where Katrina magnitude hurricane surges are more likely caused by global warming than not.”
Second, as climate scientist Kevin Trenberth has noted, global warming has also increased the amount of moisture in the air, causing more rainfall and amplifying flooding during hurricanes.
Third, warmer oceans are fuel for hurricanes. Research has shown thatthe strongest hurricanes have grown stronger in most ocean basins around the world over the past several decades, and climate models consistently project that this trend will continue. Chris Mooney recently documented the past decade’s worth of monster hurricanes around the world, and Jeff Masters estimates that 6 of the 13 strongest tropical cyclones on record at landfall have happened since 1998
Chapter 2.6.3 of the 2013 IPCC report notes,
“Time series of cyclone indices such as power dissipation … show upward trends in the North Atlantic and weaker upward trends in the western North Pacific since the late 1970s”
The hurricane intensity trend in the western North Pacific (where the Philippines are located) isn’t crystal clear. One recent paper finds that over the past few decades their intensity has slightly fallen (but has grown for the planet as a whole), while others suggest it’s slightly risen, or that there are fewer but stronger hurricanes in that western North Pacific region.
However, a paper published earlier this year in the Proceedings of the National Academy of Sciences by MIT hurricane expert Kerry Emanuel found that future “Increases in tropical cyclone activity are most prominent in the western North Pacific.” So while we’re not certain about the trend over the past few decades, the evidence indicates that hurricanes near the Philippines will both become stronger and form more frequently in a warmer world.