Model projections of tropical cyclone (TC) activity response to anthropogenic warming in climate models are assessed. Observations, theory, and models, with increasing robustness, indicate rising global TC risk for some metrics that are projected to impact multiple regions. A 2°C anthropogenic global warming is projected to impact TC activity as follows. 1) The most confident TC-related projection is that sea level rise accompanying the warming will lead to higher storm inundation levels, assuming all other factors are unchanged. 2) For TC precipitation rates, there is at least medium-to-high confidence in an increase globally, with a median projected increase of 14%, or close to the rate of tropical water vapor increase with warming, at constant relative humidity. 3) For TC intensity, 10 of 11 authors had at least medium-to-high confidence that the global average will increase. The median projected increase in lifetime maximum surface wind speeds is about 5% (range: 1%–10%) in available higher-resolution studies. 4) For the global proportion (as opposed to frequency) of TCs that reach very intense (category 4–5) levels, there is at least medium-to-high confidence in an increase, with a median projected change of +13%. Author opinion was more mixed and confidence levels lower for the following projections: 5) a further poleward expansion of the latitude of maximum TC intensity in the western North Pacific; 6) a decrease of global TC frequency, as projected in most studies; 7) an increase in global very intense TC frequency (category 4–5), seen most prominently in higher-resolution models; and 8) a slowdown in TC translation speed.

An assessment was made of whether detectable changes in tropical cyclone (TC) activity are identifiable in observations and whether any changes can be attributed to anthropogenic climate change. Overall, historical data suggest detectable TC activity changes in some regions associated with TC track changes, while data quality and quantity issues create greater challenges for analyses based on TC intensity and frequency. A number of specific published conclusions (case studies) about possible detectable anthropogenic influence on TCs were assessed using the conventional approach of preferentially avoiding type I errors (i.e., overstating anthropogenic influence or detection). We conclude there is at least low to medium confidence that the observed poleward migration of the latitude of maximum intensity in the western North Pacific is detectable, or highly unusual compared to expected natural variability. Opinion on the author team was divided on whether any observed TC changes demonstrate discernible anthropogenic influence, or whether any other observed changes represent detectable changes. The issue was then reframed by assessing evidence for detectable anthropogenic influence while seeking to reduce the chance of type II errors (i.e., missing or understating anthropogenic influence or detection). For this purpose, we used a much weaker “balance of evidence” criterion for assessment. This leads to a number of more speculative TC detection and/or attribution statements, which we recognize have substantial potential for being false alarms (i.e., overstating anthropogenic influence or detection) but which may be useful for risk assessment. Several examples of these alternative statements, derived using this approach, are presented in the report.

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This paper presents results of a comprehensive study of the relatil)nship between the movement of tropical cyclones and the large-scale chculation which surrounds them. Cyclones have been stratified by di:rection and speed of movement, latitude, intensity change, and size (u determined by the radius of the outermost closed surface isobar) in th:ree ocean basins: the northwest Pacific, the west Atlantic and the Au:stralian-south Pacific region. Twenty-one different stratifications arl~ available in the northwest Pacific. 13 in the west Atlantic, and 6 in the Australian-south Pacific area. Cyclone movement and surrounding fl,:)w relationships were studied at different pressure levels and a va:riety of radii. Pressure-weighted layer-averages were also analyzed in search for such relationships.Results show an important relationship between surrounding largescale flow and tropical cyclone movement. For all stratifications, the wi:nds in the mid-troposphere (500-700 mb) at 5-7 0 latitude radius from the cyclone center have the best correlation with cyclone movement. Tropical cyclones in the Northern Hemisphere move about 10-20 0 to the left of their surrounding mid-tropospheric flow at 5-7 0 latitude radius and those in the Southern Hemisphere move about 10 0 to the right~l It is also found that cyclones, in general, move faster by about 1 m s than this flow. These general relationships appear to be modified by the vertical shear of the environmental wind, the zonal component of the cyclone velocity, and other characteristics of the cyclone. The mean tropospheric flow (surface to 100 mb) at 5-7 0 latitude radius also correlates well with cyclone movement in most cases. For cyclones embedded in an environment with relatively small vertical wind shear, the mid-tropospheric flow is as good a descriptor of cyclone motion as the mean tropospheric flow. The average wind between the upper (200 mb) and lower (900 mb) troposphere also appears to correlate reasonably well with cyclone movement.