Topic 4.2: Possible relationships between climate change and tropical cyclone activity

Projections of Future Atlantic Hurricane Activity

Hurricane Katrina, Aug. 2005GFDL model simulation of Atlantic hurricane activityTom Knutson

NOAA / Geophysical Fluid Dynamics LabPrinceton, New Jersey

http://www.gfdl.noaa.gov/~tk1112

Climate Change Attributionare observed changes consistent with expected responses to forcings inconsistent with alternative explanations

ObservationsAll forcingSolar+volcanicSource: IPCC 4th Assessment Report. Used with permission.3Attribution of climate change to causes involves READ

Climate models are important tools for attributing and understanding climate change. Understanding observed changes is based on our best understanding of climate physics, as contained in simple to complex climate models. For the 4rth assessment report, we had a new and very comprehensive archive of 20th century simulations available. This has greatly helped.This figure gives an example.You see observed global and annual mean temperature in black over the 20th century compared to that simulated by a wide range of these models. On the top, in red, are individual model simulations and their overall mean shown fat, that are driven by external influences including increases in greenhouse gases, in aerosols, in changes in solar radiation and by volcanic eruptions. The observations rarely leave the range of model simulations. The trends and individual events like cooling in response to volcanic eruptions (POINT) are well reproduced. The fuzzy range gives an idea of uncertainty with variability in the climate system.IPCC Projections of Future Changes in Climate

IPCC best estimate for low scenario (B1) is 1.8C (likely range is 1.1C to 2.9C), and for high scenario (A1FI) is 4.0C (likely range is 2.4C to 6.4C). Broadly consistent with span quoted for SRES in TAR, but not directly comparable Source: IPCC 4th Assessment Report. Used with permission.4

There is some evidence that overall Atlantic hurricane activity may have increased since in the 1950s and 60s in association with increasing sea surface temperaturesSource: Kerry Emanuel, J. Climate (2007).PDI is proportional to the time integral of the cube of the surface wind speeds accumulated across all storms over their entire life cycles.5Increasing data uncertainty556Source: Vecchi et al. Science (2008)Projection 1: Absolute SST

~300% projected increase in Power Dissipation

Indirect attribution: CO2 SST HurricanesProjection 2: Relative SST

Projected change: sign uncertain, +/- 80%

No Attribution

Supported by dynamical models

.Two future projections of Atlantic tropical cyclone power dissipation

6Example of a regional model that reproduces the interannual variability and trend of Atlantic hurricane counts (1980-2006) 18-km grid model nudged toward large-scale (wave 0-2) NCEP Reanalyses7

Source: Knutson et al., 2007, Bull. Amer. Meteor. Soc.

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Late 21st century projected Atlantic region climate changes: 18-Model CMIP3 ensembleHigher shearHigher potential intensity

The 26.5oC threshhold temperature for Atlantic tropical storm formation: a climate dependent threshhold, which may increase to ~28.5oC by the late 21st century with climate warming.Source: Knutson et al., 2008, Nature Geoscience.9Note the decrease in overall number of storms in the warmer climate

The regional model simulates increased hurricane rainfall rates in the warmer climate (late 21st century, A1B scenario) consistent with previous studies Present Climate Warm Climate Warm Climate Present ClimateRainfall Rates (mm/day)Avg. Rainfall Rate Increases: 50 km radius: +37%100 km radius: +23%150 km radius: +17%400 km radius: +10%Average Warming: 1.72oCSource: Knutson et al., 2008, Nature Geoscience.10Example of a double-downscaling method used to explore frequencies and intensities of Atlantic hurricanes at high resolution Geophysical Fluid Dynamics Laboratory/NOAA

Late 21st Century Climate Warming Projection-- Average of 18 CMIP3 Models

(27 Simulated Hurricane Seasons) Source: Bender et al., Science, 2010

Conclusions: What can climate science tell us about late 21st century climate and Atlantic hurricanes?

Climate science provides a compelling case that human emissions of greenhouse gases have caused much of the long-term global warming over the past 50 yr and 140 yr (IPCC AR4). Climate models project substantial further global warming over the 21st century, including in the tropical Atlantic.Sea level rise has already begun and further sea level rise is expected to exacerbate storm surge impacts, even if the storms themselves do not change.There is no consensus among current models on future changes in Atlantic tropical storm frequency (+/- 60%). Globally there is a general consensus on a decrease (or little change) in global frequency of tropical storms.Most high-resolution models agree that hurricane intensities will increase (range +2 to 11% by 2100) and that hurricanes will have higher rainfall rates (~+20%). In one study, the number of very intense category 4 and 5 Atlantic hurricanes doubles by 2100, though not for all of the climate models used in the study. However, we cannot yet conclude that humans have already caused a detectable change in Atlantic hurricane activity. Note that humans may have already caused changes that are either below the 'detection threshold' or are not yet confidently modeled (e.g., aerosol effects).

13Decreasing Confidence 13