Are Hurricanes Becoming More Furious Under Global Warming?

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1 Are Hurricanes Becoming More Furious Under Global Warming? Z H A N L I U N I V E R S I T Y O F U T A H A T M O S P H E R I C S C I E N C E S D E P A R T M E N T T U E S D A Y, M A R C H 1 6,

2 OUTLINE Introduction SST, Wind Shear, RH... Absolute SST or Relative SST? Physical Thinking Wind Shear Conclusions and Concerns

Shear Wind Shear Relative Humidity Maxima Potential

3 Introduction Sea Surface Temperature (SST) & Maximum Potential Intensity (MPI) Vertical Wind Shear Wind Shear Relative Humidity Maxima Potential Intensity Genesis Potential Index Vecchi and Soden, GRL, 2007

4 How are Hurricanes related to SST? Hurricane Katrina strengthened from Category 2 to Category 5 with sustained winds of 175 mph for 3 days when it entered into the warm water of the Gulf of Mexican. Image at

5 Physical Thinking Hurricane activity depends partly on atmospheric instability, which increases with local warming but is not determined by absolute SSTs alone. Warming of remote ocean warms the upper troposphere and stabilizes the atmosphere. Relative SST warming is associated with atmospheric circulation changes that make the environment more favorable to hurricane development and intensification.

Absolute SST VS Relative SST PDI based on model Absolute SST

the absolute SST and the tropical mean SST Power Dissipation

regressed onto absolute and relative SST; High-resolution

6 Absolute SST VS Relative SST PDI based on model Absolute SST Absolute SST: over the MDR Relative SST: the difference between the absolute SST and the tropical mean SST Power Dissipation Index (PDI) Observe d PDI Method: IPCC models ensemble; PDI is regressed onto absolute and relative SST; High-resolution dynamic technique PDI based on model Relative SST Vecchi et al, Science, 2008

related with the local SST change from tropical mean SST change MPI Vecchi and Soden, Nature, 2007

7 The PI Thinking about SST What is PI? SST SST increases over the whole tropical ocean The reduction of MPI over Northern Atlantic The MPI is more related with the local SST change from tropical mean SST change MPI Vecchi and Soden, Nature, 2007 Model-projected changes in SST and PI for the 21 st century a. SST change; b. PI change (shaded) and the normalized departure of local SST change from tropical-mean SST change (contoured)

8 Time Series of Change in SST and PI SST Local SSTs increases steady in response to global warming. PI PI shows large decadal variability. For Tropical Atlantic no huge change of PI due to SST increase. Vecchi and Soden, Nature, 2007

9 Frequency under the SST Anomalies Prediction: For late in the 21 st century, the ensemble model shows a reduction in the globally averaged hurricane. (Ming Zhao et al, 2009) In the past, the contrasts of the frequencies for different SSTs patterns hints that relative SST governs Atlantic hurricane activity. High Absolute SST N= 18 High Absolute SST N= 9 High Relative SST N= 23 High Relative SST N= 6 Vecchi et al, Science, 2008 Tracks of intense hurricanes (Cat. 4 &5) during the years with the highest/lowest SST anomalies

10 How about Vertical Wind Shear?

How Wind Shear Affect Hurricanes Weak vertical wind shear is required for the genesis and development of hurricanes. Strong wind shear tear the warm core away from the surface circulation.

11 How Wind Shear Affect Hurricanes Weak vertical wind shear is required for the genesis and development of hurricanes. Strong wind shear tear the warm core away from the surface circulation. Hurricane Wilma at 00GMT October 25, 2005 Wilma had just intensified to a Category 3. Just to the west of Wilma one can see wind shear values of 120 knots, associated with the jet stream. Strong wind shear dries the middle level atmospheric environment. Image at

Correlation between Hurricanes and Wind Shear Zhao et al, JC, 2009 The projections of model members show a clear negative correlation with coefficient of -0.

12 Correlation between Hurricanes and Wind Shear Zhao et al, JC, 2009 The projections of model members show a clear negative correlation with coefficient of -0.8 between annual hurricane count and ASO wind shear anomaly; The observed annual hurricane count is as well correlated with wind shear anomaly from NCEP or AMIP model members.

Wind Shear Change under Global Warming Mean wind Shear change # Models increased shear Vecchi and Soden, GRL, 2007 Wind shear change of IPCC multi-model projections a.

13 Wind Shear Change under Global Warming Mean wind Shear change # Models increased shear Vecchi and Soden, GRL, 2007 Wind shear change of IPCC multi-model projections a. Wind shear (shaded) and ensemble mean background shear (contoured); b. Number of models showing positive change in wind shear Prominent increase in wind shear over tropical Atlantic and East Pacific. These changes are robust across the multi-model suite. Expect the decreased activity of hurricane

Relationship between Walker Circulation and Wind Shear Vecchi and Soden, GRL, 2007 The warm color of the negative correlation indicates regions where a decrease

14 Relationship between Walker Circulation and Wind Shear Vecchi and Soden, GRL, 2007 The warm color of the negative correlation indicates regions where a decrease in Pacific Walker Circulation is associated with increased shear. The strongest correlation region corresponds to the Shear Enhancement Region shown in the box.

15 Conclusions Alternative interpretation of the relationship between SST and hurricane activity imply vastly different future Atlantic hurricane activity. Changes in local SST are inadequate for characterizing PI but the long-term changes in PI are closely related to the departure of the local SST from the mean tropical SST change. Increased tropical Atlantic wind shear in model projections of global warming.

16 Concerns Other complicated factors can influence the hurricane activity and there may be some relationship among the changes of these factors under the global warming. The latest study releases that the hurricanes become fewer but more intense under global warming. (Knuston et al. Nature, 2010)

17 Reference Vecchi, G. et al, 2008: Whither hurricane activity? Science, 322, , doi: /science Vecchi, G. and B. Soden, 2007a: Effect of remote sea surface temperature change on tropic cyclone potential intensity. Nature, 450, , doi: /nature06423 Vecchi, G. and B. Soden, 2007b: Increased tropical Atlantic wind shear in model projections of global warming. Geophys. Res. Lett., 34, L08702, doi: /2009gl Zhao, M. et al, 2009: Simulations of global hurricane climatology, interannual variability, and response to global warming using a 60-km resolution GCM. J. Climate, 22, , doi: /2009jcli3049.1

Possible Effects of Global Warming on Tropical Cyclone Activity

Possible Effects of Global Warming on Tropical Cyclone Activity Johnny Chan Guy Carpenter Asia-Pacific Climate Impact Centre School of Energy and Environment City University of Hong Kong Outline Background