The Recent Increase in North Atlantic Hurricane Activity

1. The Recent Increase in North Atlantic Hurricane Activity Muthuvel Chelliah Gerald Bell Climate Prediction Center National Centers for Environmental Prediction, National Weather Service/NOAA Washington DC.

2. And …. the question is: • Does the recent increase (which started in 1995) in North Atlantic Hurricane activity represents a cyclic behavior ? OR ….. ? • Can we really answer this question with the duration/quality of data that we have at our disposal? • OR, at least can we say whether the recent increase is associated with atmospheric/oceanic conditions similar/dissimilar to those of the 1950’s and 1960’s ?

3. Our analysis is based on the NCEP Atmospheric Reanalysis data availability period 1949 + .. • For this period we have global atmospheric circulation data, so we can examine atmospheric patterns/changes associated with extended periods of enhanced/suppressed hurricane activity. - And the totally independent Hurricane data (HURDAT). Does each data set has its own problems ? May be! Does each data set has its own merits ? YES ! Let’s look at the bigger picture !

5. Conditions Related to Active Atlantic Hurricane Season

6. NOAA Atlantic Hurricane Outlook August Outlook and Verification:1998 – 2004(To see how our NOAA Forecast did for 2005: See our poster; Bell & Chelliah P2.1) 16 15 15 14 14 10 9 8 8 8 12 12 7 4 5 245 5 240 202 200 4 3 3 3 132 121 2 75 Observed Forecast Range

7. Some of what will be presented here is based on discussions in: • Chelliah and Bell (2004): Tropical Multi-decadal and Interannual Climate variability in the NCEP-NCAR reanalysis, J.Climate, 17,1777-1803. • Bell and Chelliah (2005/6): Leading Tropical Modes associated with Interannual and Multi-decadal fluctuations in North Atlantic Hurricane activity, In PRESS. J.Climate.

8. Composite: 200, 850 strm, wind Anti-cyclonic anomalies Stronger Tropical Easterly Jet Cyclonic anomalies Reduced easterly trades Inter-hemispheric symmetry of 200-hPa streamfunction anomalies, Reversal of equatorial zonal wind anomalies. Reflect global-scale patterns linked to anomalous tropical convection.

9. Leading Climate Modes Associated with Atlantic Seasonal Hurricane Extremes Variations in tropical convection represent dominant forcing of coherent circulation anomalies associated with seasonal and multi-decadal extremes in Atlantic hurricane activity. • Modes based on EOF analyses of ASO mean Low Pass (LP) and High Pass (HP) filtered 200 mb Velocity Potential anomalies from 1949-2004. • TMM 1 and TMM 2 are the leading modes from LF analysis. • ENSO is the leading mode from HF analysis.

10. Tropical Multi-decadal Modes (TMM 1 and 2) and ENSO Mode: TMM 1: A global tropical mode linked to multi-decadal fluctuations in tropical convection between West African monsoon region, Amazon Basin, and central tropical Pacific. --- Linked to leading multi-decadal EOF of tropical surface temperature (land + ocean) anomalies --- Dominates seasonal variability over eastern half of MDR and Africa --- Sign change of mode coincident with demise of Sahel rainfall and onset of drought conditions TMM 2: A global mode, but with regional emphasis – particularly impacting the N.Atlantic --- Reflecting the increase in Surface T and SST and the associated regional circulation changes. ENSO:Dominates vertical shear anomalies over western MDR Its impacts can be substantially influenced by multi-decadal signal

11. ACTIVE ACTIVE TMM 1 TMM 2 ENSO INACTIVE

12. JJA: TMM 1 TMM 2 TMM 1 ENSO

22. TOTAL zonal component of vertical wind shear (u200-u850) during the three different periods of N.Atlantic hurricane activity as measured by ACE (Figs.2,3) 1950-1971 – Active 1972-1994 – InActive 1995-Current --Active

23. ANOMALOUS (from the 1949-2004 MEAN) Zonal component of vertical wind shear for the three active/inacitve periods of North Atlantic hurricane activity. 1950-1971 – Active 1972-1994 – InActive 1995-2004 --Active Compare shear during these periods.

24. ACTIVE North Atlantic Hurricane Activity seasons Until ~ 1971 (1969 ?, definitely till late 1960’s) Relatively INACTIVE North Atlantic Hurricane seasons Until 1994 1995: ACTIVE North Atlantic Hurricane seasons begin

32. SUMMARY • The Tropical Multi-decadal Modes TMM1, TMM2 and the Interannual ENSO mode – used in issuing NOAA’s official long lead Hurricane outlooks - can together explain the the temporal behavior of N. Atlantic hurricane activity over the last five decades. • The current active North Atlantic Hurricane era is associated with ‘somewhat different’ atmospheric/SST conditions than those of the 50-60’s. • Slightly more warmer oceanic temperatures and less weaker vertical wind shear accompany the current active period which began in 1995 and is expected to continue for at least a couple of more decades (The interannual behavior will be modulated by ENSO). • The combination of cyclic behavior as well as the recent warming in the North Atlantic (part of the global signal) appears to have contributed to the ongoing active North Atlantic Hurricane activity.

33. Summary –Contd. • The same mechanisms have impacted and contributed to the inactive phase of the East tropical Pacific Hurricane activity (not shown) – NOAA issued, after 2 years of issuing Experimental Hurricane Outlooks, the first Official Outlook this year. • The very nature of ENSO and its impact on the N.Atlantic hurricane activity and other associated global circulations has changed as the background signal (TMM1 and TMM2) changes phase (not shown).