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The Active 2008 Atlantic Hurricane Season Links to Known Climate Factors

This report investigates the key features of the active 2008 Atlantic hurricane season and explores the climate factors that influenced its intensity and duration. It examines anomalies in sea surface temperatures and vertical wind shear, as well as the impact of tropical multi-decadal signals and the La Niña phenomenon. The report highlights the importance of predicting convection patterns, in addition to sea surface temperatures, for accurate seasonal hurricane outlooks.

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The Active 2008 Atlantic Hurricane Season Links to Known Climate Factors

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  1. The Active 2008 Atlantic Hurricane Season Links to Known Climate Factors Gerry Bell NOAA Lead Seasonal Hurricane Forecaster Climate Prediction Center

  2. Goals • 1. Describe key features of the active 2008 Atlantic hurricane season. • We see a strong climate influence on these features • 2. Interpret observed anomalies within context of large-scale tropical climate signals • Related Publications: • Bell and Chelliah (JCLI 2006, 15 Feb), Chelliah and Bell (JCLI, 2004) • State of the Climate (BAMS May/June issues 1999-2007)

  3. www.cpc.ncep.noaa.gov/products/hurricane

  4. 2008 Atlantic Named Storms MDR Courtesy: Unisys Weather Total Named Storms (NS): 15 Hurricanes (H): 7 Major Hurricanes (MH): 4 U.S. Landfalls 6 U.S.: (3 TS, 3H) 5 Gulf Coast: (3 TS, 2H) 1 East Coast: (1H)

  5. Historical Atlantic Seasonal Activity • Wind energy index measures seasonal activity (intensity and duration) • 2008 ACE well into above-normal range (pink shading). • 90% of ACE in 2008 from storms first named in MDR. • Active Atlantic hurricane era continues.

  6. Conditions Associated With Active 2008 Atlantic Hurricane Season

  7. July - September 2008: SSTA (C) MDR Jul-Sep. 2008 SST anomaly in MDR is 0.52oC. Warmer waters consistent with active era.

  8. 1 Jul – 13 Oct 2008 200-850 hPa Vertical Wind Shear Total Anomaly • Vertical wind shear (Red) is reduced across MDR. • 3-celled pattern results from easterly shear anomalies • Consistent with other active hurricane seasons.

  9. Conditions During 2008 Atlantic Hurricane Season • Inter-related set of conditions during 2008 is typical of other above-normal seasons. • Links to two leading tropical modes of convective variability: • Multi-decadal signal • Lingering La Niña signal

  10. Tropical Multi-Decadal SignalPhase shown for Active Atlantic Hurricane Era Warmer Warmer Wetter Drier • Leading LF modes of tropical convective variability during JJA and ASO • Based on EOF analysis applied to 5-yr running means of seasonal 200-hPa velocity potential anomalies (From Bell and Chelliah, JCLI, 2006). • Links monsoon variability to Atlantic Multi-Decadal Oscillation (AMO). • Both independently linked to multi-decadal fluctuations in Atlantic hurricane activity.

  11. Tropical Multi-Decadal Signal and ACE Observedvs. Regressed Explained variance = 82%. Current active Atlantic hurricane era associated with phase change in tropical multi-decadal signal.

  12. 200-hPa Anomaly Velocity Potential and Divergent Wind Vector 1 Jul – 13 Oct 2008 Jul-Oct: 1995-2007 minus 1971-1994 Regressed Anomalies associated with tropical multi-decadal signal 2008 conditions consistent with ongoing active era and tropical multi-decadal signal.

  13. ASO 5-year LP-Filtered Variance (%) Explained by Tropical Multi-Decadal Signal Tropical multi-decadal signal physically links key regional circulation anomalies to monsoon variability and multi-decadal fluctuations in Atlantic hurricane activity.

  14. 200-hPa Streamfunction Anomaly 1 Jul – 13 Oct 2008 Enhanced subtropical ridges in both hemispheres. 200-hPa easterly anomalies (Arrows) Regressed: Tropical Multi-decadal Signal 1995-2003 minus 1971-1994 Jul-Oct: 1995-2007 minus 1971-1994 200-hPa circulation anomalies during 2008 are consistent with active hurricane era and tropical multi-decadal signal.

  15. 1 Jul – 13 Oct 2008 200-850 hPa Vertical Wind Shear Anomaly • Pattern of anomalous vertical shear during 2008 is consistent with tropical multi-decadal signal + weak La Niña forcing.

  16. La Niña Signal Pentad OLR Anomalies: 5N-5S Suppressed Convection Enhanced Convection La Niña + active hurricane era greatly increases probability of above-normal season.

  17. Summary • Inter-related set of conditions during 2008 season reflects leading tropical convective modes. Tropical multi-decadal signal, lingering La Niña. • The tropical multi-decadal signal • Accounts for inter-related set of atmospheric anomalies known to favor active/ inactive hurricane eras. • Phase change coincides with active Atlantic era (Bell and Chelliah 2006) • Relation between warmer Atlantic SSTs and active hurricane era strongly rooted in common association with the overlying atmospheric circulation and anomalous tropical convection (Bell and Chelliah 2006). • Predicting La Niña convection (not just SSTA) is key to seasonal hurricane outlooks

  18. Above Normal 2008 Atlantic Hurricane Season NOAA Outlooks BelowNormal5% BelowNormal10% Near Normal10% Near Normal25% Above Normal65% Above Normal85% Issued 22 May Issued 7 August 67% Probability for Each Range May Outlook August Outlook Observed ACE (% Median): 100%-210% 140%-230% 154% Named Storms: 12-16 14-18 15 Hurricanes: 6-9 7-10 7 Major Hurricanes: 2-5 3-6 4

  19. 200-hPa Velocity Potential Anomalies: 5N-5S Active Atlantic Active Atlantic Active Atlantic Intra-seasonal variability in Atlantic hurricane activity linked to MJO. Atlantic especially active when MJO and La Niña forcing in phase.

  20. Anomalous 700-hPa Winds 1 Jul – 13 Oct 2008 700-hPa Relative Vorticity: ASO • 700-hPa anomalies consistent with multi-decadal signal. • Sign reversal from 200-hPa: convective signature enhanced West African monsoon system.

  21. 2008 Regional Activity • Storm formation region largely determines season type. • Historical ACE for MDR: • Averages 9 times higher in above-normal seasons than below-normal seasons. • Accounts for 95% of the difference between active/ inactive eras • Bell and Chelliah (J. Climate, 2006)

  22. Warmer Atlantic Sea-Surface Temperatures • SST anomalies range from 0.5C-1C over large portions of MDR. • Warmth is consistent with • weaker tropical easterlies, • reduced northeasterly flow into MDR • Less evaporation because air is of tropical origin instead of drier NE flow

  23. U.S. Landfalls 2002-2008Totals (Bars) and Averages (Numbers) Landfalling Named Storms 5.4 4.0 1.6 Tropical Storms Only Hurricanes Only 3.0 2.4 2.4 1.6 0.7 0.9 U.S. Gulf Coast East Coast

  24. Sea Level Pressure and Anomalies

  25. 700- hPa Potential Vorticity and Winds • Pronounced reversal in PV gradient (even in July): spans 800 hPa-500 hPa layer • Indicates linear barotropic and baroclinic instability • AEJ is well north of normal and has very strong cyclonic shear along equatorward flank

  26. 700-hPa: Relative Vorticity Total (Contours), and Anomaly (shaded) 1 Aug-13 Oct 2008 ASO: 1995-2007 minus 1971-1994 • Weaker easterlies south of African easterly jet increase cyclonic shear in MDR . • African Easterly Jet farther north. Jet location and structure very conducive to significant development of African easterly waves. • Key regional aspects of multi-decadal signal.

  27. Key Concepts • Activity to date—opposite to EPAC (large scale circulation anomalies) • Total compared to NOAA Forecast • Early season activity (in MDR) • Lots of landfalls and rainfall • Consistent with other active seasons: 3. Key conditions and Climate Factors

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