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

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 )

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

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.

Conditions Associated With Active 2008 Atlantic Hurricane Season

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

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

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

Tropical Multi-Decadal Signal Phase shown for Active Atlantic Hurricane Era Drier Warmer Wetter Warmer 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.

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

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

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.

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

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

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

Summary 1.Inter-related set of conditions during 2008 season reflects leading tropical convective modes. Tropical multi-decadal signal, lingering La Niña. 2.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) 3.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). 4.Predicting La Niña convection (not just SSTA) is key to seasonal hurricane outlooks

Above Normal 2008 Atlantic Hurricane Season NOAA Outlooks Above Normal 85% Near Normal 10% Below Normal 5% Issued 7 August Above Normal 65% Near Normal 25% Below Normal 10% May Outlook August Outlook Observed ACE (% Median): 100%-210% 140%-230% 154% Named Storms: Hurricanes: Major Hurricanes: % Probability for Each Range Issued 22 May

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

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

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)

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

U.S. Landfalls Totals (Bars) and Averages (Numbers) Tropical Storms Only Hurricanes Only Landfalling Named Storms U.S. Gulf CoastEast Coast

Sea Level Pressure and Anomalies

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

700-hPa: Relative Vorticity Total (Contours), and Anomaly (shaded) 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. 1 Aug-13 Oct 2008 ASO: minus

Key Concepts 1. 1.Activity to date—opposite to EPAC (large scale circulation anomalies) a. a.Total compared to NOAA Forecast b. b.Early season activity (in MDR) c. c.Lots of landfalls and rainfall 2. 2.Consistent with other active seasons: 3. Key conditions and Climate Factors