NOAA’s 2007 Atlantic Hurricane Outlook Date Issued: 9 August 2007

1. NOAA’s 2007 Atlantic Hurricane Outlook Date Issued: 9 August 2007 Season and 2007 Outlooks 1951-2000 Activity Type August May Normals Chance Above Normal 85% 75% Chance Near Normal 10% 20% Chance Below Normal 5% 5% Named Storms 13-16 13-17 11 Hurricanes 7-9 7-10 6 Major Hurricanes 3-5 3-5 2 ACE Index (% Medn) 140-200 125-210 100

2. Historical Atlantic Seasonal Activity The 2007 Atlantic hurricane season outlooks reflects the ongoing active Atlantic hurricane era that began in 1995, ongoing warmer Atlantic SSTs, and ENSO-neutral or La Niña conditions.

3. Expected August-October 2007 Conditions Conditions expected during August-October 2007 reflect the ongoing active Atlantic hurricane era, ongoing warmer Atlantic Ocean temperatures in the west, and La Niña-like patterns of tropical convection and wind shear.

4. Multi-Decadal Signal in Atmosphere Above-Normal Seasons feature: Reduced vertical wind shear in boxed region is critical for hurricane formation. Weaker easterly winds in boxed region are associated with northward shift of the ITCZ and a more favorable African Easterly Jet (AEJ). Enhanced cyclonic circulation is aligned along equatorward flank of AEJ (boxed region), and provides energy to developing tropical disturbances.

5. Observedvs. Multi-Decadal Signal Explained variance = 82%. The transition into the current active Atlantic hurricane era is captured by a phase change in tropical multi-decadal signal (Bell and Chelliah, J. Climate, 15 Feb. 2006).

6. Underlying Climate Pattern forActive Atlantic Hurricane Era Warmer Warmer Wetter Drier The combination of an enhanced West African monsoon system and suppressed convection over the Amazon Basin has been in place since 1995. Similar conditions were also present during the active period 1950-1970, and opposite anomaly patterns were associated with the below-normal hurricane era 1971-1994. The current convection patterns favor expanded upper-level easterlies (green arrow) and weaker trade winds (blue arrow) across the tropical Atlantic, which contributes to warmer SSTs, reduced vertical wind shear, and a more favorable African easterly Jet. 7

7. SST Departures (oC) in the Tropical Pacific Ocean During the Last 4 Weeks • Recent sea-surface temperatures (SSTs) in the equatorial Pacific Ocean were more than 1ºC below average in between 130ºW and the South American coast, and more than 0.5oC above average in portions of the western Pacific. This pattern reflects the continued evolution toward a La Niña episode.

8. Recent Pattern of Anomalous Tropical Convection The pattern of tropical convection (indicated by anomalous Outgoing Longwave Radiation) continues to be La Niña-like, with suppressed convection across the central and east-central equatorial Pacific (orange shading), and enhanced convection over the western equatorial Pacific (blue shading).

9. Atlantic Sea-Surface Temperature Departures (oC) During June-July 2007 • Sea-surface temperatures remain above average across the western tropical Atlantic and Caribbean Sea (Red box). Departures during June-July 2007 were the fifth warmest since 1950 (averaging 0.56oC). • Sea-surface temperatures in the eastern tropical Atlantic (Blue box) dropped to only slightly above-average (0.17oC) levels during June-July, but have now begun warming back up.