NOAA

1
NOAAs 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
Observed vs. 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.