The Recent Increase in North Atlantic Hurricane Activity

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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.

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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
  1950s and 1960s ?

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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 !
• Lets look at the bigger picture
  !

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Conditions Related to Active Atlantic Hurricane
Season
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NOAA Atlantic Hurricane Outlook August Outlook
and Verification1998 2004(To see how our NOAA
Forecast did for 2005 See our poster Bell
Chelliah P2.1)
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15
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14
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10
9
8
8
8
12
12
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4
5
5
245
240
202
4
200
3
3
3
132
121
2
75
Observed
Forecast Range
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• 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.

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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.
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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.

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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
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ACTIVE
ACTIVE
TMM 1
TMM 2
ENSO
INACTIVE
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JJA
TMM 2
TMM 1
ENSO
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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
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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.
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ACTIVE North Atlantic Hurricane Activity
seasons Until 1971 (1969 ?, definitely till
late 1960s)
Relatively INACTIVE North Atlantic Hurricane
seasons Until 1994
1995 ACTIVE North Atlantic Hurricane seasons
begin
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SUMMARY

• The Tropical Multi-decadal Modes TMM1, TMM2 and
  the Interannual ENSO mode used in issuing
  NOAAs 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
  50-60s.
• 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.

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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).