Global Warming, Climate Variability and

1
Global Warming, Climate Variability and Atlantic
Hurricanes
Chunzai Wang
NOAA/AOML Miami, Florida U. S. A.
International CLIVAR Workshop Guangzhou,
China November 26-28, 2007
2
Outline

• A brief review of recent hurricane/climate
  debate.
• Global warming and U.S. landfalling hurricanes.
• Atlantic warm pool (AWP) acting as a link between
  Atlantic multidecadal oscillation (AMO) and
  Atlantic hurricane activity.
• How does the AWP affect Atlantic hurricanes?
• Summary.

3
Power dissipation index (PDI winds cubed
summed for season) of TCs in the Atlantic
Number of global hurricanes
Emanuel (2005, Nature)
Webster et al. (2005, Science)
This group argues that the recent increase in
hurricane activity is attributed to global
warming.
4
Another group states that natural variability of
Atlantic Multidecadal Oscillation (AMO) is
responsible for the increase of Atlantic
hurricanes
Goldenberg et al. (2001, Science)
Cool phase of AMO
Warm phases of AMO
5
The first EOF mode of SST representing a global
warming mode
ERSST data over the past 153 years (1854-2006)
Three tropical oceans compete with one another
for affecting Atlantic hurricanes.
6
Vertical wind shear is an important factor
affecting hurricanes
Regression of vertical wind shear (June-November)
onto global warming index
Global warming of the sea surface is associated
with a secular increase of tropospheric vertical
wind shear in the main development region (MDR)
for Atlantic hurricanes.
7
Given hurricane data problems (hurricanes were
hardly detected over the open ocean before
satellite technology), the most reliable
hurricane measurement over the long term is U.S.
landfalling hurricanes.

• It shows a weak downward trend.
• However, trend is robust because it is
  independent of the beginning of linear fit as
  long as the fitted data cover at least a full
  cycle of AMO.
• It shows an upward trend from the 1970s because
  the period of 1970-2006 is from cool (1970-90) to
  warm (1995-2006) AMO phases.

8
Consistent with paleoclimatic proxy data
Nyberg et al. (2007) show an increased trend in
the wind shear (reconstructed using corals and
marine sediment cores) in the Caribbean Sea.
Major hurricanes decrease gradually from the
1760s until the early 1990s.
9
Consistent with future model projections of
global warming
Vecchi and Soden (2007, GRL)
The consistency of our result with the
paleoclimatic proxy data and future model
projections suggests that the increase of
vertical wind shear in the MDR for Atlantic
hurricanes, associated with global warming, is a
robust feature.
10
Why does global ocean warming produce an increase
of wind shear in the MDR?

• Tropical oceans compete with one another.
• Warming in tropical North Atlantic reduces
  vertical wind shear in the MDR.
• Warmings in the Pacific and Indian Oceans
  enhances vertical wind shear in the MDR.

Whether future global warming increases vertical
wind shear in the MDR for Atlantic hurricanes
will depend on the relative role induced by
secular warmings over the three tropical oceans.
11
Regression of vertical wind shear (JJASON) onto
U.S. landfalling hurricanes
Negative wind shear regression in the MDR extends
toward to the U.S. via the Gulf of Mexico,
creating a condition that favors a hurricane to
be intensified and make landfall in the U.S.
12
Maximum streamfunction at 30N
Future model projections under global warming
scenarios (IPCC models).
SST from the equator to 60N

• As the Atlantic Meridional Overturning
  Circulation (AMOC) weakens, a cooling effect will
  cause the North Atlantic to warm less than the
  Pacific Indian Oceans.
• Inducing environmental changes typified by a
  small Atlantic Warm Pool (AWP).

13
Atlantic Warm Pool (AWP)

• ERSST from 1854-2006.
• AWP variability is large.
• Large AWPs are almost three times larger than the
  small ones.

14
AWP (SST 28.5C) area anomaly indices during
the Atlantic hurricane season of June-November
The AWP shows multiscale variability that
includes interannual, multidecadal, and linear
warming trend variations.
15
(AMO)
An EOF mode of the ERSST data over the past 153
years (1854-2006)
16
AMO regimes are characterized by large or small
AWPs

• About 80, large (small) AWPs occur during warm
  (cool) phases of AMO.
• Other 20 occurs in transition phases.
• Climate response to North Atlantic SST is
  primarily forced at low latitudes.
• The AWP is the path of or a birthplace for
  Atlantic TCs.
• These imply that the AWP acts as a link between
  the AMO and Atlantic TCs and climate response.

17

• Since 1995 hurricanes have been significantly
  more active.
• However, the recent increase is not
  unprecedented.
• Multidecadal variation is consistent with phases
  of the AMO and AWP.
• Hurricane activity also shows an interannual
  variation.

18
Vertical wind shear during the Atlantic hurricane
season of June-November in the main development
region (MDR)
It shows a multiscale variability that includes
linear trend, multidecadal, and interannual
timescale variations.
19
Regression of vertical wind shear (June-November)
onto AWP indices

• Anomalously large (small) AWPs weaken
  (strengthen) vertical wind shear in the MDR and
  strengthen (weaken) wind shear in the eastern
  North Pacific.
• This suggests that a large (small) AWP favors
  (disfavors) Atlantic TCs, whereas it suppresses
  (enhances) eastern Pacific TCs.

20
NCAR Community Atmospheric Model (Version 3.1
CAM3.1)

• A global spectral model (T42 with 26 vertical
  layers equivalent to a 2.8?2.8 horizontal
  resolution).
• SST from the Hadley Centre (UK) as the
  model-forcing.
• The control (CTRL) ensemble (with 18 members)
  run Climatological SST is prescribed globally.
• The large AWP (LAWP) ensemble run SST composite
  for large AWP is used in the AWP region.
• The small AWP (SAWP) ensemble run SST composite
  for small AWP is used in the AWP region.
• The difference is taken between the LAWP and SAWP
  runs.

21
Impact of the AWP on Atlantic Hurricanes via
Wind Shear
Vertical Wind Shear (JJASON)
The AWP reduces lower tropospheric easterly flow
and upper tropospheric westerly flow, resulting
in a reduction of the vertical wind shear in main
development region (MDR) that favors Atlantic
hurricanes.
22
Impact of the AWP on Atlantic Hurricanes via
Instability
Convective available potential energy (CAPE) in
JJASON
The orientation lies along the track of
historical hurricanes.
The AWP increases CAPE that provides the fuel for
moist convection and thus increases Atlantic
hurricane activity.
23
Summary

• Global warming is associated with a secular
  increase of vertical wind shear in the MDR which
  coincides with a downward trend in U.S.
  landfalling hurricanes.
• Relative role induced by secular warmings over
  the three tropical oceans is important for
  determining whether global warming increases wind
  shear in the MDR for Atlantic hurricanes.
• The AWP acts as a link between the AMO and
  Atlantic hurricanes and climate response.
• Large (small) AWP reduces (enhances) vertical
  wind shear in the MDR and increases (decreases)
  the moist static instability of the troposphere,
  both of which favor (disfavor) Atlantic TC
  activity.

24
Summary of Summer Responses to AWP