Climate Changes and Atlantic Hurricane Activity

Climate Changes and Atlantic Hurricane Activity

Chunzai Wang

Physical Oceanography Division

NOAA/Atlantic Oceanographic & Meteorological Lab.

Miami, Florida

NOAA/Climate Prediction Center Camp Springs, Maryland

February 24, 2009

2004

2005

Because of “An Inconvenient

Truth” and his efforts in climate

change, Al Gore was awarded

the 2007 Nobel Peace Prize.

Power dissipation index (PDI; winds cubed &

summed for season) of TCs in the Atlantic

Emanuel (2005, Nature) Webster et al. (2005, Science) Number of global hurricanes

This group argues that the recent (since the 1990s) increase in

hurricane activity is attributed to global warming.

The 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

Coverage Today of Meteorological/Oceanographic Satellites Coverage Today of Meteorological/Oceanographic Satellites

Hurricane data problem: Satellite technology only begins in the mid-1960s

Open Atlantic Ocean Differences 2005 Hurricane Season

Hurricane data problem: Hurricanes were hardly detected over the open ocean before the era of satellite technology (around the mid-1960s) &

aircraft reconnaissance (around the mid-1940s).

Landsea (2007, Eos)

 Global warming and U.S. landfalling hurricanes (Wang &

Lee 2008, GRL).

 Atlantic warm pool (AWP) acting as a link between Atlantic multidecadal oscillation (AMO) and Atlantic hurricanes

(Wang et al. 2008, G

).

 How does the AWP affect Atlantic hurricanes (Wang et al.

2008, J. Clim.)?

 Summary.

Outline

Warming Occurs Almost Everywhere over the Global Ocean

The first EOF mode from the ERSST data over the past 153

years (1854-2006).

Three tropical oceans

compete with one

another for affecting

Atlantic hurricanes.

Effects of Vertical Wind Shear (VWS) on Atlantic Hurricanes

STRONG

STRONG SHEAR => UN UNFAVORABLE UPPER-LEVEL WINDS

LOWER-LEVEL WINDS

WEAK SHEAR => FAVORABLE

VWS either helps a vertically coherent storm vortex to

develop or hinders it from doing

so, depending on its magnitude.

Regression of vertical wind shear (Jun.-Nov.) 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.

Hurricane upper-level

steering pattern (Klotzbach

and Gray 2006)

As the global ocean is warmed up, the vertical wind shear in the hurricane MDR is increased.

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 hurricane main

development region (MDR), unfavorable for Atlantic hurricanes.

Given hurricane data problem, 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.

Why does global warming produce an increase of 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 (also see Vecchi et al. 2008;

Swanson 2008).

• Tropical oceans compete with one another.

• Warming in the Atlantic MDR reduces vertical wind shear in the MDR.

• Warmings in the Pacific and Indian Oceans enhance vertical wind shear in the MDR.

All oceans make contributions!

Wang & Enfield (2001, GRL) named

the Western Hemisphere warm

pool (WHWP)

SST ≥ 28.5°C

Composite of the Atlantic Warm Pool (AWP)

• ERSST from 1854-2006.

• AWP variability is large.

• Large AWPs are almost

three times larger than

the small ones.

AWP (SST ≥ 28.5°C) area anomaly indices during the Atlantic hurricane season of June-November

The AWP shows multiscale

variability: Interannual, multidecadal, and linear warming trend variations.

Wang et al. (2008, G

)

Rationale for WP Research

Why Study the Warm Pool?



ENSO impacts climate mainly in winter; need a value-added paradigm for summer climate prediction, especially for rainfall in the central/eastern U.S., and for Atlantic hurricanes

hurricanes.



Atmosphere responds to inter-basin anomalies; can’t look just at the Pacific.



Warm pool size is expression of SST anomalies, but weighted toward regions of maximum SST > 28°C where atmospheric heating occurs -- good match for tropical cyclogenesis.



WP provides physical keys to understand rainfall & hurricane

impacts of Atlantic multidecadal oscillation (AMO) -- 80% of

large WPs occur for AMO(+), 80% of small WPs for AMO(-).

WHWP (Amazonia) is summer (winter) heat source of divergent circulation in the WH

J. Climate

Why and how does the AWP affect climate/hurricanes?

 The AWP is a source of moisture. Atmospheric low-level flows carry moisture from the AWP to the central U.S. for rainfall there.

 The AWP changes the NASH which in turn affects atmospheric

circulation and then climate.

An EOF mode of the ERSST data over the past 153 years (1854- 2006)

Atlantic Multidecadal Oscillation (AMO)

• 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/climate response.

AMO regimes are characterized by large or small AWPs

54 Years of Tropical Storms (1950-2003)

Busy hurricane years

= years for which the number of season

hurricanes fall within the top tercile of all years

Of the 18 years with small warm pools

3 busy years, 23 storms

Of the 18 years with large warm pools

11 busy years, 82 storms

Atlantic Warm Pool (AWP) and Atlantic Hurricanes

Large (small) AWPs => Low (high) shear => More (less) hurricanes.

Hurricane composites for large & small AWPs

Regression of wind shear onto AWP index

Why does the AWP affect hurricanes?

Wang et al. (2006, J. Climate)

VWS = (U

− U

)

+ ( V

− V

)

Vertical wind shear (VWS) during the Atlantic hurricane season of June-

November in the MDR:

It shows a multiscale variability that

includes linear trend, multidecadal, and

interannual timescale variations. All

show that global warming is associated

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.

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 the MDR that favors Atlantic hurricanes.

VWS = ( U

− U

)

+ ( V

− V

)

How/Why Does the AWP Reduce Vertical Wind Shear in the MDR?

Gill’s (1980) physics: Baroclinic response to a heating

Anomalous anticyclone at 200 mb

Anomalous cyclone at 850 mb

LAWP – SAWP

Impact of the AWP on Atlantic Hurricanes: via Atmos. Instability

Convective Available Potential Energy (CAPE)

The higher the CAPE value, the more energy available to foster storm growth

A large AWP tends to increase CAPE due to the increased near-surface air temperature and water vapor content, which provides the fuel for moist convection and thus increases Atlantic hurricane activity.

The orientation lies along the

track of historical hurricanes.

Impact of the AWP on North Atlantic Subtropical High (NASH)

The AWP weakens the NASH (especially at its southwestern edge) and strengthens summer continental low over the North American monsoon region.

SLP’s response to AWP variability in JJA

The Atlantic warm pool (AWP), the North Atlantic subtropical high (NASH) and the hurricane track.

NASH without the AWP NASH with the AWP

The AWP’s effect is to make the NASH shrink northeastward, so hurricanes likely

would be steered around NASH’s edge to the north (instead of making landfall in

the southeast United States).

An International Program Called the Intra Americas Study of Climate Processes (IASCLIP)

A CLIVAR-VAMOS Program (FY2009 - FY2014)

The science and implementation plan is available from

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.

 Whether future global warming increases Atlantic hurricane activity will depend on relative warming role of the three tropical oceans.

 The AWP acts as a link between the AMO and hurricanes/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.

Summary of Summer Responses to AWP

The paper was covered by more than 300 newspapers worldwide (plus many TV & radio shows).

Our paper “Global warming and United States landfalling hurricanes” was

published on January 23, 2008 (Wang and Lee, GRL).

ENSO: shear is increased but moist instability (CAPE)

also increases

Warm pool: moist instability (CAPE) and shear work

together

Moist instability measuring by

Convective Available Potential Energy

(CAPE).

Regression of vertical wind shear (June-November) onto Nino3 SSTA

ENSO-induced wind shear pattern is opposite to that of the

AWP interannual variability.

An 11-year running mean is

applied to SST data before

EOF analysis. That is,

ENSO signal is removed.

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.

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.