R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Tropical eddies in a future climate

Lennart Bengtsson

ESSC, University Reading

Many thanks to Kevin Hodges

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

How may anthropogenic climate change effect atmospheric vortices?

• There is a general expectation that climate change will imply more intense cyclones both in the extra-tropics and the tropics. And this is certainly the perception of media and the laymen that this is likely to happen in a future climate. And political decisions are driven by such perceptions. Intense storms even now are seen as being a consequence of greenhouse gases and only reduced CO2 emission will prevent future disaster storms.

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

After Emanuel

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

There are recent claims that there is an increase in hurricane intensity ( e.g. Emanuel (2005), Webster et al. (2005)

• Are these findings credible?

• They are generally not supported by operational meteorologists

• According to Knutson and Tuleya (2004) any changes are probably undetectable “for decades to come”

• Results from this study and some additional work will presumably reduce the likelihood of detection further

• There are structural problems in the detection of trends

• Changes in observing systems

• Difficulties to separate a genuine change in storms from societal causes behind the huge increase in damages and damage cost

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Tropical cyclones in a future climatewhat could be expected?

• Higher SST and higher atmospheric moisture would generally favor more intense storms ( e.g. Emanuel 1988, 1999)

• This is supported by modeling results by Knutson and Tuleya (2004) driving an limited area model with CMIP2+ boundary data ( 9 different models).

• Increasing vertical wind-shear and reduced relative humidity would counteract this tendency. Such influences occur in the tropical N. Atlantic during El Nino.

• How will the number of storms change? What are the general conditions controlling the number of tropical storms?

• What are the critical conditions in modeling tropical storms? Are results from large scale models with limited resolution credible?

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Impact of CO2-induced warming on simulated hurricane intensityKnutson and Tuleya (2004, J of Climate)

• They used a high resolution limited area model driven by the SST and moisture of 9 CGCM from the CMIP 2+ project.

• CMIP2 uses 1%yr-1 increase over an 80-year period implying an increase by a factor of 2.2.

• Model calculations are undertaken in NW Pacific-, NE Pacific- and Atlantic basin

• Four different convective schemes are tested (no significant differences)

• RESULTS:

• Max. surface wind speed increases by 6%

• Min. central pressure by 14%

• Max. precipitation by 24%

• Hurricane increase by a factor of 1/2 in the Simpson-Saphire scale

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Intensification of hurricanes at 2xCO2

Knutson and Tuleya (2004)

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Tropical eddies in GCMsSome previous work

• Manabe et al., 1970 (J.Atmos. Sci.)

• Bengtsson et al., 1982 ( Tellus)

• Haarsma et al., 1993 (Climat. Dyn.)

• Bengtsson et al., 1995, 1997 (Tellus)

• Tsutsui and Kasahara, 1996 ( J. Geophys.Res.)

• Vitard et al., 1997, 1999, 2001 (J. Climate)

• Sugi et al.,2002 ( J. Meteor. Soc. Japan)

• Camargo and Sobel, 2005 (J. Climate)

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Early results, Bengtsson et al., 1995 (Tellus)

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Effect of 2xCO2 From Bengtsson et al., 1997 (Tellus) ( number of cyclones /basin)

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

ECHAM 5

• Roeckner et al., (2003), MPI-Report 349( J of Clim. 2005)

• Resolution used T63L31 (top at 10hPa)

• Water vapour, cloud liquid water and cloud ice in semi-Lagrangian flux form-scheme

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

How are transient eddies identified?

• Data sets are needed at least every 6 hour

• We use a method proposed by Hodges (Hodges, 1999, MWR)

• We use the vorticity at 850hPa (unit 10-5 s-1)

• A transient eddy must exist for >48hours and be extended over at least1000km

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Tropical storm tracks, 2005 MJJASO850 hPa ( Courtesy ECMWF)

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Tropical track density (MJJASO)ECHAM5 (top), ERA40 (bottom)

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Storm track intensity and densityECHAM5 and ERA 40 (MJJASO)

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Number of tropical vortices ( max. intensity)ERA 40 and ECHAM5 (AMIP2), 3x

Extreme storms

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Summary of results for ECHAM5 AMIP runs, NH tropics

• ECHAM5 has more eddy activity over the African continent with a slightly more northerly position

• In the Pacific ocean the eddy activity is less than in ERA40 except in the eastern Pacific.

• Some differences in the statistical distributing with more stronger storms in ECHAM5 except for a very few intense vortices ( less than one /year) where there some more in ERA40.

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Courtesy

J. O’Brien

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Tropical vortices response to ENSO, track density top ERA40, below ECHAM5 (20 year, AMIP)

ECHAM5, TRD

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Tropical vortices response to ENSO, storm intensity top ERA40, below ECHAM5 (20 year, AMIP)

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Storm track and ENSO

• There is a good agreement between ECHAM5 and ERA40 in the response to ENSO (using SST in NINO3 as a measure)

• Most marked is the storm track enhancement over southern US stretching into the Atlantic and the storm track enhancement in the northeast Pacific

• There is a weakening of the tropical Atlantic storm track and a southward transition of the Pacific storm track

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

The Climate change experiment The coupled model

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Ocean Model (MPI-OM)

• Marsland et al., 2003: Ocean Modelling, 5(2), 91-127

• 40 levels, bottom topography, partial grid cells• 1.5° resolution, grid poles over land areas• Parameterization include isopycnal diffusion,

horizontal tracer mixing, vertical eddy mixing, convective overturning, slope convection

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Climate change experiment

• Coupled model was run with pre-industrial forcing for 500 years ( negligible drift)

• 20th century runs 1860-2000 with observed anthropogenic forcing including CFCs, ozone and sulphate aerosols ( direct and indirect)

• 3 runs from different ocean and atmospheric states• The runs were continued until 2100 using IPCC

SRES scenario A1B

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

What is A1B?

• Middle of the line scenario

• Carbon emission peaking in the 2050s (16 Gt/year)

• CO2 reaching 450 ppm. in 2030

• CO2 reaching 700 ppm. in 2100

• SO2 peaking in 2020 then coming done to 20% thereof in 2100

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

SST changes 21C - 20 C

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

How will climate change affects the storm tracks?

• We compare three 30 year periods of 1961-1990 (20C) and 2071-2100 (21C)

• The 20C run agrees closely with the AMIP run

• Two different kinds of changes stand out:• (a) A broad conservation of the total number of

storms tracks except a minor reduction of the weaker storms

• (b) Geographical changes in the storm tracks

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Changes in storm track density (top) and intensity(bottom) (21C-20C), MJJASO Tropics

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Changes in storm track density (21C-20C) MJJASO

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Tropical storm tracks at 20C and 21(NH)Number/month as a function of max. intensity

Extreme storms

Total nr (90 y)

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Tropical vorticesNH(0, 35N, gen. 0, 20N) May through October

Period/number of storms

All storms Intense storms

>8x10-5s-1

20C (obs. forcing)

(1961-1990)x3

277+- 20 41 +- 5

21C ( scen. A1B)

(2071 - 2100)x3

259 +- 19 40 +- 5

ERA-40

1977 - 2002

248 +- 17 26 +- 5

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Number of eddies as a function of max. intensity, Atlantic sector: 8-35N, 50-85W

20C

21C

Unit: 10-5s-1

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Number of eddies as a function of max. intensity, Eastern Pacific: 0-35N, 90-135W

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Number of eddies as a function of max. intensity, Western Pacific: 0-35N, 110-180E

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Changes in the NH Tropics

• Slight reduction in the number of tropical storms

• No overall increase in intensity in spite of a SST warming by 2-3°C

• Reduced activity in the Atlantic sector and a southward movement of the east Pacific storm track

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Attempts towards an interpretation of the apparent contrary results between GCM results and

theoretical assessments based on local conditions

• Large scale effects:• Increasing SSTs reduce the moist adiabatic lapse rate

(because of more moisture) providing a larger warming in the upper troposphere

• This creates an enhanced northward temperature gradient through the troposphere.

• The effect of this is to increase (through the thermal wind equation) the vertical wind shear particular in the regions where tropical storm amplify

• High vertical wind shear counteracts tropical storm amplification

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

SST changes 21C - 20 C

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Attempts towards an interpretation of the apparent contrary results between GCM results and

theoretical assessments based on local conditions

• Large scale effects:• Increasing SSTs reduce the moist adiabatic lapse rate

(because of more moisture) providing a larger warming in the upper troposphere

• This creates an enhanced northward temperature gradient through the troposphere.

• The effect of this is to increase (through the thermal wind equation) the vertical wind shear particular in the regions where tropical storm amplify

• High vertical wind shear counteracts tropical storm amplification

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Tropical storm tracks (modelled and observed)at higher resolution (T159)

Number/month as a function of max. intensity

Comparison with T159 resolution

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

Extreme tropical storm tracks (modelled and observed))Number/month as a function of max. intensity

Comparison with T159

R. Met. Soc.15. March 2006

Lennart BengtssonESSC, Reading, University

END