22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Changes in extra-tropical and tropical storms in the 21st century Professor Lennart Bengtsson ESSC, University of Reading, UK

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Changes in extra-tropical and tropical storm tracks in the 21st century The change of storm tracks in the 21st century has been calculated by the ECHAM5 model using the IPCC-A1B scenario. For the present climate comparisons have been undertaken with the ERA- 40 reanalyses. Thanks to Kevin Hodges and Erich Roeckner Joint paper submitted to J of Climate (April 2005)

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Transient extra-tropical weather systems and climate Determine the local weather Regulate global transport of heat, water and momentum Interact strongly with large scale circulation anomalies( e.g. blocking)

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Transient tropical weather systems and climate Transient easterly waves dominate the weather in the Tropics During late summer and autumn some of the waves are transformed into hurricanes

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Typical cyclone storm tracks Tracks Intensities

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Changes in extra-tropical and tropical storm tracks in the 21st century Identification of storm tracks Models Storm track validation ( ERA40) Climate change scenario Extra-tropical storm tracks Tropical storm tracks General conclusions

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson From Geng and Sugi, 2002, J. of Clim.

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson How are transient eddies identified? Date sets are needed at least every 6 hour We use a method proposed by Hodges (Hodges, 1999, MWR) We use the vorticity at 850hPa A transient eddy must exist for >48hours and be extended over at least1000km

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Storm tracks DJF 2002/03 at 850 hPa ERA 40

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson ECHAM 5 Roeckner et al., (2003), MPI-Report 349 Resolution used T63L31 (top at 10hPa) Water vapour, cloud liquid water and cloud ice in semi-Lagrangian flux form-scheme

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Modelling error as a function of horisontal and vertical resolution (ECHAM climate model 2005)

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Simulation of climate Köppen climate zones Main groups A: Tropical rainy climate, all months > +18 C B: Dry climate, Evaporation > Precipitation C: Mild humid climate, coldest month +18 C - -3 C D: Snowy - forest climate, coldest month +10 E: Polar climate, warmest month < +10 C ET: Tundra climate, warmest month > 0 C Subgroups f : Moist, no dry seasons w: Dry season in winter s : Dry season in summer

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson ECHAM5 simulated ERA40 determined from analyses. Köppen climate zones

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Validation using ERA40 3 AMIP-type experiments using observed SST and sea-ice ( with AMIP-2 protocol ( WGNE,1996) ERA

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Storm track diagnostic The full storm track cycle has been calculated including cyclogenesis, storm track density, storm track intensity and cyclolysis Calculations has been done for the NH extra- tropics ( 30N-90N), SH extra-tropics (30S-90S) and for the NH tropical belt ( 0-30N) Storm tracks have been ordered as a function of the maximum intensity obtained for each individual track.

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson ERA40 Cyclogenesis, 850 hPa (DJF)

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson ERA 40 storm track density and intensity DJF

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Storm tracks ERA40 (left) ECHAM5 ( right) NH(DJF)

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Number of vortices ( max. intensity) ERA 40 and ECHAM5 (AMIPII), NH NH for DJF

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson The same for all seasons (NH) DJF JJASON MAM

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Summary of results for ECHAM5 AMIP runs, NH extra-tropics Minor differences in areas of cyclogenesis from ERA40, some reduction over Kuroshio- and the Gulfstream area. Storm track density and intensity agree well Statistical distribution of storms for the whole region is practically identical for all three realizations and with ERA40

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Storm tracks ERA40 (left) ECHAM5 ( right) SH (JJA)

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Number of vortices ( max. intensity) ERA 40 and ECHAM5 (AMIPII), SH SH for DJF

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson The same for all seasons (SH)

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Result of FGGE: ERA40 storm tracks at the SH before and after 1979

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Summary of results for ECHAM5 AMIP runs, SH extra-tropics There are some differences in areas of cyclogenesis with a reduced number in northern Patagonia compared to ERA40 and more cyclogenesis over the Indian ocean Some reduced activity ( track density) along the Antarctic front ( Pacific sector) compared to ERA40 but increased activity in the Indian ocean Systematic underestimation of cyclone intensity prior to 1979 in ERA40 ( due to lack of data) Overall good agreement in cyclone statistics but slightly more intense storms in ECHAM5 than in ERA40

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Tropical track density (MJJASO) ECHAM5 (top), ERA40 (bottom )

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Storm track intensity and density ECHAM5 and ERA 40 (MJJASO)

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Number of vortices ( max. intensity) ERA 40 and ECHAM5 (AMIPII), Tropics Extreme storms

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson 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.

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Effect of ENSO (ERA 40) Warm-Cold events

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Effect of ENSO (ERA 40) ERA 40 left/top and ECHAM5 right/bottom

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Effect of ENSO (ERA 40) ERA40 (top) ECHAM5 (bottom)

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson 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

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Ocean Model (MPI-OM) Marsland et al., 2003: Ocean Modelling, 5(2), 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

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson MPI Coupled model Jungclaus et al., (2004) J. Climate submitted SST, sea ice concentration and thickness, snow on ice, ocean surface velocity are passed on to atmosphere. Accumulated fluxes ( one day) are transferred to the ocean River runoff and glacier calving are treated interactively in the atmospheric model with fresh water fluxes passed on to the ocean Surface wind stress over ocean is calculated relative to the ocean current There is no flux adjustment

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Climate change experiment Coupled model was run with pre-industrial forcing for 500 years ( negligible drift 0.026K/century) 20th century runs 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

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson 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

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson How will climate change affects the storm tracks? We compare three 30 year periods of (20C) and (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

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Changes in storm track density and intensity (21C-20C) DJF (left), JJA (right) NH

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Changes in storm track density (21C-20C) DJF NH

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Change in storm track density

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Change in storm track intensity

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Change in storm tracks DJF (21C-20C) number( max intensity) NH

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Change in storm tracks JJA (21C-20C) number( max intensity) NH

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Changes in storm tracks (21C-20C) north British isles (left), Mediterranean area (right)

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Changes in storm track density (21C-20C) DJF, SH

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Changes in storm track density and intensity (21C-20C) DJF (left), JJA (right) SH

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Change in storm tracks DJF (21C-20C) number( max intensity) SH

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Change in storm tracks JJA (21C-20C) number( max intensity) SH

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Changes in storm track density (top) and intensity(bottom) (21C-20C), MJJASO Tropics

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Changes in storm track density (21C-20C) MJJASO

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Change in storm tracks (21C-20C) number( max intensity) Tropics

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Changes in the NH Extra-tropics A minor reduction in the number of storms ( - 4%) Intensification of the storm tracks north and west of the British Isles Reduced storm tracks in the Mediterranean and through the middle east

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Changes in the SH Extra-tropics Slight reduction in the number of storms Clear poleward movement of the storm track Intensification of storms during JJA but no change in other seasons

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson 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

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson General conclusions 1 In agreement with earlier studies ( e.g. Geng and Sugi, (2002) J. of Clim, ) it appears that is is the reduced baroclinicity which is the main cause of the reduced number of storms in the extra tropics The increase in intensity in the SH winter we suggest is due to the more poleward position of the storm tracks and the closeness to Antarctic air masses providing more available potential energy for the storms

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Condition for baroclinicity

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Thermal wind relation

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson General conclusions 2 There is a trend towards a more poleward position of the storm tracks by far most clear at the SH We suggest this is due to two different effects (a) enhanced eddy propagation out of the tropics due to enhanced convective activity (b) At the SH a poleward transition of the maximum meridional SST gradients (c) influences related to (a) leading to different orographic cyclogenesis regions

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Meridional SST gradients

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson Track density changes with latitude

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson SST gradient and storm track changes for DJF (left) and JJA (right)

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson General conclusions 3 There is a minor reduction in the number of tropical storms by some 6% There are no changes in the extremes of tropical storms in spite of increased tropical SST by 2-3°C There are marked changes in the regional tropical storm tracks which we suggest, analogues to ENSO, are driven by regional tropical SST anomalies

22 April 2005 Oslo Met. Institutt Storm tracks and Climate change Lennart Bengtsson END