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The atmospheric water cycle How will it change in a warmer climate? – Lennart Bengtsson University of Reading International Space Science Institute, Bern Uppsala University Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 1 The global temperature depends on the state of the climate system but a warming trend stands out. (source :WMO) Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 2 What is happening to the hydrological cycle? The global precipitation during100 years Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 3 Annual precipitation for Sweden 1860-2011 credit: SMHI A minor increase is indicated, some 50-75 mm Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 4 The atmospheric water cycle - Some key questions • • • • • • • Why is the global precipitation not increasing in spite of a warming temperature trend? Is this an observational problem or is it because global precipitation is not directly driven by temperature? There are indications from some regions that precipitation is indeed increasing. Is this an artefact because of unreliable reports or can it be correct? What is happening to weather systems such as tropical cyclones that are driven by release of latent heat? Are they likely to intensify and will they increase in number? Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 5 The Atmospheric water cycle - content of my talk • The greenhouse effect on water vapour and precipitation • The water cycle in polar regions • Tropical cyclones • Summary and conclusions Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 6 The role of the water cycle in the climate system Precipitation is crucial for life on the planet The largest warming factor of the atmosphere is through the release of latent heat amounting to 80-90 Wm-2 The net transport of water from ocean to the land surfaces amounts to some 40000 km3/year Water is re-circulating over land increasing land precipitation some 3 times Water vapour is the dominant greenhouse gas. Water vapour provides a positive feedback and enhances significantly the warming from the anthropogenic greenhouse gases Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 7 CO2 is a genuine forcing while H2O is a part of the climate response system • The residence time of CO2 in the atmosphere is from years to multi-millennia • The residence time of H2O in the atmosphere is 7-8 days CO2 Seoul 30.5.2012 H2O • H2O, albeit a more powerful greenhouse gas, is driven by temperature that in turn is forced by the slower components of the climate system. Lennart Bengtsson Global Water cycle and Climate 1 Clausius-Clapeyron relation Relation between temperature,T and saturated water vapor, es Atmospheric temperature determines water vapour following the C-C relation Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 9 Water vapour and temperature For a temperature change, dT , the humidity change, dq, follows the C-C relation seen as a conservation of relative humidity dT + 0.4°C Observations and model calculations from observed SST 1979-2005 dq + 3% Held and Soden, 2006 dT+ 4°C dq + 35% Model 1860-2100 Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate Semenov and 1 Bengtsson, 2002 The atmospheric water cycle • The atmospheric water cycle follows closely Clausius-Clapeyrons (C-C) relation. (6-7%/ K) • It also follows that transport of water vapour scales with the C-C relation. • That means more precipitation in areas of convergence • The global precipitation increases much slower than global water vapor. (1-2%/ K) Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 11 Why is water vapour increasing faster than precipitation in global mean and what is the reason? • Water vapour is controlled by the 3-dimensional atmospheric circulation. • Precipitation = Evaporation is determined by the surface energy balance. • While water vapour always will increase in a warmer climate, global precipitation can under certain conditions, such as enhanced aerosol load, even decrease! Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 12 What is happening to the regional water cycle? • Transport of water vapour is scaled by the CC relation • That means increased convergence and divergence of water vapour • A crucial consequence is less precipitation in areas of divergence and more in areas of convergence. • Increased transport of water vapour (latent heat) implies a reduced transport of dry energy (weakening of the large scale circulation) Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 13 IPCC 4th assessment , Changes in the hydrological cycle 2007 Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 14 What might happen in a warmer climate? A modeling approach We have used the AIB scenario And the coupled MPI model at T63 resolution used in the IPCC 4th assessment Higher resolution experiments use the transient SST from T63 (time - window) We study C20 (1961- 1990) C21 ( 2071-2100) Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 15 The Cryosphere ( picture by Frederic Edwin Church) Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 16 Transport of water vapour across 60°N Annual mean calculated for every 6 hrs. T213 resolution ( ca 50 km) ERA-Interim re-analysis 1989-2009 ECHAM5 (T213) for the period 1959-1990 ECHAM5 (IPCC scenario A1B) 2069-2100 Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 1 Changed hydro production in Sweden in a warmer climate. Credit: SMHI (S. Bergström) Increased precipitation. More generation during winter due to more rain Estimated increase by 5-20 TWh/year at the end of 21st century More energy available when the need is largest Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 18 Mass balance change C21-‐C20 GREENLAND - 519 km3/year Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 19 Change in sea level from SMB only. Contribution from Greenland (red), from Antarctica (blue).Total contribution (black). ECHAM5 model, IPCC Scenario A1B, MPI, Hamburg Greenland Does not include changes in calving glaciers Total Antarctica Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 20 What might happen to tropical cyclones? Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 21 Hurricane Katrina Intensity at Landfall 29 Aug 2005 14 Z 4 km WRF, 62 h forecast Mobile Radar IVA, 6 oktober 2010 Klimatmodellering Lennart Bengtsson Courtesy of P. Fox (NCAR) Hurricane Katrina August 2005 ECMWF operational analyses, 850 hPa vorticity Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 23 Super Typhoon 21 (1991) in ERA-40 (left) and a selected simulated typhoon in ECHAM5 (right) Intensity (vorticity at 850 hPa) Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 24 Tangential (left) and Radial winds (right) for the T319 resolution. Negative values inflow. Average of 100 tropical cyclones. Radius 5 degrees. Unit m/s The flow is predominantly inward to the rear and left of the storm and outward to the front and right (Frank 1977 MWR) Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 25 Number of tropical cyclones simulated C19 (black), C20 (red) and C21(blue) Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 26 Number of TCs/year (T213) for C 20 and C21 for wind speed and vorticity All (6, 6, 4) >2x10-4 s-1 >5x10-4 s-1 >1x10-3 s-1 20C (1961-1990) 104 97 40 6.0 21C (2071-2100) 94 90 49 9.8 T213 >18ms-1 >33ms-1 >50ms-1 20C (1961-1990) 100 33 3.7 21C (2071-2100) 92 36 4.9 T213 Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 27 Conclusions from the T319 resolution (40 km) • Results confirm the finding from the T213 experiment • The number of tropical cylones are reduced by some 10% at the end of the 21st century • The stronger storms are becoming more intense (Cyclones with wind speeds >50m/s increase from 12 to 17 per year compared to the end of the 20th century) Intensification occurs in all tropical storm regions Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 28 Why do we see a reduced number of tropical cyclones in a warmer climate? • We suggest the following mechanism • There is a reduction in the large-scale tropical circulation at climate warming due to increased static stability and a rapid increase in the amount of water vapor in the atmosphere. • This generally leads to less favorable conditions to generate organized onset vortices which are seen as key atmospheric conditions for the generation of tropical storms. Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 29 Why do we see an intensification of the tropical storms in a warmer climate? • We suggest the following process • Due to chance favorable conditions are likely to evolve • In such situations the mechanisms proposed by Emanuel may be applied, that means that the maximum wind speed is proportional to the sqr. of the available latent energy provided to the storm • To demonstrate this with a GCM requires sufficient resolution ( say 50-100 km or less) Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 30 END Thanks for your attention! Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 31 Result from present study: More intense hurricanes • Given favorable atmospheric condition we suggest the ideas put forward by Emanuel and Holland comparing a hurricane with a Carnot cycle can be applied. • This will provide an energy input broadly proportional to the specific humidity at a higher temperature (following SST) • The intensification of the tropical cyclones depends on sufficient model resolution to accurately describe the convergence of momentum which generate the very high wind speeds at the core of the vortex. • For TCs reaching 33ms-1 PDI (power dissipation index) increases by 16% Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 32 Climate change experiment using ECHAM5 (MPI, Hamburg) • We have investigated two periods: • • 20 C: 1959-1990 using observed/estimated greenhouse gases and aerosols 21 C: 2069-2100 using scenario A1B • A1B is a 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 Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 33 Mass balance change C21-‐C20 ANTARCTICA +289 km3/year Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 34 Tropical cyclones in different regions, T319 resolution Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 35 Number of tropical cyclones Gualdi et al (2008) 30-years PREIND 2CO2 4CO2 Total 2196 1839 1229 Mean 73.2 61.3 41.0 STD 6.8 8.3 7.6 Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 36 Result from present study: Reduced number of hurricanes • There is s reduction in the number of tropical cyclones in agreement with most previous studies. • We suggest this is due to a weakening of the tropical circulation. This can best be seen as a slowing down in the circulation of water vapor by 16% • The radiative cooling of the tropics increases due to more water vapor in the upper troposphere and dynamical cooling due to increased static stability. This processes can compensate warming from release of latent heat (6%) with a less active tropical circulation. Seoul 30.5.2012 Lennart Bengtsson Global Water cycle and Climate 37