Global climate and the hydrologic cycle

Transcription

1 Global climate and the hydrologic cycle

2 Key points The climate system in ultimately driven by the sun; through latent heat exchanges, the global hydrologic cycle is an important component of the climate system. Differential solar heating gives rise to an equator-to-pole temperature gradient, resulting in a gradient in the height of atmospheric pressure surfaces that drives the global atmospheric circulation. Patterns of precipitation and evapotranspiration reflect the effects of latitude and regional aspects of atmospheric circulation linked to orography, the distribution of continents and ocean, ocean currents and other factors.

3 The hydrologic system is ultimately driven by the sun, which radiates approximately as a 6000K blackbody (ε = 1). About 99.9% of solar energy is emitted between wavelengths of μm (μm = 10-6 m). From Wien s law, the wavelength λ of maximum intensity is at about 0.5 μm in the visible spectrum λ max = 2.897x10 3 μm K/K K is temperature in Kelvin. Net incoming solar radiation at the top of the atmosphere S net is S net = [(1-A).S con π.r 2 ]/[4. π.r 2 ]= 239 W m -2 Where A is planetary albedo (approximately 0.3), S con is the solar constant (1367 W m -2 ) and R is the earth s radius (6371 km) Stefan-Boltzmann equation for blackbody emission E: E = ε.σ.t 4, T in Kelvin, emissivity ε = 1, σ = Stefan Boltzmann constant The earth radiates back to space in longwave radiation. Most of the radiation emission to space is from the atmosphere and not the surface. This is because of absorbtion and emission by greenhouse gases like CO 2 and H 2 0, i.e., the natural (and critical to life!) greenhouse effect.

4 Global radiative balance and the greenhouse effect The earth as a whole is close to a state of radiative balance, where net solar radiation received at the top of the atmosphere is balanced by longwave radiation emission to space: Incoming = outgoing, or: σ.t e4.4.π.r 2 = (1-A).S con π.r 2 where σ is the Stefan-Boltzman constant (5.7x10-8 W m -2 K -4 ) and T e is the effective radiation emission temperature (or effective temperature) of the earth of approximately 255 K. The emission to space peaks at about 10 μm. From Wikipedia As mentioned, because of absorbtion and emission by greenhouse gases, most of the emission to space is from the atmosphere. Because of the counter-radiation back to the surface from greenhouse gases (H 2 0, CO 2, CH 4 and others) the earth s average surface temperature is a habitable 288 K, 33K higher than the radiation emission temperature.

5 Average global energy flows (courtesy of Trenberth, Fasullo and Kiehl, BAMS, 2009). Net solar radiation at the top of the atmosphere is the sum of the incoming solar and reflected solar radiation, which balances the outgoing longwave radiation to space (radiative balance). The above global mean view masks strong differences in all terms by related to latitude, atmospheric and oceanic circulation and characteristics of the surface.

6 Top of atmosphere incoming solar radiation Dingman 2002, Figure 3-6 The seasonal distribution of solar radiation at the top of the atmosphere (at left, in MJ m -2 ) is a function of earth orbital geometry (top)

7 The global heat engine Differential solar heating between low and high latitudes gives rise to a circulation of the atmosphere and ocean that transports sensible heat, latent heat and geopotential poleward. As a result of these transports, poleward of about 38 deg. in both hemispheres, longwave radiation emission to space exceeds shortwave (solar) radiation gain. Without this transport, the polar regions would be colder and the equatorial regions warmer than observed. Key point from the left-hand panel: radiative balance holds only for the globe as a whole and not for individual latitudes [courtesy Kevin Trenberth, NCAR].

8 Northward energy transports Zonal averages of the mean annual northward energy transport required by the net radiation budget at the top of the atmosphere (RT). The total atmospheric transport is AT and the ocean transport is OT. Units are petawatts (PW) [from Trenberth and Caron, 2001]. Negative values in the southern hemisphere mean transport to the south. Atmospheric transport dominates.

9 Average surface air temperature

10 Top of atmosphere net radiation budget The global pattern of the annual mean net radiation budget at the top of the atmosphere [from Trenberth et al., 2001, by permission of Springer-Verlag]. Note that along with the basic latitudinal gradients, there are asymmetries that relate to regional difference in albedo, ocean temperature and other factors.

11 Northward transport of total atmospheric energy The zonal mean annual cycle of the northward atmospheric energy transport, averaged for (PW). Negative values in the Southern Hemisphere mean transport to the south (towards the South Pole) [from Trenberth and Stepaniak, 2003, by permission of AMS]. Note the strong seasonality in the transport; the transport is strongest in the winter of each hemisphere.

12 Global atmospheric circulation Dingman 2002, Figure The global atmospheric circulation is shaped primarily by the Coriolis Force associated with the earth s rotation. large-scale orographic barriers such as the Rocky Mountains and the Himalayas, and large-scale temperature contrasts between continents and ocean. In middle and higher latitudes, most of the poleward atmospheric energy transport is associated with transient eddies, better known as extratropical cyclones and anticyclones. The atmospheric circulation has key controls on global, regional and local patterns of precipitation and evapotranspiration.

13 Annual mean sea level pressure Mean annual sea-level pressure calculated from the ECMWF 40-year reanalysis (from Kallberg et al. 2005). Note the mean subtropical highs (anticyclones) and mean subpolar lows in each hemisphere.

14 Global distribution of annual precipitation Global Precipitation Climatology Program The zonal mean pattern (averaging precipitation for each latitude across all longitudes) is one of high precipitation in the equatorial regions associated with the ITCZ, lower precipitation in the subtropics associated with the Hadley Cells, fairly high precipitation in the middle latitudes linked to extratropical cyclones, and low precipitation in the cold high latitudes. However, the spatial pattern is very complex, reflecting things like orographic uplift, rain shadows, monsoonal circulations and ocean currents. The figure at left is from the JRA-25 reanalysis.

15 Arctic precipitation Mean precipitation north of 60 N for the four mid-season months, based on data from land stations and the Arctic Ocean with bias adjustments, the NCEP/NCAR reanalysis and satellite retrievals (over open ocean). The fields are based primarily on data for Contours are at every 10 mm up to 80 mm and at every 50 mm (dashed) for amounts of 100 mm and higher [from Serreze and Barry, 2005]. The Atlantic sector of the Arctic is quite wet. Precipitation over other parts of the Arctic is quite low. Some land areas are classified as polar desert. Precipitation peaks over the Atlantic sector during the cold months and in summer over most land areas.

16 Global evapotranspiration The global pattern of evapotranspiration, from Dingman 2002 (Figure 3-24). Evapotranspiration is in general largest over the equatorial regions and smallest in high latitudes, reflecting the availability of energy to evaporate water. Evapotranspiration also tends to be larger over ocean than land. However, values also reflect land precipitation (over deserts, there is little water available to evaporate), wind speed and vertical gradients in atmospheric humidity.

17 Soils are formed by the physical and chemical breakdown of rock. The global distribution of soils is hence strongly determined by patterns of temperature and precipitation

18 Biomes of the globe Vegetation across the globe can be divided into different biomes, and while the distribution of biomes is strongly controlled by ranges in precipitation and temperature, vegetation also affects climate through impacts on albedo and evapotranspiration.

19 Storages and fluxes Two schematics of the global hydrologic cycle showing key storages and fluxes, the one of the left is courtesy of UCAR, and the one of the right is from Dingman 2002 (Figure 3-16). Storages are in 10 3 km 3, and fluxes are in 10 3 km 3 yr -1. Note that the numbers don t match between the figures it is hard to get estimates for some terms. Fluxes into the atmosphere represent evapotranspiration, and fluxes out of the atmosphere represent condensation; these represent strong energy transfers in the global climate system.

20 Northward transport of latent heat energy The zonal mean annual cycle of northward latent heat energy transport (water vapor), averaged for (PW). Negative values in the Southern Hemisphere mean transport to the south (towards the South Pole) [from Trenberth and Stepaniak, 2003, by permission of AMS]. Transports are strongest in the tropics.

21 Precipitable water is the equivalent liquid water depth of water vapor in the atmosphere. The above figure is based on the NASA Atmospheric Infrared Sounder (AIRS) aboard the Terra satellite. instrument. Over the tropics, precipitable water may exceed 70 mm. Over the Antarctic continent it is only a few mm. The global average is about 25 mm. See previous slide compared to the global ocean, water storage in the atmosphere is very small.

22 A few basic considerations Globally: P = ET However, also globally, Oceans: E T > P Land: P > ET Hence there is a net transfer of water from the ocean to the land, and this must appear as runoff, R Considering all land masses together: P = ET + R 75 cm 48 cm 27 cm 100% 64% 36% Africa = 80% + 20% N.A. = 55% + 45% Antarctica = 17% + 83% Residence time (S/I) from above numbers: Global Atmosphere= 12,900/(71, , ,000) = years = 8.2 days Global Ocean = 1,338,000,000/(458, , ,200 +2,700 = 2,636 years Atmospheric storage is small, with big fluxes into or out of it. Ocean storage is very big compared to the fluxes into or out of it.