The global atmospheric water cycle

By Lennart Bengtsson. 2010

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Water vapour plays a key role in the Earth's energy balance. Almost 50% of the absorbed solar radiation at the surface is used to cool the surface, through evaporation, and warm the atmosphere, through release of latent heat. Latent heat is the single largest factor in warming the atmosphere and in transporting heat from low to high latitudes. Water vapour is also the dominant greenhouse gas and contributes to a warming of the climate system by some 24°C (Kondratev 1972). However, water vapour is a passive component in the troposphere as it is uniquely determined by temperature and should therefore be seen as a part of the climate feedback system. In this short overview, we will first describe the water on planet Earth and the role of the hydrological cycle: the way water vapour is transported between oceans and continents and the return of water via rivers to the oceans. Generally water vapour is well observed and analysed; however, there are considerable obstacles to observing precipitation, in particular over the oceans. The response of the hydrological cycle to global warming is far reaching. Because different physical processes control the change in water vapour and evaporation/precipitation, this leads to a more extreme distribution of precipitation making, in general, wet areas wetter and dry areas dryer. Another consequence is a transition towards more intense precipitation. It is to be expected that the changes in the hydrological cycle as a consequence of climate warming may be more severe that the temperature changes. Water on planet Earth The total amount of available water on the Earth amounts to some 1.5 x 109 km3. The dominant part of this, 1.4 x 109 km3, resides in the oceans. About 29 x 106 km3 are locked up in land ice and glaciers and some 15 x 106 km3 are estimated to exist as groundwater. If all land ice and glaciers were to melt the sea level would rise some 80 m (Baumgartner and Reichel 1975). 13 x 103 km3 of water vapour are found in the atmosphere corresponding to a global average of 26 kg m-2 or 26 mm m-2 of water for each column of air on the surface of the Earth. There are large geographical differences such as between low and high latitudes. Figure 1 shows an estimate of the global water exchange between ocean and land, an annual average in units of 103 km3 (Baumgartner and Reichel 1975). An updated version can be found in Trenberth et al 2007 (their figure 1) showing broadly similar results. There is a net transport of some 38 units from ocean to land with about the same amount returning by the rivers to the ocean. However the amount of precipitation over the continents is almost three times as high, indicating a considerable recirculation of water over land. As shown by Trenberth et al (2007) the recirculation has a marked annual cycle as well as having large variations between continents. The recirculation is larger during the summer and for tropical land areas. The hydrological cycle of the world's oceans interacts differently with that of the continents. Most of the water from the Pacific Ocean recirculates between different parts of the Pacific itself, as and there is little net transport towards land. The pattern of water exchange between ocean and land is different in the Atlantic and Indian Oceans. Two thirds of the total net transport of water towards the continents comes from the Atlantic Ocean, with the rest essentially from the Indian Ocean. Most of the continental water for North and South America, Europe and Africa emanates from the Atlantic and is also returned to the Atlantic by the rivers. Figure 1. The global water cycle following Baumgartner and Reichel (1975). Annual values are in units of 103 km3 year-1. There is widespread evaporation (maximum some 2 m year-1) on each side of the inter-tropical convergence zone (ITCZ), transporting water vapour into the ITCZ and into the storm tracks of high latitudes. Conversely, in regions of net evaporation ocean salinity is increasing, leading to increased ocean vertical mixing. In the ITCZ and in the extra-tropical storm tracks ocean salinity is decreasing due to high precipitation thus reducing ocean vertical mixing. This means that moisture convergence in the atmosphere is compensated for by moisture divergence (salinity convergence) and vice versa in the oceans, which can be seen as a counterpart to the atmospheric circulation (Webster 1994). Atmospheric water vapour is measured both from space and from in situ observations and analysed in detail both by the operational weather services and through different re-analysis programs (e.g. Uppala et al 2005, Onogi et al 2007). However, while data for atmospheric water vapour are generally well observed we are much less comfortable with observations of precipitation. Precipitation has a large variance on small time and space scales and current observing systems cannot sample this sufficiently. This is especially the case over ocean areas where measurements from ships are notoriously poor as the collection of precipitation is strongly influenced by wind, including the local wind effect caused by the movements of the ship itself. Different proxy methods have been developed using space and rain gauge data and combining these with model data. The most commonly used are the NOAA Climate Prediction Centre Merged Analysis of Precipitation (CMAP; Xie and Arkin 1997) and the Global Precipitation Climatology Project (GPCP; Adler et al 2003). GPCP consists of combined satellite and in situ precipitation measurements and provides global coverage of monthly averages. As with most climate data sets, great caution should be exercised in trying to identify trends in precipitation (e.g. Allan and Soden 2007). In addition to the severe sampling problems we also have to identify the effect caused by ongoing changes in the global observing systems. Hagemann et al (2006) has undertaken a study using the ECHAM5 model (Roeckner et al 2003, 2006) where they compared the hydrological cycle over 20 years using 11 different model versions having resolutions from T21/L19 (spectral transform model until wave number 21 and with 19 vertical levels) to T156/L31. There is a minor increase in global precipitation with resolution by some 7% (between T21/L19 and T156/L31) with a systematic increase of large-scale precipitation and reduced convective precipitation with resolution. There is generally good agreement between model results and CMAP and GPCP over land but the modelled precipitation is larger over the oceans by some 70 x 103 km3 year-1 on average. Whether this implies that the model overestimates precipitation over oceans cannot be concluded as it might equally well imply that the CMAP and GPCP underestimate. Another way of evaluating the hydrological cycle is to measure the river outflow and compare this with the net water flux of the river catchments. A detailed study has been conducted for the different sub-regions of the Baltic Sea covering some 10 years with excellent agreement between observed river outflow and calculations of the atmospheric water cycle from a limited area model (figure 2; D Jacob 2001, private communication). This suggests that, when reliable river outflow data exist, this is an alternative way to validate the model-generated atmospheric hydrological cycle for a larger scale area. Figure 2. Model simulation of the water balance of the Baltic Sea (1979–1988). Units in km3 year-1. (Courtesy D Jacob (2001) Max-Planck-Institute for Meteorology, Hamburg.) The estimated total river run off agrees broadly with measured data (Raschke et al (2001). The net outflow from the Baltic Sea is close to various estimates (e.g. Omstedt and Rutgersson 2000). Net water vapour inflow into the region is calculated under the assumption of long-term water balance. Water vapour as a greenhouse gas Water vapour is the dominant greenhouse gas and accounts for around 75% of the total greenhouse effect on Earth (Kondratev 1972). Water vapour in the atmosphere is strongly controlled by the atmospheric circulation. If, for example, we follow a trajectory of water vapour in the atmosphere it shows that it undergoes a complex series of vertical and horizontal motions that can include several events of condensation. At any given time the saturation mixing ratio is determined by the temperature of the latest event of condensation. For this reason it follows that the mixing ratio will increase with temperature broadly following the Clausius–Clapeyron relation (Pierrehumbert et al 2007). The residence time of water vapour in the atmosphere is about a week, as compared with the very long residence time of the greenhouse gases, in particular CO2which includes multi-centennial time scales. Water vapour is, of course, continuously replenished by evaporation from the oceans and the land surfaces but is nevertheless regulated by temperature. In that sense, water vapour should be seen as a part of the response of the climate system to external forcing. Such a change in climate forcing can be due to changes in the concentration of the more persistent greenhouse gases such as CO2. Alternatively it can be due to changes in solar radiation, or to aerosols from volcanic eruptions or due to anthropogenic emissions. The absorption bands in the water vapour spectra are close to saturation, similar to those of CO2, with the absorption mainly occurring in the wings of the spectral bands. The total effect on the radiation balance can be approximated by a logarithmic function of the mixing ratio. This is how the effect of atmospheric water vapour is included in the code for the calculation of long-wave radiation in weather and climate models. In contrast to the well-mixed greenhouse gases, water vapour varies strongly in the atmosphere and this on a small scale in both time and space. The reason is strong vertical motion of different signs that implies sharp gradients in the moisture field. Climate models that have coarse resolu

Subfields: Atmospheric Science, Global and Planetary Change

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