Water is a naturally circulating resource that is constantly recharged. Therefore, even though the stocks of water in natural and artificial reservoirs are helpful to increase the available water resources for human society, the flow of water should be the main focus in water resources assessments. The climate system puts an upper limit on the circulation rate of available renewable freshwater resources (RFWR). Although current global withdrawals are well below the upper limit, more than two billion people live in highly water-stressed areas because of the uneven distribution of RFWR in time and space. Climate change is expected to accelerate water cycles and thereby increase the available RFWR. This would slow down the increase of people living under water stress; however, changes in seasonal patterns and increasing probability of extreme events may offset this effect. Reducing current vulnerability will be the first step to prepare for such anticipated changes.

/pdf/global-hydrological-cycles-and-world-water-resources-52g3wp6cnp.pdf

Global Hydrological Cycles and World Water Resources

Global hydrological cycles and world water resources

Salinity stratification is critical to the vertical circulation of the high-latitude ocean. We here examine the control of the vertical circulation in the northern seas, and the potential for altering it, by considering the budgets and storage of fresh water in the Arctic Ocean and in the convective regions to the south. We find that the present-day Greenland and Iceland seas, and probably also the Labrador Sea, are rather delicately poised with respect to their ability to sustain convection. Small variations in the fresh water supplied to the convective gyres from the Arctic Ocean via the East Greenland Current can alter or stop the convection in what may be a modern analog to the halocline catastrophes proposed for the distant past. The North Atlantic salinity anomaly of the 1960s and 1970s is a recent example; it must have had its origin in an increased fresh water discharge from the Arctic Ocean. Similarly, the freshening and cooling of the deep North Atlantic in recent years is a likely manifestation of the increased transfer of fresh water from the Arctic Ocean into the convective gyres. Finally, we note that because of the temperature dependence of compressibility, a slight salinity stratification in the convective gyres is required to efficiently ventilate the deep ocean.

The role of sea ice and other fresh water in the Arctic circulation

Formation of North Atlantic Deep Water (NADW) represents a transfer of upper layer water to abyssal depths at a rate of 15 to 20 x 10 6 m3/s. NADW spreads throughout the Atlantic Ocean and is exported to the Indian and Pacific Oceans by the Antarctic Circumpolar Current and deep western boundary currents. Naturally, there must be a compensating flow of upper layer water toward the northern North Atlantic to feed NADW production. It is proposed that this return flow is accomplished primarily within the ocean's warm water thermocline layer. In this way the main thermoclines of the ocean are linked as they participate in a thermohaline-driven global scale circulation cell associated with NADW formation. The path of the return flow of warm water is as follows: Pacific to Indian flow within the Indonesian Seas, advection across the Indian Ocean in the 10o-15oS latitude belt, southward transfer in the Mozambique Channel, entry into the South Atlantic by a branch of the Agulhas Current that does not complete the retroflection pattern, northward advection within the subtropical gyre of the South Atlantic (which on balance with the southward flux of colder North Atlantic Deep Water supports the northward oceanic heat flux characteristic of the South Atlantic), and cross-equatorial flow into the western North Atlantic. The magnitude of the return flow increases along its path as more NADW is incorporated into the upper layer of the ocean. Additionally, the water mass characteristics of the return flow are gradually altered by regional ocean-atmosphere interaction and mixing processes. Within the Indonesian seas there is evidence of strong vertical mixing across the thermocline. The cold water route, Pacific to Atlantic transport of Subantarctic water within the Drake Passage, is of secondary importance, amounting to perhaps 25% of the warm water route transport. The continuity or vigor of the warm water route is vulnerable to change not only as the thermohaline forcing in the northern North Atlantic varies but also as the larger-scale wind-driven criculation factors vary. The interocean links within the Indonesian seas and at the Agulhas retroflection may be particularly responsive to such variability. Changes in the warn: water route continuity may in turn influence formation characteristics of NADW.

/pdf/interocean-exchange-of-thermocline-water-5797lm6od2.pdf

Interocean Exchange of Thermocline Water

Land-surface evapotranspiration is shown to strongly influence global fields of rainfall, temperature and motion by calculations using a numerical model of the atmosphere, confirming the general belief in the importance of evapotranspiration-producing surface vegetation for the earth's climate. The current version of the Goddard Laboratory atmospheric general circulation model is used in the present experiment, in which conservation equations for mass, momentum, moisture and energy are expressed in finite-difference form for a spherical grid to calculate (1) surface pressure field evolution, and (2) the wind, temperature, and water vapor fields at nine levels between the surface and a 20 km height.

https://science.sciencemag.org/content/sci/215/4539/1498.full.pdf

Influence of land-surface evapotranspiration on the earth's climate

The world water balance: Mean annual global, continental and maritime precipitation, evaporation and run-off

Der Wasserhaushalt der Alpen : Niederschlag, Verdunstung, Abfluß und Gletscherspende im Gesamtgebiet der Alpen im Jahresdurchschnitt für die Normalperiode 1931-1960

Albert Baumgartner

Papers

The world water balance: Mean annual global, continental and maritime precipitation, evaporation and run-off

Der Wasserhaushalt der Alpen : Niederschlag, Verdunstung, Abfluß und Gletscherspende im Gesamtgebiet der Alpen im Jahresdurchschnitt für die Normalperiode 1931-1960

Die Weltwasserbilanz : Niederschlag, Verdunstung und Abfluß über Land und Meer sowie auf der Erde im Jahresdurchschnitt