Oceanic freshwater fluxes are defined as the transport of non saline water between the oceans and the other components of the Earth's system (the lands, the atmosphere and the cryosphere). These fluxes have an impact on the local ocean properties (on sea surface salinity, temperature and elevation), as well as on the large scale circulation patterns (such as the thermohaline circulation).
Introduction
Freshwater fluxes in general describe how freshwater is transported between and stored in the earth's systems: oceans, land, the atmosphere and the cryosphere. While the total amount of water on Earth has remained virtually constant over human timescales, the relative distribution of that total mass between the four reservoirs has been influenced by past climate states, such as glacial cycles. Since the oceans account for 71% of the Earth's surface area, 86% of evaporation (E) and 78% of precipitation (P) occur over the ocean, the oceanic freshwater fluxes represent a large part of the world's freshwater fluxes.
There are five major freshwater fluxes into and out of the ocean, namely:
Precipitation Evaporation Riverine discharge Ice freezing or melting (Sea ice freezing or melting, ice shelf melting, iceberg melting) Groundwater discharge
whereby the 1., 3. and 5. are all inputs, adding freshwater to the ocean, while 2. is an output, i.e. a negative freshwater flux and 4. can be either a freshwater loss (freezing) or gain (melting).
The quantity and the spatial distribution of those fluxes determine the ocean salinity (the salt concentration of the ocean water). A positive freshwater flux leads to mixing of water with low to zero salinity with the salty ocean water, resulting in a decrease of the water salinity. This is for example the case in regions, where precipitation is greater than evaporation. On the contrary, if evaporation gets greater than precipitation, the ocean salinity increases, since only water (H2O) evaporates, but not the ions (e.g. Na+, Cl+) which make up salt.
Estimates of the annual mean freshwater fluxes into the ocean are for precipitation (88% of total freshwater input), for riverine discharge from land (9%), for ice discharge from land (<1%) and and for saline and fresh groundwater discharge respectively (<1%). The annual mean freshwater fluxes out of the ocean via evaporation is estimated to be .
The salinity, along with temperature and pressure, determines the density of the water. Higher salinity and cooler water results in a higher water density (see also spiciness of ocean water). Since differences in water density drive large-scale ocean circulation, freshwater fluxes are most important for ocean circulation patterns like the Thermohaline Circulation (THC).
Freshwater fluxes into the ocean
Evaporation and precipitation There are large spatial and temporal variations in precipitation and evaporation patterns. The dominant reason for precipitation is adiabatic cooling when moist air rises, whose water vapor then becomes supersaturated above a certain altitude and condenses out. Areas of large precipitation are therefore areas of convection, which is most prominent in the Intertropical Convergence Zone (ITCZ), a band of latitudes around the equator.
Evaporation describes the process when surface water changes its phase from liquid to gaseous. This process requires a high amount of energy, due to the strong hydrogen bonds between the water molecules. This results in a global evaporation pattern, where high evaporation rates can be observed mostly in warm tropical and subtropical regions, where the surface was heated by solar radiation which can provide the necessary amount of energy. At higher latitudes the evaporation rate decreases. Additionally, the evaporation rate is influenced by the relative humidity of the air overlying the water surface. Approaching the saturation of the air with water vapour, the evaporation rate decreases, i.e. a lower air–sea humidity gradient decreases evaporation.
The actual freshwater flux that the ocean experiences in a certain timeframe is the net amount of precipitation and evaporation in this time interval. This means, if evaporation minus precipitation (E-P) is positive, the ocean experiences a net loss of freshwater, while the opposite is true for a negative value for E-P. On a global scale, the subtropical gyres and western boundary currents of the Atlantic, Pacific and Indian Oceans are regions where evaporation exceeds precipitation. In contrast, the ITCZ as well as high latitudes (> 40° N/S) are regions of net precipitation, although the ITCZ exceeds the high latitudes in terms of quantity of rainfall. The equatorial region of net precipitation is centered north of the equator in the Atlantic and Pacific Oceans but is broader and extends further south in the Indian Ocean. An additional center of strong net precipitation is located over the western Pacific-Indonesian region.
Both Atlantic subtropical gyres are net evaporative, as well as the Pacific subtropical gyres, although they show an east–west transition with increased evaporation near the eastern boundaries. This spatial pattern can be attributed to the fact that the overlying air becomes saturated in humidity, subsequently leading to decreasing evaporation rates as the air is driven westward by the trade winds.
An estimation of the annual mean freshwater flux into the ocean is for precipitation, while annual mean freshwater flux out of the ocean via evaporation is estimated to be .
When considering all the ocean basins, the only ocean basin which experiences net precipitation averaged over the year, is the North Pacific. The other ocean basins, namely the South Pacific, the North and South Atlantic and the Indian Ocean are areas of net evaporation, albeit with varying strength. The net evaporation over the South Pacific Ocean is distinctly smaller than over the other ocean basins, although the South Pacific Ocean covers an area as large the whole Atlantic Ocean and one third larger than the Indian Ocean.
It is very likely that the energy increase (heat flux) observed in the upper 700 m of the global oceans can be attributed to anthropogenic climate change and increased radiative forcing due to greenhouse gas emissions. Although observed trends in evaporation minus precipitation suggest that the Atlantic Ocean will become saltier, while the Indian Ocean will become fresher in the coming decades, it is easier to project global patterns of air-sea flux based on changes in heat content and salinity while regional trends are rarely robust.
Seasonal cycle
The amount and even the sign of the net total freshwater flux E-P from an ocean basin can change throughout the year.
The net evaporation over much of the subtropics is most pronounced during winter season due to the increased strength of the easterly trades in winter. This applies for both hemispheres. The wind impacts evaporation in two ways. Firstly directly, whereby a greater wind speed carries water vapour faster away from the evaporating surface, leading to a faster reestablishment of the air–sea humidity gradient, which was reduced by the evaporation beforehand and is necessary for high evaporation rates. Secondly indirectly, since enhanced surface wind strengthens the wind-driven subtropical gyre. Since the subtropical gyres drive a northwards heat transport via the western boundary currents, the sea surface temperatures warm up along the paths of the currents and cause more evaporation by providing more energy and enlarging the air–sea humidity gradients. In the extratropics the net precipitation is not explainable by a simple seasonal cycle. In the Atlantic and Pacific Oceans the net mid-latitude precipitation reaches its peak during June–August synchronously in the northern and the southern hemisphere, i.e. in different seasons.
In the North and South Atlantic Oceans and in the North Pacific Ocean evaporation exceeds precipitation in winter and spring. During summer and autumn the sign of E-P changes for all ocean basins but the South Atlantic Ocean, which is always net evaporative. When considering the Atlantic as a whole, the constant net loss of freshwater in the South Atlantic Ocean determines the sign of the total freshwater flux and cancels the net precipitation from the North Atlantic in summer out. This means, the Atlantic in total is net-evaporative during the whole year due to the prominent influence of the South Atlantic Ocean. The opposite can be stated about the Pacific Ocean as a whole, which shows an excess of precipitation over evaporation for every season. This pattern of evaporation minus precipitation is consistent with the observed higher salinity in the Atlantic compared to the Pacific Ocean. In the Indian Ocean a net evaporation rules most of the year, except during December–February.
Changes due to climate change Past
The report from Working Group 1 in the IPCC 2021 AR6 concluded that patterns of evaporation minus precipitation (E-P) over the ocean have enhanced the present mean pattern of wetting and drying. In general, saline surface waters had become saltier (especially in the Atlantic Ocean) while relatively fresh surface waters had become fresher (especially in the Indian Ocean). However, AR6 assessed only low confidence in globally averaged trends in E-P over the 20th century due to observational uncertainty, with a spatial dominated by evaporation increases over the ocean.Gulev, S.K., P.W. Thorne, J. Ahn, F.J. Dentener, C.M. Domingues, S. Gerland, D. Gong, D.S. Kaufman, H.C. Nnamchi, J. Quaas, J.A. Rivera, S. Sathyendranath, S.L. Smith, B. Trewin, K. von Schuckmann, and R.S. Vose, 2021: Changing State of the Climate System. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press. In Press. Even coarse-resolution models show that mean SST and variability in SST are sensitive to changes in flux forcing.
Future