Although everyone knows that seawater is salty, few know that even small variations in Sea Surface Salinity (SSS) can have dramatic effects on the water cycle and ocean circulation. SSS tells us the about the concentration of dissolved salts in the upper centimeter of the ocean surface. Throughout Earth’s history, certain processes have served to make the ocean salty. The weathering of rocks delivers minerals, including salt, into the ocean. Evaporation of ocean water and formation of sea ice both increase the salinity of the ocean. However these “salinity raising” factors are continually counterbalanced by processes that decrease salinity such as the continuous input of fresh water from rivers, precipitation of rain and snow, and melting of ice.
Illustration of the global distribution of sea surface salinity. Highest concentrations (over 37 practical salinity units) of salt water are present the mid-Atlantic Ocean and lower-Atlantic off the coast of Brazil, the Mediterranean Sea and the Red Sea. Lower concentrations are found near the Arctic and Antarctic and the coastal regions of east Asia and western North America. Surface salinities of the ocean. The distributions of salinity are quite different from temperature. High concentrations are usually in the center of the ocean basins away from the mouths of rivers, which input fresh water. High concentrations are also in sub-tropical regions due to high rates of evaporation (clear skies, little rain, and prevailing winds) and in landlocked seas in arid regions. At high latitudes, salinity is low. This can be attributed to lower elevation rates and the melting of ice that dilutes seawater. To sum up, salinity is low where precipitation is greater than evaporation, mainly in coastal or equatorial regions. Credit: NASA, Jet Propulsion Laboratory.
Salinity & The Water Cycle
Understanding why the sea is salty begins with knowing how water cycles among the ocean’s physical states: liquid, vapor, and ice. As a liquid, water dissolves rocks and sediments and reacts with emissions from volcanoes and hydrothermal vents. This creates a complex solution of mineral salts in our ocean basins. Conversely, in other states of ocean water such as vapor and ice, water and salt are incompatible: water vapor and ice are essentially salt free.
Since 86% of global evaporation and 78% of global precipitation occur over the ocean, SSS is the key variable for understanding how fresh water input and output affects ocean dynamics. By tracking SSS we can directly monitor variations in the water cycle: land runoff, sea ice freezing and melting, and evaporation and precipitation over the oceans.
Yet, at present, our knowledge of salinity on a global scale is extremely limited because we cannot measure salinity from a satellite and are currently restricted to measuring salinity at the planet’s surface via ships and buoys.
Salinity & Ocean Circulation
Changes in salt concentration at the ocean surface affect the weight of surface waters. Fresh water is light and floats on the surface, while salty water is heavy and sinks. Together, salinity and temperature determine seawater density and buoyancy, driving the extent of ocean stratification, mixing, and water mass formation. Greater salinity, like colder temperatures, results in an increase in ocean density with a corresponding depression of the sea surface height. In warmer, fresher waters, the density is lower resulting in an elevation of the sea surface. These height differences are related to the circulation of the ocean. The changes in density bring warm water poleward on the surface to replace the sinking water driving the global thermohaline (heat & salt) circulation within the ocean called the Global Conveyor Belt.
Generalized model of the thermohaline circulation: ‘Global Conveyor Belt’ This illustration shows cold deep high salinity currents circulating from the north Atlantic Ocean to the southern Atlantic Ocean and east to the Indian Ocean. Deep water returns to the surface in the Indian and Pacific Oceans through the process of upwelling. The warm shallow current then returns west past the Indian Ocean, round South Africa and up to the North Atlantic where the water becomes saltier and colder and sinks starting the process all over again.
“The Global Conveyer Belt for Heat” represents in a simple way how ocean currents carry warm surface waters from the equator toward the poles and moderate global climate. This global circuit takes up to 1,000 years to complete.
Salinity & Climate
The Global Conveyor Belt is the principal mechanism by which the oceans store and transport heat. The ocean stores more heat in the uppermost 3 meters than that of the entire atmosphere and acts as a “global heat engine.” Since salinity is a key ingredient in the global thermohaline circulation, SSS will help us discover how climate variation induces change in global ocean circulation.
Despite all the progress in understanding our ocean-atmosphere system, Sea Surface Salinity (SSS) – the principal surface tracer of fresh water input and output from the ocean and a direct contributor to seawater density — is not currently measured remotely from space.
A global understanding of SSS has been difficult because sampling by ships, buoys, drifters, and moorings has been extremely limited. Between 300 and 600 AD, awareness of changes in salinity, temperature, and smell helped Polynesians explore the southern Pacific Ocean. In the 1870s, scientists aboard H.M.S. Challenger systematically measured salinity, temperature, and water density in the world’s oceans. Over the years, techniques for measuring such ocean water properties have changed drastically in method and accuracy.
Eight days & 100 years of Salinity Data. Salinity has been sparsely detected at sea, limited mostly to summertime observations in shipping lanes. Looking at a map of Earth with 100 years of sea surface data, about 25% of ice-free oceans have never been sampled. During its first two months in space, the Aquarius mission will acquire as many SSS measurements as had been collected from ships and buoys during the previous 125 years. During its three-year mission, it will provide maps of seasonal and year-to-year variation in global SSS; this pioneering information will be used to discern longer-term changes in our oceans and climate. The left image shows a prediction of a complete global data set which could be obtained in just eight days of satellite data collection. The entire ice free ocean is measured. The right image shows the sparse amount of sea surface salinity data collected over a period of 100 years. Since data collection has been limited to in situ collection, like shipping routes, large gaps exist in the data.
Starting in 2008, the Aquarius mission will measure global SSS with unprecedented resolution. The science instruments
will include a set of three radiometers that are sensitive to salinity (1.413 GHz; L-band) and a scatterometer that corrects for the ocean’s surface roughness. The spacecraft will be contributed by Argentina’s Comisión Nacional de Actividades Espaciales (CONAE).
Seafarers through history have discovered that SSS varies spatially. Today scientists know that SSS in the open ocean generally ranges between 32 and 37 practical salinity units, but may be much lower near fresh water sources or as high as 42 in the Red Sea. The SSS mission will provide the first precise global, space-based observations and data products that provide:
- Global salinity maps (0.2 psu) produced on a monthly basis
- Charts of seasonal and year-to-year variations of SSS
- Observations & models of the processes that relate salinity variations and to climate changes in the global cycling of water
- Understanding how SSS variations influence ocean circulation
As computer models evolve, Aquarius will provide the essential SSS data needed to link the two major components of the climate system: the water cycle and ocean circulation.
What can salinity tell us?
By observing states and changes of the concentrations of ocean salinity we are able to learn something new about ocean circulation, the water cycle, and climate change. The Aquarius satellite mission will provide monthly maps of global SSS over a three-year period. These maps will allow us to witness small variations in SSS that may have a dramatic impact on the water cycle and ocean circulation. Further, long-term accurate global maps of SSS are crucial to increase understanding of Earth’s climate.
SSS will tell us how global precipitation, evaporation, and the water cycle are changing. As salinity is a key surface tracer of fresh water input to output from the ocean, SSS provides much-needed information for global water cycle research. Thus Aquarius data will augment spaceborne measurements of:
- Soil moisture
- Atmospheric water vapor
- Sea ice extent
By tracking SSS we can directly monitor variations in the water cycle: land runoff, sea ice freezing and melting, evaporation and precipitation over the oceans. These changes may reflect changes in the global Earth system or other natural or human-induced changes.
To track changes in SSS patterns over time, scientists monitor the relationship between two primary processes in the oceans: evaporation, which controls the loss of water; and precipitation which governs the gain of water. Data from the Aquarius mission will enable scientists to produce accurate maps of global balance of evaporation and precipitation. Thus, for the first time we will observe how the ocean responds to variability in the water cycle, from season-to-season and year-to-year.
A Global View of Salinity
The cycling of water and energy through the atmosphere and oceans is crucial to life on Earth, but the ties among the water cycle, ocean circulation, and climate are poorly understood. Global measurement of SSS over time will provide insight to interactions among these relationships.
For example, in the tropics, increased precipitation can lead to fresh water surface layers on the ocean, which heat up and modify the energy exchange with the atmosphere, affecting El Niño and Monsoon processes. At high latitudes, melting sea ice, increased precipitation, and/or river inputs will also make ocean surface water less salty. This density change could disrupt thermohaline circulation, which could restrict the ocean-atmosphere heat pump that normally warms the atmosphere, leading to possible dramatic changes in climate.
Another important mission objective is demonstrating how monitoring salinity-driven ocean circulation — and its subsequent feedback on climate and events such as El Niño and La Niña — can benefit society as whole. For example, observations of salinity can significantly improve predictions of an El Niño event. When changes in salinity occur, they affect the El Niño event for the next 6 to 12 months. In this lag time, salinity changes have the potential to modify the layers of the ocean and affect the heat content of the western Pacific Ocean. This is the region where the unusual atmospheric and oceanic behavior associated to El Niño first develops. Thus, including SSS measurements in forecast models could help predict El Niño events 6 to 12 months in advance.
Global SSS data will allow us to create unprecedented computer models that bridge ocean-atmosphere-land-ice systems, with the goal of predicting future climate conditions.
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