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Sea Surface Temperature (SST)

Sea Surface Temperature ExampleThe temperature of the sea surface (or sea surface temperature - SST) is an important indicator of conditions that may affect many types of activity on the water. Information about SST can be used to assist decision making where water temperature is a major factor influencing operations on or near the ocean’s surface, or where it can be used to infer properties of the ocean environment in the zone just below the surface.

The BLUElink SST products depict the temperature of the sea surface commencing at a given start date and stepping forward by 24 hours each step out to 7 days. The first chart is based on actual observations of temperature sourced from a variety of temperature sensors based on ships, floats, drifting and moored buoys, manned stations and satellites, which have been incorporated into the BLUElink system, to produce the base analysis of the SST prevailing at that time. Subsequent charts are produced as forecasts of SST at daily intervals, based on the information provided from the base analysis, using the BLUElink ocean prediction system to step forward in time.

The graduated colour scale indicates the temperature values in degrees Celsius.

Sea Surface Salinity (SSS)

Sea Surface Salinity ExampleThe sea-surface salinity (SSS) is the measure of the concentration of dissolved salt in the sea water at or very close to the ocean surface. The SSS is an important variable indicating how fresh water input and output in the ocean affects ocean dynamics. Variations in SSS can be anticipated from variations in the flux of fresh water into the oceans (fresh water from river discharge and evaporation and precipitation over the oceans) which can be directly or indirectly monitored. For example evaporation of ocean water removes fresh water from the ocean, thereby locally increasing the salinity of the ocean. Rainfall and snow over the ocean and discharge from river systems add fresh water, and thereby locally decrease the salinity of the ocean.

It is estimated that about 85 per cent of evaporation across the globe occurs over the oceans. Around 80 per cent of precipitation (rain and snow) also occurs over the oceans. Thus SSS is a key variable for understanding the global water cycle. In addition, changes in salt concentration at the ocean surface affect the density of surface waters, for example at the same temperature fresh water is more buoyant than saltier water, and can float on the surface.

Salinity and temperature determine the density and buoyancy of sea water. High salinity and cool temperatures decreases the specific volume of the water thereby increasing the density (through the "thermal contraction" of the volume of water caused by cooling and the increase in the amount of dissolved salts, which both act to increase the density). The increase in density can lead to a depression of the sea-surface height (as the denser water is heavier and will tend to sink).

The density is lower in warmer, fresher waters, where "thermal expansion" of the water (due to heating of the water) and lower salinity give rise to a higher specific volume for the water and hence water of lower density. The less dense water leads to an increase in the elevation of the sea surface (the less dense water is lighter and will tend to rise). These height differences (relating directly to the internal pressure of the ocean waters) represent a force that drives ocean circulation.

On the global scale, changes in density of the ocean waters drives the transport of warm water poleward on the surface, to be replaced by cold water from deeper levels from the polar regions. This transport of water drives the global thermohaline circulation (i.e. the circulation of heat and salt) within the ocean. This has been called the Global Conveyor Belt.

The Global Converyor Belt

Graphic illustrating the global conveyor belt, courtesy of NASA (USA).

SSS in the open ocean generally ranges between 32 and 37 practical salinity units (psu), but may be much lower near fresh water sources and can be as high as 42 in the Red Sea.

Sea Surface Salinity Values

Illustration of the typical values of SSS in the global oceans, courtesy of NASA (USA).

Knowledge of the detailed nature of salinity from in situ measurements has dramatically improved in recent years through the Argo program.

Observations will be further enhanced with remote sensing satellites for SSS. A number of satellites which will enable observations of SSS from space on the global scale, are planned to be in operation in the next few years, including the Aquarius satellite (a collaboration between Argentina and USA) which is planned to become operational in 2008-09.

Sampling Salinity

Illustration of sampling of SSS from the global oceans, showing a composite over 100 years (on the right) and from early data from the Aquarius satellite (on the left). Courtesy NASA (USA).

The graduated colour scale used in the BLUElink SSS products indicates the surface salinity values in psu.

Sea Level Anomaly (SLA)

Sea Level Anomaly ExampleThe surface of the ocean is very 'bumpy' comprising a very large range of phenomena which arise from wave processes and as a result of variations in the steric height (the specific volume) of the ocean waters. The kind of waves and steric height variations that are modelled by a general circulation model, like that used by BLUElink, are those affected by the earth's rotation and changes in water density at depth.

Familiar waves, like sea-swell and breaking waves, are too short to be influenced by the earth's rotation. The influence of tides on the ocean's general circulation is largely limited to mixing near the bottom in shallow water, such as shelves and reefs. Specialised models are already used by the Bureau to produce forecasts of these wave types.

A phenomenon that has a dominant influence on the general circulation of the ocean is the so-called 'eddy'. This term refers to a cyclonic motion (in the form of a vortex) on the scale 10 to 200 km in diameter that forms, propagates throughout the ocean and later decays. Throughout this cycle the eddies redistribute heat and salt. Common forms of eddies found in the Australian region are 'warm-core' and 'cold-core' eddies. These terms describe how the interior of the eddy is either warmer or colder than the surrounding ocean. A warmer, less dense water column has a larger specific volume leading to an increase in surface height and pressure compared to the surrounding ocean. In the presence of the earth's rotation, this pressure gradient radiating out from the warm core of the eddy is 'geostrophically' balanced by vortical currents circulating around the core. The term 'geostrophic' refers to the situation where, for an observer on the earth's surface, the vortical currents (the circulating currents flowing around the eddy) induce an equal and opposite Coriolis force to balance the gradient in pressure. A warm-core eddy, which has a pressure gradient radiating out, leads to anticyclonic motion, which is anticlockwise in the southern hemisphere. A cold-core eddy has a pressure gradient force that is focused toward the core, which leads to cyclonic motion that is clockwise in the southern hemisphere. Good examples of these can be found in the Tasman Sea as part of the East Australian Current. BLUElink has been specifically designed to model eddy motion in the Australian region.

The change in density between the eddy core and the surrounding ocean can be detected as a change in the surface height of the ocean of the order of one metre. Observing the surface height changes of eddies in the presence of all the other wave motion on the sea surface has been one of the most remarkable scientific achievements of the past decade. This has been made possible through satellite altimetry which measures the height of the ocean to great precision (see Jason). The BLUElink system uses this information to infer the correct position of eddies, their geostrophic motion as well as their temperature and salinity structure below the surface.

Sea Surface Temperature (SST) & Currents

Sea Surface Temperature and Currents ExampleOcean currents at any one location can be influenced by a large array of phenomena. At the largest scale, basin-wide circulations develop in response to large-scale atmospheric surface winds. The pattern of circulation is determined by the position of atmospheric winds and the shapes of the ocean basins. This 'gyre' circulation is closed along western boundaries by strong narrow currents (typically more than 2 knots). The so-called western boundary currents often start in warm tropical regions and reach the mid to high latitudes carrying warm water, which can be detected in the SST patterns. A good example of this is the East Australian Current. Currents can also occur along eastern as well as southern and northern boundaries through other large-scale mechanisms.

In the Australian region there are several significant currents that are very prominent features of the oceanic environment. They have major impact on the oceanography, weather and climate, and the biological characteristics of the waters through which the currents flow. They also have significant impact on human activities that take place in our marine environment.

East Australian Current (EAC)

From its source in the Coral Sea, the EAC tracks down off the east coast of Australia, sometimes coming to within five or six kilometres of the coast. It is well known for forming eddies which may be as large as 200 km across and which persist for several weeks.

The EAC has a significant influence on the lives of Australians on the eastern seaboard. By bringing warmer ocean water southward, it adds several degrees of temperature to the coastal waters, enlarges the stock of fish and other marine animals by sweeping additional species down from the north, and helps flush effluent from the waterways and coastal areas which it touches along its journey south.

Major Current Systems

Schematic representation of the major current systems in the Australian region (Courtesy CSIRO).

Leeuwin current

The Leeuwin current sweeps down Australia's west coast, from about the North West Cape and can extend as far as the Great Australian Bight and the southwest of Tasmania. It turns east at Cape Leeuwin, where it is called the South Australian Current, and flows eastwards below most of South Australia to Tasmania, where it is known as the Zeehan Current. During winter this entire system can act as a single current. BLUElink reanalyses have impacted our understanding of this remarkable system.

Compared to the EAC, it transports only about a quarter of the volume of water, but nevertheless has a profound effect on the Western Australian and South Australian coasts and as far east and south as Tasmania.

It brings warm tropical water to Western Australia, raising the ocean temperatures several degrees more than would otherwise be experienced. The current is responsible for the transportation of tropical species south along the Western Australian coast and further east around the corner of Western Australia and into the waters of South Australia. For example, the life cycles and abundance in Australian waters of salmon and tuna are dependent on the current. Swept by the current, salmon avoid the warm waters, instead circling around into South Australia seeking cooler waters. The current transports juvenile tuna after they have spawned in the area between Australia and Indonesia, to an area west of Broome. The tuna ride the current south and eastwards off South Australia as they grow.

The Leeuwin current can sometimes reach very close to the coast. It can sometimes be as close as several kilometres from the coast at, for example, Ningaloo (north of Perth).

Sea Level Anomaly (SLA) & Currents

Sea Level Anomaly and Currents ExampleThis product depicts the pattern of sea- level elevation (shown here as anomalies, which are departures above or below the long-term average sea level for that location and time of year) overlaid with the sea-surface currents. The direction of the current tends to flow along contours of SLA or normal to the gradient in sea level (which represents the internal pressure field in the water). An eddy with a positive sea-level anomaly is associated with an anticlockwise direction in the southern hemisphere (clockwise in the northern hemisphere).

You can readily correlate the patterns of the anomalies with the flow of the current vectors (the arrows depicting the direction and strength of the current) in this product.

Ocean Regions

These products are available for the following ocean regions around Australia:

  • Queensland
  • New South Wales
  • Victoria/Tasmania
  • South Australia
  • Southern Western Australia
  • North West Shelf
  • Northern territory
  • Christmas Island
  • Cocos Island
  • Lord Howe Island
  • Norfolk Island
  • Macquarie Island

 

Sea Temperature
Analysis

Links to ocean temperature maps for the Sea Surface and Subsurface. Daily, weekly and monthly periods covering Australia, nearby regions and the globe.