2.4 - Density Profiles
Objectives:
- Know that density depends on both temperature and salinity.
- Understand how the salinity of the surface layer can change depending on the rate of input/output of freshwater.
- Be able to explain changes in the sea surface salinity based on geographical location.
- Know that the sea surface temperature is highly variable and important measurement for weather forecasting.
- To know how the temperature and salinity vary as a function of depth in various locations.
- Know and understand the terms; thermocline, halocline and pycnocline.
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*** add the wind mixed layer ***
In the previous topic it was shown that the surface density of seawater is variable as it depends on both the salinity and the temperature. The density of the ocean is also variable in location and depth. This is of great importance as water tends to flow from high density to low density as denser water sinks. Changes that occur at the surface can also affect the deep ocean.
Conduction and diffusion are slow processes and can be ignored.
*** add the wind mixed layer ***
In the previous topic it was shown that the surface density of seawater is variable as it depends on both the salinity and the temperature. The density of the ocean is also variable in location and depth. This is of great importance as water tends to flow from high density to low density as denser water sinks. Changes that occur at the surface can also affect the deep ocean.
Conduction and diffusion are slow processes and can be ignored.
Sea Surface Salinity
The salinity of the surface layers of the oceans depends on the rate that fresh water is either leaving or arriving. The most important loss of fresh water, which increases the salinity, is evaporation. Another way is the formation of sea ice. Warmer seas have a high rate of evaporation. There are a number of ways that fresh water can be added to the sea: rivers, melting ice and rainfall.
If the rate of adding fresh water exceeds the evaporation, the salinity decreases. If the rate of evaporation exceeds the rate of adding fresh water the salinity increases.
The salinity of the surface layers of the oceans depends on the rate that fresh water is either leaving or arriving. The most important loss of fresh water, which increases the salinity, is evaporation. Another way is the formation of sea ice. Warmer seas have a high rate of evaporation. There are a number of ways that fresh water can be added to the sea: rivers, melting ice and rainfall.
If the rate of adding fresh water exceeds the evaporation, the salinity decreases. If the rate of evaporation exceeds the rate of adding fresh water the salinity increases.
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ACTIVITY - look carefully at the chart of mean sea surface salinity of the oceans. Consider the following locations and explain why the salinity is higher or lower than the mean value.
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Sea Surface Temperature
The other major variable that affects the density of the ocean is the temperature. The deep ocean temperatures are pretty stable but, as everyone in Bermuda knows, the surface temperatures can vary a great deal. No Bermudian would dream of going swimming before May 24th! Jumping in the sea in either the UK or New England during the winter is only for the terminally insane. Originally the sea surface temperature was measured by hauling a bucket of water from the sea and sticking a thermometer into it. This meant that the data was somewhat spotty and prone to errors. Nowadays the temperature is measured by sensors mounted buoys and in the seawater cooling intakes of a huge number of ships which regularly transmit the data, along with their positions, to the World Meteorological Organisation (WMO) and several NASA satellites use the wavelength of the infrared radiation emitted from the surface to produce worldwide charts. The ship and buoy data help to calibrate the satellite data and both are fed daily into the huge supercomputers that run the weather forecasting models. (see Weather Forecasting)
Sea surface temperature (SST) has a massive effect on the path and strength of tropical cyclones (hurricanes).
The other major variable that affects the density of the ocean is the temperature. The deep ocean temperatures are pretty stable but, as everyone in Bermuda knows, the surface temperatures can vary a great deal. No Bermudian would dream of going swimming before May 24th! Jumping in the sea in either the UK or New England during the winter is only for the terminally insane. Originally the sea surface temperature was measured by hauling a bucket of water from the sea and sticking a thermometer into it. This meant that the data was somewhat spotty and prone to errors. Nowadays the temperature is measured by sensors mounted buoys and in the seawater cooling intakes of a huge number of ships which regularly transmit the data, along with their positions, to the World Meteorological Organisation (WMO) and several NASA satellites use the wavelength of the infrared radiation emitted from the surface to produce worldwide charts. The ship and buoy data help to calibrate the satellite data and both are fed daily into the huge supercomputers that run the weather forecasting models. (see Weather Forecasting)
Sea surface temperature (SST) has a massive effect on the path and strength of tropical cyclones (hurricanes).
Mean SST of the world ocean (Image: NASA)
ACTIVITY - go online and find current charts of the SST of the North Atlantic. Can you find the Gulf Stream? Does the temperature at Bermuda match with what you would expect to find if you went to the beach after school?
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ACTIVITY - research the link between the SST and hurricanes. Two good examples are Hurricanes Gonzalo and Katrina
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Vertical Profiles
The temperature and salinity (and hence density) changes with depth as well. The deep ocean tends to be relatively stable, although the surface layers are prone to significant seasonal variability. There is a marked change around \(50 - 100\,\text{m}\) depth in all three variables. These are the "clines".
The temperature and salinity (and hence density) changes with depth as well. The deep ocean tends to be relatively stable, although the surface layers are prone to significant seasonal variability. There is a marked change around \(50 - 100\,\text{m}\) depth in all three variables. These are the "clines".
- Rapid temperature drop - THERMOCLINE
- Rapid salinity change - HALOCLINE
- Rapid density increase - PYCNOCLINE
A - Temperature
The surface layer of the ocean changes with the seasons, as discussed in the SST section. The surface layer is warmer and less dense than the deeper ocean. There exists a sudden drop in temperature, which is known as the THERMOCLINE. The depth and severity of the thermocline is variable. It is more pronounced in the tropics.
The surface layer of the ocean changes with the seasons, as discussed in the SST section. The surface layer is warmer and less dense than the deeper ocean. There exists a sudden drop in temperature, which is known as the THERMOCLINE. The depth and severity of the thermocline is variable. It is more pronounced in the tropics.
B - Salinity
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ACTIVITY - While aboard the Atlantic Explorer, we sent down a CTD to measure the vertical profile of the ocean at that point in space and time. I took a photo of the live data stream from the computer attached to the CTD. The yellow line on the left shows the flourescence, which relates to the photosynthesis occuring. The thermocline and halocline are not terribly obvious as this was only the first \(50\,\text{m}\) of the tropical ocean. The data file is attached. Use excel to produce the vertical temperature and salinity profiles and label the "clines".
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