Sea temp falling- removed from Learner thread

[aswade]

Not an expert myself, but my boss has an active interest in the health of the world's oceans and is quite involved with many organizations, oceanographers and scientists who study these things. I have seen some of the stuff he gets from the various organizations re: ocean currents, temperature changes etc.

Almost all surface currents globally are generated by winds. The temperature drop in this case was almost certainly caused by the high winds changing prevailing currents. It does not take much to affect surface temperature locally because there is a tremendous amount of water movement in the vertical plane as well as the horizontal: upwelling and downwelling caused by the surface currents and deeper currents. The ocean is very stratified and one layer can be much colder than the one directly above- any change that allows the colder layer to rise would do the trick.

The prevailing high winds could have easily caused enough of a change in the normal water flow to allow much cooler water to come in from below or another area, it does not take much of a change to cause an upwelling that could drop the water temperature dramatically. I have seen no evidence that there is any other effect that could cause this kind of drastic temperature drop.

The length of time for the water temp to return to "normal" (whether it is a matter od days or weeks) is a natural effect of how the ocean currents recover from short term, wind-induced changes in flow.

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[EME]

Ummhh...

As you may have gathered we are all amatures on this subject... most of the co-respondents being mech. eng. background.

However there have been a number of 'thermo-graphs' of the med in recent papers clearly showing the whole of the med in satellite view at 28-30 degrees ... as in WHOLE.

So a simple moving of the surface would not suffice.... Do ask the learned man what happened though as I am confused...and more importantly when does it get hot again?

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[tcm]

nah. The thermograph will only show the temp of the surface. The water u nderneath is dead cold, so the wind stirs it up like I said (only obviously, with more scientific and longer words like aswade says)

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[DavidJ]

Yep, you notice it when diving in the Med. It can easily be a lovely 24C on the surface but 18C just 13 metres down (taken from my dive computer in Bonifacio)

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[aswade]

Matt is correct, the thermograph only measures surface temp. As I said, I am not an expert, but here is how I understand it from what has been explained to me and from what I've read:

The ocean is not one homogenous body of water. It tends to stratify into different "blobs" based on temperature, salinity and density. This stratification can occur in the horizontal x and y axis, which is called a front (just like a meteorological front is a line between higher/lower areas of pressure/temperature); or in the vertical spatial scale from the surface to the bottom of the ocean, the z-axis, which is called a cline. Thermoclines are areas of rapid temperature change in this vertical axis. As anyone who has done any scuba diving can attest, the water can be layered into defined zones of temperature- you can be at a depth of 20 feet with the water temp at 65 degrees F, sink down just 5 feet and pass through a thermocline where the temperature at a depth of 25 feet is 50 degrees F.

Wind driven upwelling and downwelling of ocean water can cause a colder layer to be brought to the surface in a small area or a wider area. This is the most likely cause of the change you experienced, given the strong winds you describe.

In theory, it is possible for a body of water to experience a temperature drop through an increase in salinity and density, since both of those factors have an inverse relationship with temperature, but that is less likely than the wind causing an upwelling of colder water. For instance, it is possible that heavy rains causing a major increase in runoff would lower the salinity and density in a bay where the runoff entered the sea. This would cause the temperature of the water to rise since temperature has an inverse relationship to salinity and density. If the runoff stopped and the salinity and density increased back to normal levels, that could cause the water temperature to decrease markedly at that point. Does not fit the facts in this case though.

Cheers,

Alex



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[tcm]

Re: Cor!

This is very impressive guff er i mean geoscientific information. I am off to write up my notes and do a nice diagram.

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[jfm]

Aswade, thanks for info and an informative post. My latent heat theory was total rubbish it seems and I will order extra humble pie for lunch. Two thoughts though:

1. It is not correct to say that thermographs can show only surface temperature. The temperature can be measured many ways including from space. It is perfectly possible for a radiant-heat measuring method to compute the temperature at points much lower than the surface because radiant measurement can intrinsically do that. So, you have to get behind the thermograph data to find out what exactly it claims to be telling you

2. If last week's fast cooling is just a mixing up of deep cold water with hot surface water, partly caused by wind, then surely the sea will now remain cold for rest of year. I mean, when the wind stops, the hot and cold layers of water will not separate out and re-stratify will they? The main thing that will make the sea warm to swim in again will be the sun heating the surface, which takes say 1 month +. So, will be interesing to see if the sea now stays cold or if it warms up fast.....

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