A bolide did it! No... not really. (2)

[MariusRomanus@aol.com]

In as few a words as possible, here is the break downâ

The oceans carry roughly one half of the heat due to solar insolation in the 
tropics up to high latitudes, maintaining the Earthâs temperature. For what 
concerns us, itâs the South Equatorial Current, known also as the Gulf 
Stream, that transports 40 Sv (Sv = Sverdrup = 10^6 m^3/s, a unit of transport 
used in oceanography) of warm 18 degree Celsius waters up to the North 
Atlantic, waters that warm Europe. Of this, 14 Sv return southward via the deep 
western boundary current with a temperature of +2 degrees Celsius. Therefore, 
the flow must lose roughly 1.2 +/- 0.2 petawatts (1 petawatt = 10^15 watts) 
north of 24 degrees N in the North Atlantic. Palm trees actually grow on the 
west coast of Ireland, but none are found in Newfoundland, which is actually 
further south. Norway, at 60 degrees N, is far warmer than southern Greenland 
or northern Labrador, both being at the very same latitude. This is the effect 
of the ocean component of heat transport known as the Global Conve!
 yo!
r Belt. Warm surface waters release heat and water into the atmosphere, and the 
water becomes sufficiently dense enough so that it then sinks to the bottom in 
the Norwegian and Greenland Seas. This deep water, the North Atlantic Deep 
Water, later upwells in other regions of the oceans, eventually making its way 
back up to the North Atlantic via the Gulf Streamâ over and over and over 
again.

Now, the production of this bottom water, as already noted, depends upon the 
Thermohaline Circulation. Itâs all about the changes in salinity in the North 
Atlantic. More saline surface waters form denser water in winter than less 
saline water. You might think that temperature is important, but it really is 
not since at high latitudes, water in all the oceans gets cold enough to 
freeze, producing â 2 degrees Celsius water at the surface.  Of this water, 
only the most salty will sink, with the saltiest waters of all being found in 
the Atlantic and also under the ice surrounding the continental shelves around 
Antarctica.

The production of deep water is terribly sensitive to salinity, so much so that 
numerical models of the Thermohaline Circulation have demonstrated that a +/- 
0.1 Sv variation in the flow of fresh water into the North Atlantic can switch 
on or off the deep circulation of 14 Sv. If this deep water production is shut 
off during times of low salinity, then the 1 petawatt of heat may also be shut 
off. I say âmayâ because the ocean is a complex system. We donât know if 
other processes will work to increase heat transport if the deep circulation is 
disturbed. A possibility could be the increase in intermediate depth 
circulation, and thus heat trasport, when the deep circulation is reduced.

The production of bottom water is also heavily modulated by the rate of 
upwelling due to mixing in other oceanic areas. In fact, it is remarkably 
sensitive to small changes in mixing in the deep ocean. Itâs been calculated 
that 2.1 TW (1 terawatt = 10^12 watts) are required to drive the deep 
circulation. This small source of mechanical mixing actually drives a poleward 
heat flux of 2000 TW. Most of the energy for the mixing comes from the 
dissipation of tidal currents, which in turn depend on the distribution of the 
continents. Therefore, during events such as the last ice age, when sea level 
was much lower, the tides, tidal currents, tidal dissipation, and deep 
circulation, were all different from present day values.

Profound implications for the ocean circulation during the Mesozoic come to 
mind, but thatâs a topic for another day. :-)

On to post number 3...