Upwelling: closing the conveyor belt cycle

As mentioned in earlier posts, upwelling describes the process of ocean deep water changing its density and rising to the top. Global upwelling is not only necessary to close the conveyor belt system, but also essential for most marine life on earth.

Major upwelling areas around the world

http://oceanservice.noaa.gov/education/kits/currents/media/supp_cur04a.html

Upwelling may occur either in the open ocean or on the coast. In both cases, upwelling is induced by wind stress on the upper ocean layer, referred to as Ekman transport [info box, Price et al., 1987]. 

Wind pushes away water which needs to be replaced immediately. During an upwelling event, the water replacing the blown away surface water is ocean deep water. So, cold bottom water is forced to the top of the ocean. Once in contact with the atmosphere and surrounding water masses, it will change its temperature/salinity/density characteristics and become part of the warmer surface currents.

The upwelling phenomenon may be especially strong on coast lines (Figueroa & Moffat, 2012), due to the coriolis force [see post on Deep Water Formation]. 

Depending on the direction of the wind and the earth “spinning away from the water” an especially large “hole” is left behind to be filled with cold bottom water. Hence, most large upwelling areas are close to the world’s coastlines.

http://www.nwfsc.noaa.gov/research/divisions/fe/estuarine/oeip/db-coastal-upwelling-index.cfm

Why is the process of upwelling so important for our marine life:

Cold ocean bottom water is known to be high in oxygen and nutrient content (see info box), while warm surface waters often lack this richness. Now imagine yourself on a hot summer day, when suddenly someone hands you a wonderful cold and fresh drink! That’s what it must be like as a marine living being in the (sub)tropics during an upwelling event. In other words, marine life explodes. The large excess in nutrients and oxygen lead to excessive algal blooms with in turn activate the whole food chain !

(Widman & Smith, 2003; Gruber et al., 2011; Chavez & Messie, 2009; broader information also available at: Edyvane, 1999; Cheung et al., 2009;

http://wordquests.info/cgi/ice2-for.cgi?file=/hsphere/local/home/scribejo/wordquests.info/htm/L-Gk-plankton-phyto-zooPt2.htm&HIGHLIGHT=japan

[All of the above named processes are well described in van Aken (2007) The OceanicThermohaline Circulation, An Introduction. Atmospheric and Oceanic SciencesLibrary, Vol. 39 or Talley et al. (2011) Descriptive Physical Oceanography: An Introduction, 6.Ed.]

What drives Ocean Circulation: The Thermohaline Circulation

We were interested in the factors that drive and control Broecker's ocean conveyor belt. The most influencial process is called...

The Thermohaline Circulation (THC):

What is that? If you've come across some latin terms you might have figured out that thermo obviously refers to temperature, while haline refers to salinity. Thus, you have just named the two most important factors that drive the ocean conveyor belt.

Here is how it works:

Density of salty water happens to be positively correlated with salt content and negatively correlated with temperature. This means that the density of cold and salty water is higher than that of warm and fresh water and the less dense water will swim ontop of the denser water.
In the oceans, this phenomenon leads to large water masses being "sorted" by their density, with coldest and saltiest waters on the bottom of the ocean and warmest, freshest waters at the surface (Broecker, 1997).

http://omp.gso.uri.edu/ompweb/doee/science/physical/cipatt1.htm

Are you the kind who doesn't like jumping into the salty ocean during your beach holidays??... well be happy you're not a diver ;)

The upper figure shows that under normal circumstances there is no mixing between the layers. However, imagine you change the density of one water parcel in the corner of the figure by making it colder, then the parcel would sink and push away the last water parcel in the bottom layer which is forced to rise. Due to the rise it heats up slightly and becomes less dense, rising even better. And just like that you end up with a cycle that gets your water layers to rotate. In principle, the ocean conveyor works just like that. At upwelling areas bottom deep water comes to the surface, while downwelling areas surface waters sink to the bottom (Broecker, 1997).

graph produced by the author


To fully understand the process, let's take a trip to the most presigious downwelling areas on Earth: ARCTICA and ANTARCTICA

wooooooo.... you'll need your mittens!

http://www.pinterest.com/tristess/aleut-inuit-pantheon-people-of-the-pacific-northwe/