Cold Waters Run Deep

by srosengard

At midnight in the Indian Ocean (037°29.8’ S and 039°58.6’ E) on February 22 I touched the coldest seawater I have yet encountered on this trip–about 50 degrees Fahrenheit. By cold, I mean relative to the shallow, subtropical water I have thus far handled at our one other collection site (a little below room temperature), as we have not ventured too far south towards Antarctica yet. This midnight collection retrieved waters from 2500 meters (~1.5 miles) below surface.

Rumors among the science crew suggest that this was North Atlantic Deep Water (NADW), a seawater mass famous among oceanographers for its long, meandering path and its role in the global climate system. In fact, if you aren’t an oceanographer but have at least studied general environmental science, you might have read about this water mass. NADW is indeed famous because it helps transport heat to the North Atlantic, effectively regulating global climate patterns across the Northern Hemisphere.

CTD

The CTD is connected to a rosette of 12 bottles that collect seawater at various depths.

One fascinating fact of the ocean is that even the subtlest differences can cause striking patterns. Just as one off-beat drummer may shift the entire rhythm of the marching band, small differences in saltiness or temperature of seawater shift movements in the entire ocean. Amazingly, these slight variations become global patterns that govern climate, weather and can even affect the movement of one accidentally discarded soda bottle as it drifts across the ocean. North Atlantic Deep Water is a phenomenon of such proportions. It originates as surface water in the North Atlantic, which sinks to the deep because it is colder and saltier than surrounding water. Once in the deep, NADW flows south and then east before welling up again in the Indian and Pacific Oceans and eventually returning to the Atlantic, making it one of the largest seawater migrations on Earth. This extensive path enables oceanographers even in the Indian Ocean to encounter this abyssal treasure.

Meeting deep waters is not a simple task, but oceanographers have fine-tuned ways to collect them. A common way to do it is through a “CTD cast”, as we do on the Revelle. CTD stands for conductivity, temperature and depth; suspended from a wire, the instrument it describes physical characteristics of seawater (conductivity, temperature and depth–hence the name) as it travels beneath the surface. CTDs do not collect the water themselves. This is the job of the rosette attached to the CTD. The rosette comprises a ring of 12 bottles—each a little over half my height—that collect seawater at target depths. As the bottles (actually tubes with caps on either end) descend in open position, a diligent observer sits in front a computer and, at proper depth, sends an impulse for one bottle to close, capturing a seawater at that location. Early yesterday, after 3 hours and thousands of meters of underwater travel, the Revelle science team had its own collection of deep water on board.

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