Guest Blogger: Dr. Ben Hodges (WHOI)
Just as oil floats on top of vinegar, lighter water floats on top of heavier water in the ocean. What determines the heaviness of seawater? Two properties: temperature and salinity. As water gets colder or saltier, it gets heavier.
In the deep ocean, isolated from the warming of the sun’s rays and the stirring caused by the wind, these two properties (temperature and salinity) don’t change very quickly. At the surface though, they do change. When it’s exposed to cold air, seawater can cool; when exposed to dry air, it can get saltier as the evaporating fresh water leaves its salt behind. Seawater can also get saltier as ice forms: the ice contains less salt than the liquid water that froze to make it, leaving excess salt in the surrounding (still liquid) water. These three processes can make seawater at the surface get heavier. If it gets heavy enough, it sinks into the quiet depths of the ocean.
Every bit of water in the deep ocean is on a journey that began at the surface some time in the past, and will end at the surface again some time in the future. Interestingly, most of these journeys begin in a few relatively small areas of the ocean surface, and each such location has its own unique combination of temperature and salinity. Just by measuring these two properties far below the surface, scientists can trace the journey of seawater back to where it began, often decades ago and thousands of miles away.
Just as oil floats on top of vinegar, lighter water floats on top of heavier water in the ocean. What determines the heaviness of seawater? Two properties: temperature and salinity. As water gets colder or saltier, it gets heavier.
In the deep ocean, isolated from the warming of the sun’s rays and the stirring caused by the wind, these two properties (temperature and salinity) don’t change very quickly. At the surface though, they do change. When it’s exposed to cold air, seawater can cool; when exposed to dry air, it can get saltier as the evaporating fresh water leaves its salt behind. Seawater can also get saltier as ice forms: the ice contains less salt than the liquid water that froze to make it, leaving excess salt in the surrounding (still liquid) water. These three processes can make seawater at the surface get heavier. If it gets heavy enough, it sinks into the quiet depths of the ocean.
Every bit of water in the deep ocean is on a journey that began at the surface some time in the past, and will end at the surface again some time in the future. Interestingly, most of these journeys begin in a few relatively small areas of the ocean surface, and each such location has its own unique combination of temperature and salinity. Just by measuring these two properties far below the surface, scientists can trace the journey of seawater back to where it began, often decades ago and thousands of miles away.
As we head northward along the 38° W meridian, we stop every 60 nautical miles to
lower an instrument package more than a mile into the sea and haul it back up again.
Besides measuring temperature and salinity (and dissolved oxygen), the package has an array of bottles to bring seawater from various depths back onboard. Jason
Smith (above) uses a sensitive instrument called a salinometer to double-check the
salinity of the water from each bottle, so any error or drift in the lowered electronic sensors
can be accounted for.
Far below us, as we float here in the middle of the Atlantic Ocean, there’s “North Atlantic Deep Water” that left the surface near Greenland long ago; we capture some of it in bottle, 1500 meters deep. A little above that, there’s slightly fresher water that was last at the surface somewhere between Cape Horn (the southern tip of South America) and Antarctica; the bottle from 1000 meters brings some of that up. In less than two hours, we can collect samples of water that began their undersea journeys on opposite sides of the globe. The graph below shows the salinity profiles from our first four casts along 38° W, with water masses labeled.
Far below us, as we float here in the middle of the Atlantic Ocean, there’s “North Atlantic Deep Water” that left the surface near Greenland long ago; we capture some of it in bottle, 1500 meters deep. A little above that, there’s slightly fresher water that was last at the surface somewhere between Cape Horn (the southern tip of South America) and Antarctica; the bottle from 1000 meters brings some of that up. In less than two hours, we can collect samples of water that began their undersea journeys on opposite sides of the globe. The graph below shows the salinity profiles from our first four casts along 38° W, with water masses labeled.
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