Discerning ocean currents at current

Bering Strait, satellite view / Courtesy NASA
Bering Strait, satellite view / Courtesy NASA

Instruments made to measure currents tug against their moorings on the sea floor. Others bob and whirl, catching currents, winds and tides with their rectangular wings spread just under wavetops in the Bering Strait west of Alaska. Ocean water is on the move.

“There’s a strong connection between the world’s ocean currents and what comes through Bering Strait; that’s one of the controls on climate that Bering Strait offers for the global weather,” said Seth Danielson. Danielson is a research oceanographer at the Institute of Marine Science, School of Fisheries and Ocean Sciences, University of Alaska Fairbanks.

The Bering Strait spatially is a shallow and narrow juncture joining two oceans, measuring approximately 164 feet [50 meters] deep and 50 miles [80 kilometers] across. The current that carries water from the Pacific Ocean through Bering Strait to the Arctic Ocean actually flows downhill; locally the Arctic Ocean’s sea floor is roughly 1.64 feet [0.5 meters] lower than the North Pacific’s. Danielson told FrontierScientists “Bering Strait is the choke point between the Pacific Ocean and the Atlantic Ocean and it’s a critical juncture for earth climate processes. It’s a really important point for the globe as a whole.”

Oceans are interconnected systems. The water flowing through Bering Strait is the only connection between the Pacific and Arctic Oceans. “That current is enormous. It’s like having 50 or 100 Missippippi Rivers flowing northward between the Bering Sea and the Chukchi Sea,” said Danielson. While strong enough winds have the potential to temporarily reverse the flow, the current traveling through Bering Strait generally carries sea water and its characteristics and contents north. That means chemicals, nutrients, phytoplankton and zooplankton, as well as water with a certain temperature, salinity, and acidity, are carried into the Arctic Ocean. The water moves at speeds driven by sea level height difference, prevailing winds, currents, and tides. Two of the especially impactful things it brings to the Arctic are heat and freshwater.

Polarstern in the Central Arctic (position approx. 83° N, 130° O). One-year thin sea ice predominated in the Arctic in the summer of 2012. The ice cover is permeated by open water areas and melting ponds. / Courtesy: Stefan Hendricks, Alfred Wegener Institute
Polarstern in the Central Arctic (position approx. 83° N, 130° O). One-year thin sea ice predominated in the Arctic in the summer of 2012. The ice cover is permeated by open water areas and melting ponds. / Courtesy Stefan Hendricks, Alfred Wegener Institute

Danielson stated “The flow heading northward through Bering Strait into the Arctic Ocean is a first-order contribution to the Arctic Ocean freshwater budget.” A lot of the water flowing through the strait is freshwater. An influx of water from snow and ice melt, from melting glaciers, from rivers discharging into the sea and from sea ice that has melted in warmer southern waters means that the ocean’s salt water is diluted; its salinity (dissolved salt content) is lowered. Fresh water has a big impact on sea ice and on how water circulates through the word’s oceans. Danielson: “The water which is relatively fresh coming northward through Bering Strait comes into the Arctic and eventually it makes its way into the North Atlantic and the North Atlantic is where the global ocean circulation is primarily modulated from.”

Saltier water is heavier and more dense than less salty water. So freshwater stays near the top of the Arctic Ocean. That helps sea ice to grow because freshwater freezes at 32°F [0°C]. It takes colder temperatures to freeze salt water; that’s why salt can be thrown on sidewalks to de-ice them (as long as it’s not too cold). The saltiest of salt water requires temperatures of -5.98°F [-21.1°C] to freeze. Most sea water is not fully saturated by salt, but sits somewhere in between. As sea ice starts to slowly form on the sea’s surface salt tends to leech out of the forming ice and escape into the water below, changing the water’s salinity and leaving the sea ice composed of mostly fresh water.

Danielson explained that the large amount of freshwater from “Pacific water coming through Bering Strait helps to cap the sea ice in the Arctic Ocean from the warm Atlantic water down below, so that helps keep the sea ice on top of the Arctic ocean.” Freshwater, lighter and less dense than super salty water, stays near the surface of the Arctic Ocean. It makes it easier for sea ice to grow by freezing at less extreme temperatures. Like a buffer zone, it insulates and protects existing sea ice from touching water encroaching from the North Atlantic which is generally saltier and warmer than Arctic Ocean water.

The sea ice cap cools the air and reflects sunlight away from earth, keeping the Arctic cold. That temperature difference is important in many ways. It helps to regulate thermohaline circulation or ocean circulation, the complex system of currents that cycle water through the world’s oceans. Warm water from near Earth’s equator flows north to the North Atlantic, where it is cooled before being shuttled back south. This system has been compared to conveyor belts that move heat around the planet. Ocean circulation helps regulate Earth’s climate; without it equator regions might be blazing deserts and pole regions places of unrelenting ice. Changing sea ice conditions, warmer air temperatures, and greater-than-usual freshwater concentrations from melting ice can potentially impact and slow the speed of the conveyor belts. Danielson said “The North Atlantic is where the global ocean circulation is primarily modulated from and that flow can be slowed down with the export of fresh water from the Arctic, a large portion of which originally came from Bering Strait.” Arctic changes alter large-scale oceanic circulation trends, which impact global climate trends.

High-resolution models run on supercomputers help simulate Arctic ocean realities, but they’re not easy to build. They require careful calibration combining data about ocean conditions, atmospheric conditions, and sea ice conditions. Watch this FrontierScientists video – part of a series on modeling Arctic waters – featuring Danielson and other scientists as they work together to measure ocean conditions, accurately simulate sea ice trends and heat circulation, and help to predict future climate conditions.

Laura Nielsen 2015

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