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Sea of Change: How our Oceans Affect the Climate, Part 2


This is the second half to a blog posted on 25 March 2016. To see part one, click here. We are pleased that our guest blogger, Jacob Spivey, is back to share more information about oceans and climate. Jacob also blogs at Weatherbolt

If part of the ocean has a lower salinity, then it’s going to be less dense and there won’t be as much sinking water there. This can trigger the slowdown of another circulation, the Atlantic Meridional Overturning Circulation, or AMOC (thank goodness scientists abbreviate some of the names that they come up with!). Like the GTC, this is another circulation where warm, salty water flows northward on the surface of the Atlantic before it sinks and flows southward in the depths of the Atlantic. We can think about this more easily as if it was a pinwheel; a pinwheel needs wind to spin, right? If the wind slows down, then the spinning slows down. Same principle goes for the AMOC; if the North Atlantic becomes less salty, then the water will not sink and flow back southward, and the circulation “spins” less. The picture below gives you an idea of what the AMOC looks like through the ocean depths; the arrows that run through the area labeled “NADW” represent the pinwheel in the above analogy.

ocean circulations
Diagram of ocean circulations, looking at the depths of the oceans from a side view. Latitudes are at the top, gray areas represent the solid ocean floor. The area to focus on here is the flow labeled “NADW” where the cold water in the North Atlantic is sinking to the ocean depths and is flowing southward. Credit: http://www.seos-project.eu 
 

Almost there! Over time, a slowing of the AMOC is predicted to raise ocean temperatures off the East Coast of the United States and lower ocean temperatures to the south of Greenland, a trend that has already begun over the last few years. Recent research supports this idea, including a newly-published paper that looks at how the northwestern Atlantic Ocean will undergo enhanced warming in the decades to come.

climate model projection
High-resolution climate model projection of how ocean surface temperatures will change in response to doubling carbon dioxide levels in the atmosphere. Yellow and orange colors represent higher temperatures, blue colors signify colder temperatures, and green colors are neutral. Credit: Saba et al. (2015)

Now, the big question: How are warmer temperatures off the East Coast going to affect our weather going forward? Sometimes, the best way to think about the future is to look at the past. The blizzard that we just had in January encountered much warmer-than-normal temperatures near the Gulf Stream, and one of the highlights of that storm was the heavy bands of snow that plowed into New York City and northeastern Pennsylvania. These bands of snow contained moisture that was enhanced by the warm ocean temperatures, since warmer air can hold more moisture. The warm ocean temperatures also created an even larger temperature difference between cold, wintry land air and the air right above the water off the East Coast. That is significant, since one of the most fundamental rules in meteorology is that large-scale storms, such as this blizzard, can get much stronger in areas where there are big temperature differences.

chart of sea surface temperatures from January 2016
Chart of sea surface temperature departures from normal from January 22-23, when the Blizzard of 2016 was burying the Mid-Atlantic region with snow. When the storm moved off the coast of North Carolina, where ocean temperatures were at least 5 degrees above normal, it quickly strengthened. Credit: Vincent Saba, NOAA

Go back to 2012, and you have a similar situation when Superstorm Sandy was moving northward along the East Coast. Although I discussed the details of Sandy’s life in a previous post, it wasn’t mentioned that the ocean temperatures off the East Coast were much higher than usual, as shown here. It was these warm ocean temperatures that helped to keep Sandy very strong as it moved into an area where tropical systems usually weaken.

The bottom line is that if circulations like the AMOC continue to slow down, then it’s very possible that storm systems could become more extreme along the East Coast. While that may scare some people (since a disappearing AMOC was the motivation behind the apocalyptic storms in the 2004 movie The Day After Tomorrow), there won’t be any kind of changes in our climate system even close to being that dramatic. However, there would still be changes all over the globe if the circulation slowed; the oceans and atmosphere are all connected, and it is a fantastically complicated system. The good news is that there is plenty of new research being conducted on this topic and others that are similar, such as the paper from Saba et al. that I’ve linked to above. This research will lead to interesting discoveries in the years to come, which will help us to better understand the climate system as a whole.

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