Ocean Dynamics and Sea Level

So we all know about the thermohaline circulation redistributing heat and salt around the World, it's easy to imagine how it can have an important role in climate through heat transport, but perhaps a little more difficult to imagine how changes in the ocean currents can affect sea level. Struggle no longer!


Until three months ago I didn't even know that changes in ocean dynamics could affect sea level, but now I do, so I'm going to tell you all about it. I'm going to talk about the Gulf Stream, part of the Atlantic Meridional Overturning Circulation, firstly because I think as ocean currents go, it's quite famous, and secondly it is causing serious problems on the American east coast.

Satellite altimetry image of the Gulf Stream (NOAA)

The Gulf Stream flows along the American east coast until Cape Hatteras where it changes direction and starts moving in the towards Europe where it has a warming influence - European countries are a lot warmer than land at equivalent latitudes in North America (Bryden et al, 2005). The Gulf Stream flows close to the surface, this means that its surface velocity controls the sea surface gradient on either side. This means that sea level on the American Atlantic Coast can is up to 1.5 metres lower than you might expect without the effects of the Gulf stream (Atkinson et al.,2013).


But guess what? The speed of the Gulf Steam is highly variable, and its distance from the coast also changes. Both of these have a direct influence on sea level - the faster the Gulf Stream the lower the sea level, and increased distance from the coast also contributes to lower sea level (Ezer et al, 2013). At the moment sea level along the coast is rising up to three times faster than global mean sea level, and this has partly been attributed to changing ocean dynamics  - the weakening Gulf Stream (Ezer et al, 2013).


So why is the Gulf Stream weakening? The main reason is thought to be climate change. As we know the warming climate is encouraging ice to melt. The North Atlantic is an important site of deep water formation, and increasing melting of glaciers and ice sheets are adding fresh water to the North Atlantic and making the water less dense, and therefore harder to sink (Leverman et al, 2005). This is causing a slowdown of the Gulf Stream, decreasing the sea surface gradient, and therefore causing a relative sea level rise along the coast. Research by Bryden et al (2005) at 25 degrees north, suggests that the Gulf Stream has slowed by as much as 30% between 1957 and 2004. This sounds really high, but it is important to remember that they do not have any data prior to 1957, and for such a short time period it is difficult to say whether this could just be part of natural variability. However, models of Gulf Stream strength and sea level along the US east coast with additions of freshwater in the North Atlantic have backed up their findings (Leverman et al, 2005).


But the Earth is continuing to warm, and as we have looked at previously, forecasts do suggest a reduction in land ice in the Arctic Circle. So continual ice melting will have a secondary effect on sea level - in addition to adding water to the oceans it can also change the ocean currents and therefore the sea level. Everything is connected. For the American east coast - it is all connected in a really bad way.

And this is what the Gulf Stream looked like 200 years ago, so as you can see it really has undergone some serious changes...! (NOAA)