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Physical oceanography

SURFO Special: The Power of a Swirl

 

Each summer, the University of Rhode Island Graduate School of Oceanography (GSO) hosts undergraduate students from all over the country to participate in oceanographic research. These Summer Undergraduate Research Fellows (SURFOs) have not only been working with GSO scientists, but they have spent part of their time learning how to communicate this science to the public. Read on to find out what they have been up to, and why they everyone should be as excited as they are about their work.

This post was written by Madeline Mamer. Madeline is a rising senior at the University of Washington in Seattle majoring in Earth and Space Sciences. She spent the quarantined summer of 2020 with Dr. Jaime Palter and Afonso Goncalves investigating the Mid Atlantic Bight. She hopes to continue her education and pursue a graduate school program centered around ices and planetary cryospheres. The remote research experience, while challenging bouncing from East Coast to West Coast time, gave her an opportunity to do something she loves and spend with her family!


Figure 1: Schematic of Western Atlantic continental shelf. Inspired by Zhang and Vallis, 2007.

 

A swirling vortex of doom is responsible for the devastation from hurricanes on the eastern seaboard and changes in fish reserves. Okay, but not really – well, kind of. The continental shelf off of the East Coast of the United States has been changing for at least the last 10 years. Temperature is increasing, sea surface height is becoming higher, and current movement is meandering. It is important because this coast plays a crucial role in fisheries and the global climate system. Not to mention the Gulf and Atlantic Coasts are home to just under 60 million Americans.

The Question

So the simple question gets asked by scientists, ‘What is causing these changes?’ The circulation off of the East Coast is incredibly dynamic, powerful, and important in the ocean’s circulation. It is where the warm waters from the Caribbean get transported to the cold, nutrient rich polar waters. This event influences how the North Atlantic Current heading east towards Europe behaves, which in turn influences Earth’s global climate system. There is no quick and dirty explanation to the changes on the East Coast’s continental shelf, so we answer this question by breaking it down. Specifically, we will be Atlantic Bight (MAB), Northern Recirculation Gyre (NRG), and Deep Western Boundary Current (DWBC).

looking at three major players in the western Atlantic: the Mid Atlantic Bight (MAB), Northern Recirculation Gyre (NRG), and Deep Western Boundary Current (DWBC) (see Figure 1).

The king of them all is the Gulf Stream, which behaves like a hose that no one has a hold of. It meanders like a river. The Gulf Stream can bend and move, and occasionally this movement chokes off the cold southward flowing water from the Arctic. Looking closely at Figure 2, you can see red curves that get ‘cut’ off and flow away in smaller circles—these are called eddies. Scientists discovered that one of these ‘choking’ off events happened in 2008, causing a warming that propagated south from Newfoundland all the way to Cape Hatteras. The area in between Cape Hatteras and Cape Cod (aka the MAB) however, took an odd amount of time to get these warm waters relative to nearby areas.

This is where the NRG and DWBC come into the picture. The NRG is a huge gyre in the North Atlantic that is elusive since it circulates at a depth, meaning we cannot visualize it with sea surface satellite data. The DWBC is an example of how cold, salty water sinks below warmer waters. The hypothesis is that weaker southward DWBC flow from the cold Arctic waters weakens the NRG and MAB Gyres, which increases the influence of the Gulf Stream on the MAB, elevating the sea surface height and slowing down the warm water flowing from the 2008 ‘choking’ event. The Gulf Stream’s influence increases because there is less of a barrier between the two water masses, which lets the Gulf Stream invade the MAB.

Figure 2: A movie of sea surface height (in centimeters) over a period of time in the northwest Atlantic. The Slope Sea Gyre is represented by a black outline, with the lowest point outlined by a circle. The dotted line stretching from Long Island to Bermuda is the Oleander Line (Rossby et al. 2019).

The Investigation

To test if this was the case, I started by asking, ‘How do the physical characteristics of the MAB area change throughout time?’ Physical characteristics such as current perimeter, area, and strength are difficult to measure when there is no structure to the current. Luckily, the MAB is home to the Slope Sea Gyre, so named because the heart of it overlies the drop-off zone of the continental shelf. Using satellite sea surface height data, I isolated this gyre from the rest of the MAB and measured the three S’s: shape, size, strength. What I ended up finding was that the size and shape of the gyre was not really changing in the last quarter century. But in fact, the strength and height of the gyre was changing. The strength of the gyre is measured by the difference between the sea surface height of the outermost circle and the innermost, smallest circle. Looking at Figure 2, you can see how the gyre moves and plays around the MAB. The Slope Sea Gyre changes with the seasons, but there is no noticeable long term trend in size. 

Figure 3: Sea Surface Height Anomalies in cm from 1993 to 2018.

Figure 3 shows the ‘anomalies’ (the difference from the average) of the sea surface height of the edge of the gyre. There is a jump in mid-2009 showing further deviance from normal conditions. This supports the idea that the sea surface height in this region is increasing, and just by qualitatively looking at Figure 2, you can visualize the region becoming redder, meaning a higher sea surface. This could be because of increased intrusion from the Gulf Stream, but that remains to be tested.

Final Thoughts

The Slope Sea Gyre dominates the MAB since it blankets the region, so it is crucial to understand how other oceanic structures impact it. The US East Coast not only governs components of the global climate system, but also serves as a cornerstone in fisheries and is the home to millions, its behavior can change lives and communities. So while the swirling vortex of the Slope Sea Gyre isn’t solely responsible for all of the doom, it can alleviate or agitate the destruction. 

References:

Zhang, R., and G. K. Vallis, 2007: The Role of Bottom Vortex Stretching on the Path of the North Atlantic Western Boundary Current and on the Northern Recirculation Gyre. J. Phys. Oceanogr.37, 2053–2080, https://doi.org/10.1175/JPO3102.1.

Rossby, T., C.N. Flagg, K. Donohue, S. Fontana, R. Curry, M. Andres, and J. Forsyth. 2019. Oleander is more than a flower: Twenty-five years of oceanography aboard a merchant vessel. Oceanography 32(3):126–137, https://doi.org/10.5670/oceanog.2019.319.

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