Researchers often see sea level rise on the US East Coast at the same time as the Gulf Stream flow decreases.
(With considerable help from Dr. Tal Ezer)
Water level along the US east coast varies because of tides, winds, land motion (e.g., subsidence), global sea level rise and many other processes. One process that just recently has been more recognized is the contribution to variations in sea level from changes in the strength and position of the Gulf Stream. In this blog we’ll explain what the role of the Gulf Stream the Gulf Stream plays.First let’s look at the Gulf Stream
The Gulf Stream is called a Western Boundary current being on the western side of the North Atlantic Ocean. The analogous current in the North Pacific is the Kuroshio off Taiwan and Japan. These currents transport heat in the ocean water from the lower latitudes to higher latitudes. That heat is what causes northeasters to intensify off Cape Hatteras.
Figure of Gulf Stream Sea Surface Temperature in winter. The temperature of the ocean surface is easily measured by satellite. This image shows the warm water (colored red) flowing north off Florida, past Cape Hatteras and then northeast into the North Atlantic. Cold water (colored blue) is coming from the north and spread southward along the coast.
The transport of the Gulf Stream when it leaves the Florida Straits (i.e., the “Florida Current”) is about 25-35 Sverdrup (Sv is a popular unit used by oceanographers equal to 1 million cubic meter per second or 264 million USgal/s). However, downstream in the Mid-Atlantic its flow increases to about 100-150 Sv when recirculation currents north and south of the Gulf Stream join the main current. For comparison the Mississippi River transport is about 0.02 Sv (5000 Mississippi Rivers equal 1 Gulf Stream).
Now you should be asking “How do you measure the Gulf Stream?”
The first way I’ve always found amazing. The Gulf Stream is a flowing conductive fluid (salt water) and it is flowing through the magnetic field of the earth. Recalling your high school physics an electric current will be generated. But how do you measure it?
Luckily there is an abandoned telephone cable between Florida (West Palm Beach and Vero Beach) and Grand Bahamas Island. The voltage in the circuit formed by the cable and the seawater is proportional to the transport of the Gulf Stream. (more info at Link
Figure. Location of cables used for measuring the Florida Current flow (source: NOAA/AOML Link
And here is a plot of the transport obtained from these cables.
Figure. Florida Current transport in millions of cubic meters per second. Envision thousands Mississippi Rivers flowing past Miami. From same site as previous figure.
A second way to capture the Gulf Stream is by measuring the surface height of the ocean using satellite altimetry. The altimeter measures the height of the surface of the ocean, and after tides, geoid, atmospheric pressure and other components are removed, surface ocean currents can be detected. The reason this works is that the intensity (speed) of a surface ocean current is proportional to the sea level slope across the current (this is called a “geostrophic balance” in oceanography and very similar to winds around high and low pressure areas in the atmosphere). In particular, there is a change of sea level across the Gulf Stream, whereas sea level on the shore side of the Gulf Stream (say in the Mid-Atlantic coast) is lower than the offshore side (say near Bermuda) by about 1-1.5 m (~3-5 feet) – see figure below. Therefore, any change in the strength and position of the Gulf Stream can affect the coastal sea level (in an extreme and unlikely case that the Gulf Stream completely shut down, sea level along the U.S. East Coast might rise by up to 3-5 feet!).
Figure. A schematic diagram of the North Atlantic circulation- the northward flowing Gulf Stream is the upper branch of AMOC, then water cooled down in polar and sub-polar regions, become denser and sink down to form the southward flowing part of the circulation (source: T. Ezer, ODU). (More information on AMOC is available from the international measuring program called RAPID which has monitored the AMOC since 2004: Link
Note that the Gulf Stream is not a steady, straight-flowing current, instead, it is continuously changing its position and strength- it meanders like a snake and occasionally eddies are separated from it and travel away from the main current. These are variations that can be seen over short periods of days, weeks and several months. On the other hand the Gulf Stream also experienced long-term changes over periods of years, decades and longer due to variations in the climate over the Atlantic Ocean (some natural oscillations and some related to the global warming trend). So let’s look at two different aspects of the Gulf Stream- long-term and short term.Long-term variations in the Gulf Stream
So how will the Gulf Stream change if climate is changing? Well, for a long time oceanographers predicted that under a warmer future climate the Atlantic Ocean circulation (represented by AMOC) will slow down. In high latitudes water will become warmer and fresher (from melting ice), so they will be less dense and will not sink as fast as before. Therefore, if the Gulf Stream will also weaken, sea level along the coast will rise. Recent research in fact found evidence in data that this Gulf Stream slowdown may have started already and the result can be seen in accelerated rate of sea level rise (Link
) The figure below demonstrates that during periods of slowdown in the Gulf Stream, coastal sea level is often higher than normal, causing increase in flooding (Link
Figure. Coherent long-term oscillations of sea level from tide gauges data (thin color lines) are anti-correlated with the Gulf Stream strength index (wide red line) from satellite altimeter data (from publication by Ezer et al., 2013; LinkShort-term fluctuations in the Gulf Stream
The top figure shows the hourly observed water level in Norfolk (red) relative to the predicted tide (blue). Green is the observed anomaly relative to predicted. The bottom figure shows the daily water level anomaly (green) and the Florida Current measurement (blue). (Figure source: T. Ezer, ODU)
Keep in mind that research on the changes of the Gulf Stream, the AMOC and the overall North Atlantic circulation and the relation of these changes to coastal sea level is an area of very active research so stay tuned for updates on our understanding. We also would like to point out that continuous observations of the GS after it separated from the coast and moves eastward (ship board current measurements and satellites) cover just over 20 years, so detecting a statistically significant long-term trend in the GS flow is not clear in all data (for example, see a study of the stability of the GS by a group headed by Tom Rossby of the University of Rhode Island; Link
While it may appear that Rossby’s data contradicts several other studies that consistently show a significant weakening trend in the GS and AMOC, a later study (Link
) compared Rossby’s data with two other independent measurements, showing that there is no real discrepancy between different data- all showing the same periods when the Gulf Stream and AMOC declined while coastal sea level rose.
As for detecting the long-term trend in these processes and the causes, one point is clear- we need more data, so we should continue the monitoring programs, and also continue this new area of research to better understand how large-scale currents like the Gulf Stream might affect local coastal sea level.
Since direct measurements are often sparse, computer ocean circulation models (representing the ocean’s physics) can help us to better understand the observations. For example, a new study (Ezer, 2016 Link
) demonstrates how short-term variations in the Gulf Stream near Florida can result in coherent sea level variations along the entire U.S. East Coast. This model results explain why we see unexpected coastal flooding when the Gulf stream suddenly weakens.
Some scientific publications on the topic:
Sweet, W. V., and C. Zervas (2011), Cool-season sea level anomalies and storm surges along the U.S. East Coast: Climatology and comparison with the 2009/10El Niño, Mon. Weather Rev., 139, 2290–2299 Link
Ezer, T. and W. B. Corlett (2012), Is sea level rise accelerating in the Chesapeake Bay? A demonstration of a novel new approach for analyzing sea level data, Geophysical Research Letters, Vol. 39, L19605. Link
Sallenger, A. H., K.S. Doran, P. Howd (2012), Hotspot of accelerated sea-level rise on the Atlantic coast of North America. Nature Climate Change, 2, pp. 884–888. Link
Smeed, D. A., G. McCarthy, S.A. Cunningham, E. Frajka-Williams, D. Rayner, W.E. Johns, C.S. Meinen, M.O. Baringer, B.I. Moat, A. Duchez, H.L. Bryden (2013), Observed decline of the Atlantic Meridional Overturning Circulation 2004 to 2012. Ocean Science Discussion, 10, pp. 1619–1645. Link
Ezer, T. (2013), Sea level rise, spatially uneven and temporally unsteady: why the U. S. east coast, the global tide gauge record and the global altimeter data show different trends, Geophysical Research Letters, 40(20), 5439-5444. Link
Ezer, T. L. P. Atkinson, W. B. Corlett and J. L. Blanco (2013), Gulf Stream's induced sea level rise and variability along the U.S. mid-Atlantic coast, Journal of Geophysical Research, 118(2), 685-697.Link
Ezer, T. and L. P. Atkinson (2014), Accelerated flooding along the U. S. East Coast: On the impact of sea level rise, tides, storms, the Gulf Stream and the North Atlantic Oscillations. Earth's Future, 2(8), 362-382. Link
Sweet, W. V., & Park, J. (2014). From the extreme to the mean: Acceleration and tipping points of coastal inundation from sea level rise. Earth's Future, 2(12), 579-600. Link
Rossby, T., C. N. Flagg, K. Donohue, A. Sanchez-Franks, and J. Lillibridge (2014), On the long-term stability of Gulf Stream transport based on 20 years of direct measurements, Geophysical Research Letters, 41, 114–120. Link
Ezer, T. (2015), Detecting changes in the transport of the Gulf Stream and the Atlantic overturning circulation from coastal sea level data: The extreme decline in 2009-2010 and estimated variations for 1935-2012, Global and Planetary Change, 129, 23-36. Link
Ezer, T., (2016), Can the Gulf Stream induce coherent short-term fluctuations in sea level along the U.S. East Coast?: A modeling study, Ocean Dynamics, 66(2), 207-220. Link