The Atlantic Meridional Oceanic Current and its Role in the Global Climate System

The Atlantic Meridional Oceanic Current and its Role in the Global Climate System

Olivia Kellner, PhD

Climate Impact Company Research Scientist

Note: All images within the document were obtained from online sources for educational purposes only and are not property of Climate Impact Company. Sources of the images are provided at the end of the document.

Executive Summary: The Atlantic Meridional Oceanic Current (AMOC, also referred to as the Atlantic Meridional Overturning Circulation) in a smaller oceanic current within the Global Thermohaline Circulation (THC) that traverses northward from the Equatorial Atlantic Ocean to the North Atlantic Ocean. It is responsible for transporting warm equatorial surface ocean waters northward along the Gulf Coast and East Coast of the United States northeast across the North Atlantic Ocean where the water cools, releases heat to the atmosphere west of Europe, and then sinks due to its greater density.  The Gulf Stream (GS) is a defined portion of the AMOC off of the East coast of the United States that moderates climate, acts as fuel source for tropical and mid-latitude low pressure systems, and with its higher heat energy, can directly influence the motion storm systems.  The rate of transport of the AMOC directly impacts global weather and climate.

Overview of the Global Climate System (GCS) and the role of the THC

Earth’s GCS exists due to the uneven distribution of solar radiation across latitudes on a rotating planet.  Radiation from the sun strikes the equator at a direct 90°angle over a smaller surface area compared to the polar latitudes where the same amount of radiation is spread across a larger surface area at a less intense angle (Fig. 1).

Figure 1: Diagram showing the difference in solar radiation intensity and surface area distribution between the equator and poles.

The uneven distribution of solar radiation results in the atmosphere, oceans, and landmasses at tropic and subtropic latitudes to be warmer than the atmosphere, oceans, and landmasses at the midlatitude, and polar latitudes.  Because Earth’s climate system works to redistribute energy, mass, and momentum globally, the uneven distribution of solar radiation results in global atmospheric circulation cells (GACCs) and the THC (Fig. 2). Both the GACs and the THC work to redistribute energy, mass, and momentum globally to achieve a state of thermal equilibrium.

Figure 2: Global Atmospheric Circulation Cells (GACCs) resulting from the uneven distribution of solar radiation on Earth that work to redistribute heat in the atmosphere (left), and the global thermohaline circulation (THC) that is driven by atmospheric winds (GACCs) and ocean temperature and salinity gradients to redistribute heat in ocean waters (right).

The GCS is impacted by natural and anthropogenic components.  Natural components include solar radiation (relatively constant through time) and latitude (as just explained), Milankovitch Cycles, natural greenhouse gas (GHG) concentrations, and variation in Earth’s wobble.  Anthropogenic components that influence the balance of the GCS include emission of GHGs that alter natural GHG concentrations and land use and land cover change (LULC).  LULC subsequently impacts surface albedo and Earth’s radiation budget.  As the THC is the other half of the GCS (remember GACCs are the other half), anthropogenic impacts to Earth’s atmosphere will also impact how heat is redistributed throughout oceans and the ocean-atmosphere interface (and vice versa).

Latest Research on the AMOC

Research within the last several decades has documented a slowing in the AMOC.  However, recent research published in Nature Geoscience further emphasizes and builds on past research findings utilizing 11 proxy sources of climate data.  The study provides a more precise measurement of AMOC flow rates, and better identifies the influence of different flow rates on Earth’s climate during those times. According to this newest publication and its findings, the consensus that the AMOC is currently at its weakest (slowest) state in over a millennium is further solidified, with 15% of its observed weakening occurring since 1950.  Weather and climate records also show one of the most distinct signatures of anthropogenic influence on weather and climate during this time as well.

What Does the Slowing of the AMOC and THC Mean for Global Weather and Climate?

In a natural, balanced state, cold and dense salty water that drives the deep currents within the THC originates off the coast of Greenland in the North Atlantic Ocean. As ocean water freezes into sea ice and glaciers, salt is left behind in the unfrozen water, increasing the water’s salinity and density.  Once the AMOC reaches northern latitudes and releases heat to the atmosphere near western Europe, the cooler waters of the AMOC circulate westward and encounter the colder ocean waters of higher salinity.  The two waters mix, becoming heavier than warmer surrounding ocean waters and sink to the deep ocean. The sinking water drives the continuous current as warmer surface waters move into the place where colder and more dense water has sunk to lower depths.

If the momentum of the AMOC (and subsequently, the THC) slows, heat within the GCS will not be redistributed quickly enough from the equator to the poles.  Subsequently, atmospheric and ocean water temperatures at tropic, subtropic, and mid-latitudes will get warmer, and climate change impacts will be exacerbated beyond levels already projected by current climate models.  Impacts include:

• Gulf Coast and East Coast will see stronger tropical, subtropical, and mid-latitude storms systems as the heat within the AMOC (Florida Current and Gulf Stream) increases (Fig. 3).
• Greater levels of sea-level rise along the Gulf and East Coasts.
• Western Europe will see shifts in temperature and precipitation
• A decrease in momentum of the THC will move the GCS to a state of imbalance and a cascade of high-impact weather and climate events will ensue.

Figure 3: The Gulf Stream and its loop currents through the Caribbean and Gulf of Mexico are components of the AMOC, which is a larger current of the global THC.  The Gulf Stream and loop currents are common heat and energy source for tropical, subtropical, and midlatitude storm system

Components of the AMOC

The North Atlantic Warm Hole (NAWH) is a sea surface temperature (SST) anomaly of cooler ocean waters across the midlatitudes of the North Atlantic Ocean southeast of Greenland.  With mean global temperature increasing in recent decades and climate projections showing increasing temperatures in both the atmosphere and oceans, one would deduce that ocean temperatures should warm across the Midlatitudes as well. The NAWH is not actually a region of ocean waters that is cooling, but a region of ocean water not experiencing the same amount of warming as that of the surrounding waters (Figure 4).

Figure 4: 2019 0-700m ocean heat content.  The NAWH is noted within the yellow box for clarity.

The NAWH exists due to the complex interactions between freshwater flux (melting sea ice, freshwater stream and river discharge into the Labrador Sea and surrounding ocean), the AMOC, and the North Atlantic subpolar gyre (SPG).  The SPG is a key component of the THC, and assists in deep ocean convection and deep-water formation that keeps the THC moving.  Weakening (strengthening) of the AMOC lends to SPG expansion (contraction), which solicits changes in salinity(density) and heat transport from the subtropics, manifesting as the NAWH.  Numerous research articles demonstrate that the magnitude of the change of the anomaly of the NAWH in recent years supports the slowing of the AMOC.  As the ocean-atmosphere interface is a continuous exchange of energy, mass, and momentum, the atmosphere above the NAWH will respond impacting weather downstream across Europe/Eurasia (Chemke et al. 2020, Drijfhout et al. 2012, Gervais et al. 2018, Gervais et al. 2020, and references therein).

The AMOC and Antarctic Sea Ice Loss, Are They Related?

While the momentum of the AMOC contributes to the overall momentum of the THC, the anthropogenic slowing of the AMOC as exemplified in research cannot be linked to Antarctica sea ice loss.  Antarctic sea is influenced by geography, Southern Ocean dynamics, absence of Polar Amplification, the presence of the Southern Annular Mode, and the Antarctic Circumpolar Current.  These acting forces are different than the drivers of the AMOC and Arctic sea ice.  Despite noted anthropogenic influence on global climate, research shows that for approximately the last 40 years, Antarctic sea ice shows a slightly positive trend overall (opposite of that in the Arctic), despite some decline in smaller regions and larger decline across the Antarctic Peninsula.  While 2016-2019 experienced markedly lower sea ice extent across the Antarctic compared to 1981-2020 normals, sea ice extent for spring 2020 were near the long-term average, indicating no shift in the long-term trend (Michon 2020, Parkinson 2014, Parkinson et al. 2019; Pritchard et al. 2012, and references therein).

References

Figures/Graphics:

Figure 1: https://www.theweatherclub.org.uk/index.php/node/373

Figure 2: https://www.theweatherclub.org.uk/index.php/node/373 and http://www.pik-potsdam.de/~stefan/thc_fact_sheet.html.

Figure 3: https://scijinks.gov/gulf-stream/

Figure 4: https://www.ncei.noaa.gov/access/global-ocean-heat-content/

Research Articles:

Chemke, R., Zanna, L. & Polvani, L.M. Identifying a human signal in the North Atlantic warming hole. Nat Commun 11, 1540 (2020). https://doi.org/10.1038/s41467-020-15285-x.

Comiso, J.C., Gersten, R.A., Stock, L.V., Turner, J., Perez, G.J. and Cho, K. Positive trend in the Antarctic sea ice cover and associated changes in surface temperature. J. Climate 30 2251-2267 (2017).

Drijfhout, S., van Oldenborgh, G.J., & Cimatoribus, A. Is a decline of the AMOC causing the warming hole above the North Atlantic observed in modeled warming patterns? J. Climate 14:24, 8373-8379 (2012). https://doi.org/10.1175/JCLI-D-12-00490.1.

Gervais, M., Shaman, J. & Kushnir, Y. Mechanisms governing the development of the North Atlantic Warming Hole in the CSM-LE future climate simulations. J. Climate 31:15, 5927-5946 (2018). https://doi.org/10.1175/JCLI-D-17-0635.1.

Gervais, M., Shaman, J. & Kushnir, Y. Impact of the North Atlantic Warming Hole on sensible weather. J. Climate 33:10, 4255-4271 (2020). https://doi.org/10.1175/JCLI-D-19-0636.1.

Michon, Scott: Understanding climate: Antarctic aea ice extent. Climate Watch. 28 April 2020 https://www.climate.gov/news-features/understanding-climate/understanding-climate-antarctic-sea-ice-extent.

Parkinson, C.L. A 40-y record reveals gradual Antarctic sea ice increases followed by decreases at rates far exceeding the rates seen in the Arctic. Proceedings of the National Academy of Sciences 116 (29) 14414-14423 (Jul 2019); DOI: 10.1073/pnas.1906556116

Parkinson, C.L. Global sea ice coverage from satellite data: Annual cycle and 35-yr trends. J. Climate 27:24 9377-9382 (2014). https://doi.org/10.1175/JCLI-D-14-00605.1

Pritchard, H.D., Ligtenberg, S.R.M., Fricker, H.A., Vaughan, D.G., van den Broeke, M.R. and Padman L. Antarctic ice-sheet loss driven by basal melting of ice shelves. Nature 484, 502-505 (2012). Doi:10.1038/nature10968