Atlantic Ocean measures

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Atlantic Ocean measures

The Atlantic meridional overturning circulation (AMOC) is a measure of the volume transport northwards at the surface and southwards at depth in the ocean. It is measured through multi-instrument arrays at particular latitudes.

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AMOC across the RAPID array at 26.5°N in the Atlantic

AMOC across the RAPID array at 26.5°N in the Atlantic

Monthly and 12-month running mean AMOC across the RAPID array at 26.5°N. Two datasets are shown; RAPID array observational data and GloSea5 global ocean reanalysis data. They are in good agreement on the inter-annual and longer-term variability of the AMOC.

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AMOC across the OSNAP array in the subpolar North Atlantic

AMOC across the OSNAP array in the subpolar North Atlantic

Monthly and 12-month running mean AMOC across the OSNAP array in the subpolar North Atlantic. Two datasets are shown; OSNAP array observational data and GloSea5 global ocean reanalysis data. They agree on the monthly variability (not shown), but the OSNAP dataset is too short to robustly determine longer term variability.

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Subpolar North Atlantic ocean heat content (SpNA-OHC) to a depth of 1000m expressed as a difference from 1991-2020 average

Subpolar North Atlantic ocean heat content (SpNA-OHC) to a depth of 1000m expressed as a difference from 1991-2020 average

Monthly-mean subpolar North Atlantic ocean heat content (SpNA-OHC) anomaly in the upper 1000m of the North Atlantic Ocean (latitude range 45°-67°N), relative to the long term average (1991-2020). Two observational datasets are shown; EN4 and Argo. There is good agreement in their long-term trends and inter-annual variability.

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Why are the AMOC and SpNA-OHC important?

The Atlantic Meridional Overturning Circulation (AMOC) is a measure of the movement of water in the North Atlantic: warm water is transported northward into the subpolar North Atlantic (high northern latitudes), where it cools and sinks before returning southward in the deep ocean. This northward transport of heat is responsible for the mild climate of Western Europe compared to North-Eastern North America. Changes in the AMOC change the sea temperature in the North Atlantic and hence have a wide range of impacts, including on temperature and precipitation in Europe, sea level along the US East Coast, Atlantic hurricane activity and tropical monsoons. Therefore, monitoring changes in the AMOC is crucial to understand future changes in the climate of these regions.

Graphical representation of Ocean Circulation
Figure 1: Representation of the circulation of water in the global ocean. Warmer, fresher, surface water is represented by the red arrows, with colder, saltier, deep water by the blue arrows. In the North Atlantic, warm surface waters travel north-east, before cooling and sinking and returning southwards. This is the Atlantic Meridional (north-south) Overturning Circulation, or AMOC.

The AMOC transports heat northwards, and so it can have a significant impact on the subpolar North Atlantic ocean heat content (SpNA-OHC), although this is also affected by the heat transport from wind-driven flow and heat exchanged between the ocean and atmosphere. The SpNA-OHC captures the long-term changes of heat transport into the subpolar regions of the North Atlantic.

The Subpolar North Atlantic Ocean Heat Content (SpNA-OHC) is a measure of the amount of heat stored in the upper 1000m of the ocean in the North Atlantic region (45°-67°N). It measures changes in the heat supply to this region, and is linked to the AMOC and other measures of Atlantic variability.

How have the AMOC and SpNA-OHC changed?

The AMOC has been continually measured across the North Atlantic at two latitudes: the RAPID array is at 26.5°N (in the subtropical North Atlantic), and the OSNAP array at 50-60°N (in the subpolar North Atlantic). However, only relatively short timeseries are available from these two, allowing only multi-annual (and shorter timescale) variability to be determined rather than multi-decadal variability.

There is significant inter-annual variability in the subtropical North Atlantic overturning circulation from the RAPID measurements, with a large decline in 2009/10. Much of this variability is driven by changes in local winds. There was a weakening of the subtropical AMOC from 2004-2014, although a longer timeseries is needed to put this into context. An estimate of the subtropical AMOC from the GloSea5 reanalysis suggests it was stronger in the late 90's, declining around 2000 before increasing to the mid-2000's.

The subpolar AMOC observed across the OSNAP array also has large inter-annual variability and a strong seasonal cycle. Only four years of data are available from the OSNAP array observations. An estimate of the AMOC at the OSNAP array from the GloSea5 reanalysis has had no long-term trend since 1993, although it has significant inter-annual variations. However, changes earlier in the timeseries are more uncertain as subsurface observations are sparse during this period and the trend differs among reanalysis products.

The SpNA-OHC increased through the late 90's and early 2000's, before declining after 2011. Since 2013, the OHC has been below the 1991-2020 average, so the northern North Atlantic is colder than normal.

How are the AMOC and SpNA-OHC measured?

Continuous observations of the AMOC are obtained by deploying basin-wide observing arrays across the Atlantic Ocean that are used to infer the volume flow rate across the array in units of Sverdrups (1Sv = 106 m3 s-1). The arrays measure ocean velocity, pressure, temperature and salinity using various sensors. Full depth moorings are used to estimate the density profiles across the basin, allowing the net transport in the ocean interior to be calculated. Transports of water by surface winds (known as Ekman transports) and transports from boundary currents obtained from velocity measurements are added to obtain the total transport.

A reconstruction of the AMOC strength from the GloSea5 ocean reanalysis is also shown at both latitudes. Ocean reanalyses combine observations with an ocean model based on physical equations and properties to infer past changes in climate. The GloSea5 reanalysis uses observations of ocean temperature, salinity, sea ice concentration and sea level from both in situ and satellite observations.

The SpNA-OHC is the heat content of the upper 1000m of the subpolar North Atlantic (45°-67°N) calculated from temperature observations (using Argo or EN4 datasets) and is presented as the difference from the long-term (1991-2020) average.

Why have the AMOC and SpNA-OHC changed?

The AMOC varies significantly on many timescales. Changes in the strength of the wind has an important impact on monthly and inter-annual variations, while decadal changes are caused by internal variability of the ocean and longer-term climate forcings such as increases in greenhouse gases.

The AMOC weakens in response to increases in atmospheric greenhouse gas concentrations because it leads to warming and freshening (from increased precipitation and the melting of Arctic sea ice and the Greenland ice cap) of the North Atlantic. Detecting human influence on changes of the AMOC is challenging, because it requires long timeseries to distinguish it from internal variability.

Changes in the subpolar North Atlantic OHC (SpNA-OHC) are caused by both changes in the surface heat fluxes and ocean circulation, although it is thought ocean circulation changes dominate on timescales greater than about a decade.

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AMOC

RAPID (12 month mean)
  • Information reference: Moat B.I.; Smeed D.A.; Rayner D.; Johns W.E.; Smith, R.; Volkov, D.; Elipot S.; Petit T.; Kajtar J.; Baringer M. O.; and Collins, J. (2024). Atlantic meridional overturning circulation observed by the RAPID-MOCHA-WBTS (RAPID-Meridional Overturning Circulation and Heatflux Array-Western Boundary Time Series) array at 26N from 2004 to 2023 (v2023.1), British Oceanographic Data Centre - Natural Environment Research Council, UK.

RAPID
  • Information reference: Moat B.I.; Smeed D.A.; Rayner D.; Johns W.E.; Smith, R.; Volkov, D.; Elipot S.; Petit T.; Kajtar J.; Baringer M. O.; and Collins, J. (2024). Atlantic meridional overturning circulation observed by the RAPID-MOCHA-WBTS (RAPID-Meridional Overturning Circulation and Heatflux Array-Western Boundary Time Series) array at 26N from 2004 to 2023 (v2023.1), British Oceanographic Data Centre - Natural Environment Research Council, UK.

GloSea5 (12 month mean)

AMOC

OSNAP (12 month mean)

OSNAP

GloSea5 (12 month mean)

OHC

EN4 SpNA-OHC

Argo SpNA-OHC

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AMOC

AMOC

OHC