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RAPID 26°N ARRAY

The RAPID/MOCHA/WBTS (hereafter, “RAPID”) project has pioneered the use of
arrays of moorings to measure the variability of the meridional overturning
circulation (MOC), sustaining the observations for more than 20 years.


THE ATLANTIC MERIDIONAL OVERTURNING CIRCULATION (AMOC): WHAT IS IT AND WHY IS IT
SO IMPORTANT?



 

The Atlantic meridional overturning circulation (AMOC) carries 1.2PW (a petaWatt
is a million-billion Watts) of heat northwards in the ocean – a quarter of the
combined global atmosphere-ocean poleward transport and equivalent to the power
output from 1.2 million power plants. The MOC is both a regulator of and
vulnerable to global climate change. In the past 100,000 years, rapid variations
in Earth’s climate (e.g., 10°C changes in Greenland temperatures within a 5 to
25 year period) have been attributed to MOC shutdown.

In the present climate, multi-decadal fluctuations in the Atlantic have been
attributed to MOC variations, and correspond to periods with more intense
hurricanes and droughts in the Sahel, variations in central England
temperatures, and sea level change along the US East Coast. Future projections
of the MOC strength show that this large-scale circulation pattern is very
likely to weaken over the 21st century; there is medium confidence that the MOC
will not shutdown before 2100, though physically plausible, and the likelihood
increases after 2100 under higher emissions scenarios (IPCC SROCC).

 


Transport timeseries obtained from the first 18.8 years of observations at
26.5°N. The different curves show the MOC (red line) and its constituents, i.e.
the transport through the Florida Straits (blue line), the Ekman transport
(green line), and the density driven transport obtained from the mooring data
(pink line). The transport units are Sverdrups (Sv, 1Sv = 106 m3 s-1). The mean
and standard deviations for the different transports are 17.1 ± 4.64v (MOC),
31.7 ± 2.8Sv (Florida Straits), 3.7 ± 3.4Sv (Ekman), and -18.3 ± 3.4Sv
(transport from mooring densities). Brief descriptions of how the transports are
calculated are given below

The MOC strength is a key indicator of the state and tendency of the climate and
is commonly used as a metric for numerical models–hindcasts, forecasts and
predictions. Continuous observations of the MOC strength at 26°N have
underpinned new understanding of the large-scale ocean circulation. These new
discoveries have included:

 1. The MOC varies on timescales of days to a decade, where previously the
    expectation was for variability on timescales of decades to centuries.
 2. Wind-forcing plays a dominant role in MOC variability, where previously it
    was believed to primarily respond to buoyancy forcing at high latitudes, and
 3. With the further installation of additional Atlantic arrays, meridional
    coherence of the MOC appears much more complicated than previously expected.
    The schematic representation of the MOC is as a 'great ocean conveyor' where
    the expectation is that, like a conveyor belt in a supermarket, when the
    conveyor speeds up at one location, it speeds up by the same amount at every
    other location.

These new discoveries have fundamentally changed our view of the large-scale
ocean circulation and variability, and our approach to measuring this key
component of the Earth's climate system.

Over the 2004 – 2022 observing period, this new method of observing ocean
transports has revealed unexpected variations at every measured timescale.
Besides the large variability discovered on short time scales, the array
revealed a major downturn of the MOC in 2010–11 that marked the beginning of a
sustained decline since that time. From the data through 2012, the ocean
community learned that the MOC at 26°N could reverse, albeit for short periods,
and that interannual variations in associated meridional heat transports could
result in measurable basin-average temperature changes.

 


RAPID-EVOLUTION

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