Climate simulations predict a slowing of the Atlantic meridional overturning circulation (MOC), a key oceanic component of the climate system, while continuous observations of the MOC from boundary ...arrays demonstrate substantial variability on weekly to interannual time scales. These arrays are necessarily limited to individual latitudes. A potential proxy for the MOC covering longer time scales and larger spatial scales is desirable. Here we use sea surface height data from satellites to estimate the interannual variability of transbasin ocean transports at 26°N. Combining this estimate with surface Ekman transport and cable measurements of the Florida Current, we construct a time series of the MOC from 1993 to 2014. This satellite‐based estimate recovers over 90% of the interannual variability of the MOC measured by the RAPID 26°N array. This analysis complements in situ observational efforts to measure the MOC at multiple latitudes and opens the door to a broader spatial understanding of the Atlantic circulation variability.
Key Points
Western boundary dynamic height governs interannual variability of transports
Sea level anomalies in the west anticorrelate transbasin transports at 26°N
The MOC 1993–2014 weakened (~1 Sv) due to a weakening of the Florida Current
The Atlantic Meridional Overturning Circulation (AMOC) is responsible for a variable and climatically important northward transport of heat. Using data from an array of instruments that span the ...Atlantic at 26°N, we show that the AMOC has been in a state of reduced overturning since 2008 as compared to 2004–2008. This change of AMOC state is concurrent with other changes in the North Atlantic such as a northward shift and broadening of the Gulf Stream and altered patterns of heat content and sea surface temperature. These changes resemble the response to a declining AMOC predicted by coupled climate models. Concurrent changes in air‐sea fluxes close to the western boundary reveal that the changes in ocean heat transport and sea surface temperature have altered the pattern of ocean‐atmosphere heat exchange over the North Atlantic. These results provide strong observational evidence that the AMOC is a major factor in decadal‐scale variability of North Atlantic climate.
Key Points
New data from the RAPID 26°N array show that the AMOC has been in a state of reduced overturning since mid‐2008
Observations of heat content and SSH indicate that the impact of the reduction in the AMOC is similar to that predicted by climate models
The results indicate that changes in ocean heat transport have altered ocean‐atmosphere heat exchange over the North Atlantic
The Atlantic meridional overturning circulation (MOC) plays a critical role in the climate system and is responsible for much of the heat transported by the ocean. A mooring array, nominally at 26°N ...between the Bahamas and the Canary Islands, deployed in Apr 2004 provides continuous measurements of the strength and variability of this circulation. With seven full years of measurements, we now examine the interannual variability of the MOC. While earlier results highlighted substantial seasonal and shorter timescale variability, there had not been significant interannual variability. The mean MOC from 1 Apr 2004 to the 31 March 2009 was 18.5 Sv with the annual means having a standard deviation of only 1.0 Sv. From 1 April 2009 to 31 March 2010, the annually averaged MOC strength was just 12.8 Sv, representing a 30% decline. This downturn persisted from early 2009 to mid‐2010. We show that the cause of the decline was not only an anomalous wind‐driven event from Dec 2009–Mar 2010 but also a strengthening of the geostrophic flow. In particular, the southward flow in the top 1100 m intensified, while the deep southward return transport—particularly in the deepest layer from 3000–5000 m—weakened. This rebalancing of the transport from the deep overturning to the upper gyre has implications for the heat transported by the Atlantic.
Key Points
New observations of the interannual variability of the Atlantic MOC in 2009‐10
The 30% weakening of the MOC driven by extreme winds and increased upper ocean flow
This variability has a large impact on the heat transported in the Atlantic
Determining whether the Atlantic Meridional Overturning Circulation (AMOC)'s transport is in decline is challenging due to the short duration of continuous observations. To estimate how many years ...are needed to detect a decline, we conduct a simulation study using synthetic data that mimics an AMOC time series. The time series' characteristics are reproduced using the trend, variance, and autocorrelation coefficient of the AMOC strength at 26.5°N from 20 Coupled Model Intercomparison Project Phase 5 (CMIP5) models under the RCP8.5 future scenario, and from RAPID observations (2004–2018). Our results suggest that the 14‐year RAPID length has just entered the lower limits of the trend's “detection window” based on synthetic data generated using CMIP5 trends and variability (14–42 years; median
= 24 years), but twice the length is required for detectability based on RAPID variability (29–67 years; median
= 43 years). The annual RAPID trend is currently not statistically significant (−0.11 Sv yr−1, p > 0.05).
Plain Language Summary
There are ongoing discussions in the scientific community about whether the Atlantic Meridional Overturning Circulation transport is slowing down. This is of interest due to the importance of this circulation in transporting heat from the tropics to the northern latitudes. A consensus about its decline is hard to reach due to the limited direct observational data available; with the longest continuous data being 14 years long from 2004 to 2018. We therefore conduct a simulation experiment to examine how many years of data are required to detect a decline in the circulation. We create simulations of the North Atlantic transport based on statistical properties from 20 general circulation models with future climate change projections (until 2100) and from the RAPID array observations (since 2004). Our results demonstrate that the length of data we currently have from observations has just entered the “detection window” of 14–42 years (based on model simulations). However, the RAPID observations do not currently exhibit a statistically significant trend.
Key Points
The estimated 14‐year annually averaged RAPID AMOC trend of −0.1 Sv yr−1 is not statistically significant
The detection window for a long‐term AMOC decline is 14–42 years based on future scenario CMIP5 output statistical properties
Autocorrelation of annually averaged AMOC data is weak and has a limited impact on detection of a long‐term decline
The response of the subpolar Southern Ocean (sSO) to wind forcing is assessed using satellite radar altimetry. sSO sea level exhibits a phased, zonally coherent, bimodal adjustment to circumpolar ...wind changes, involving comparable seasonal and interannual variations. The adjustment is effected via a quasi‐instantaneous exchange of mass between the Antarctic continental shelf and the sSO to the north, and a 2‐month‐delayed transfer of mass between the wider Southern Ocean and the subtropics. Both adjustment modes are consistent with an Ekman‐mediated response to variations in surface stress. Only the fast mode projects significantly onto the surface geostrophic flow of the sSO; thus, the regional circulation varies in phase with the leading edge of sSO sea level variability. The surface forcing of changes in the sSO system is partly associated with variations of surface winds linked to the Southern Annular Mode and is modulated by sea ice cover near Antarctica.
Key Points
Variability in subpolar Southern Ocean sea level is documented using satellite radar altimetry, including in ice‐covered areas
Subpolar Southern Ocean sea level exhibits a phased, bimodal response to circumpolar wind changes, entailing a strong seasonal cycle
The relevant wind forcing is partially described by the Southern Annular Mode and is modulated by sea ice cover near Antarctica
Should AMOC observations continue: how and why? Frajka-Williams, E.; Foukal, N.; Danabasoglu, G.
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
12/2023, Letnik:
381, Številka:
2262
Journal Article
Recenzirano
Odprti dostop
The Atlantic meridional overturning circulation (AMOC) is a large-scale circulation pattern responsible for northward heat transport in the Atlantic and is associated with climate variations on a ...wide range of time scales. Observing the time-varying AMOC has fundamentally changed our understanding of the large-scale ocean circulation and its interaction with the climate system, as well as identified shortcomings in numerical simulations. With a wide range of gains already achieved, some now ask whether AMOC observations should continue. A measured approach is required for a future observing system that addresses identified gaps in understanding, accounts for shortcomings in observing methods and maximizes the potential to guide improvements in ocean and climate models. Here, we outline a perspective on future AMOC observing and steps that the community should consider to move forward.
This article is part of a discussion meeting issue ‘Atlantic overturning: new observations and challenges’.
The overturning circulation of the global ocean is critically shaped by deep-ocean mixing, which transforms cold waters sinking at high latitudes into warmer, shallower waters. The effectiveness of ...mixing in driving this transformation is jointly set by two factors: the intensity of turbulence near topography and the rate at which well-mixed boundary waters are exchanged with the stratified ocean interior. Here, we use innovative observations of a major branch of the overturning circulation—an abyssal boundary current in the Southern Ocean—to identify a previously undocumented mixing mechanism, by which deep-ocean waters are efficiently laundered through intensified near-boundary turbulence and boundary–interior exchange. The linchpin of the mechanism is the generation of submesoscale dynamical instabilities by the flow of deepocean waters along a steep topographic boundary. As the conditions conducive to this mode of mixing are common to many abyssal boundary currents, our findings highlight an imperative for its representation in models of oceanic overturning.
A recent hydrographic section at 24.5°N in the Atlantic and 6 months of observations from a moored array show that Antarctic Bottom Water (AABW), the densest and deepest water mass in the world ...oceans, has been warming. While Johnson et al. (2008) showed that northward AABW transport at 24.5°N has been declining from 1981 to 2004, suggesting that the lower cell of the overturning circulation could halt in the near future, estimates from the latest hydrographic section in 2010 indicate a partial recovery of northward AABW transport. From 6 months of temperature and salinity observations at a deep moored array at 24–26°N, we find that short‐term variability between April and November 2009 is of the same magnitude as the changes observed from hydrographic sections between 1981 and 2004. These observations highlight the possibility that transport changes estimated from hydrographic sections may be aliased by short‐term variability. The observed AABW transport variability affects present estimates of the upper meridional overturning circulation by ±0.4 Sv (1 Sv = 106 m3s−1).
Key Points
Antarctic Bottom Water transport at 24.5N is variable on subannual timescales
AABW transport had a recent resurgence in 2010
Hydrographic estimates of AABW transport are aliased by short‐term variations
Time series of the observational estimate of the Atlantic meridional overturning circulation (AMOC) have recently become available, but so far, no contemporaneous relation has been documented between ...them. Here, we analyze the variability of the 26°N Rapid Climate Change programme (RAPID) and the 41°N Argo‐based AMOC estimates on seasonal timescales, and we compare them to a simulation from a high‐resolution National Centers for Environmental Prediction (NCEP)‐forced ocean model. In our analysis of the observed time series, we find that the seasonal cycles of the non‐Ekman component of the AMOC between 26°N and 41°N are 180‐degrees out‐of‐phase. Removing the mean seasonal cycle from each time series, the residuals have a non‐stationary covariability. Our results demonstrate that the AMOC is meridionally covariable between 26°N and 41°N at seasonal timescales. We find the same covariability in the model, although the phasing differs from the observed phasing. This may offer the possibility of inferring AMOC variations and associated climate anomalies throughout the North Atlantic from discontinuous observations.
Key Points
First joint analysis of observed and modeled Atlantic overturning timeseries
Find meridional coherence at seasonal timescales
Find non‐stationary covariability after removing the mean seasonal cycle
The Atlantic meridional overturning circulation (AMOC) has been observed at 26.5°N since 2004. During 2009/2010, there was a transient 30% weakening of the AMOC driven by anomalies in geostrophic and ...Ekman transports. Here, we use simulations based on the Met Office Forecast Ocean Assimilation Model (FOAM) to diagnose the relative importance of atmospheric forcings and internal ocean dynamics in driving the anomalous geostrophic circulation of 2009/2010. Data‐assimilating experiments with FOAM accurately reproduce the mean strength and depth of the AMOC at 26.5°N. In addition, agreement between simulated and observed stream functions in the deep ocean is improved when we calculate the AMOC using a method that approximates the observing array at 26.5°N. The main features of the geostrophic circulation anomaly are captured by an ensemble of simulations without data assimilation. These model results suggest that the atmosphere played a dominant role in driving recent interannual variability of the AMOC.
Key Points
Recent interannual variability of the AMOC dominated by mid‐ocean anomalies
Most of this variability can be simulated as a response to atmospheric forcings
Model‐data agreement improved when AMOC calculated using observational method
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