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Stuart A. Cunningham and Thomas W. N. Haine

Abstract

A synoptic distribution of Labrador Sea Water (LSW) in the eastern North Atlantic is determined from a regularly sampled, but sparse (3° resolution), survey covering 39° to 54°N, 11° to 34°W during spring 1991.

The core of LSW can be defined by a minimum in potential vorticity (PV). Using property values at this minimum the authors infer the circulation of LSW. In addition, using a known source function for salinities at the core of LSW, estimates are able to be made of LSW vintages. The authors then compared the synoptic circulation to historical data.

Youngest, 1986 vintage, LSW crosses the Mid-Atlantic Ridge to the eastern basin between 48° and 51°N at 34°W. This water then flows north to the Iceland Basin and eastward to the Rockall Trough, where it was found to be of 1978 vintage. Tongues of low salinity, low temperature, and high oxygen extend southward on the eastern side of the Mid-Atlantic Ridge, indicating that LSW also flows southward in the eastern basin. At the southern edge of the survey the salinity and density of LSW increases.

Compared to historical data of Talley and McCartney for the years 1957–1964 the authors found 1) no coincident values of PV, with LSW now having much lower PV and 2) that the core of LSW is significantly fresher. These differences show that climate variability, which affect these properties at the source, has a dramatic impact on tracer distribution at middepth in the eastern North Atlantic.

Mediterranean Water is shown to overlap the LSW in a band 600 km wide spanning the eastern North Atlantic. Staircase structures on salinity profiles are not observed in the region, indicating that salt fingering if present, must be intermittent. This is contrasted with the work of Schmitz and McCartney who show that salt fingering is active south of 39°N.

In Part II of this paper, the authors examine the anomalies inherited from the boundary condition variability and examine the advective/diffusive balance for LSW.

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Stuart A. Cunningham and Thomas W. N. Haine

Abstract

Deep wintertime convection in the Labrador Basin produces Labrador Sea Water (LSW), which spreads throughout the North Atlantic. The LSW is not formed with constant characteristics but varies on decadal timescale. These changes in the source of LSW propagate throughout the North Atlantic, having a dramatic impact on middepth properties in regions far removed from the Labrador Basin. Here, by means of a property variance correlation analysis on isopycnal surfaces through the core of LSW, the authors show that the boundary anomalies of LSW survive the effects of mixing over the length and timescales taken for LSW to reach the eastern North Atlantic. Typically, the anomaly amplitude is reduced by a factor of 5 during transit, and from a simple estimate the integrated timescale for this reduction is seven years. Uncertainties in the oxygen anomaly analysis exclude specific interpretation, but constrain the boundary condition variability to about ±5%. The lateral effect of unresolved processes resembles isopycnic Fickian diffusion; and an estimate of the mean isopycnal eddy diffusion coefficient through the LSW core is 950±300 m2 s−1, with a mixing length scale of 166 ±30 km.

The authors examine the advective-diffusive balance for salinity and oxygen and calculate the horizontal advective flux divergence and diffusion. Contributions from the vertical advection and diffusion terms are small and can be neglected. The uncertainties in the oxygen balance prohibit any firm conclusions regarding the time rate of change of this property. However, for salinity an unsteady contribution up to ∼0.008±0.005 psu yr−1 is needed to close the budget. On σ1.5=34.64 kg m−3, comparing the unsteady contribution to the source function in the Labrador Basin the age range of LSW was found to be 10±3 years.

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Peter M. Saunders, Stuart A. Cunningham, Beverly A. de Cuevas, and Andrew C. Coward

Abstract

It is demonstrated that current level models, both with and without assimilation, generate too shallow an overturning in the North Atlantic (just like their predecessors) because they do not reproduce the descent of plumes of cold water from the Greenland and Norwegian Seas. Consequently, the prediction of decadal change from one such model reported by C. Wunsch and P. Heimbach is queried.

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Johanna Baehr, Helmuth Haak, Steven Alderson, Stuart A. Cunningham, Johann H. Jungclaus, and Jochem Marotzke

Abstract

It is investigated how changes in the North Atlantic meridional overturning circulation (MOC) might be reliably detected within a few decades, using the observations provided by the RAPID-MOC 26°N array. Previously, detectability of MOC changes had been investigated with a univariate MOC time series exhibiting strong internal variability, which would prohibit the detection of MOC changes within a few decades. Here, a modification of K. Hasselmann’s fingerprint technique is used: (simulated) observations are projected onto a time-independent spatial pattern of natural variability to derive a time-dependent detection variable. The fixed spatial pattern of natural variability is derived by regressing the zonal density gradient along 26°N against the strength of the MOC at 26°N within the coupled ECHAM5/Max Planck Institute Ocean Model’s (MPI-OM) control climate simulation. This pattern is confirmed against the observed anomalies found between the 1957 and the 2004 hydrographic occupations of the section. Onto this fixed spatial pattern of natural variability, both the existing hydrographic observations and simulated observations mimicking the RAPID-MOC 26°N array in three realizations of the Intergovernmental Panel on Climate Change (IPCC) scenario A1B are projected. For a random observation error of 0.01 kg m−3, and only using zonal density gradients between 1700- and 3100-m depth, statistically significant detection occurs with 95% reliability after about 30 yr, in the model and climate change scenario analyzed here. Compared to using a single MOC time series as the detection variable, continuous observations of zonal density gradients reduce the detection time by 50%. For the five hydrographic occupations of the 26°N transect, none of the analyzed depth ranges shows a significant trend between 1957 and 2004, implying that there was no MOC trend over the past 50 yr.

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Elaine L. McDonagh, Harry L. Bryden, Brian A. King, Richard J. Sanders, Stuart A. Cunningham, and Robert Marsh

Abstract

A significant change in properties of the thermocline is observed across the whole Indian Ocean 32°S section between 1987 and 2002. This change represents a reversal of the pre-1987 freshening and decreasing oxygen concentrations of the upper thermocline that had been interpreted as a fingerprint of anthropogenic climate change. The thermocline at the western end of the section (40°–70°E) is occupied by a single variety of mode water with a potential temperature of around 13°C. The thermocline at the eastern end of the 32°S section is occupied by mode waters with a range of properties cooling from ∼11°C at 80°E to ∼9°C near the Australian coast. The change in θS properties between 1987 and 2002 is zonally coherent east of 80°E, with a maximum change on isopycnals at 11.6°C. Ages derived from helium–tritium data imply that the mode waters at all longitudes take about the same time to reach 32°S from their respective ventilation sites. Dissolved oxygen concentration changes imply that all of the mode water reached the section ∼20% faster in 2002 than in 1987.

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James A. Carton, Stuart A. Cunningham, Eleanor Frajka-Williams, Young-Oh Kwon, David P. Marshall, and Rym Msadek
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Elaine L. McDonagh, Brian A. King, Harry L. Bryden, Peggy Courtois, Zoltan Szuts, Molly Baringer, Stuart A. Cunningham, Chris Atkinson, and Gerard McCarthy

Abstract

The first continuous estimates of freshwater flux across 26.5°N are calculated using observations from the RAPID–MOCHA–Western Boundary Time Series (WBTS) and Argo floats every 10 days between April 2004 and October 2012. The mean plus or minus the standard deviation of the freshwater flux (F W) is −1.17 ± 0.20 Sv (1 Sv ≡ 106 m3 s−1; negative flux is southward), implying a freshwater divergence of −0.37 ± 0.20 Sv between the Bering Strait and 26.5°N. This is in the sense of an input of 0.37 Sv of freshwater into the ocean, consistent with a region where precipitation dominates over evaporation. The sign and the variability of the freshwater divergence are dominated by the overturning component (−0.78 ± 0.21 Sv). The horizontal component of the freshwater divergence is smaller, associated with little variability and positive (0.35 ± 0.04 Sv). A linear relationship, describing 91% of the variance, exists between the strength of the meridional overturning circulation (MOC) and the freshwater flux (−0.37 − 0.047 Sv of F W per Sverdrups of MOC). The time series of the residual to this relationship shows a small (0.02 Sv in 8.5 yr) but detectable decrease in the freshwater flux (i.e., an increase in the southward freshwater flux) for a given MOC strength. Historical analyses of observations at 24.5°N are consistent with a more negative freshwater divergence from −0.03 to −0.37 Sv since 1974. This change is associated with an increased southward freshwater flux at this latitude due to an increase in the Florida Straits salinity (and therefore the northward salinity flux).

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M. Susan Lozier, Sheldon Bacon, Amy S. Bower, Stuart A. Cunningham, M. Femke de Jong, Laura de Steur, Brad deYoung, Jürgen Fischer, Stefan F. Gary, Blair J. W. Greenan, Patrick Heimbach, Naomi P. Holliday, Loïc Houpert, Mark E. Inall, William E. Johns, Helen L. Johnson, Johannes Karstensen, Feili Li, Xiaopei Lin, Neill Mackay, David P. Marshall, Herlé Mercier, Paul G. Myers, Robert S. Pickart, Helen R. Pillar, Fiammetta Straneo, Virginie Thierry, Robert A. Weller, Richard G. Williams, Chris Wilson, Jiayan Yang, Jian Zhao, and Jan D. Zika

Abstract

For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017.

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