The Mean State and Variability of the North Atlantic Circulation: A Perspective From Ocean Reanalyses

The observational network around the North Atlantic has improved significantly over the last few decades with the advent of Argo and satellite observations, and the more recent efforts to monitor the Atlantic Meridional Overturning Circulation (AMOC) using arrays such as RAPID and OSNAP. These have shown decadal timescale changes across the North Atlantic including in heat content, heat transport and the circulation.&#160;However there are still significant gaps in the observational coverage, and significant uncertainties around some observational products. Ocean reanalyses integrate the observations with a dynamically consistent ocean model and are potentially tools that can be used to understand the observed changes. However the suitability of the reanalyses for the task must also be assessed. We use an ensemble of global ocean reanalyses in comparison with observations in order to examine the mean state and interannual-decadal variability of the North Atlantic ocean since 1993. We assess how well the reanalyses are able to capture different processes and whether any understanding can be inferred. In particular we look at ocean heat content, transports, the AMOC and gyre strengths, water masses and convection.&#160;&#160;

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The Rapid Warming of the North Atlantic Ocean in the Mid-1990s in an Eddy-Permitting Ocean Reanalysis (1982–2013)

Journal of Climate ◽

10.1175/jcli-d-15-0438.1 ◽

2016 ◽

Vol 29 (15) ◽

pp. 5417-5430 ◽

Cited By ~ 5

Author(s):

Chunxue Yang ◽

Simona Masina ◽

Alessio Bellucci ◽

Andrea Storto

Keyword(s):

Atlantic Ocean ◽

Heat Transport ◽

North Atlantic ◽

North Atlantic Ocean ◽

Ocean Reanalysis ◽

The North ◽

Mean Temperature ◽

The Mean ◽

The North Atlantic ◽

Mean State

Abstract The rapid warming in the mid-1990s in the North Atlantic Ocean is investigated by means of an eddy-permitting ocean reanalysis. Both the mean state and variability, including the mid-1990s warming event, are well captured by the reanalysis. An ocean heat budget applied to the subpolar gyre (SPG) region (50°–66°N, 60°–10°W) shows that the 1995–99 rapid warming is primarily dictated by changes in the heat transport convergence term while the surface heat fluxes appear to play a minor role. The mean negative temperature increment suggests a warm bias in the model and data assimilation corrects the mean state of the model, but it is not crucial to reconstruct the time variability of the upper-ocean temperature. The decomposition of the heat transport across the southern edge of the SPG into time-mean and time-varying components shows that the SPG warming is mainly associated with both the anomalous advection of mean temperature and the mean advection of temperature anomalies across the 50°N zonal section. The relative contributions of the Atlantic meridional overturning circulation (AMOC) and gyre circulation to the heat transport are also analyzed. It is shown that both the overturning and gyre components are relevant to the mid-1990s warming. In particular, the fast adjustment of the barotropic circulation response to the NAO drives the anomalous transport of mean temperature at the subtropical/subpolar boundary, while the slowly evolving AMOC feeds the large-scale advection of thermal anomalies across 50°N. The persistently positive phase of the NAO during the years prior to the rapid warming likely favored the cross-gyre heat transfer and the following SPG warming.

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The North Atlantic Oscillation, Surface Current Velocities, and SST Changes in the Subpolar North Atlantic

Journal of Climate ◽

10.1175/2787.1 ◽

2003 ◽

Vol 16 (14) ◽

pp. 2355-2369 ◽

Cited By ~ 116

Author(s):

Maria K. Flatau ◽

Lynne Talley ◽

Pearn P. Niiler

Keyword(s):

North Atlantic Oscillation ◽

North Atlantic ◽

Surface Current ◽

North Atlantic Current ◽

Reykjanes Ridge ◽

The North ◽

The Mean ◽

The North Atlantic ◽

The North Atlantic Oscillation ◽

Atlantic Current

Abstract Changes in surface circulation in the subpolar North Atlantic are documented for the recent interannual switch in the North Atlantic Oscillation (NAO) index from positive values in the early 1990s to negative values in 1995/96. Data from Lagrangian drifters, which were deployed in the North Atlantic from 1992 to 1998, were used to compute the mean and varying surface currents. NCEP winds were used to calculate the Ekman component, allowing isolation of the geostrophic currents. The mean Ekman velocities are considerably smaller than the mean total velocities that resemble historical analyses. The northeastward flow of the North Atlantic Current is organized into three strong cores associated with topography: along the eastern boundary in Rockall Trough, in the Iceland Basin (the subpolar front), and on the western flank of the Reykjanes Ridge (Irminger Current). The last is isolated in this Eulerian mean from the rest of the North Atlantic Current by a region of weak velocities on the east side of the Reykjanes Ridge. The drifter results during the two different NAO periods are compared with geostrophic flow changes calculated from the NASA/Pathfinder monthly gridded sea surface height (SSH) variability products and the Advanced Very High Resolution Radiometer (AVHRR) SST data. During the positive NAO years the northeastward flow in the North Atlantic Current appeared stronger and the circulation in the cyclonic gyre in the Irminger Basin became more intense. This was consistent with the geostrophic velocities calculated from altimetry data and surface temperature changes from AVHRR SST data, which show that during the positive NAO years, with stronger westerlies, the subpolar front was sharper and located farther east. SST gradients intensified in the North Atlantic Current, Irminger Basin, and east of the Shetland Islands during the positive NAO phase, associated with stronger currents. SST differences between positive and negative NAO years were consistent with changes in air–sea heat flux and the eastward shift of the subpolar front. SST advection, as diagnosed from the drifters, likely acted to reduce the SST differences.

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XXVII. On Holtenia, a genus of vitreous sponges

Philosophical Transactions of the Royal Society of London ◽

10.1098/rstl.1869.0030 ◽

1869 ◽

Vol 159 ◽

pp. 701-720 ◽

Cited By ~ 8

Keyword(s):

Surface Temperature ◽

North Atlantic ◽

Small Proportion ◽

Gulf Stream ◽

Deep Sea ◽

Minimum Temperature ◽

The North ◽

The Mean ◽

The North Atlantic ◽

Amorphous Particles

During the deep-sea dredging cruise of Her Majesty’s Ship 'Lightning' in the autumn of the year 1868, the 6th of September was occupied in dredging at the depth of 530 fathoms in latitude 59° 36' N., and longitude 7° 20' W., only about 20 miles beyond the 100-fathom line of the Coast Survey of Scotland, slightly to the westward of north of the Butt of the Lews. The minimum temperature indicated by the mean of three thermometers (which registered 47°, 47°∙5, and 47°∙5 Fahr. respectively) was 47°∙3 Fahr, the surface-temperature being 52°5 Fahr. During the day there were four successful hauls of the dredge, which came up each time full of a pale-grey tenacious mud, consisting in a great measure of minute amorphous particles of carbonate of lime mixed with “coccoliths” and “coccospheres.” There was only a small proportion of the Globigerinæ and other minute Rhizopods which are so abundant and characteristic over the whole of the warm or “Gulf-stream” area of the North Atlantic. The mud was glairy, as if it had been mixed with white of egg; and it contained disseminated through it an immense quantity of extremely delicate siliceous organisms, spicules of sponges, and the shells of Radiolarians and Diatoms. Large Rhizopods of the genera Astrorhiza, Rhabdammina, Cristellaria, Cornuspira , and others were abundant; and there was a somewhat scanty sprinkling of small forms belonging to the higher groups, Echinoderms, Annulosa, and Mollusca. Besides a number of dead shells, chiefly of the Boreal or Scandinavian type, and several undescribed Echinoderms and Crustaceans, the following species were procured living.

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Predictability of the Atlantic Overturning Circulation and Associated Surface Patterns in Two CCSM3 Climate Change Ensemble Experiments

Journal of Climate ◽

10.1175/2011jcli4207.1 ◽

2011 ◽

Vol 24 (23) ◽

pp. 6054-6076 ◽

Cited By ~ 27

Author(s):

Haiyan Teng ◽

Grant Branstator ◽

Gerald A. Meehl

Keyword(s):

Climate Change ◽

North Atlantic ◽

Meridional Overturning Circulation ◽

Subpolar Gyre ◽

Overturning Circulation ◽

The North ◽

Surface Patterns ◽

The Mean ◽

The North Atlantic ◽

Sst Anomalies

Abstract Predictability of the Atlantic meridional overturning circulation (AMOC) and associated oceanic and atmospheric fields on decadal time scales in the Community Climate System Model, version 3 (CCSM3) at T42 resolution is quantified with a 700-yr control run and two 40-member “perfect model” climate change experiments. After taking into account both the mean and spread about the mean of the forecast distributions and allowing for the possibility of time-evolving modes, the natural variability of the AMOC is found to be predictable for about a decade; beyond that range the forced predictability resulting from greenhouse gas forcing becomes dominant. The upper 500-m temperature in the North Atlantic is even more predictable than the AMOC by several years. This predictability is associated with subsurface and sea surface temperature (SST) anomalies that propagate in an anticlockwise direction along the subpolar gyre and tend to be prominent during the 10 yr following peaks in the amplitude of AMOC anomalies. Predictability in the North Atlantic SST mainly resides in the ensemble mean signals after three to four forecast years. Analysis suggests that in the CCSM3 the subpolar gyre SST anomalies associated with the AMOC variability can influence the atmosphere and produce surface climate predictability that goes beyond the ENSO time scale. However, the resulting initial-value predictability in the atmosphere is very weak.

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Investigating the Impact of Reemerging Sea Surface Temperature Anomalies on the Winter Atmospheric Circulation over the North Atlantic

Journal of Climate ◽

10.1175/jcli4202.1 ◽

2007 ◽

Vol 20 (14) ◽

pp. 3510-3526 ◽

Cited By ~ 57

Author(s):

Christophe Cassou ◽

Clara Deser ◽

Michael A. Alexander

Keyword(s):

Mixed Layer ◽

North Atlantic ◽

Mean Flow ◽

Thermal Anomalies ◽

Temperature Anomalies ◽

The North ◽

The Mean ◽

The North Atlantic ◽

The Impact ◽

Control Integration

Abstract Extratropical SSTs can be influenced by the “reemergence mechanism,” whereby thermal anomalies in the deep winter mixed layer persist at depth through summer and are then reentrained into the mixed layer in the following winter. The impact of reemergence in the North Atlantic Ocean (NAO) upon the climate system is investigated using an atmospheric general circulation model coupled to a mixed layer ocean/thermodynamic sea ice model. The dominant pattern of thermal anomalies below the mixed layer in summer in a 150-yr control integration is associated with the North Atlantic SST tripole forced by the NAO in the previous winter as indicated by singular value decomposition (SVD). To isolate the reemerging signal, two additional 60-member ensemble experiments were conducted in which temperature anomalies below 40 m obtained from the SVD analysis are added to or subtracted from the control integration. The reemerging signal, given by the mean difference between the two 60-member ensembles, causes the SST anomaly tripole to recur, beginning in fall, amplifying through January, and persisting through the following spring. The atmospheric response to these SST anomalies resembles the circulation that created them the previous winter but with reduced amplitude (10–20 m at 500 mb per °C), modestly enhancing the winter-to-winter persistence of the NAO. Changes in the transient eddies and their interactions with the mean flow contribute to the large-scale equivalent barotropic response throughout the troposphere. The latter can also be attributed to the change in occurrence of intrinsic weather regimes.

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Impact of Freshwater Release in the North Atlantic under Different Climate Conditions in an OAGCM

Journal of Climate ◽

10.1175/2009jcli3028.1 ◽

2009 ◽

Vol 22 (23) ◽

pp. 6377-6403 ◽

Cited By ~ 81

Author(s):

Didier Swingedouw ◽

Juliette Mignot ◽

Pascale Braconnot ◽

Eloi Mosquet ◽

Masa Kageyama ◽

...

Keyword(s):

Northern Hemisphere ◽

North Atlantic ◽

Freshwater Input ◽

Climatic Response ◽

Last Glacial ◽

The North ◽

The Mean ◽

The North Atlantic ◽

Mean Climate ◽

Freshwater Release

Abstract The response of climate to freshwater input in the North Atlantic (NA) has raised a lot of concern about the issue of climate stability since the discovery of abrupt coolings during the last glacial period. Such coolings have usually been related to a weakening of the Atlantic meridional overturning circulation (AMOC), probably associated with massive iceberg surges or meltwater pulses. Additionally, the recent increase in greenhouse gases in the atmosphere has also raised the possibility of a melting of the Greenland ice sheet, which may impact the future AMOC, and thereby the climate. In this study, the extent to which the mean climate influences the freshwater release linked to ice sheet melting in the NA and the associated climatic response is explored. For this purpose the simulations of several climatic states [last interglacial, Last Glacial Maximum, mid-Holocene, preindustrial, and future (2 × CO2)] are considered, and the climatic response to a freshwater input computed interactively according to a surface heat flux budget over the ice sheets is analyzed. It is shown that the AMOC response is not linear with the freshwater input and depends on the mean climate state. The climatic responses to these different AMOC changes share qualitative similarities for the general picture, notably a cooling in the Northern Hemisphere and a southward shift of the intertropical convergence zone (ITCZ) in the Atlantic and across the Panama Isthmus. The cooling in the Northern Hemisphere is related to the sea ice cover response, which strongly depends on the responses of the atmospheric circulation, the local oceanic heat transport, and the density threshold of the oceanic convection sites. These feedbacks and the magnitude of temperature and precipitation changes outside the North Atlantic depend on the mean climate.

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How Much Has the North Atlantic Ocean Overturning Circulation Changed in the Last 50 Years?

Journal of Climate ◽

10.1175/jcli-d-12-00095.1 ◽

2014 ◽

Vol 27 (16) ◽

pp. 6325-6342 ◽

Cited By ~ 15

Author(s):

Simon F. B. Tett ◽

Toby J. Sherwin ◽

Amrita Shravat ◽

Oliver Browne

Keyword(s):

North Atlantic ◽

Mixed Layer Depth ◽

Meridional Overturning Circulation ◽

Western Boundary ◽

Boundary Current ◽

Overturning Circulation ◽

The North ◽

Ocean Reanalyses ◽

The North Atlantic

Abstract Volume transports from six ocean reanalyses are compared with four sets of in situ observations: across the Greenland–Scotland ridge (GSR), in the Labrador Sea boundary current, in the deep western boundary current at 43°N, and in the Atlantic meridional overturning circulation (AMOC) at 26°N in the North Atlantic. The higher-resolution reanalyses (on the order of ¼° × ¼°) are better at reproducing the circulation pattern in the subpolar gyre than those with lower resolution (on the order of 1°). Simple Ocean Data Assimilation (SODA) and Estimating the Circulation and Climate of the Ocean (ECCO)–Jet Propulsion Laboratory (JPL) produce transports at 26°N that are close to those observed [17 Sv (1 Sv ≡ 106 m3 s−1)]. ECCO, version 2, and SODA produce northward transports across the GSR (observed transport of 8.2 Sv) that are 22% and 29% too big, respectively. By contrast, the low-resolution reanalyses have transports that are either too small [by 31% for ECCO-JPL and 49% for Ocean Reanalysis, system 3 (ORA-S3)] or much too large [Decadal Prediction System (DePreSys)]. SODA had the best simulations of mixed layer depth and with two coarse grid long-term reanalyses (DePreSys and ORA-S3) is used to examine changes in North Atlantic circulation from 1960 to 2008. Its results suggest that the AMOC increased by about 20% at 26°N while transport across the GSR hardly altered. The other (less reliable) long-term reanalyses also had small changes across the GSR but changes of +10% and −20%, respectively, at 26°N. Thus, it appears that changes in the overturning circulation at 26°N are decoupled from the flow across the GSR. It is recommended that transport observations should not be assimilated in ocean reanalyses but used for validation instead.

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Heat content variability in the North Atlantic Ocean in ocean reanalyses

Geophysical Research Letters ◽

10.1002/2015gl063299 ◽

2015 ◽

Vol 42 (8) ◽

pp. 2901-2909 ◽

Cited By ~ 26

Author(s):

Sirpa Häkkinen ◽

Peter B. Rhines ◽

Denise L. Worthen

Keyword(s):

Atlantic Ocean ◽

North Atlantic ◽

North Atlantic Ocean ◽

Heat Content ◽

The North ◽

Ocean Reanalyses ◽

The North Atlantic

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An assessment of the role of the North Atlantic as a CO 2 sink

Philosophical Transactions of the Royal Society B Biological Sciences ◽

10.1098/rstb.1995.0056 ◽

1995 ◽

Vol 348 (1324) ◽

pp. 143-152 ◽

Cited By ~ 51

Keyword(s):

North Atlantic ◽

Surface Waters ◽

Time Lag ◽

Gas Transfer ◽

Advective Transport ◽

Transfer Coefficients ◽

The North ◽

The Mean ◽

The North Atlantic ◽

Secular Increase

A numerical interpolation scheme based upon the lateral diffusive and advective transport of ocean surface waters has been developed to interpolate measurements made in irregular time and space over the oceans. This has been applied to about 2700 surface ocean P CO 2 measurements observed during the period 1972-1992 in the North Atlantic to give the distribution of sea-air P CO 2 difference (Δ P CO 2 ) over the Atlantic. Although the atmospheric CO 2 concentration has increased by about 28 ppm over this period, the P CO 2 values in the surface waters of subarctic regions have increased little because they are dictated primarily by the properties of underlying deep waters through vertical mixing. Accordingly, Δ P CO 2 values measured north of 50° N have been corrected to the year 1990 using the secular increase of atmospheric CO 2 . Because the surface water P CO 2 value in temperate waters tracks the secular increase in atmospheric CO 2 with a time lag of about two years, no correction was applied to the warm water Δ p CO 2 data. It has been assumed that seasonal variations are the same for each year. The net CO2 flux across the sea surface has been computed over a 4° latitude x 5° longitude grid using the mean monthly A p CO 2 values and the gas transfer coefficients estimated using the mean monthly wind speed. It has been found that the areas of the high latitude North Atlantic and the adjacent seas north of 42° N are net CO 2 sinks of 0.2 to 0.5 Gt G a -1 . The total sink flux of CO 2 over the temperate North Atlantic areas between 18° N and 42° N is balanced approximately by the source flux over the tropical Atlantic between 18° N and 18° S.

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