Salinity Trends within the Upper Layers of the Subpolar North Atlantic

Examination of a range of salinity products collectively suggests widespread freshening of the North Atlantic from the mid-2000s to the present. Monthly salinity fields reveal negative trends that differ in magnitude and significance between western and eastern regions of the North Atlantic. These differences can be attributed to the large negative interannual excursions in salinity in the western subpolar gyre and the Labrador Sea, which are not apparent in the central or eastern subpolar gyre. This study demonstrates that temporal trends in salinity in the northwest (including the Labrador Sea) are subject to mechanisms that are distinct from those responsible for the salinity trends in the central and eastern North Atlantic. In the western subpolar gyre a negative correlation between near-surface salinity and the circulation strength of the subpolar gyre suggests that negative salinity anomalies are connected to an intensification of the subpolar gyre, which is causing increased flux of freshwater from the East Greenland Current and subsequent transport into the Labrador Sea during the melting season. Analyses of sea surface wind fields suggest that the strength of the subpolar gyre is linked to the North Atlantic Oscillation– and Arctic Oscillation–driven changes in wind stress curl in the eastern subpolar gyre. If this trend of decreasing salinity continues, it has the potential to enhance water column stratification, reduce vertical fluxes of nutrients, and cause a decline in biological production and carbon export in the North Atlantic Ocean.

Download Full-text

Subpolar Gyre – AMOC – Atmosphere Interactions on Multidecadal Timescales in a Version of the Kiel Climate Model

10.5194/egusphere-egu21-4608 ◽

2021 ◽

Author(s):

Jing Sun ◽

Mojib Latif ◽

Wonsun Park

Keyword(s):

North Atlantic ◽

Climate Model ◽

Deep Convection ◽

Meridional Overturning Circulation ◽

Subpolar Gyre ◽

Surface Salinity ◽

The North ◽

Ocean Coupling ◽

Meridional Overturning ◽

The North Atlantic

There is a controversy about the nature of multidecadal climate variability in the North Atlantic (NA) region, concerning the roles of ocean circulation and atmosphere-ocean coupling. Here we describe NA multidecadal variability from a version of the Kiel Climate Model, in which both subpolar gyre (SPG)-Atlantic Meridional Overturning Circulation (AMOC) and atmosphere-ocean coupling are essential. The oceanic barotropic streamfuntions, meridional overturning streamfunctions, and sea level pressure are jointly analyzed to derive the leading mode of Atlantic variability. This mode accounting for about 23.7 % of the total combined variance is oscillatory with an irregular periodicity of 25-50 years and an e-folding time of about a decade. SPG and AMOC mutually influence each other and together provide the delayed negative feedback necessary for maintaining the oscillation. An anomalously strong SPG, for example, drives higher surface salinity and density in the NA&#8217;s sinking region. In response, oceanic deep convection and AMOC intensify, which, with a time delay of about a decade, reduces SPG strength by enhancing upper-ocean heat content. The weaker gyre circulation leads to lower surface salinity and density in the sinking region, which eventually reduces deep convection and AMOC strength. There is a positive ocean-atmosphere feedback between the sea surface temperature and low-level atmospheric circulation over the Southern Greenland area, with related wind stress changes reinforcing SPG changes, thereby maintaining the (damped) multidecadal oscillation against dissipation. Stochastic surface heat-flux forcing associated with the North Atlantic Oscillation drives the eigenmode.

Download Full-text

The recent AMOC variability in the Subpolar Gyre: results across the OVIDE section

10.5194/egusphere-egu2020-19663 ◽

2020 ◽

Author(s):

Pascale Lherminier ◽

Herlé Mercier ◽

Fiz F. Perez ◽

Marcos Fontela

Keyword(s):

North Atlantic ◽

Lower Limb ◽

Deep Convection ◽

Labrador Sea ◽

Subpolar Gyre ◽

North Atlantic Current ◽

The North ◽

Subarctic Front ◽

The North Atlantic ◽

Atlantic Current

According to the subpolar AMOC index built from ARGO and altimetry, the AMOC amplitude across the OVIDE section (from Greenland to Portugal) was similar to that of the mid-1990s between 2014 and 2017, i.e. 4-5 Sv above the level of the 2000s. It then returned to average values in 2018. The same index computed independently from the biennial summer cruises over 2002-2018 confirms this statement. Interestingly, despite the concomitant cold and fresh anomaly in the subpolar Atlantic, the heat flux across OVIDE remains correlated with the AMOC amplitude. This can be explained by the paths taken by the North Atlantic Current and the transport anomalies in the subarctic front. In 2014, the OVIDE section was complemented by a section from Greenland to Newfoundland (GA01), showing how the water of the lower limb of the AMOC was densified by deep convection in the Labrador Sea. The spatial patterns of volume, heat, salt and oxygen transport anomalies after 2014 will be discussed at the light of the 2000s average.

Download Full-text

Near-surface wind speeds in ERA5: Climatology, decadal variability and long-term trends

10.5194/egusphere-egu21-4472 ◽

2021 ◽

Author(s):

Terhi K. Laurila ◽

Victoria A. Sinclair ◽

Hilppa Gregow

Keyword(s):

Iberian Peninsula ◽

North Atlantic ◽

Decadal Variability ◽

Surface Wind ◽

Northern Europe ◽

Near Surface ◽

Wind Speeds ◽

The North ◽

The North Atlantic

The knowledge of long-term climate and variability of near-surface wind speeds is essential and widely used among meteorologists, climate scientists and in industries such as wind energy and forestry. The new high-resolution ERA5 reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF) will likely be used as a reference in future climate projections and in many wind-related applications. Hence, it is important to know what is the mean climate and variability of wind speeds in ERA5.We present the monthly 10-m wind speed climate and decadal variability in the North Atlantic and Europe during the 40-year period (1979-2018) based on ERA5. In addition, we examine temporal time series and possible trends in three locations: the central North Atlantic, Finland and Iberian Peninsula. Moreover, we investigate what are the physical reasons for the decadal changes in 10-m wind speeds.The 40-year mean and the 98th percentile wind speeds show a distinct contrast between land and sea with the strongest winds over the ocean and a seasonal variation with the strongest winds during winter time. The winds have the highest values and variabilities associated with storm tracks and local wind phenomena such as the mistral. To investigate the extremeness of the winds, we defined an extreme find factor (EWF) which is the ratio between the 98th percentile and mean wind speeds. The EWF is higher in southern Europe than in northern Europe during all months. Mostly no statistically significant linear trends of 10-m wind speeds were found in the 40-year period in the three locations and the annual and decadal variability was large.The windiest decade in northern Europe was the 1990s and in southern Europe the 1980s and 2010s. The decadal changes in 10-m wind speeds were largely explained by the position of the jet stream and storm tracks and the strength of the north-south pressure gradient over the North Atlantic. In addition, we investigated the correlation between the North Atlantic Oscillation (NAO) and the Atlantic Multi-decadal Oscillation (AMO) in the three locations. The NAO has a positive correlation in the central North Atlantic and Finland and a negative correlation in Iberian Peninsula. The AMO correlates moderately with the winds in the central North Atlantic but no correlation was found in Finland or the Iberian Peninsula. Overall, our study highlights that rather than just using long-term linear trends in wind speeds it is more informative to consider inter-annual or decadal variability.

Download Full-text

Near-Surface Transport Pathways in the North Atlantic Ocean: Looking for Throughput from the Subtropical to the Subpolar Gyre

Journal of Physical Oceanography ◽

10.1175/2010jpo4498.1 ◽

2011 ◽

Vol 41 (5) ◽

pp. 911-925 ◽

Cited By ~ 6

Author(s):

Irina I. Rypina ◽

Lawrence J. Pratt ◽

M. Susan Lozier

Keyword(s):

Atlantic Ocean ◽

North Atlantic ◽

North Atlantic Ocean ◽

Subpolar Gyre ◽

Transport Pathways ◽

Near Surface ◽

Surface Transport ◽

The North ◽

The North Atlantic

Download Full-text

Climatology, variability, and trends in near‐surface wind speeds over the North Atlantic and Europe during 1979–2018 based on ERA5

International Journal of Climatology ◽

10.1002/joc.6957 ◽

2020 ◽

Author(s):

Terhi K. Laurila ◽

Victoria A. Sinclair ◽

Hilppa Gregow

Keyword(s):

North Atlantic ◽

Surface Wind ◽

Near Surface ◽

Wind Speeds ◽

The North ◽

The North Atlantic

Download Full-text

Size Matters: Another Reason Why the Atlantic Is Saltier than the Pacific

Journal of Physical Oceanography ◽

10.1175/jpo-d-17-0075.1 ◽

2017 ◽

Vol 47 (11) ◽

pp. 2843-2859 ◽

Cited By ~ 6

Author(s):

C. S. Jones ◽

Paola Cessi

Keyword(s):

North Atlantic ◽

Deep Water ◽

Western Boundary Current ◽

Meridional Overturning Circulation ◽

Western Boundary ◽

Subpolar Gyre ◽

Boundary Current ◽

Surface Salinity ◽

The North ◽

The North Atlantic

AbstractThe surface salinity in the North Atlantic controls the position of the sinking branch of the meridional overturning circulation (MOC); the North Atlantic has higher salinity, so deep-water formation occurs there rather than in the North Pacific. Here, it is shown that in a 3D primitive equation model of two basins of different widths connected by a reentrant channel, there is a preference for sinking in the narrow basin even under zonally uniform surface forcing. This preference is linked to the details of the velocity and salinity fields in the “sinking” basin. The southward western boundary current associated with the wind-driven subpolar gyre has higher velocity in the wide basin than in the narrow basin. It overwhelms the northward western boundary current associated with the MOC for wide-basin sinking, so freshwater is brought from the far north of the domain southward and forms a pool on the western boundary in the wide basin. The fresh pool suppresses local convection and spreads eastward, leading to low salinities in the north of the wide basin for wide-basin sinking. This pool of freshwater is much less prominent in the narrow basin for narrow-basin sinking, where the northward MOC western boundary current overcomes the southward western boundary current associated with the wind-driven subpolar gyre, bringing salty water from lower latitudes northward and enabling deep-water mass formation.

Download Full-text

Near-Surface Transport Pathways in the North Atlantic Ocean: Looking for Throughput from the Subtropical to the Subpolar Gyre

Journal of Physical Oceanography ◽

10.1175/2011jpo4498.1 ◽

2011 ◽

Vol 41 (5) ◽

pp. 911-925 ◽

Cited By ~ 29

Author(s):

Irina I. Rypina ◽

Lawrence J. Pratt ◽

M. Susan Lozier

Keyword(s):

North Atlantic ◽

Gulf Stream ◽

Subpolar Gyre ◽

Fluid Exchange ◽

Transport Barrier ◽

Transport Pathways ◽

Near Surface ◽

Surface Transport ◽

The North ◽

The North Atlantic

Abstract Motivated by discrepancies between Eulerian transport estimates and the behavior of Lagrangian surface drifters, near-surface transport pathways and processes in the North Atlantic are studied using a combination of data, altimetric surface heights, statistical analysis of trajectories, and dynamical systems techniques. Particular attention is paid to the issue of the subtropical-to-subpolar intergyre fluid exchange. The velocity field used in this study is composed of a steady drifter-derived background flow, upon which a time-dependent altimeter-based perturbation is superimposed. This analysis suggests that most of the fluid entering the subpolar gyre from the subtropical gyre within two years comes from a narrow region lying inshore of the Gulf Stream core, whereas fluid on the offshore side of the Gulf Stream is largely prevented from doing so by the Gulf Stream core, which acts as a strong transport barrier, in agreement with past studies. The transport barrier near the Gulf Stream core is robust and persistent from 1992 until 2008. The qualitative behavior is found to be largely independent of the Ekman drift.

Download Full-text

On the Temporally Varying Northward Penetration of Mediterranean Overflow Water and Eastward Penetration of Labrador Sea Water

Journal of Physical Oceanography ◽

10.1175/2008jpo3908.1 ◽

2008 ◽

Vol 38 (9) ◽

pp. 2097-2103 ◽

Cited By ~ 74

Author(s):

M. Susan Lozier ◽

Nicole M. Stewart

Keyword(s):

North Atlantic ◽

Water Mass ◽

Sea Water ◽

Labrador Sea ◽

Subpolar Gyre ◽

The North ◽

The North Atlantic ◽

Eastern North Atlantic ◽

The North Atlantic Oscillation ◽

Water Mass Transformations

Abstract Historical hydrographic data in the eastern North Atlantic are used to suggest a connection between the northward penetration of Mediterranean Overflow Water (MOW) and the location of the subpolar front, the latter of which is shown to vary with the North Atlantic Oscillation (NAO). During persistent high-NAO periods, when the subpolar front moves eastward, waters in the subpolar gyre essentially block the northward-flowing MOW, preventing its entry into the subpolar gyre. Conversely, during low NAO periods, the subpolar front moves westward, allowing MOW to penetrate past Porcupine Bank into the subpolar gyre. The impacts of an intermittent penetration of MOW into the subpolar gyre, including the possible effect on water mass transformations, remain to be investigated.

Download Full-text

Mercury distribution and transport in the North Atlantic Ocean along the GEOTRACES-GA01 transect

Biogeosciences ◽

10.5194/bg-15-2309-2018 ◽

2018 ◽

Vol 15 (8) ◽

pp. 2309-2323 ◽

Cited By ~ 12

Author(s):

Daniel Cossa ◽

Lars-Eric Heimbürger ◽

Fiz F. Pérez ◽

Maribel I. García-Ibáñez ◽

Jeroen E. Sonke ◽

...

Keyword(s):

Atlantic Ocean ◽

North Atlantic ◽

North Atlantic Ocean ◽

Geometric Mean ◽

Meridional Overturning Circulation ◽

Labrador Sea ◽

Subpolar Gyre ◽

Data Set ◽

The North ◽

The North Atlantic

Abstract. We report here the results of total mercury (HgT) determinations along the 2014 Geotraces Geovide cruise (GA01 transect) in the North Atlantic Ocean (NA) from Lisbon (Portugal) to the coast of Labrador (Canada). HgT concentrations in unfiltered samples (HgTUNF) were log-normally distributed and ranged between 0.16 and 1.54 pmol L−1, with a geometric mean of 0.51 pmol L−1 for the 535 samples analysed. The dissolved fraction (< 0.45 µm) of HgT (HgTF), determined on 141 samples, averaged 78 % of the HgTUNF for the entire data set, 84 % for open seawaters (below 100 m) and 91 % if the Labrador Sea data are excluded, where the primary production was high (with a winter convection down to 1400 m). HgTUNF concentrations increased eastwards and with depth from Greenland to Europe and from subsurface to bottom waters. The HgTUNF concentrations were similarly low in the subpolar gyre waters ( ∼  0.45 pmol L−1), whereas they exceeded 0.60 pmol L−1 in the subtropical gyre waters. The HgTUNF distribution mirrored that of dissolved oxygen concentration, with highest concentration levels associated with oxygen-depleted zones. The relationship between HgTF and the apparent oxygen utilization confirms the nutrient-like behaviour of Hg in the NA. An extended optimum multiparameter analysis allowed us to characterize HgTUNF concentrations in the different source water types (SWTs) present along the transect. The distribution pattern of HgTUNF, modelled by the mixing of SWTs, show Hg enrichment in Mediterranean waters and North East Atlantic Deep Water and low concentrations in young waters formed in the subpolar gyre and Nordic seas. The change in anthropogenic Hg concentrations in the Labrador Sea Water during its eastward journey suggests a continuous decrease in Hg content in this water mass over the last decades. Calculation of the water transport driven by the Atlantic Meridional Overturning Circulation across the Portugal–Greenland transect indicates northward Hg transport within the upper limb and southward Hg transport within the lower limb, with resulting net northward transport of about 97.2 kmol yr−1.

Download Full-text