17 research outputs found
A continuous pathway for fresh water along the East Greenland shelf
No full text© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Foukal, N. P., Gelderloos, R., & Pickart, R. S. A continuous pathway for fresh water along the East Greenland shelf. Science Advances, 6(43), (2020): eabc4254, doi:10.1126/sciadv.abc4254.Export from the Arctic and meltwater from the Greenland Ice Sheet together form a southward-flowing coastal current along the East Greenland shelf. This current transports enough fresh water to substantially alter the large-scale circulation of the North Atlantic, yet the coastal current’s origin and fate are poorly known due to our lack of knowledge concerning its north-south connectivity. Here, we demonstrate how the current negotiates the complex topography of Denmark Strait using in situ data and output from an ocean circulation model. We determine that the coastal current north of the strait supplies half of the transport to the coastal current south of the strait, while the other half is sourced from offshore via the shelfbreak jet, with little input from the Greenland Ice Sheet. These results indicate that there is a continuous pathway for Arctic-sourced fresh water along the entire East Greenland shelf from Fram Strait to Cape Farewell.Funding for this work comes from the NSF under grant numbers OCE-1756361 and OCE-1558742 (N.P.F. and R.S.P.) and grant numbers OCE-1756863 and OAC-1835640 (R.G.)
Subsurface Heat Channel Drove Sea Surface Warming in the High‐Latitude North Atlantic During the Mid‐Pleistocene Transition
No full text© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Catunda, M. C. A., Bahr, A., Kaboth-Bahr, S., Zhang, X., Foukal, N. P., & Friedrich, O. Subsurface heat channel drove sea surface warming in the high-latitude North Atlantic during the Mid-Pleistocene Transition. Geophysical Research Letters, 48(11), (2021): e2020GL091899, https://doi.org/10.1029/2020GL091899.The Mid-Pleistocene Transition (MPT, 1,200–600 ka) marks the rapid expansion of Northern Hemisphere (NH) continental ice sheets and stronger precession pacing of glacial/interglacial cyclicity. Here, we investigate the relationship between thermocline depth in the central North Atlantic, subsurface northward heat transport and the initiation of the 100-kyr cyclicity during the MPT. To reconstruct deep-thermocline temperatures, we generated a Mg/Ca-based temperature record of deep-dwelling (∼800 m) planktonic foraminifera from mid-latitude North Atlantic at Site U1313. This record shows phases of pronounced heat accumulation at subsurface levels during the mid-MPT glacial driven by increased outflow of the Mediterranean Sea. Concurrent warming of the subtropical thermocline and subpolar surface waters indicates enhanced (subsurface) inter-gyre transport of warm water to the subpolar North Atlantic, which provided moisture for ice-sheet growth. Precession-modulated variability in the northward transport of subtropical waters imprinted this orbital cyclicity into NH ice-sheets after Marine Isotope Stage 24.Catunda and A. Bahr were funded by DFG project BA 3809/8, O.F. by DFG project FR 2544/11. S. Kaboth-Bahr acknowledges an Open-Topic Post-Doc Grant from the University of Potsdam. X.Z. was funded via the Lanzhou University (project 225000–830006) and National Science Foundation of China (Grant 42075047). N.F. was funded by the NSF Grant 1756361. Open access funding enabled and organized by Projekt DEAL
Lagrangian Decomposition of the Meridional Heat Transport at 26.5N - Water Parcel Crossings of the RAPID 26.5N Array
No full text<p>This dataset contains the initial and final positions and properties of Lagrangian trajectories evaluated using 5-day mean velocity and tracer fields output from the ORCA0083-N06 ocean sea-ice model hindcast (1958-2015). Numerical water parcels are initialised to sample the full-depth southward transport across the RAPID 26.5N array every month during 2004-2015. Water parcels are advected backwards-in-time using a bespoke version of TRACMASS v7.1 Lagrangian particle tracking tool which enables users to specify a custom domain using a mask netCDF file.</p><p>Particles are initialised on the first-available day of each month (based on the centre of the model 5-day mean field windows) between 2004 and 2015 (inclusive) before being advected backwards-in-time within the North Atlantic Ocean until any one of four termination conditions are met: (1) water parcels reach the RAPID 26.5N array, (2) water parcels reach the OSNAP (West or East) arrays in the subpolar North Atlantic, (3) water parcels reach either the English Channel or Gibraltar Strait, or (4) particles reach the maximum advection time of 25-years. The 25-year maximum advection time ensures that we adequately resolve the subtropical gyre circulation north to the RAPID 26.5N array. The pathway transporting dense North Atlantic Deep Water from the OSNAP arrays to RAPID at 26.5N is not fully resolved in this Lagrangian experiment since these water parcels transit on multi-decadal timescales.</p><p>The number of water parcels initialised in each model-grid cell scales with the total northward transport through that cell, such that the maximum possible transport conveyed by any single particle is 5.0 mSv (mSv == 10-3 Sv), enabling the calculation of robust Lagrangian statistics. In reality, the average. water parcel has an associated volume transport of 3.3 mSv which is conserved throughout its circulation.</p><p>Water parcel locations (converted to geographical coordinates) and properties (conservative temperature, absolute salinity, potential density [TEOS-10]) are output on every model-grid cell crossing. TRACMASS determines particle properties on grid-cell crossings by taking the average of the properties stored at the nearest two T-grid points. Here, we provide the initial and final locations and properties of all water parcels initialised from RAPID 26.5N.</p><p>All Lagrangian experiments were completed using the JASMIN High-Performance Computing facility (<a href="https://jasmin.ac.uk">https://jasmin.ac.uk</a>).</p><p><strong>For a complete description of the ORCA0083-N06 hindcast configuration see:</strong> Moat et al. (2016).</p><p><strong>For a complete description of TRACMASS v7.1 see</strong>: <a href="https://www.tracmass.org">https://www.tracmass.org</a></p>
Biogeography and Phenology of Satellite-Measured Phytoplankton Seasonality in the California Current
No full textThirteen years (1998-2010) of satellite-measured chlorophyll a (CHL) quantify spatial patterns in climatological phytoplankton biomass seasonality across the California Current System (CCS) and its interannual variability. Multivariate clustering divides the study area based on the shape of the local climatological seasonal cycle into four cycle groups: two with spring-summer maxima representing the coastal upwelling zones, one with a summer minimum offshore in mid-latitudes and a fourth with very weak seasonality in between. Multivariate clustering on the individual seasonal cycles from all thirteen years provides a view of interannual variability in seasonal biogeography. Our resulting seasonal cycles are similar to, and appear in relatively similar locations as the climatological clusters. However, strong interannual variability in the geography of the seasonal cycles is evident across the CCS, including changes associated with the 1997- 1999 El Nino-Southern Oscillation (ENSO) signal as well as the 2005 delayed spring transition off the Oregon and northern and central California coasts. We quantify linear trends over the study period in the seasonal timing of the two seasonal cycles that represent the biologically productive coastal upwelling zones. In the northern upwelling region, the date of the spring maximum is delaying and the central tendency of the summer elevated period is advancing. In the southern coastal upwelling region, both the initiation and cessation of the spring maximum are delaying and the maximum is increasing in duration over the study period. Connections between shifts in phytoplankton seasonality and physical forcing expressed as either basin-scale climate signals or local forcing show phytoplankton seasonality in the CCS to be strongly influenced by the seasonality of the wind mixing power offshore and coastal upwelling in the near-shore regions; large-scale patterns in local winds in the CCS are often driven by climate signals such as ENSO, PDO and NPGO
Consensus Around a Common Definition of Atlantic Overturning Will Promote Progress
No full textQuantifying the strength of the Atlantic Meridional Overturning Circulation (AMOC) involves separating the north-ward and southward limbs and calculating their volume transports. The limbs can be distinguished either by depth level or by density class, but recent results have indicated that this choice of coordinate system leads to divergent results, both in terms of the AMOC mean state and its variability. Here, we demonstrate that the AMOC in density coordinates is more informative of the large-scale, three-dimensional AMOC structure, is more closely aligned with the AMOC’s climatic impact via oceanic meridional heat trans-port, and retains more information about future AMOC pathways than the depth space definition. Adopting a commonly accepted definition of the AMOC in density coordinates will unify a divided literature and promote progress in the field. This commentary thus highlights that the coordinate system used to define the AMOC matters, not only for understanding physical processes and past variations that remain elusive, but also for physically appropriate monitoring of its future evolution.</p
Examining the origins of ocean heat content variability in the eastern North Atlantic subpolar gyre
No full textAuthor Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 11,275-11,283, doi:10.1029/2018GL079122.We analyze sources of ocean heat content (OHC) variability in the eastern North Atlantic subpolar gyre from both Eulerian and Lagrangian perspectives within two ocean simulations from 1990 to 2015. Heat budgets reveal that while the OHC seasonal cycle is driven by air‐sea fluxes, interannual OHC variability is driven by both air‐sea fluxes and the divergence of ocean heat transport, the latter of which is dominated by the oceanic flux through the southern face of the study area. Lagrangian trajectories initialized along the southern face and run backward in time indicate that interannual variability in the subtropical‐origin volume flux (i.e., the upper limb of the overturning circulation) drives variability in the temperature flux through the southern face. As such, the heat carried by the imported subtropical waters is an important component of the eastern subpolar gyre heat budget on interannual time scales.NSF. Grant Number NSF‐OCE‐12‐59102;
NASA Grant Number: NNX13AO21H2019-04-2
Evaluating altimetry-derived surface currents on the south Greenland shelf with surface drifters
No full textThe pathways and fate of freshwater in the East Greenland Coastal Current (EGCC) are crucial to the climate system. The EGCC transports large amounts of freshwater in close proximity to sites of deep open-ocean convection in the Labrador and Irminger seas. Many studies have attempted to analyze this system from models and various observational platforms, but the modeling results largely disagree with one another, and observations are limited due to the harsh conditions typical of the region. Altimetry-derived surface currents, constructed from remote-sensing observations and applying geostrophic equations, provide a continuous observational data set beginning in 1993. However, these products have historically encountered difficulties in coastal regions, and thus their validity must be checked. In this work, we use a comprehensive methodology to compare these Eulerian data to a Lagrangian data set of 34 surface drifter trajectories and demonstrate that the altimetry-derived surface currents are surprisingly capable of recovering the spatial structure of the flow field on the south Greenland shelf and can mimic the Lagrangian nature of the flow as observed from surface drifters
Ocean Heat Transport from the Subtropical Gyre to the Subpolar Gyre in the North Atlantic
No full textThe North Atlantic Ocean transports on the order of 1 petawatt of heat poleward from the Equator toward the high-latitude Arctic via an exchange of warm, upper-layer water for cold bottom water, reducing the Equator-to-pole temperature gradient and providing an opportunity for regional climate predictability at seasonal to decadal time scales. As more high-quality observations and numerical models of the ocean have become available in the past decade, oceanographers have realized that the variability in this oceanic meridional heat transport is not coherent across latitudes, with a prominent break in the meridional coherence between the subtropical gyre and the subpolar gyre. In this dissertation, I demonstrate the progress I have made on understanding how heat is conveyed from the subtropical gyre to the subpolar gyre in the North Atlantic at these critical inter-gyre latitudes (35°N-50°N). I use a suite of data including databases of in situ measurements of oceanic temperature and salinity, satellite observations of sea-surface temperature and height, and ocean model output of ocean current velocities, temperature and salinity. I conclude that a majority of the inter-gyre heat transport in the North Atlantic can be explained by variability in the strength of the sub-surface transport between the gyres. In this work, I also test whether pathways for propagating sea-surface temperature anomalies exist between the gyres as has been previously suggested and find that there is no evidence for this pathway in more modern satellite measurements. In addition, I show that variability in the sub-surface pathway cannot be explained by dynamics in the size and strength of the subpolar gyre as has long been assumed. Finally, I do a detailed analysis within two ocean circulation models and conclude that the oceanic heat fluxes are as important or more important as the surface atmospheric forcing to the temperature variability in the northeastern Atlantic, even at high frequencies. I then find that the origin of these oceanic heat fluxes stem from variability in the upper limb of the overturning circulation at inter-gyre latitudes. These results impact how we might expect to track ocean heat fluxes and thus where to look for climate predictability in the North Atlantic sector.</p
Atlantic overturning inferred from air-sea heat fluxes indicates no decline since the 1960s
No full textAbstract The Atlantic Meridional Overturning Circulation (AMOC) is crucial for global ocean carbon and heat uptake, and controls the climate around the North Atlantic. Despite its importance, quantifying the AMOC’s past changes and assessing its vulnerability to climate change remains highly uncertain. Understanding past AMOC changes has relied on proxies, most notably sea surface temperature anomalies over the subpolar North Atlantic. Here, we use 24 Earth System Models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to demonstrate that these temperature anomalies cannot robustly reconstruct the AMOC. Instead, we find that air-sea heat flux anomalies north of any given latitude in the North Atlantic between 26.5°N and 50°N are tightly linked to the AMOC anomaly at that latitude on decadal and centennial timescales. On these timescales, air-sea heat flux anomalies are strongly linked to AMOC-driven northward heat flux anomalies through the conservation of energy. On annual timescales, however, air-sea heat flux anomalies are mostly altered by atmospheric variability and less by AMOC anomalies. Based on the here identified relationship and observation-based estimates of the past air-sea heat flux in the North Atlantic from reanalysis products, the decadal averaged AMOC at 26.5°N has not weakened from 1963 to 2017 although substantial variability exists at all latitudes
Extreme wind events responsible for an outsized role in shelf-basin exchange around the southern tip of Greenland
No full textThe coastal circulation around Southern Greenland transports fresh, buoyant water masses from the Arctic and Greenland Ice Sheet near regions of convection, sinking, and deep-water formation in the Irminger and Labrador Seas. Here, we track the pathways and fate of these fresh water masses by initializing synthetic particles in the East Greenland Coastal Current on the Southeast Greenland shelf and running them through altimetry-derived surface currents from 1993 to 2021. We report that the majority of waters (83%) remain on the shelf around the southern tip of Greenland. Variability in the shelf-basin exchange of the remaining particles closely follows the number of tip jet wind events on seasonal and interannual timescales. The probability of a particle exiting the shelf increases almost fivefold during a tip jet event. These results indicate that the number of tip jets is a close proxy of the shelf-basin exchange around Southern Greenland
