April 23-28, 2017 in Vienna, Austria
Amy Bower, Heather Furey, and Susan Lozier
Overflow Water Pathways in the Subpolar North Atlantic Observed with Deep Floats
Loïc Houpert, Mark Inall, Estelle Dumont, Stefan Gary, Marie Porter, William Johns, and Stuart Cunningham
Transport Structure and Energetic of the North Atlantic Current in Subpolar Gyre from Observations
William Johns, Adam Houk, Greg Koman, Sijia Zou, and Susan Lozier
Transport of Iceland-Scotland Overflow waters in the Deep Western Boundary Current along the Reykjanes Ridge
OSNAP Update: Measuring the AMOC in the subpolar North Atlantic
Robert Pickart, Wilken von Appen, Dana Mastropole, Hedinn Valdimarsson, Kjetil Vage, Steingriumur Jonsson, Kerstin Jochumsen, and James Girton
On the Nature of the Mesoscale Variability in Denmark Strait
Chris Wilson, Yi Liu, Melissa Green, and Chris Hughes
Gulf Stream transport and mixing processes via coherent structure dynamics
Jian Zhao, Amy Bower, Jiayan Yang, Xiaopei Lin, and Chun Zhou
Mesoscale eddies control meridional heat flux variability in the subpolar North Atlantic
Sijia Zou, Susan Lozier, Walter Zenk, Amy Bower, and William Johns
Observed and Modeled Pathways of the Iceland Scotland Overflow Water in the eastern North Atlantic
Femke de Jong, Laura de Steur, Stelios Kritsotalakis
Volume, heat and freshwater transport in the Irminger Current
Nick Foukal and Susan Lozier
Assessing variability in the size and strength of the North Atlantic subpolar gyre
Greg Koman, Adam Houk, Cobi Christiansen, and Bill Johns
Transport and seasonal variability of the East Reykjanes Ridge Current
Feili Li and Susan Lozier
On the Linkage between Labrador Sea Water Volume and Overturning Circulation in the Labrador Sea
Neill Mackay, Chris Wilson, and Jan Zika
Application of a Regional Thermohaline Inverse Method to observational reanalyses in an Arctic domain
Ryan Peabody and Susan Lozier
The AMOC as a mechanism for nutrient supply to the Eastern North Atlantic
Anne Piron, Virginie Thierry, Herlé Mercier, and Guy Caniaux
Gyre scale deep convection in the subpolar North Atlantic Ocean during winter 2014-2015
Petit Tillys, Mercier Herle, and Thierry Virginie
Circulation in the region of the Reykjanes Ridge in June-July 2015
December 12-16, 2016 in San Francisco, California
Lozier, Brown, Zhang and Li
The necessity of cloud feedback for a basin-scale Atlantic Multidecadal Oscillation (Invited)
The Atlantic Multidecadal Oscillation (AMO), characterized by basin-scale multidecadal variability in North Atlantic sea surface temperatures (SSTs), has traditionally been interpreted as the surface signature of variability in oceanic heat convergence (OHC) associated with the Atlantic Meridional Overturning Circulation (AMOC). This view has been challenged by recent studies which show that AMOC variability is not simultaneously meridionally coherent over the North Atlantic and that AMOC-induced low frequency variability of OHC is weak in the tropical North Atlantic. Here we present modeling evidence that the AMO-related SST variability over the extratropical North Atlantic results directly from anomalous OHC associated with the AMOC, but that the emergence of the coherent multidecadal SST variability over the tropical North Atlantic requires cloud feedback. Our study identifies atmospheric processes as a necessary component for the existence of a basin-scale AMO, thus amending the canonical view that the AMOC-AMO connection is solely attributable to oceanic processes. The implications of our work for the AMOC-AMO relationship on other time scales will be discussed, as will observational efforts focused on studying AMOC coherence.
Laura de Steur and Femke de Jong
Variability in the Irminger Sea: new results from continuous ocean measurements between 2014-2015
The Irminger Current along the Reykjanes Ridge transports warm and saline Atlantic Water northward in the subpolar gyre and hence forms an important contribution to the upper warm limb of the AMOC. Volume and heat transport estimates have up to present principally been based on summer hydrographic data combined with satellite surface velocities. Here we present the first year-round volume and heat transports based on the full-depth mooring array on the western flank of the Reykjanes Ridge between 2014 and 2015. These estimates are compared with results based on shipboard data from the early 1990s and 2000s when two different modes of transport variability were observed through the appearance of a second deep core of the Irminger Current. The recently obtained continuous measurements show a distinct change in the shape and strength of the Irminger Current during the one-year deployment period. This change occurred during the winter of 2014-2015 concomitantly with record deep convection observed in the central Irminger Gyre. The convection, observed by a moored CTD-profiler, was associated with very strong sustained surface buoyancy forcing, leading to mixed layer depths of 1200 m. This
oxygen-rich, recently ventilated water was observed basin wide in the Irminger Sea in 2015 and contrasted the stratified situation seen in 2014. The Irminger Current and the Irminger basin hydrography are reminiscent of the conditions that were seen in the early 1990s.
Helen Pillar, Patrick Heimbach, Helen Johnson and David Marshall
Dynamical Attribution of Recent Variability in Atlantic Overturning
Attributing observed variability of the Atlantic Meridional Overturning Circulation (AMOC) to past changes in surface forcing is challenging but essential for detecting any influence of anthropogenic forcing and reducing uncertainty in future climate predictions. Here we obtain quantitative estimates of wind and buoyancy-driven AMOC variations at 25N by projecting observed atmospheric anomalies onto model-based dynamical patterns
of AMOC sensitivity to surface wind, thermal and freshwater forcing over the preceding 15 years. We show that local wind forcing dominates AMOC variability on short timescales, whereas subpolar heat fluxes dominate on decadal timescales. The reconstructed transport time series successfully reproduces most of the interannual variability observed by the RAPID-MOCHA array. However, the apparent decadal trend in the RAPID-MOCHA time
series is not captured, requiring improved model representation of ocean adjustment to subpolar heat fluxes over at least the past two decades, and highlighting the importance of sustained monitoring of the high latitude North Atlantic.
Ocean Sciences 2016
New Orleans, LA
Stuart Cunningham et al
O43A-05: The Subpolar AMOC: Dynamic Response of the Horizontal and Overturning Circulations due to Ocean Heat Content Changes between 1990 and 2014
Ocean heat content (OHC) in the subpolar region of the North Atlantic varies on interannual to decadal timescales and with spatial variations between its sub-basins as large as the temporal variability. In 2014 the Overturning in the Subpolar North Atlantic Programme (OSNAP) installed a mooring array across the Labrador Sea and from Greenland to Scotland. The objective of the array is to measure volume, heat and fresh-water fluxes. By combining Argo and altimeter data for the period 1990 to 2014 we describe and quantify the anomalous horizontal and overturning circulations and fluxes of heat and fresh-water driven by the long-term OHC changes. We thus provide a longer-term context for the new observations being made as part of OSNAP. Changes to the horizontal circulation involve deceleration of the gyre rim currents, lateral shifts of major open ocean current features and increased exchanges in the eastern intergyre region. These changes impact the Atlantic Meridional Overturning Circulation (AMOC) in density space causing a rich vertical anomalous structure. The net impact over this 24 year period is a reduction in northward heat-flux and decrease in southward fresh-water flux.
Patricia Handmann et al
PO41A-07 North Atlantic Deep Western Boundary Current Dynamics as Simulated by the VIKING20 Model Compared with Labrador Sea Observations
The connection of dynamic and hydrographic properties simulated by the VIKING20 model driven by CORE2 atmospheric forcing will be presented and compared to more than decade-long observations at the exit of the Labrador Sea near 53°N. VIKING20 is a high resolution (1/20°) nest, implemented by two-way nesting in a global configuration of the NEMO-LIM2 ocean-sea ice model in the North Atlantic (ORCA25). The exit of the Labrador Sea is the place where water masses from different origins and pathways meet and which are collectively called North Atlantic Deep Water (NADW). The VIKING20 flow field on average reproduces the observed structure as well as the bottom intensification of the western boundary current at 53°N. Here, we investigate the properties of the observed and modeled deep western boundary current by comparing North Atlantic water masses and currents simulated by the high resolution model with moored and hydrographic data from almost 20 year-long observations at 53°N. As comparable density fields in the model in comparison to the observations are found at shallower depths, we will present an evaluation of dynamic and hydrographic changes connected to each other and to atmospheric forcing in the model and observed data. In addition the following key questions will be addressed: How is energy distributed in baroclinic and barotropic components in observations and model in comparison to each other? The seasonal cycle can be found in the shallow Labrador Current in the model and the observations, but how deep is it reaching and causing dynamic and hydrographic changes?
PO41A-04 Dynamical Attribution of Recent Variability in Atlantic Overturning
Attributing observed variability of the Atlantic Meridional Overturning Circulation (AMOC) to past changes in surface forcing is challenging but essential for detecting any influence of anthropogenic forcing and reducing uncertainty in future climate predictions. Here we obtain quantitative estimates of wind and buoyancy-driven AMOC variations at 25◦N by projecting observed atmospheric anomalies onto model-based dynamical patterns of AMOC sensitivity to surface wind, thermal and freshwater forcing over the preceding 15 years. We show that local wind forcing dominates AMOC variability on short timescales, whereas subpolar heat fluxes dominate on decadal timescales. The reconstructed transport time series successfully reproduces most of the interannual variability observed by the RAPID-MOCHA array. However, the apparent decadal trend in the RAPID-MOCHA time series is not captured, requiring improved model representation of ocean adjustment to subpolar heat fluxes over at least the past two decades, and highlighting the importance of sustained monitoring of the high latitude North Atlantic.
Johannes Karstensen et al
PO53A-05: Observations and causes of hydrographic variability in den deep western boundary current at the exit of the Labrador Sea.
The hydrographic variability of the Deep Western Boundary Current (DWBC) in the Labrador Sea is discussed using observational data from the period 1997 to 2014. This variability of the DWBC occurs on time scales from a few days to multiannual. The hydrographic data is analyzed in terms of signals originating from different “behavioral modes” of the DWBC, including the re-positioning of the core along the sloping topography, the pulsing of the core, and the advection of watermass anomalies within the core. Cross-correlation spectra show that the hydrographic variability on time scales of a few days can be explained by the periodic re-location of the core due to topographic waves. Variability on longer time scales can be interpreted by long-term re-location of the core, potentially related to an adjustment of the core to circulation changes on gyre scale. However, along-flow advection of anomalies is likely another source for this long-term variability. Possible scenarios for the generation of hydrographic variability in the source regions of the DWBC are discussed.
A Decade after The Day After Tomorrow: Our Current Understanding of the Ocean’s Overturning Circulation
In 1800 Count Rumford ascertained the ocean’s meridional overturning circulation from a single profile of ocean temperature constructed with the use of a rope, a wooden bucket and a rudimentary thermometer. Over two centuries later, data from floats, gliders and moorings deployed across the North Atlantic has transformed our understanding of the temporal and spatial variability of the meridional overturning: the component of the climate system responsible for sequestering heat and anthropogenic carbon dioxide in the deep ocean. In this talk I will review our current understanding of the overturning circulation with a particular focus on what we currently do and don’t understand about the mechanisms controlling its temporal change.
PO13E-06: Circulation and mixing in the subpolar North Atlantic diagnosed from climatology using a Regional Thermohaline Inverse Method (RTHIM)
The Overturning in the Subpolar North Atlantic Program (OSNAP) aims to quantify the subpolar Atlantic Meridional Overturning Circulation (AMOC), including associated advective and diffusive transport of heat and freshwater. The OSNAP observational array will provide a continuous subpolar record of the AMOC from Labrador-Greenland-Scotland during 2014-2018. To understand the significance of high- and low- frequency changes measured by the array, including changes to AMOC metrics, water mass transformation and transports, Argo observations provide a useful complementary constraint for an inverse method, with the aim of resolving intra-seasonal timescales.
A novel inverse method in thermohaline coordinates has recently been demonstrated as being able to diagnose aspects of the global overturning circulation and mixing from model data. Here we have further developed a Regional Thermohaline Inverse Method, (RTHIM) and have validated it with the NEMO model in the OSNAP region, before applying it to a seasonal Argo climatology.
In an ocean basin there exists a balance between surface heat and freshwater fluxes, advective fluxes at an open boundary and interior diffusive mixing. RTHIM makes use of this balance to determine unknown velocities at the open boundary and diffusive fluxes of heat and salt within the domain volume. We identify key transport and mixing regions and events, relevant to the subpolar AMOC, and discuss the robustness of the inverse solutions. RTHIM is also able to identify the particular contributions to AMOC volume transport changes from temperature and salinity components.
PC41A-01 Climate sensitivity to ocean sequestration of heat and carbon.
Ocean ventilation is a crucial process leading to heat and anthropogenic carbon being sequestered from the atmosphere. The rate by which the global ocean sequesters heat and carbon has a profound effect on the transient global warming. This climate response is empirically defined in terms of a climate index, the transient climate response to emissions (TCRE). Here, we provide a theoretical framework to understand how the TCRE can be interpreted in terms of a product of three differential terms: the dependence of surface warming on radiative forcing, the fractional radiative forcing contribution from atmospheric CO2 and the dependence of radiative forcing from atmospheric CO2 on cumulative carbon emissions. This framework is used to diagnose two models, an Earth System Model of Intermediate Complexity, configured as an idealised coupled atmosphere and ocean, and an IPCC-class Earth System Model. In both models, the centennial trends in the TCRE are controlled by the response of the ocean, which acts to sequester both heat and carbon; there is a decrease in the dependence of radiative forcing from CO2 on carbon emissions, which is partly compensated by an increase in the dependence of surface warming on radiative forcing. On decadal timescales, there are larger changes in the TCRE due to changes in ocean heat uptake and changes in non-CO2 radiative forcing linked to other greenhouse gases and aerosols. Our framework may be used to interpret the response of different climate models and used to provide traceability between simple and complex climate models.
PO54A-3225: The Charlie-Gibbs Fracture Zone: A Crossroads of the Atlantic Meridional Overturning Circulation
The Charlie-Gibbs Fracture Zone (CGFZ), a deep gap in the Mid-Atlantic Ridge at ~52N, is the primary conduit for westward-flowing Iceland-Scotland Overflow Water (ISOW), which merges with Denmark Strait Overflow Water to form the Deep Western Boundary Current. The CGFZ has also been shown to “funnel” the path of the northern branch of the eastward-flowing North Atlantic Current (NAC), thereby bringing these two branches of the AMOC into close proximity. A recent two-year time series of hydrographic properties and currents from eight tall moorings across the CGFZ offers the first opportunity to investigate the NAC as a source of variability for ISOW transport. The two-year mean and standard deviation of ISOW transport was -1.7 ± 1.5 Sv, compared to -2.4 ± 3.0 Sv reported by Saunders for a 13-month period in 1988-1989. Differences in the two estimates are partly explained by limitations of the Saunders array, but more importantly reflect the strong low-frequency variability in ISOW transport through CGFZ (which includes complete reversals). Both the observations and output from a multi-decadal simulation of the North Atlantic using the Hybrid Coordinate Ocean Model (HYCOM) forced with interannually varying wind and buoyancy fields indicate a strong positive correlation between ISOW transport and the strength of the NAC through the CGFZ (stronger eastward NAC related to weaker westward ISOW transport). Vertical structure of the low-frequency current variability and water mass structure in the CGFZ will also be discussed. The results have implications regarding the interaction of the upper and lower limbs of the AMOC, and downstream propagation of ISOW transport variability in the Deep Western Boundary Current.
Femke de Jong & Laura de Steur
PO54B-3241: Record deep convection in the Irminger Sea: Observations from the LOCO mooring during winter 2014-2015.
Anomalously strong cooling over the Irminger Sea during the winter of 2014-2015 caused record depths convective mixing. Active mixed layer depths at the LOCO mooring site, near the center of the Irminger Gyre, reached down to 1200 m. A further reduction of stratification suggests mixed layers down to 1500 m. The deep mixing eroded the intermediate salinity minimum associated with Labrador Sea Water and replaced it with a cold, fresh homogeneous layer rich in oxygen. This layer was seen to extend across the basin in the hydrographic section of summer 2015, suggesting that a significant part of the basin participated in the mixing. The winter 2014-2015 convective event exceeded the previous maximum of 1000 m in the winter of 2007-2008. The main cause is the surface buoyancy forcing. Strong surface buoyancy loss (monthly mean > 125 W/m2) lasted for an additional month. The 2015 hydrography is reminiscent of the situation in the 1990s when a large volume of homogeneous water filled the Irminger basin.
Laura de Steur & Femke de Jong
PO54B-3242: Transport variability of the Irminger Current: First year-round results from a mooring array on the Reykjanes Ridge.
The Irminger Current on the Reykjanes Ridge transports warm and saline Atlantic Water northward in the subpolar gyre and hence forms an important component of the upper warm limb of the AMOC. Volume and heat transports have – up to now – principally been based on analysis of summer hydrographic data combined with satellite surface velocities. Here we present the first year-round volume and heat transport estimates based on the full-depth mooring array on the western flank of the Reykjanes Ridge between 2014 and 2015. These estimates are compared with results based on shipboard data from the early 1990s and the early 2000s when two contrasting modes of transport variability were seen through the appearance of a second deep core of the Irminger Current. The results of the newly obtained continuous measurements initially show two clear bottom intensified cores in the flow field. However, during the deployment period the Irminger Current showed increased variability in shape and strength during the winter 2014-2015. This change happened in concert with a return from a highly stratified Irminger basin in 2014 to a basin that was filled with cold and fresh LSW-like water in 2015. All in all the situation in 2015 was very reminiscent of the conditions that were seen in the early 1990s in the Irminger basin. These results are further explored in light of atmospheric circulation, a strong positive NAO and the strong winter of 2014-2015 causing record deep convection in the central Irminger Gyre.
Nicholas Foukal and Susan Lozier
PO54A-3229: Variability in Lagrangian-derived througput from the subtropical to the subpolar gyres in the North Atlantic and its impact on inter-gyre heat transport.
Variability in the strength of the inter-gyre throughput of water from the subtropical to the subpolar gyres in the North Atlantic as part of the upper limb of the Atlantic Meridional Overturning Circulation has been hypothesized to control the variability in the inter-gyre heat transport. Here, we first quantify the variability in the inter-gyre throughput by tracking backwards-run Lagrangian trajectories in a high-resolution ocean circulation model and secondly determine the controlling mechanisms of this variability by analyzing the strength and spatial extent of the subtropical and subpolar gyres as measured by satellite altimetry. Backwards-run Lagrangian trajectories provide an accurate measure of the origin of water masses without the subjective designations of water mass classes and assumptions on the mixing along pathways. Similarly, by explicitly tracking the strength (sea-surface height of gyre center – sea-surface height of gyre boundary) and spatial extent (area enclosed by the largest closed sea-surface height contour), no assumptions are necessary on the role of statistical modes that change through time. Results from these analyses will be discussed as they pertain to the observed warming of the subpolar gyre during the past two decades.
PO54A-3222: The AMOC and subpolar gyre circulation at the OSNAP section in summer 2014.
The overturning and gyre circulation in the subpolar North Atlantic is being measured by the international observing array deployed by OSNAP (Overturning in the Subpolar North Atlantic Programme, www.o-snap.org). The OSNAP line crosses the Labrador Sea from 52N to the southern end of Greenland, and lies across the Irminger Sea, Iceland Basin and Rockall Trough at approximately 60-57°N. The array, deployed 2014-2018, uses moored instruments, gliders and floats to measure the surface to seafloor circulation. OSNAP is novel and exciting because it is making the first ever continuous measurements of the Atlantic Meridional Overturning Circulation in the subpolar region. In time, OSNAP will generate products analogous to the RAPID time series at 26N.
In this study we use a high resolution CTD/LADCP OSNAP section from summer 2014 to provide a synoptic view of the circulation, heat and freshwater fluxes at the time that the array was deployed. The data represent the first time that the entire subpolar region including the Labrador Sea has been measured in a single cruise (GO-SHIP section AR07, cruise JR302). We will present estimates of both the horizontal and overturning circulation, and the latter will be examined both in pressure and in density co-ordinates. We will show volume, heat and freshwater transports for the whole section and key components, including the deep boundary currents and shallow shelf currents containing freshwater exported from the Arctic.
PO54B-3234: Glider Observations of the Properties, Circulation and Formation of Water Masses on the Rockall Plateau in the North Atlantic.
The Overturning in Subpolar North Atlantic Program (OSNAP) is an international collaboration with the overarching goal of measuring the full-depth mass fluxes associated with the AMOC (Atlantic Meridional Overturning Circulation), as well as meridional heat and fresh-water fluxes. Through the deployment of moorings and gliders, UK-OSNAP is part of this international partnership to maintain a transoceanic observing system in the subpolar north Atlantic (the OSNAP array).
We present here the first year and a half of UK-OSNAP glider missions on the Rockall Plateau in the North Atlantic, along the section located at 58°N, between 22°W and 15°W. Between July 2014 and September 2015, 10 gliders sections were realized on the Rockall Plateau. The depth-averaged current estimated from gliders shows very strong values (up to 45cm.s-1) associated with meso-scale variability due particularly to eddies and water mass formation. Glider data also reveal a deep mixed layer in February/March 2015 up to 600m associated with the formation of the 27.3σθ and 27.4σθ Subpolar Mode Waters.
The variability of the meridional transport of heat, salt and mass on the Rockall Plateau are also discussed. Relative and absolute geostrophic transports are calculated from the glider data and from the combination of the glider data and the data from mooring M4 located in the Iceland Basin (58°N, 21°W).
Marilena Oltmanns et al
HE14B-1415: The Role of Local and Regional Atmospheric Forcing for Convection in the Subpolar North Atlantic
Variabilities in upper ocean hydrography in the subpolar North Atlantic and Nordic Seas can have large implications for deep water formation, often seen as the downward branch of the Atlantic meridional overturning circulation. These variabilities encompass a wide range of spatial and temporal scales and their causes may vary between different years and basins. Specific causes on short time scales of the order of days include local air-sea heat fluxes during intense wind events and transient eddies. Freshwater fluxes associated with discharges from land and sea ice and vertical transports related to the regional wind stress curl can cause variations in stratification on time scales of the order of weeks to months. On even longer time scales, changes in ocean currents, subsurface water mass anomalies and large-scale climate variability may alter the upper ocean hydrography.
In this presentation, we focus on relatively short time scales of the order of days up to several weeks. First, we will characterize the atmospheric forcing on these time scales over the convection centers in the subpolar North Atlantic with a reanalysis product, remote sensing data and observations from nearby weather stations. Using observations from moored instruments in the subpolar North Atlantic, we will then trace the spectral characteristics of the local heat and freshwater fluxes, as well as of the regional momentum fluxes, in salinity and temperature variabilities in the upper ocean. Finally, we will explore the coherence among the different fluxes, relate them to specific forcing mechanisms, such as cyclones, polar lows and blocking anti-cyclones, and evaluate their contribution to convection.
PO54B-3239: Argo float observations of basin-scale deep convection in the Irminger Sea during winter 2011-2012.
An analysis of Argo data during the 2011-2012 winter revealed the presence of an exceptionally large number of profiles over the Irminger Basin with mixed layer depths (MLD) exceeding 700 m, which was deep enough to reach the pool of the intermediate Labrador Sea Water located in the Irminger Sea. Among them, 4 profiles exhibited an MLD of 1000 m, which was the maximum value observed this winter. Owing to the exceptional Argo sampling in the Irminger Sea during that winter the different phases of the mixed layer deepening down to 1000 m and their spatial extents were observed for the first time in the Irminger Sea. Two intense convective periods occurred in late January south of Cape Farewell and in late February-early March east of Greenland. A final deepening period was observed in mid-March during which the deepest mixed layers were observed. This long deepening period occurred in large regional areas and was followed by a rapid restratification phase. A mixed layer heat budget along the trajectories of the 4 floats that sampled the deepest mixed layers showed that heat loss at the air-sea interface was mainly responsible for heat content variations in the mixed layer. Greenland Tip Jets were of primary importance for the development of deep convection in the Irminger Sea in the 2011-2012 winter. They enhanced the winter heat loss and two long (more than 24 hours), intense and close in time late events boosted the mixed layer deepening down to 1000m. Net air-sea fluxes, the number of Greenland Tip Jets, the stratification of the water column, the NAO index and Ekman-induced heat flux are pertinent indicators to assess the favorable conditions for the development of deep convection in the Irminger Sea. When considering each of those indicators, we concluded that the 2011-2012 event was not significantly different compared to the three other documented occurrences of deep convection in the Irminger Sea.This work is a contribution to the NAOS project.
PO44A-3130: Gyre-specific Ocean Heat Content Changes Controlled by the Meridional Overturning in the North Atlantic
In the North Atlantic, there are pronounced gyre-scale changes in ocean heat content on interannual to decadal time scales. This climate variability is investigated using a semi-diagnostic dynamical analysis of historical temperature and salinity data from 1962 to 2011 together with idealised isopycnic model experiments. On timescales of typically five years, the tendencies in upper ocean heat content are not simply explained by the area-averaged atmospheric forcing for each gyre, but instead dominated by heat convergences associated with the meridional overturning circulation (MOC). In the subtropics, the most pronounced warming events are associated with an increased influx of tropical heat driven by stronger Trade winds. In the subpolar gyre, the warming and cooling events are associated with changes in western boundary density, where increasing boundary density in the Labrador Sea leads to an enhanced overturning and an influx of subtropical heat. The different effects of the meridional overturning are a consequence of how the poleward heat transport is achieved in a different manner over the basin: the heat carried mainly by the MOC over the upper 100m at low latitudes and instead by the MOC from 100m to 1300m at mid and high latitudes, augmented by the gyre transport at high latitudes. In summary, upper ocean heat content anomalies are formed in a different manner in the subtropical and subpolar gyres, with different components of the meridional overturning circulation probably excited by the local imprint of atmospheric forcing.
PO24B-2949: An Update to the ‘Barrier or Blender’ Model of the Gulf Stream, Based on Lagrangian Analysis of Aviso Altimetry.
Finite-time Lyapunov exponent (FTLE) is calculated from 22 years of Aviso geostrophic velocity to identify Lagrangian coherent structure (LCS) in the Gulf Stream region. The coherent structures in and around the Gulf Stream are delineated by the both positive- and negative-time FTLE ridges, and represent boundaries between dynamically distinct regions with characteristic transport and mixing processes. Alternating positive- and negative-time FTLE ridge patterns are found to line the meandering jet, which indicate the regions of entrainment and detrainment along the jet.
This LCS pattern compares well with the Bower kinematic model of a meandering jet, although it is clear that the kinematic model is an over-simplification of the jet dynamics, and studying the dynamics of vortex interaction with the jet is important for more fully understanding fluid transfer in the Gulf Stream region. A new conceptual model for the Gulf Stream is proposed, including a mechanism for the generation of the observed region of largest mean mixing efficiency. There is large variability in mixing efficiency in the ‘wavemaker’ region, where standing Rossby waves are important.
PO44A-3118: Subpolar North Atlantic glider observations for OSNAP
OSNAP is an international program designed to provide a continuous record of the full-water-column, trans-basin fluxes of heat, mass, and freshwater in the subpolar North Atlantic. The observational efforts of this program are focused largely along lines connecting Labrador to Greenland, and Greenland to Scotland. The OSNAP experimental plan includes continuous sampling by Slocum G2 gliders along the latter (easternmost) of these two sections, specifically across the northeastward-flowing North Atlantic Current in the Iceland Basin. The glider observations, a collaboration between the Ocean University of China and Woods Hole Oceanographic Institution, provide higher spatial resolution of water properties than is possible from moorings alone. These observations commenced in June 2015 with a mission to fly back and forth along a section between two OSNAP moorings, profiling from the surface to 1000-m depth. As of September 2015, five sections (including over 240 profiles) have been recorded. As expected, the data indicates energetic intraseasonal variability at smaller scales than can be captured by the OSNAP mooring array. We are investigating how this variability may impact calculated fluxes of heat, mass, and freshwater. The glider repeatedly crossed a cyclonic eddy between the two moorings, enabling study of fine thermohaline structure during the development and dissipation of mesoscale eddies in the subpolar North Atlantic. With additional sensors measuring fluorescence, dissolved oxygen, nitrate, and multispectral light, the dataset also has the potential to significantly advance our understanding of the biogeochemical processes of mesoscale and submesoscale eddies in the subpolar North Atlantic.
PO54A-3224: Contradictory Pathways between Labrador Sea Water Advection and Property Propagation.
Past observational studies have shown a strong relationship between Labrador Sea Water (LSW) property variability and property anomalies in the western subtropical gyre, with the former leading the latter by around 10 years. This time scale stands in contradiction to recent studies that have revealed a much longer advective time scale for LSW to enter the subtropical gyre. Using simulated floats from an ocean general circulation model, we show that LSW is not directly exported to the subtropical gyre, but rather recirculates within the subpolar gyre before it crosses the inter-gyre boundaries, primarily through interior pathways. The average age of LSW upon entering the subtropical basin is 22 (± 10) years. Once in the subtropical basin, LSW is advected from the central and eastern regions to the western region, where it joins the Deep Western Boundary Current with an average age of 30 (± 8) years. This spreading pattern of LSW trajectories differs markedly from the apparent pathway of LSW salinity anomalies: a cross correlation map of observational salinity anomalies in the Labrador Sea with those across the entire North Atlantic, reveals a direct and relatively fast propagation pathway along the western boundary, which takes 10-12 years to reach 30°N. Ongoing research to understand the mechanisms of LSW trajectory and property pathways will be discussed.