OSNAP at Ocean Sciences 2016


Monday, 22 February

Neill Mackay
03:15-03:30 PM
Room 228-230
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 http://www.buypropeciaonline.org 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.

Tuesday, 23 February

Susan Lozier
Plenary lecture
10:30–11:30 AM
Great Hall A&B
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.

Thursday, February 25, 2016

Ric Williams
08:00-08:15 AM
Rooms 211-213
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.

Helen Johnson
08:45 – 09:00am
Rooms 203-205
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.

Patricia Handmann et al
09:30 – 09:45 AM
Rooms 203-205
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?

Stuart Cunningham et al
03:00 – 03:15 PM
Rooms 203-205
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.

Friday, February 26, 2016

Johannes Karstensen et al
03:00 – 03:15 PM
Rooms 203-205
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.

Poster Presentations

Monday, February 22, 2016 04:00 PM – 06:00 PM
Ernest N. Morial Convention Center, Poster Hall

HE14B High Latitude Air-Sea-Ice Interactions  in a Changing Climate II Posters

Marilena Oltmanns et al
HE14B-1415: The Role of Local and Regional Atmospheric Forcing for Convection in the Subpolar North Atlantic

Tuesday, February 22, 2016 04:00 PM – 06:00 PM
Ernest N. Morial Convention Center, Poster Hall

 PO24B: Mesoscale and Submesoscale Processes: Characterization, Dynamics, and Representation VI Posters

Chris Wilson 
An Update to the ‘Barrier or Blender’ Model of the Gulf Stream, Based on Lagrangian Analysis of Aviso Altimetry.

Thursday, February 25, 2016 04:00 PM – 06:00 PM
Ernest N. Morial Convention Center, Poster Hall

PO44 Atlantic Meridional Overturning Circulation: Past, Present, and Future III Posters

Chun Zhou
PO44A-3118: Subpolar North Atlantic glider observations for OSNAP

Friday, February 26, 2016 04:00 PM – 06:00 PM
Ernest N. Morial Convention Center, Poster Hall

PO54A: Atlantic Meridional Overturning Circulation: Past, Present, and Future V Posters

Amy Bower
PO54A-3225: The Charlie-Gibbs Fracture Zone: A Crossroads of the Atlantic Meridional Overturning Circulation

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.

Penny Holliday
PO54A-3222: The AMOC and subpolar gyre circulation at the OSNAP section in summer 2014.

Ric Williams
PO44A-3130: Gyre-specific Ocean Heat Content Changes Controlled by the Meridional Overturning in the North Atlantic

Sijia Zou
PO54A-3224: Contradictory Pathways between Labrador Sea Water Advection and Property Propagation.

PO54B: Climate Trends, Hydrographic Variability, Circulation, and Air-Land-Sea Interactions in the Marginal Seas of the North Atlantic III Posters

Femke de Jong & Laura de Steur
PO54B-3241: Record deep convection in the Irminger Sea: Observations from the LOCO mooring during winter 2014-2015.

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.

Loïc Houpert
PO54B-3234: Glider Observations of the Properties, Circulation and Formation of Water Masses on the Rockall Plateau in the North Atlantic.

Virginie Thierry
PO54B-3239: Argo float observations of basin-scale deep convection in the Irminger Sea during winter 2011-2012.

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