Atlantic Water Circulation

1^st FAMOS Workshop: Status of Experiment in 2012

Several projects are focused on the simulation and analysis of the Atlantic water (AW) circulation in the Arctic Ocean.

• First, Y. Aksenov is continuing to lead a multi-model study of AW circulation, using numerical tracers released at key inflow locations such as the Fram Strait and Barents Sea Opening.  The idea is to determine the inter-model variability in AW residence time and circulation pathways which has implications for model mixing, bathymetric steering, and heat/salt budgets.
• Second, a new project was started at the 2012 FAMOS workshop, to be led by M. Karcher, which would extend the single-model study of Karcher et al. (2012) to compare observed AW iodine concentrations with a suite of models and with a special focus on the AW circulation response to atmospheric circulation modes.
• Third, another project was started in 2012, to be led by M. Steele, which will examine slope-basin exchange of AW in the Eurasian Basin.  The idea is to determine the dominant terms in the models’ tracer evolution equations that mix AW properties across the slope, to be compared with mooring observations taken by NABOS and a newly funded project led by R. Pickart (WHOI) who recently deployed moorings across the slope northeast of Svalbard.

2^nd FAMOS Workshop: Updates and concrete recommendations/instructions (2013)

Leaders: Y. Aksenov and M. Karcher

Participants (open for joining; contact Y. Aksenov and M. Karcher)

1. Yevgeny Aksenov (National Oceanography Centre, Southampton, U.K.)
2. Elena Golubeva (Institute of Computational Mathematics and Mathematical Geophysics, RAS, Novosibirsk, Russia)
3. Christophe Herbaut (ILOCEAN-IPSL, Paris, France)
4. Marie-Noelle Houssais (ILOCEAN-IPSL, Paris, France)
5. Pal Isachsen (Norwegian Meteorological Institute, Norway)
6. Michael Karcher  (Alfred Wegener Institute, Bremerhaven, Germany)
7. Camille Lique (University of Oxford, Oxford, UK)
8. George Nurser (National Oceanography Centre, Southampton, U.K.)
9. Gennady A. Platov (Institute of Computational Mathematics and Mathematical Geophysics, RAS, Novosibirsk, Russia)
10. Andrey Proshutinsky (Woods Hole Oceanographic Institution, MA, USA)
11. Mike Spall (Woods Hole Oceanographic Institution, MA, USA)

Models (open for joining; contact Y. Aksenov and M. Karcher)

Models listed below are involved in this experiment:

• ICMMG (Russian Academy of Science, Russia)
• NAOSIM (Alfred Wegener Institute, Germany)
• NEMO-IPSL (Institute Pierre Simone Laplace, France)
• NEMO-NOCS (National Oceanography Centre, UK).
• ROMS (Norwegian Meteorological Institute, Norway)

All the participating models are regional, except for the NEMO-NOCS, which is a global model. The model horizontal resolution in the Arctic is between 10 km and 40 km; the details of the model setup are at the FAMOS website (http://www.whoi.edu/page.do?pid=30895).

1. Introduction

The distribution of the Atlantic Water (AW) in the Arctic Ocean has implications for mixing, ocean dynamics, heat and salt budgets, as well as for nutrients and biology. The principal goals of the coordinated tracer experiments are to:

• Determine and explain the inter-model variations in distribution, residence time and circulation pathways of the AW in the Arctic Ocean.
• Compare model results with observations to identify errors and their causes.
• Improve models to reduce uncertainties in the dynamics of the AW circulation in the Arctic Ocean

Using passive on-line “colour” tracers to track the inflow of the AW in the Arctic Ocean via the Fram Strait and the Barents Sea, we will examine the spread of AW entering the ocean through the eastern Fram Strait and through the Barents Sea and the contribution of the two AW branches to the intermediate and deep waters in the Arctic Ocean. We will analyze variability of the AW circulation and the mechanisms, including bathymetric steering of the flow.

2. Methods

2.1 Integrations

The integrations cover the period 1958-2010, including spin-up 1958-1978, completed without tracers, and continuous tracers release during 1979-2010. The Barents Sea tracer is released in the Barents Sea Opening (BSO) on the section along~19°E from the Bear Island to the coast of Norway, full depth and the Fram Strait (FS) tracer is released on the section along 78.5°N, longitude range 5E to the coast, full depth. The tracers’ redistribution is calculated during the model integrations “on-line”, thus the evolution of the tracers is fully consistent with the model dynamics. The 20-year spin-up allows us to minimize impact of the initial conditions. The models are forced either with atmospheric re-analyses from the National Center for Environmental Prediction/Coordinated Ocean Reference Experiments (NCEP/CORE) or from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA).

2.2 Output fields

We need monthly means of the model fields and their products, as listed below. For time-varying vertical coordinate (e.g., z*, z~ or sigma), layer-weighted variables should be outputted together with depth of the layers. We require 4-D (x, y, z, time) T,S,U,V and vertical diffusive heat and FW fluxes. We require 3-D (x, y, time) SSH, ice thickness, ice concentration and velocity, heat and FW fluxes at upper ocean interface with separate fields for ice covered areas and open ocean areas. We require 4-D (x, y ,z, time) fields of the following products T*U,T*V, S*U, S*V. All fields are on the model grids; we also require model grid information.

2.3 Verification of model skills

We compare model and observational fields focusing on the Atlantic layer. For that several regions the Arctic Ocean are defined: North of Fram Strait, Laptev Sea, Chukchi Sea, shelf slope north of the Barents and Kara Seas. We use 2007 summer (June-August) mean as a “snapshot”. Observed temperature and salinity in the top 1000m of the Arctic Ocean is gridded in the geographical grid with horizontal resolution of 1/2° grid (alternatively gridded in the 55.5×55.5 km grid) with 10m vertical resolution. We compare total volume and heat advective transports and those for the AW fraction through Fram Strait and BSO. The set of the model sections in use are from “Arctic Ice and Ocean Heat and Freshwater Fluxes”. The temperature and salinity structure on these sections are to be compared with the observations.  The current meter data from NABOS is used to assess the strength of the AW flow along the Siberian shelves.

2.4 Model diagnostics

Position of the temperature maximum within the Atlantic water layer, fractions of the Barents Sea branch and Fram Strait branch waters in different areas of the Arctic Ocean and heat storage and transports associated with Atlantic inflow in the Arctic is diagnosed. We diagnose depth of the Tmax within the AW layer, upper and lower boundaries of the layer (>10% of the tracer) and heat content. From the tracer fractions we compute the FSBW and BSBW components of volume and heat transports across the section in Fram Strait, BSO and SAT and across the Northern Barents Sea shelf; AW transport across sections (FS, BSO) for different density classes are also diagnosed. We decompose AW heat transports into the temperature and flow anomalies and also estimate vertical AW heat loss through vertical mixing. We apply watermass Walin-type transformation technique extended by Nurser to differentiate between vertical and horizontal mixing and surface processes of the AW modification. We estimate transit/resident times for the different branches of the Atlantic water in the Arctic from advective timescales of the AW tracers.

3. Progress and future work

So far, we have performed the AW tracer release in the four models, ICMMG, NAOSIM, NEMO-IPSL and NEMO-NOCS. The models are not eddy resolving in the Arctic and were of comparable horizontal resolution (~12-40 km). The model demonstrated a realistic spread of the Fram Strait and Barents Sea modes of AW (FSW and BSW) in the Arctic Ocean, the FSW occupied depth range of 200-700m and BSW maximum was found around 1000m. We also established a difference in the deep ocean circulation in the models. In NAOSIM, there was a deep (at ca. 1000m depth) inflow in the Arctic on the eastern side of Fram Strait and farther along the northern slope of the Barents Sea, whereas the inflow was absent in NEMO-NOCS and the deep circulation in the Nordic Sea and the Arcitc Ocean was decoupled. The light mode of the shelf waters with the origin in the Barents Sea was also detected in the Canadian Basin of the Arctic Ocean, supporting the conclusion regarding the continuous flow of the intermediate waters through the Arctic Ocean. The other two models, ICMMG and NEMO-IPSL have competed the tracer integrations and the results are under analysis.

The next step is to compare model simulations with data and analyze differences in model results in the full set of the participating models. We plan to have results ready for publication in ca. 18 months time. Aksenov and Karcher are going to lead the paper.