Preprints
https://doi.org/10.5194/gmd-2020-421
https://doi.org/10.5194/gmd-2020-421

Submitted as: development and technical paper 17 Feb 2021

Submitted as: development and technical paper | 17 Feb 2021

Review status: a revised version of this preprint is currently under review for the journal GMD.

Development of adjoint-based ocean state estimation for the Amundsen and Bellingshausen Seas and ice shelf cavities using MITgcm/ECCO

Yoshihiro Nakayama1,2, Dimitris Menemenlis1, Ou Wang1, Hong Zhang1, Ian Fenty1, and An T. Nguyen3 Yoshihiro Nakayama et al.
  • 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
  • 2Institute of Low Temperature Science, Hokkaido University, Sapporo, Hokkaido, Japan
  • 3The University of Texas at Austin, Austin, Texas, USA

Abstract. The Antarctic coastal ocean is impacting sea level rise, deep-ocean circulation, marine ecosystems, and global carbon cycle. To better describe and understand these processes and their variability, it is necessary to combine the sparse available observations with best-possible numerical descriptions of ocean circulation. In particular, high ice-shelf melting rates in the Amundsen Sea have attracted many observational campaigns and we now have some limited oceanographic data that capture seasonal and interannual variability during the past decade. One method to combine observations with numerical models that can maximize the information extracted from the sparse observations is the adjoint method, as developed and implemented for global ocean state estimation by the Estimating the Circulation and Climate of the Ocean (ECCO) project. Here, for the first time, we apply the adjoint-model estimation method to a regional configuration of the Amundsen and Bellingshausen Seas, Antarctica, including explicit representation of sub-ice shelf cavities. We utilize observations available during 2010–2014, including ship-based and seal-tagged CTD measurements, moorings, and satellite sea-ice concentration estimates. After 20 iterations of the adjoint-method minimization algorithm, the cost function, here defined as a sum of weighted model-data difference, is reduced by 65 % relative to the baseline simulation by adjusting initial conditions, atmospheric forcing, and vertical diffusivity. The sea-ice and ocean components of the cost function are reduced by 59 % and 70 %, respectively. Major improvements include better representations of (1) Winter Water (WW) characteristics and (2) intrusions of modified Circumpolar Deep Water (mCDW) towards the Pine Island Glacier. Sensitivity experiments show that ~40 % and ~10 % of improvements in sea ice and ocean state, respectively, can be attributed to the adjustment of air temperature and wind. This study is a preliminary demonstration of adjoint-method optimization with explicit representation of ice-shelf cavity circulation. Despite the 65 % cost reduction, substantial model-data discrepancies remain, in particular with annual and interannual variability observed by moorings in front of the Pine Island Ice Shelf. We list a series of possible causes for these residuals, including limitations of the model, the optimization methodology, and observational sampling. In particular, we hypothesize that that residuals could be further reduced if the model could more accurately represent sea ice concentration and coastal polynyas.

Yoshihiro Nakayama et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on gmd-2020-421', Dan Jones, 09 Mar 2021
    • AC2: 'Reply on RC1', Yoshihiro Nakayama, 31 May 2021
    • AC4: 'Reply on RC1', Yoshihiro Nakayama, 07 Jun 2021
  • CEC1: 'Comment on gmd-2020-421', Astrid Kerkweg, 29 Mar 2021
    • AC1: 'Reply on CEC1', Yoshihiro Nakayama, 31 May 2021
  • RC2: 'Comment on gmd-2020-421', Anonymous Referee #2, 09 Apr 2021
    • AC3: 'Reply on RC2', Yoshihiro Nakayama, 31 May 2021
    • AC5: 'Reply on RC2', Yoshihiro Nakayama, 07 Jun 2021

Yoshihiro Nakayama et al.

Data sets

MITgcm model setup and output for "Development of adjoint-based ocean state estimation for the Amundsen and Bellingshausen Seas and ice shelf cavities using MITgcm/ECCO" Yoshihiro Nakayama https://doi.org/10.5281/zenodo.4541036

Yoshihiro Nakayama et al.

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Short summary
High ice-shelf melting in the Amundsen Sea attracted many observational campaigns in the past decade. One method to combine observations with numerical models is the adjoint method. After 20 iterations, the cost function, defined as a sum of weighted model-data difference, is reduced by 65 % by adjusting initial conditions, atmospheric forcing, and vertical diffusivity. This study is a demonstration of adjoint-method optimization with explicit representation of ice-shelf cavity circulation.