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Discussion papers | Copyright
https://doi.org/10.5194/gmd-2018-228
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Model description paper 01 Oct 2018

Model description paper | 01 Oct 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Geoscientific Model Development (GMD).

SEAS5: The new ECMWF seasonal forecast system

Stephanie J. Johnson, Timothy N. Stockdale, Laura Ferranti, Magdalena Alonso Balmaseda, Franco Molteni, Linus Magnusson, Steffen Tietsche, Damien Decremer, Antje Weisheimer, Gianpaolo Balsamo, Sarah Keeley, Kristian Mogensen, Hao Zuo, and Beatriz Monge-Sanz Stephanie J. Johnson et al.
  • ECMWF, Shinfield Park, Reading RG2 9AX, UK

Abstract. In this paper we describe SEAS5, ECMWF’s fifth generation seasonal forecast system, which became operational in November 2017. Compared to its predecessor, System 4, SEAS5 is a substantially changed forecast system. It includes upgraded versions of the atmosphere and ocean models at higher resolutions, and adds a prognostic sea ice model. Here, we describe the configuration of SEAS5 and summarise the most noticeable results from a set of diagnostics including biases, variability, teleconnections and forecast skill.

An important improvement in SEAS5 is the reduction of the Equatorial Pacific cold tongue bias, which is accompanied by a more realistic ENSO amplitude and an improvement in ENSO prediction skill over the central-west Pacific. Improvements in two-metre temperature skill are also clear over the tropical Pacific. SST biases in the northern extratropics change due to increased ocean resolution, especially in regions associated with western boundary currents. The increased ocean resolution exposes a new problem in the northwest Atlantic, where SEAS5 fails to capture decadal variability of the North Atlantic subpolar gyre, resulting in a degradation of DJF two-metre temperature prediction skill in this region. The prognostic sea ice model improves seasonal predictions of sea ice cover, although some regions and seasons suffer from biases introduced by employing a fully dynamical model rather than the simple, empirical scheme used in System 4. There are also improvements in two-metre temperature skill in the vicinity of the Arctic sea-ice edge. Cold temperature biases in the troposphere improve, but increase at the tropopause. Biases in the extratropical jets are larger than in System 4: extratropical jets are too strong, and displaced northwards in summer. In summary, development and added complexity since System 4 has ensured SEAS5 is a state-of-the-art seasonal forecast system which continues to display a particular strength in ENSO prediction.

Stephanie J. Johnson et al.
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Status: open (until 26 Nov 2018)
Status: open (until 26 Nov 2018)
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Stephanie J. Johnson et al.
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Latest update: 18 Oct 2018
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Short summary
In this article, we describe the new ECMWF seasonal forecast system, SEAS5, which replaced its predecessor in November 2017. We describe the forecast methodology used in SEAS5 and compare results from SEAS5 to results from the previous seasonal forecast system, highlighting the strengths and weaknesses of SEAS5. SEAS5 data is publicly available through the Copernicus Climate Change Service's multi-system seasonal forecast.
In this article, we describe the new ECMWF seasonal forecast system, SEAS5, which replaced its...
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