A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF models

Shonk, Jonathan K. P.; Demissie, Teferi D.; Toniazzo, Thomas

doi:https://doi.org/10.5194/acp-19-11383-2019

Articles | Volume 19, issue 17

https://doi.org/10.5194/acp-19-11383-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-19-11383-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 19, issue 17

Research article

|

10 Sep 2019

Research article |

| 10 Sep 2019

A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF models

Jonathan K. P. Shonk, Teferi D. Demissie, and Thomas Toniazzo

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Final revised paper (published on 10 Sep 2019)
Preprint (discussion started on 03 Jan 2019)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'review of acp-2018-1316', Anonymous Referee #1, 10 Feb 2019
RC2: 'Reviewer's comments', Anonymous Referee #2, 27 Feb 2019
AC1: 'Responses to Both Reviewers' Comments', Jon Shonk, 17 May 2019

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Jon Shonk on behalf of the Authors (17 May 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (06 Jun 2019) by Timothy J. Dunkerton

RR by Anonymous Referee #1 (14 Jun 2019)

Suggestions for revision or reasons for rejection

The authors have made substantial efforts in addressing the concerns raised by my earlier comments. To this reviewer, it is clear that the study does much more than “benefit(ing) the community by increasing the number of citations” (authors’ reply to previous Comment #8). Once the authors resolve several issues that arise from the previous revision (see below), I would be happy to recommend the publication of this study.

1. Line 16-17 on Page 1: The tone of “alleged causes ... must be discarded” might need to be softer or balanced with a remark on caveats. As the authors mentioned in the last section, the findings of the study are based on specific ECMWF systems but may not necessarily apply to other models.

2. Line 1-5 on Page 19: It is confusing why the authors considered “circulation and rainfall associated with … warm SST biases (are) small”. Notably, the SST biases near 10N (Figs. 3 and 4c) can reach 0.5~1 K, and the warm biases appear to slightly lead or coincide with the manifestation of precipitation biases (e.g., Fig. 4c’s the double ITCZ feature at ~75 days after February 1st). Another issue that confuses me is how the spatial relation between “the strongest part of the northern ITCZ” (climatology) and the “warm SST biases” (bias) justify the assertion that the SST biases “only provides a small contribution to the overall bias development”. In addition, the impacts of SST anomalies/biases are often non-local. An example is the precipitation anomalies associated with the Atlantic meridional mode (Fig. 1 in Chiang and Vimont 2004).
- Chiang, J.C. and D.J. Vimont, 2004: Analogous Pacific and Atlantic Meridional Modes of Tropical Atmosphere–Ocean Variability. J. Climate, 17, 4143–4158, https://doi.org/10.1175/JCLI4953.1

3. Line 8-9 on Page 20: The decision of using the geopotential thickness between 700 and 1000 hPa as a proxy of boundary layer depth might need to be better justified. The 700-hPa level is likely well above the top of marine boundary layer, which is generally less than 2 km above the sea level (von Engeln and Teixeira 2013). Even with limited data, it might still be helpful to conduct some sensitivity tests (e.g., replacing 700 hPa with 850 hPa in calculations).
- von Engeln, A. and J. Teixeira, 2013: A Planetary Boundary Layer Height Climatology Derived from ECMWF Reanalysis Data. J. Climate, 26, 6575–6590, https://doi.org/10.1175/JCLI-D-12-00385.1

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RR by Anonymous Referee #2 (25 Jun 2019)

ED: Publish as is (10 Aug 2019) by Timothy J. Dunkerton

AR by Jon Shonk on behalf of the Authors (12 Aug 2019)

Short summary

Modern climate models are affected by systematic biases that harm their ability to produce reliable seasonal forecasts and climate projections. In this study, we investigate causes of biases in wind patterns over the tropical Atlantic during northern spring in three related models. We find that the wind biases are associated with an increase in excess rainfall and convergence in the tropical western Atlantic at the start of April, leading to the redirection of trade winds away from the Equator.