Abstract
We investigate the quality of simulating tropical Atlantic (TA) sector climatology and interannual variability in integrations of the Kiel climate model (KCM) with varying atmosphere model resolution. The ocean model resolution is kept fixed. A reasonable simulation of TA sector annual-mean climate, seasonal cycle and interannual variability can only be achieved at sufficiently high horizontal and vertical atmospheric resolution. Two major reasons for the improvements are identified. First, the western equatorial Atlantic westerly surface wind bias in spring can be largely eliminated, which is explained by a better representation of meridional and especially vertical zonal momentum transport. The enhanced atmospheric circulation along the equator in turn greatly improves the thermal structure of the upper equatorial Atlantic with much reduced warm sea surface temperature (SST) biases. Second, the coastline in the southeastern TA and steep orography are better resolved at high resolution, which improves wind structure and in turn reduces warm SST biases in the Benguela upwelling region. The strongly diminished wind and SST biases at high atmosphere model resolution allow for a more realistic latitudinal position of the intertropical convergence zone. Resulting stronger cross-equatorial winds, in conjunction with a shallower thermocline, enable a rapid cold tongue development in the eastern TA in boreal spring. This enables simulation of realistic interannual SST variability and its seasonal phase locking in the KCM, which primarily is the result of a stronger thermocline feedback. Our findings suggest that enhanced atmospheric resolution, both vertical and horizontal, could be a key to achieving more realistic simulation of TA climatology and interannual variability in climate models.
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Acknowledgements
We thank two anonymous reviewers for their thoughtful comments. This work was supported by the Bundesministerium für Bildung und Forschung grant SACUS (03G0837A) and EU FP7/2007–2013 under Grant agreement no. 603521, project PREFACE. Model integrations were performed at the Norddeutscher Verbund für Hoch- und Höchstleistungsrechnen and the Rechenzentrum der Universität Kiel.
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382_2017_3760_MOESM1_ESM.pdf
Fig. S1 Regression of SST on ATL3 SST anomalies (°C/°C). Stippling denotes 95% significance level. a) HadISST (1982-2009), b) NOAA-OISST (1982-2009), c) T42 L31 (L), d) T159 L31 (M), e) T159 L62 (M-V), f) T255 L62 (H-V) (PDF 398 KB)
382_2017_3760_MOESM2_ESM.pdf
Fig. S2 Regression of 10m winds (vectors, m/s per °C) and total precipitation (shading, mm/day per °C) on ATL3 SST for uncoupled simulations. Stippling denotes 95% significance level, only significant vectors depicted. a) T42 L31 L(A), b) T159 L31 M(A), c) T159 L62 M-V(A) (PDF 266 KB)
382_2017_3760_MOESM3_ESM.pdf
Fig. S3 Lines denote green: T42 L31 (L/L(A)), red: T159 L31 (M/M(A)), blue: T159 L62 (M-V/M-V(A)), purple: T255 L62 (H-V), black crossed: observations. a) zonal wind at 850hPa (m/s) in WTA (40°W-10°W, 3°S-3°N) for coupled models and b) for uncoupled models, obs: ERA-interim, c) standard deviation (STD) of 23°C isotherm depth in ATL3 (20°W-0°W, 3°S-3°N) for coupled models, obs: SODA (PDF 72 KB)
382_2017_3760_MOESM4_ESM.pdf
Fig. S4 Regression of net surface short wave radiation (SW, a-c) and latent heat flux (LH, d-f) anomalies on ATL3 SST anomalies (W/m2 per °C) for uncoupled simulations a/d) T42 L31 L(A), b/e) T159 L31 M(A), c/f) T159 L62 M-V(A).. Stippling denotes 95% significance level (PDF 461 KB)
382_2017_3760_MOESM5_ESM.pdf
Fig. S5 Regression of upper ocean temperature (averaged over 3°S-3°N) on ATL3 SST (°C/°C). Stippling denotes 95% significance level. a) T42 L31 (L), b) T159 L31 (M), c) T159 L62 (M-V), d) T255 L62 (H-V), e) SODA (1958-2001) (PDF 635 KB)
382_2017_3760_MOESM6_ESM.pdf
Fig. S6 Seasonally stratified Bjerknes index. Abbreviations are as in Fig. 18. Lines denote: black crossed: SODA (1958-2001), green: T42 L31 (L), red: T159 L31 (M), blue: T159 L62 (M-V), purple: T255 L62 (H-V) (PDF 59 KB)
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Harlaß, J., Latif, M. & Park, W. Alleviating tropical Atlantic sector biases in the Kiel climate model by enhancing horizontal and vertical atmosphere model resolution: climatology and interannual variability. Clim Dyn 50, 2605–2635 (2018). https://doi.org/10.1007/s00382-017-3760-4
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DOI: https://doi.org/10.1007/s00382-017-3760-4