[A-OS19] Physical Oceanography (General)

[AOS19-04] Decadal variation of the asymmetry of SSTA in the tropical Atlantic and its relations with midlatitude atmosphere circulation

*Yoshinobu Furukawa¹, Shusaku Sugimoto¹ (1.Graduate School of Science, Tohoku University)

Keywords:tropical Atlantic, atmospheric-ocean interaction, long period variability

A north-south asymmetric pattern of SSTA, whose boundary lies along the equator in the Atlantic exists, and this has a decadal (about 10 years) timescale as pointed out by past work (Tanimoto and Xie, 2002). Past works denoted that this asymmetric pattern develops due to the Wind-Evaporation-SST (WES) feedback mechanism. On the other hand, it has been implied that the asymmetric pattern could be influenced by the mid-latitude large-scale atmospheric pattern such as NAO. In this study, we examine the dominant timescale of the asymmetric pattern by using the long-term SST dataset, and then investigate a relationship between the asymmetric pattern and the large-scale atmospheric pattern such as NAO and EA using atmospheric reanalysis data. Two data were used for the indicator of north-south asymmetric pattern of the Atlantic Equator region: ERA5 SST (January 1978 to August 2019) and HadISST (January 1870 to September 2019). The indicator was calculated as follows; (1) we calculate SSTA regional-mean values over the northern equator (5 ° -15 ° N, 20 ° -60 ° W) and the southern equator (5 ° -15 ° S, 0 ° -40 ° W), (2) we obtain the 37-month running mean values of the difference between northern equation and southern equation, and then (3) we regard the smoothed time series as the asymmetry index. In addition, the meridional wind speed and SLP of ERA5 were used. We calculate monthly climatology for 30 years from 1978 to 2007. The NAO and EA index produced by NOAA. The asymmetric index by ERA5 SST showed a marked decadal-scale variations as a result of a wavelet analysis,. Identical timescale features were obtained from HadISST; the decadal timescale can be traced back to 1870. To investigated whether the WES feedback worked on this asymmetric pattern, we performed correlation analyses of the asymmetric index versus the north-south wind speed and SLP of ERA5. Results indicated a strong positive correlation with the north-south wind over the equator, and negative SLP correlation coefficients in the North Atlantic Ocean and positive SLP coefficients in the South Atlantic Ocean. It can be concluded that the WES feedback works for this asymmetric pattern. We focused on the winter (December-February) and investigated the relationship between the asymmetric index and the NAO index / EA index. Resultantly, significant correlation coefficients were not obtained, and it is pointed out that the asymmetric pattern is almost independent of NAO / EA. In the presentation, we will also discuss results of CMIP6 under future climate.