[A-CG30] Extratropical oceans and atmosphere

[ACG30-P17] Effect of SST-front on the sub-seasonal prediction in North Atlantic winter circulation using the JMA operational seasonal prediction system

*Hiroaki Naoe¹, Yukimasa Adachi¹, Yutaro Kubo² (1.Meteorological Research Institute / Japan Meteorological Agency, 2.Office of Earth System Modeling / Numerical Prediction Division / Japan Meteorological Agency)

Keywords:mid-latitude SST front, seasonal forecast

Warm currents such as the Gulf Stream associated with mid-latitude oceanic fronts have influences not only on the lower atmosphere but also the upper troposphere, further downstream to wider areas in the troposphere. Previous studies using atmospheric models driven by SST with different resolutions have shown that the mid-latitude ocean front has a considerable impact on the local climate and that the representation of the atmospheric structure is well reproduced for simulations using finer SST resolutions. Although the mid-latitude oceanic front would play a role in improving the accuracy of seasonal forecasts, the impact on forecast accuracy is still poorly understood. Thus, in this study, we investigate the influence of the mid-latitude oceanic front on forecasting using the latest seasonal forecasting system of the Japan Meteorological Agency. Here, we focus on intra-seasonal predictions (mainly 3-4th week predictions) in the North Atlantic region in winter, by comparing hindcast experiments with different ocean model resolutions in 30 years of 1991-2020. It is found that the low-resolution model shows SST cooling south of the mid-latitude oceanic front and the weakening of local lower tropospheric circulation in response to the SST, consistent with previous studies. The accuracy of the predicted SST is high for the low-resolution model at the 100-km order scale, whereas the high-resolution model is more accurate for the region-averaged SST at the 1000-km scale. This accuracy deterioration for the finer scales is probably caused by the phase shift of oceanic eddies. There are high correlations of variabilities between the mid-latitude oceanic frontal region-wide averaged SST (40-75W, 35-45N) and atmospheric variables near the frontal zone. We also confirm that their ACCs are improved by about 0.1-0.2.