Tropospheric and Stratospheric Boreal Winter Jet Response to Eddying Ocean in a Seasonal Forecast System

Understanding the impacts of high-resolution ocean model provides valuable insights for future research. However, the outcomes of sea surface state changes in both the tropics and mid-latitudes remain unclear, and initialized seasonal forecasts have not been studied extensively. This study investigates the impact of ocean model resolution with the first long-term hindcast experiment of an eddy-resolving (0.1 degrees) ocean model used for global seasonal forecasting. We show that using the high-resolution ocean model significantly changes boreal winter jet streams in the atmosphere, based on the comparison of 30-year hindcasts with ocean resolutions ranging from 1 degrees to 0.1 degrees for the Japan Meteorological Agency/Meteorological Research Institute Coupled Prediction System version 3. In boreal winters, the cold sea surface bias in the equatorial Pacific is significantly reduced, leading to an equatorward shift in the intertropical convergence zone (ITCZ) and enhanced convective activity in the western equatorial Pacific. The subtropical jet shifts equatorward due to the ITCZ shift and the weakening of equatorward propagation of mid-latitude atmospheric eddies. The enhanced convective activity in the tropics has a remote influence in the mid-latitudes, significantly reducing the upward eddy propagation of zonal wavenumber 1. Sea surface warm-up in the mid-latitudes partially cancels the reduction impact by enhancing the zonal wavenumber 2. Overall, the polar night jet accelerates due to the reduced supply of eddy forcing. Atmospheric and oceanic eddies play an important role in shaping the climate. In numerical climate simulations, the Earth is divided into a grid of discrete boxes, and the exchange of energy and mass between the boxes is computed step-by-step. Coarse grids are often used to reduce the computational cost, but recent studies have suggested that resolving fine ocean phenomena can make a significant difference. In this study, numerical seasonal forecasting experiments with ocean resolutions of 100 km, 25 km, and 10 km are compared to investigate the high-resolution effects on the atmospheric flow for boreal winter. This is the first 30-year-long, retrospective seasonal forecast experiment for the past winters using a global eddy-resolving (10 km) ocean model. The high-resolution ocean model greatly warms the surface seawater in the tropical Pacific and in the mid-latitude oceans. These changes in the sea surface influence atmospheric convection and eddies, with far-reaching effects on distant regions and altitudes. Such effects on the westerly jet streams, which flow several kilometers to tens of kilometers above the sea surface, are observed in the southward shift of the subtropical jet and the strengthening of the polar night jet. Altering the ocean model resolution from 1 degrees to 0.1 degrees in a seasonal forecast model shifts and enhances the subtropical and polar night jets Atmospheric responses to both tropical and mid-latitude oceans are observed Wave-mean flow diagnostics indicates that the jet stream responses are more strongly influenced by the tropics