Combined Effect of ENSO and AO on Winter Temperatures of the Korean Peninsula on Subseasonal Time Scales

This study investigates the combined impact of the El Niño–Southern Oscillation (ENSO) and Arctic Oscillation (AO) on the variability of winter temperatures in South Korea at the subseasonal time scale. This is achieved by analyzing hindcast data from the European Centre for Medium-Range Weather Forecasts (ECMWF) database. Six composite cases are selected based on thresholds for each index: El Niño with positive AO (EP), El Niño with negative AO (EN), La Niña with positive AO (LP), La Niña with negative AO (LN), positive AO only (PA), and negative AO only (NA). Results from reanalysis data indicate that these climate factors can serve as predictors for predicting South Korea's temperature up to 4 weeks in advance. The study confirms that ENSO significantly influences the strength of AO's impact on temperature anomalies, depending on whether they are in phase or out of phase. For instance, during El Niño periods, there's a notable increase in mean temperature anomalies due to positive geopotential height (GPH) anomalies and warm temperature advection over South Korea in the EP case. Conversely, during La Niña periods, there's a significant decrease in mean temperature anomalies due to negative GPH anomalies and cold temperature advection over South Korea in the LN case. The ECMWF S2S hindcast exhibits reasonable ability to replicate circulation patterns over East Asia up to 3 weeks in advance. It also adequately predicts weekly mean temperature anomalies over South Korea in the EP, LN, and PA cases. This suggests that the combination of ENSO and AO indices can contribute to improved subseasonal forecasting of winter temperatures in South Korea.