Role of Madden-Julian Oscillation in predicting the 2020 East Asian summer precipitation in subseasonal-to-seasonal models.

The 2020 summer monsoon season in East Asia was unusually long and intense, and the Madden-Julian Oscillation (MJO) has been proposed as an underlying reason. This study analyzes the role of the MJO in the 2020 East Asian precipitation forecasts of the subseasonal-to-seasonal (S2S) model. The S2S models underestimated the cumulative precipitation over East Asia, and the models with good forecast performance yielded a distinct precipitation band over East Asia and a western pacific subtropical high (WPSH) during the analysis period. East Asian precipitation forecast performance was more closely related to the location of the center than the strength of the WPSH, with precipitation increasing with a decrease in the latitude at the center. MJO Phases 1-3 activation intensified the WPSH and shifted the center of WPSH to lower latitudes. Our results confirm that the strong East Asian precipitation in summer 2020 was partly due to changes in the characteristics of the MJO and indicate the importance of accurately estimating the MJO-WPSH coupling for reliable East Asian precipitation forecasts.

Seasonal changes in surface ozone over South Korea.

Recently, the surface ozone concentration in the Korean peninsula has been increasing more rapidly than in the past, and seasonal changes are appearing such as increases in the number of ozone alerts in springtime. We examined changes in the timing of annual maximum South Korean O3 levels by fitting a sine function to data from 54 air-quality monitoring sites over a 10-year period (2005-2014). The analytical results show that the date of maximum ozone concentration at 23 points in the last 10 years has been advanced by about 2.1 days per year (E-sites), while the remaining 31 points have been delayed by about 2.5 days per year (L-sites). We attribute these differences to seasonal O3 changes: E-sites show a larger increase in O3 level in March-April (MA) than in June-July (JJ), while L-sites show a larger increase in JJ than in MA. Furthermore, these shifts are significantly larger in magnitude than those reported for Europe and North America. We also examined one possible reason for these seasonal differences: the relationship between O3 and precursors such as NO2 and CO. E-sites showed a rapid decrease in NO2 (NO) concentration in MA over the last decade. As a result, the ozone concentration at E-sites seems to have increased due to the absence of ozone destruction by NOx titration in early spring. In L-Sites, the concentrations of ozone precursors such as NO2 and CO in JJ showed a smaller decrease than those at other sites. Therefore, in L-sites, relatively large amounts of ozone precursors were distributed in JJ, implying that more ozone was generated. We suggest that shifts in the South Korean O3 seasonal cycle are due to changes in early spring and summer NO2 (NO) and CO levels; this should be tested further by modeling studies.