Recent changes of tropical night in the South Korea metropolitan area

To investigate the recent change of tropical night in South Korea, the duration and intensity of tropical nights (daily minimum temperature , Korea Meteorological Administration; KMA) were quantitatively analyzed for 40 years (1979-2018). As a result of spatiotemporal analysis, There were stronger and longer-lasting tropical nights in the Seoul metropolitan area compared to other regions of South Korea. In particular, the tropical nights over the region tended to increase more prominently in terms of intensity, frequency, and duration.

The tropical night event in the metropolitan area was classified into pure-TN (no heatwave prior to tropical night) and HWTN (tropical night following heatwave). In the analysis of climatological synoptic conditions based on 40-year ERA5 data, two types of tropical night events (pure-TN and HWTN) occurred. Pure-TN occurred when a mid-level anticyclone circulation existed over the Korean Peninsula with the southwesterly which induced a positive temperature advection anomaly over the metropolitan area. Moreover, a positive low cloud cover anomaly with enhanced downward longwave radiation also prevailed. On the other hand, HWTN mainly occurred when a mid-level cyclone circulation was located over the Korean Peninsula with a positive downward shortwave radiation anomaly and a descending motion anomaly that induced adiabatic heating over the metropolitan area.

Significant increasing trends in tropical night events (pure-TN: 0.143 day/year, HWTN: 0.077 day/year) appeared at 95% confidence level. The regression analysis was also conducted for three sub-analysis periods with 10 days (21-31 July;P1, 1-10 August;P2, 11-20 August;P3). Consequently, the favorable atmospheric conditions for HWTN (pure-TN) have been frequently constructed during P2 (P1, and P3).