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Temperature and precipitation changes over South Korea have been studied since 1954, both in terms of means and extreme events, using observational station data. Recurrence intervals are defined to examine trends in extreme events at each station. Generally, it is possible to describe trends in temperature and precipitation over South Korea as follows. The annual mean temperature shows an upward trend at a rate of 0.23°C decade −1 in the past four to five decades. There are some indications that climatic extremes have increased during recent decades. The diurnal temperature range increased in the later part of the time series (except for summer) as a result of a faster increase in maximum temperature than in minimum temperature. This is not consistent with the results reported elsewhere in the world. A wavelet analysis of wintertime temperatures indicates that the rapid warming in the recent decade may be associated with the warm phase of a decadal-interdecadal variation. The frequency of occurrence of extreme maximum temperature events shows an increasing trend, with higher values in the 1980s and 1990s. The frequency of occurrence of extreme minimum temperature events shows the opposite, with a statistically significant decreasing trend.

This letter reports on a possible delayed impact of the winter North Atlantic Oscillation (NAO) on the following east Asian summer monsoon precipitation. An analysis of weather station data shows significant correlations between the December NAO index and precipitation over Korea and China in the subsequent summer. It appears that the correlation may be related to a wave train pattern which originates from the North Atlantic. The east Asian branch of this wave train can affect large‐scale circulation and the precipitation over east Asia in early summer. We also found a significant interdecadal change of this relationship, which is possibly linked to a climatological change of the east Asian jet stream.

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SUMMARYThe midlatitude to Madden-Julian (MJO) teleconnection in the northern hemisphere wintertime was investigated using twenty years of outgoing long-wave radiation and National Center for Atmospheric Research/National Centers for Environmental Prediction re-analysis data. It is revealed through empirical orthogonal functions and regression analyses that the tropical upper-level divergence (convergence) associated with enhanced (reduced) tropical convection has a subtropical counterpart of upper-level convergence (divergence). As the convective region of MJO moves eastward from the Indian Ocean to the western Pacific, the divergent circulation connecting the Tropics and subtropics moves eastward. From an analysis of vertical motion diagnosed by the generalized omega equation, it was revealed that the position of the subtropical Rossby gyres of the MJO with respect to the AsianPacific jet was the most important factor affecting the existence of the localized divergence (convergence) and associated lower-level vertical motion in the subtropics. As the subtropical Rossby gyres pass by the longitudinal position of the Asian-Pacific jet, vertical motion occurs to meet the balance required by quasi-geostrophic theory. Since this anomalous vertical motion is large enough to cause convection at that region, the diabatic heating process reinforces the vertical motion. Furthermore, other dynamical processes neglected in quasi-geostrophic theory are activated and contribute to the vertical motion balancing the diabatic heating. As a result of the vertical motion near the jet entrance region in the subtropics, the vorticity advection by vertical wind and the tilting of vorticity become important. Since these two terms are similar in shape and magnitude, they reinforce each other during the development phase of the midlatitude-MJO teleconnection. Combining these two terms, we define the baroclinic jet source (BJS). In order to evaluate the BJS as a source for the midlatitude-MJO teleconnection, a numerical experiment is carried out. The linear barotropic model is forced by only two terms, yet the model captured clearly the main feature of the midlatitude-MJO teleconnection.

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.[1] Decadal changes in surface air temperature (SAT) variability and cold surge characteristics over Northeast Asia during late winter (January-March) are analyzed for the past three decades. Power spectrum analysis of SAT reveals that the low-frequency variabilities with a period longer than 10 days are significantly enhanced, while the high-frequency variabilities with a period shorter than 10 days are weakened in the 1980s and 2000s. Moreover, cold surges were stronger and lasted longer during the 1980s and 2000s compared to those that occurred in the 1990s. Here, we propose that large-scale atmospheric conditions manifested by a different phase of the Arctic Oscillation (AO) provide preconditioning for a cold surge event, which showed a prominent decadal fluctuation. The more (less) frequent strong and long-lasting cold surge occurrences in the 1980s and 2000s (1990s) are preceded by the more dominant negative (positive) phase of the AO. Lag-composite analyses for cold surge events categorized by the AO phases indicate that stronger and longer-lasting cold air advection dominates at the lower-level, when upper-level wave train and coastal trough are developed over East Asia under the strong negative AO phase. These results suggest that the decadal changes in SAT variability and cold surge characteristics are strongly associated with the decadal changes in the phase distribution of the AO.

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