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This study investigates the contributing factors of East Asian heatwaves (EAHWs) linked to the Arctic-Siberian Plain (ASP) over the past 42 years (1979–2020). EAHWs are mainly affected by two time scales of variabilities: long-term externally forced and interannual variabilities. The externally forced EAHWs are attributed to the increasing global warming trend, while their interannual variability is related to the circumglobal teleconnection (CGT) and the ASP teleconnection patterns. In addition to the CGT, the Rossby wave energy originating from the ASP propagates to East Asia through the upper troposphere, amplifying the EAHWs. The stationary high pressure in the ASP is generated by vorticity advection in the upper troposphere. Enhanced surface radiative heating and evaporation on the ASP surface increase the specific humidity and temperature, amplifying the thermal high pressure via positive water vapor feedback. Thermal high-pressure amplified by land–atmosphere interactions in the ASP during the peak summer season leads to EAHWs by the propagation of stationary Rossby wave energy. The results indicate that our enhanced understanding of the ASP teleconnection can improve forecasting of the EAHWs not only on a sub-seasonal time scale but also in future projections of global climate models.

Here we show that Atlantic windstorms of extreme category in northern winter tend to follow a well-defined route toward the Atlantic sector of Arctic, and that heat and moisture transported by these extreme storms significantly warm the Arctic. A positive North Atlantic Oscillation (NAO) condition and the associated intensified upper-level Atlantic jet provide favorable conditions for those extreme storm developments through enhanced vertical wind shear. These extreme windstorms lead to two discernible impacts on the Arctic: 1) enhanced poleward energy transport by moisture intrusion to the Arctic, which accompanies increased longwave downward radiation and 2) the occurrence of blocking-like patterns after the storm break-up. During these periods, significant Arctic warming was observed of a 10-fold increase versus normal and weak storms. The poleward deflections of extreme storms, and the Arctic warming driven by such storms, are well simulated in numerical experiments with ocean-atmosphere coupled models.

Changma, which is a vital part of East Asian summer monsoon (EASM) system, plays a critical role in modulating water and energy cycles in Korea. Better understanding of its long-term variability and change is therefore a matter of scientific and societal importance. It has been indicated that characteristics of Changma have undergone significant interdecadal changes in association with the mid-1970s global-scale climate shift and the mid-1990s EASM shift. This paper reviews and revisits the characteristics on the long-term changes of Changma focusing on the underlying mechanisms for the changes. The four important features are manifested mainly during the last few decades: 1) mean and extreme rainfalls during Changma period from June to September have been increased with the amplification of diurnal cycle of rainfall, 2) the dry spell between the first and second rainy periods has become shorter, 3) the rainfall amount as well as the number of rainy days during August have significantly increased, probably due to the increase in typhoon landfalls, and 4) the relationship between the Changma rainfall and Western Pacific Subtropical High on interannual time scale has been enhanced. The typhoon contribution to the increase in heavy rainfall is attributable to enhanced interaction between typhoons and midlatitude baroclinic environment. It is noted that the change in the relationship between Changma and the tropical sea surface temperature (SST) over the Indian, Pacific, and Atlantic Oceans is a key factor in the long-term changes of Changma and EASM. Possible sources for the recent mid-1990s change include 1) the tropical dipole-like SST pattern between the central Pacific and Indo-Pacific region (the global warming hiatus pattern), 2) the recent intensification of tropical SST gradients among the Indian Ocean, the western Pacific, and the eastern Pacific, and 3) the tropical Atlantic SST warming.

High-impact cold extremes have continued to bring devastating socioeconomic losses in recent years. In order to explain the exposure to cold extremes more comprehensively, this study investigates multiple aspects of boreal winter cold extremes, i.e., frequency, persistence, and entropy (Markovian descriptors). Cold extremes are defined by the bottom 10th percentile of daily minimum temperatures during 1950-2014 over the northern hemisphere. The spatial and temporal distributions of Markovian descriptors during 65 years are examined. Climatological mean fields show the spatial coincidence of higher frequency, shorter persistence, and higher entropy of cold extremes, and vice versa. In regard to the temporal variations over six representative regions of North America, Europe, and Asia, all regions share a decreasing tendency of frequency with the increases in regional winter mean temperature. By contrast, persistence and entropy show their intrinsic decadal variability depending on regions irrespective of the regional temperature variability, which give different information from frequency. Therefore, the exposure to cold extremes would not simply decrease with regional warming. Rather these results indicate that the descriptors with multiple aspects of the extremes would be needed to embrace the topical features as well as the holistic nature of cold extremes.

Tropical cyclones (TCs) over the western North Pacific (WNP) are simulated for the 29 TC seasons of July–October from 1982 to 2010 using the regional Weather Research and Forecasting (WRF) model nested within global WRF model simulations. Averaged over the entire 29-year period, the nested global–regional WRF has reasonably simulated the climatology of key TC features such as the location/frequency of genesis and tracks. The dynamical and thermal structures of the simulated TCs are weaker than observations owing to the coarse spatial resolution of the regional WRF (50 km × 50 km). TC frequencies are somewhat underestimated over the East China Sea but are substantially overestimated over the South China Sea and the Philippine Sea with neighboring oceans between 10°N and 15°N. Categorization of the simulated TCs into six clusters based on the observed TC clusters and the associated large-scale circulation show that the nested simulation depicts the observed TC characteristics well except for two clusters associated with TCs traveling from the Philippine Sea to the East China Sea. Errors in the simulated TC genesis and tracks are mostly related to these two clusters. In the simulation, the monsoon confluent zone over the Philippine Sea is too strong, and the mid-latitude jet stream expands farther south than that in the observations. Overall results from this study suggest that the nested global–regional WRF can be useful for studying the TC climatology over the WNP.

This study examines antecedent mid-tropospheric frontogenesis (AMF) resulting from the interaction between Typhoon Rusa (2002) and a midlatitude trough over the Korean Peninsula. In this event, the AMF contributed to the first peak in the time series of rainfall in Gangneung (37.75°N, 128.90°E), occurring about 12 h before the time of the extratropical transition (ET) process of the tropical cyclone (TC). Using observations and high-resolution model outputs, we showed that the AMF contributed to the antecedent rainfall in Gangneung during the first rainfall period when Gangneung was located outside of Rusa's sphere of direct influence. A Weather Research Forecasting (WRF) model experiment was conducted to diagnose the frontogenetical features and associated precipitation processes in detail. The experiment revealed that the AMF was mainly forced by the horizontal deformation forcing (HDF). The direction of the HDF was oriented from southwest to northeast in the middle part of the peninsula. The HDF increased positively due to the confluence of the southeasterlies from the TC and the northwesterlies emanating from the midlatitude trough. The experiment also suggested that the mid-tropospheric moisture originated from the subtropical ocean and deposited into the frontal region by the southerlies on the eastern periphery of the TC, which enhanced the convergence of moisture flux in the frontal region during the first rainfall period. The thermally direct circulation associated with the AMF lead to the mid-tropospheric saturation, which enhanced the precipitation of the first rainfall event together with the orographically forced convection at the low level above Gangneung.

Recently, the National Typhoon Center (NTC) at the Korea Meteorological Administration launched a track-pattern-based model that predicts the horizontal distribution of tropical cyclone (TC) track density from June to October. This model is the first approach to target seasonal TC track clusters covering the entire western North Pacific (WNP) basin, and may represent a milestone for seasonal TC forecasting, using a simple statistical method that can be applied at weather operation centers. In this note, we describe the procedure of the track-pattern-based model with brief technical background to provide practical information on the use and operation of the model. The model comprises three major steps. First, long-term data of WNP TC tracks reveal seven climatological track clusters. Second, the TC counts for each cluster are predicted using a hybrid statistical-dynamical method, using the seasonal prediction of large-scale environments. Third, the final forecast map of track density is constructed by merging the spatial probabilities of the seven clusters and applying necessary bias corrections. Although the model is developed to issue the seasonal forecast in mid-May, it can be applied to alternative dates and target seasons following the procedure described in this note. Work continues on establishing an automatic system for this model at the NTC.

This study identifies favorable synoptic backgrounds for indirect precipitation events over the Korean Peninsula that occur well in advance of tropical cyclone (TC) landfall. Two TCs, i.e., Rammasun (2002) and Maemi (2003) that made landfall and produced heavy rainfall over the Peninsula are compared. Although both had a remarkably similar accumulated rainfall pattern over the peninsula, the temporal evolutions of hourly rainfall were different. Only Maemi had an indirect precipitation event in conjunction with a midlatitude trough to its north. The confluent flows at middle-to-upper levels were strengthened due to the increased pressure gradient between the midlatitude trough and the subtropical high, and the warm advection by the confluent flows also became stronger near the confluent zone. By contrast, Rammasun encountered the subtropical ridge while moving northward, which results in slow recurvature and reduction of the thermal gradient over the peninsula. The highly baroclinic synoptic backgrounds in the Maemi case lead to the midlevel frontogenesis. Budget analyses using the three-dimensional frontogenesis equation revealed that the horizontal deformation forcing had a primary role in generating the front. The front was associated with a thermally direct circulation that contributed to strong ascent and indirect precipitation over the peninsula well in advance of the landfall of Maemi. Moreover, the indirect precipitation could intensify due to the abundant low-level moisture supply to the frontal zone by the southerly wind on the east side of the TC.

The global and regional projected changes in tropical cyclone (TC) genesis due to increased CO2 concentrations has been investigated through a large-scale TC genesis parameter (convective seasonal genesis parameter, ConvGP) in two perturbed physics ensembles. The ensembles are based on the third generation Hadley Centre atmosphere–ocean general circulation model with the first ensemble using a coupled fully dynamic ocean (HadCM3) and the second coupled to a simplified mixed layer thermodynamic ocean (HadSM3) both consisting of 17 members. In each ensemble, parameters are identically perturbed to provide a wide range of climate sensitivity whilst retaining a credible present-day climate simulation. It is found, by comparing the ConvGP climatology from reanalysis data with the best track genesis, that it is possible to reproduce the observed genesis distribution. Future changes in the spatial ConvGP distribution are explored with respect to each tropical ocean basin. Whilst there is a similarity in the gross pattern of the ensemble-mean projected ConvGP change between HadCM3 and HadSM3, there is a non-trivial difference in the tropical Pacific Ocean, arising from different patterns of tropical Pacific sea surface temperature change. This indicates that ocean representation can be important for regional scale projections. The quantitative contribution of individual constituent parameters (i.e. vorticity parameter, shear parameter and convective potential) to the projected ConvGP change is estimated. It is found that all three large-scale parameters generally contribute constructively, but with different magnitude, in the regions where a large doubled CO2 response is found.

This study examined the impact of tropical cyclone (TC) landfalls on the spatiotemporal variations in the rainfall over South China for the period 1957–2005. The target region was selected to show the noteworthy contribution of TC landfalls to the total rainfall during the typhoon season (July–October). Two prevailing spatial variations in the rainfall were obtained from an EOF analysis. The first EOF mode displays single-sign variability over South China with an explained variance of 23.4%. The associated time series of this mode fluctuates on a decadal timescale and was found to be correlated with TC genesis in the South China Sea. The second EOF mode shows a seesaw pattern between Hainan Island/Guangdong Province and the remaining regions with an explained variance of 11.4%. This seesaw pattern results from an anti-correlation in seasonal TC landfalls between the two regions, which was found in previous studies. This is related to the strengthening (weakening) of the upper tropospheric jets and the corresponding development of a massive anticyclonic (cyclonic) circulation over East Asia. The EOF analysis was also conducted using just the data for rainfall caused by landfalling TCs. This revealed that the first EOF mode using just the TC-induced rainfall is nearly identical to the second mode from the total rainfall. The obvious seesaw pattern of the first mode when employing just the TC-induced rainfall in the EOF analysis implies that this pattern has larger temporal variability than the single-signed pattern (i.e., the first EOF mode using the total rainfall) in terms of TC landfalls. This study suggests that TC landfalls over South China and the accompanying rainfall significantly modulate the spatial variation of the typhoon season rainfall there.

In this study, we examine the relationship between the number of tropical cyclones (TCs) in the western North Pacific and the tropical Pacific sea surface temperature (SST) during the main TC season (July–November) for the period of 1965–2006. Results show that there are periods when TC frequency and the tropical Pacific SST are well correlated and periods when the relationship breaks down. Therefore, decadal variation is readily apparent in the relationship between the TC frequency and the SST variations in the tropical Pacific. We further examine the oceanic and atmospheric states in the two periods (i.e., 1979–1989 vs. 1990–2000) when the marked contrast in the correlation between the TC frequency and the tropical Pacific SST is observed. Before 1990, the analysis indicates that oceanic conditions largely influenced anomalous TC frequency, whereas atmospheric conditions had little impact. After 1990, there the reverse appears to be the case, i.e., atmospheric conditions drive anomalous TC frequency and oceanic conditions are relatively unimportant. A role of atmosphere and ocean in relation to the TC development in the western North Pacific changes, which is consistent with the change of the correlations between the TC frequency and the tropical Pacific SST.

This study investigated the tropical cyclone (TC) rainfall (P(subscript TC)) contribution to the interdecadal change in summer (June, July and August) rainfall (P(subscript Total)) over South China between 1981 - 1992 (ID1) and 1993 - 2002 (ID2). In an area-averaged sense, the interdecadal change in P(subscript Total)was largely attributed to non-TC rainfall for the summer total and months of June and July, while P(subscript TC) became comparable in August. When the month-to-month spatial variability was considered, noticeable negative P(subscript TC) contributions showed up over the southeastern coast, Hainan Island, and Taiwan in June and over the southern coastal regions in July. In contrast, a positive P(subscript TC) contribution spread over South China with its maxima over the southern coastal regions in August, a pattern which appeared to be diametrically opposed to that of the negative P(subscript TC) contribution in July, though the latter was less significant. The negative P(subscript TC) contribution over the coastal and insular regions in June and July corresponded to less TC activity there. In June, it was attributed to reduced basin-wide TC activity due to prevailing unfavorable large-scale environments in ID2, whereas, in July, to less TC approaches from the Philippine Sea due to an enhanced cyclonic circulation centered on Taiwan in ID2. Conversely, in August, the overall enhanced positive P(subscript TC)contribution was mainly by the direct influences of increased TC formations over the South China Sea in ID2.