The Synergistic Effect of The Preceding Winter Northern Hemisphere Annular Mode and Spring Tropical North Atlantic SST On Spring Extreme Cold Events in The Mid-high Latitudes of East Asia

In this paper, the synergistic effect of the preceding winter positive Northern Hemisphere annular mode (pNAM) and spring negative tropical North Atlantic (nTNA) sea surface temperature anomaly (SSTA) on spring extreme cold events in the mid-high latitudes of East Asia (MHEA) is investigated. The results show that the co-occurrence of the two factors is unfavorable for extreme cold events during spring in the MHEA via the snow cover and atmospheric bridges. Over the Atlantic, the spring nTNA SSTA can lead to an atmospheric response that is similar to the North Atlantic Oscillation (NAO), which enhances the persistence of the pNAM and in turn amplifies the negative spring Eurasian snow cover extent (EASCE) anomaly caused by the preceding winter pNAM. Meanwhile, the spring EASCE is closely related to the spring MHEA anomalous anticyclone. In addition to storing its signal in the spring EASCE, the spring nTNA SSTA can also lead to the spring MHEA anomalous anticyclone via the eastward Rossby wave train. The evidence shows that the Rossby wave energy can propagate eastward to the MHEA as a result of the enhanced negative spring EASCE anomaly and Rossby wave induced by the spring nTNA SSTA, and the two factors have an obvious synergistic effect on the spring MHEA anomalous anticyclone. This anomalous MHEA anticyclone becomes a barrier that can hinder the intrusion of cold air from the polar region and can increase the thickness of the atmospheric layer. The anomalous sinking motion of the spring MHEA anomalous anticyclone can also lead to an increase in net radiation received at the surface and increase the air temperature through the vertical motion of air. The southerly wind over the west side of the spring MHEA anomalous anticyclone leads to horizontal warm advection. All of the above processes favor an increase in air temperature and dampen extreme cold events, implying the synergistic effect of the preceding winter pNAM and spring nTNA SSTA on spring extreme cold events in the MHEA.

Response of western North Pacific anomalous anticyclone in the summer of decaying El Niño to global warming: Diverse projections based on CMIP6 and CMIP5 models

The anomalous anticyclone over the western North Pacific (WNPAC) is a key atmospheric bridge through which El Niño-Southern Oscillation (ENSO) affects East Asian climate. In this study, the response of the anomalous WNPAC to global warming under the high-emission scenario is investigated based on 40 models from CMIP6 and 30 models from CMIP5. Despite low inter-model consensus, the multi-model median (MMM) of CMIP6 models projects an enhanced anomalous WNPAC but the MMM of CMIP5 models projects a weakened anomalous WNPAC, both of which reach about 0.5 standard deviation of the decadal internal variability derived from the pre-industrial control experiment. As consistently projected by CMIP6 and CMIP5 models, a same magnitude of sea surface temperature anomaly (SSTA) over the tropical Indian Ocean (TIO) stimulates a weaker anomalous WNPAC under a warmer climate, and this mechanism is responsible for the weakened anomalous WNPAC based on the CMIP5-MMM. However, the above mechanism is overwhelmed by another mechanism related to the changes in tropical SSTA based on the CMIP6-MMM. As a result of the enhanced warm SSTA over the TIO and the eastward shift of the warm SSTA over the equatorial Pacific during the decaying El Niño, the warm Kelvin wave emanating from the TIO is enhanced along with the stronger zonal SSTA gradient based on the CMIP6-MMM, enhancing the anomalous WNPAC. The diverse changes in the zonal SSTA gradient between the TIO and the equatorial western Pacific also explain the inter-model diversity of the changes in anomalous WNPAC.

Effects of Whole SST Anomaly in the Tropical Indian Ocean on Summer rainfall Over Central Asia

The effects of sea surface temperature (SST) anomaly in the tropical Indian Ocean (IO) on summer rainfall over central Asia (CA) are investigated using NCEP/NCAR reanalysis circulation data, Hadley Centre SST data, and GPCC gridded precipitation data for 1971–2016. Results show that the SST anomalies over the whole tropical IO play important roles in modulating summer rainfall over southeast CA via the subtropical westerly jet. When the SSTs in the tropical IO are in positive phases, the south Asian monsoon is weakened, which reduces summer rainfall in the Indian monsoon regions corresponding to less release of latent heat. There is an anomalous anticyclone over the Indian Peninsula and an anomalous cyclone in the upper troposphere over CA, corresponding to a shift of the subtropical westerly jet farther south over CA. The southward shift of westerly jet would be responded to anomalous cyclone at 500 hPa over CA and water vapor transported into CA through two steps from the Arabian Sea, above both contribute to more summer rainfall over CA.

Impacts of CP- and EP-El Niño events on the Antarctic sea ice in austral spring

Based on observational data analyses and idealized modeling experiments, we investigated the distinctive impacts of central Pacific (CP-) El Niño and eastern Pacific (EP-) El Niño on the Antarctic sea ice concentration (SIC) in austral spring (September to November). The tropical heat sources associated with EP-El Niño and the co-occurred positive phase of Indian Ocean Dipole (IOD) excite two branches of Rossby wave trains that propagate southeastward, causing an anomalous anticyclone over the eastern Ross-Amundsen-Bellingshausen Seas. Anomalous northerly (southerly) wind west (east) of the anomalous anticyclone favor poleward (offshore) movements of sea ice, resulting in a sea ice loss (growth) in the eastern Ross-Amundsen Seas (the Bellingshausen-Weddell Seas). Meanwhile, the anomalous northerly (southerly) wind also advected warmer and wetter (colder and drier) air into the eastern Ross-Amundsen Seas (the Bellingshausen-Weddell Seas), causing surface warming (cooling) through the enhanced (reduced) surface heat fluxes and thus contributing to the sea ice melting (growth). CP-El Niño, however, forces a Rossby wave train that generates an anomalous anticyclone in the eastern Ross-Amundsen Seas, 20° west of that caused by EP-El Niño. Consequently, a positive SIC anomaly occurs in the Bellingshausen Sea. A dry version of the Princeton atmospheric general circulation model was applied to verify the roles of anomalous heating in the tropics. The result showed that EP-El Niño can remotely induce an anomalous anticyclone and associated dipole temperature pattern in the Antarctic region, whereas CP-El Niño generates a similar anticyclone pattern with its location shift westward by 20° in longitudes.

Possible Relationship between Korean Heat Wave and Western North Pacific Tropical Cyclone Genesis Frequency

This study conducted a correlation analysis between tropical cyclone genesis frequency (TCGF) in the western North Pacific (WNP) and heat wave days (HWD) in Korea during July and August for 46 years (1973–2018) and we found a strong positive correlation between them. This implied that the higher the TCGF in the WNP during July and August, the higher the HWD in Korea becomes. To examine the cause of the statistically significant positive correlation between the TCGF during July and August in the WNP and the HWD in Korea, 15 years with the highest frequency and the lowest frequency out of the 46 years in the TCGF time series were selected and defined as high TCGF years and low TCGF years, respectively. An analysis of the difference in 2m air temperature (Air2m) between the two groups showed that in the mid-latitude region of Asia, the Air2m was higher during the high TCGF years. Thus, it could be seen from this analysis that the increase of HWD during the high TCGF years is likely to occur in the entire mid-latitude region of East Asia as well as in Korea. According to the difference in atmospheric circulations between the two groups, in all layers of the troposphere, anomalous anticyclonic and cyclonic circulations were strengthened in the mid-latitude region of East Asia and in the WNP, respectively, which was similar to the Pacific-Japan (PJ) teleconnection pattern. Furthermore, the anomalous anticyclone strengthened in the mid-latitude region of East Asia was associated with the weakening of the East Asian summer monsoon, and the anomalous cyclone strengthened in the WNP was associated with the WNP summer monsoon. The difference in the vertical meridional circulation averaged over the longitude range where Korea is located showed that anomalous upward and downward flows were strengthened in the WNP and in the latitude where Korea is located, respectively. This implied that the local Hadley circulation was strengthened during the high TCGF years. An analysis of the difference in the mean sea surface temperature during July and August showed that the eastern Pacific (EP) La Niña was strengthened during the high TCGF years. To determine the cause of the formations of anomalous anticyclones in the mid-latitude region of East Asia and in the WNP during the high TCGF years, the 500 hPa wave activity flux was analyzed. The wave activity flux originated from the North Atlantic, passed through the Scandinavian Peninsula, the North coast of Russia, and East Siberia before reaching Korea and the WNP. This spatial distribution was similar to the Scandinavia teleconnection pattern. Therefore, we conclude that the anomalous anticyclone formed in the mid-latitude region of East Asia and the anomalous cyclone formed in the WNP during the high TCGF years are associated with the Scandinavia teleconnection pattern.

Impact of the mean state on El Niño induced western North Pacific anomalous anticyclone during its decaying summer in AMIP models

Through the diagnosis of 29 Atmospheric Model Inter-comparison Project (AMIP) experiments from the CMIP5 inter-comparison project, we investigate the impact of the mean state on simulated western North Pacific anomalous anticyclone (WNPAC) during El Niño decaying summer. The result indicates that the inter-model difference of the JJA mean precipitation in the Indo-western Pacific warm pool is responsible for the difference of the WNPAC. During the decaying summer of an Eastern Pacific (EP) type El Niño, a model that simulates excessive mean rainfall over the western North Pacific (WNP) reproduces a stronger WNPAC response, through an enhanced local convection-circulation-moisture feedback. The intensity of the simulated WNPAC during the decay summer of a Central Pacific (CP) type El Niño, on the other hand, depends on the mean precipitation over the tropical Indian Ocean. The distinctive WNPAC-mean precipitation relationships between the EP and CP El Niño result from different anomalous SST patterns in the WNP. While the local SST anomaly plays an active role in maintaining the WNPAC during the EP El Niño, it plays a passive role during the CP El Niño. As a result, only the mean-state precipitation/moisture field in the tropical Indian Ocean modulates the circulation anomaly in the WNP in the latter case.

Possible Relationship Between Korean Heat Wave and Western North Pacific Tropical Cyclone Genesis Frequency

This study conducted a correlation analysis between tropical cyclone genesis frequency (TCGF) in the western North Pacific (WNP) and heat wave days (HWD) in Korea during July and August for 46 years (1973-2018) and we found showed a strong positive correlation between them. This implied that the higher the TCGF in the WNP during July and August, the higher the HWD in Korea becomes. To examine the cause of the statistically significant positive correlation between the TCGF during July and August in the WNP and the HWD in Korea, 15 years with the highest frequency and the lowest frequency out of the 46 years in the TCGF time series were selected and defined as high TCGF years and low TCGF years, respectively. An analysis of the difference in 2m air temperature (Air2m) between the two groups showed that in the mid-latitude region of Asia, the Air2m was higher during the high TCGF years. Thus, it could be seen from this analysis that the increase of HWD during the high TCGF years is likely to occur in the entire mid-latitude region of East Asia as well as in Korea. According to the difference in atmospheric circulations between the two groups, in all layers of the troposphere, anomalous anticyclonic and cycloninc circulations were strengthened in the mid-latitude region of East Asia and in the WNP, respectively, which was similar to the Pacific-Japan (PJ) teleconnection pattern. Furthermore, the anomalous anticyclone strengthened in the mid-latitude region of East Asia was associated with the weakening of the East Asian summer monsoon, and the anomalous cyclone strengthened in the WNP was associated with the WNP summer monsoon. The difference in the vertical meridional circulation averaged over the longitude range where Korea is located showed that anomalous upward and downward flows were strengthened in the WNP and in the latitude where Korea is located, respectively. This implied that the local Hadley circulation was strengthened during the high TCGF years. An analysis of the difference in the mean sea surface temperature during July and August showed that the eastern Pacific (EP) La Niña was strengthened during the high TCGF years. To determine the cause of the formations of anomalous anticyclones in the mid-latitude region of East Asia and in the WNP during the high TCGF years, the 500 hPa wave activity flux was analyzed. The wave activity flux originated from the North Atlantic, passed through the Scandinavian Peninsula, the North coast of Russia, and East Siberia before reaching Korea and the WNP. This spatial distribution was similar to the Scandinavia teleconnection pattern. Therefore, we conclude that the anomalous anticyclone formed in the mid-latitude region of East Asia and the anomalous cyclone formed in the WNP during the high TCGF years are associated with the Scandinavia teleconnection pattern.