[1] The present study examines variation of tropical cyclone (TC) activity in the South Indian Ocean (SIO) during TC seasons (December-March) for the period 1979-2004. The impact of the El Niño-Southern Oscillation and the Madden-Julian Oscillation (MJO) on the variation is revealed through a composite analysis. During El Niño periods TC genesis was shifted westward, enhancing the formation west of 75°E and reducing it east of 75°E. These changes in the genesis correspond to a westward shift of convection. It may be explained by a remote effect on the SIO; that is, the increase in sea surface temperature in the central eastern Pacific alters the Walker circulation and forms an anomalous anticyclonic circulation in the east SIO during El Niño. The spatial difference in TC passages between El Niño and La Niña shows a significant decrease to the southeast of Madagascar but a moderate increase in the central midlatitude SIO, indicating that TCs move farther east during El Niño. This change is possibly due to the anomalous southwesterlies east of Madagascar. Variation of TC activity also depends on various MJO phases: frequent TC passages for phases 2-4 (strong convective activity straddles along the equatorial Indian Ocean) versus infrequent TC passages for other phases. TC tracks tend to be more south oriented in phase 3 compared with those in phases 2 and 4. This is possibly caused by the increased steering northerlies which are a part of the anticyclonic Rossby wave of Gill type in response to the suppressed MJO-related convection in the maritime continent.

The threat of intense tropical cyclones (TCs) to East Asia has increased in recent decades. Integrated analyses of five available TC data sets for the period 1977-2010 revealed that the growing threat of TCs primarily results from the significant shift that the spatial positions of the maximum intensity of TCs moved closer to East Asian coastlines from Vietnam to Japan. This shift incurs a robust increase in landfall intensity over east China, Korea and Japan. In contrast, an increase of TC genesis frequency over the northern part of the South China Sea leads to a reduction in the maximum TC intensity before landfall, because of their short lifetime; thus, there are no clear tendencies in the landfall intensity across Vietnam, south China and Taiwan. All changes are related to the strengthening of the Pacific Walker circulation, closely linked with the recent manifestation that the warming trend of sea surface temperature in the tropical western Pacific is much higher than that in the central to eastern Pacific.

In January 2016, the Arctic experienced an extremely anomalous warming event after an extraordinary increase in air temperature at the end of 2015. During this event, a strong intrusion of warm and moist air and an increase in downward longwave radiation, as well as a loss of sea ice in the Barents and Kara seas, were observed. Observational analyses revealed that the abrupt warming was triggered by the entry of a strong Atlantic windstorm into the Arctic in late December 2015, which brought enormous moist and warm air masses to the Arctic. Although the storm terminated at the eastern coast of Greenland in late December, it was followed by a prolonged blocking period in early 2016 that sustained the extreme Arctic warming. Numerical experiments indicate that the warming effect of sea ice loss and associated upward turbulent heat fluxes are relatively minor in this event. This result suggests the importance of the synoptically driven warm and moist air intrusion into the Arctic as a primary contributing factor of this extreme Arctic warming event.

This study presents a new demonstration of the abrupt increase in the heavy rainfall events (≥100 mm day−1) during August–September in Korea around the late 1970s. The accumulated heavy rainfall averaged for the two months over 12 stations was 57 mm during 1954–77 (ID1); however, it changed to 103 mm during 1978–2005 (ID2). This change is found to be associated with landfalling tropical cyclones (TCs). The most plausible mechanism that accounts for the TC–heavy rainfall relationship is an enhanced TC–upper‐tropospheric trough (UTT) interaction, which results from a southward shift of the upper‐tropospheric jet in East Asia during ID2. While the intensity and duration of the landfalling TCs in Korea does not appear to exhibit such an interdecadal change based on the data available, the enhanced TC–UTT interaction increases the upper(lower)‐tropospheric divergence (convergence) and coherent ascending motion, which strengthen the frontal zone around Korea.

[1] The present study investigates how large-scale atmospheric circulation in the Southern Hemisphere (SH) modulates tropical cyclone (TC) activity in the western North Pacific (WNP) during a typhoon season (July, August, and September; boreal summer). The variation of the SH circulation of interest is the Antarctic Oscillation (AAO). In the positive phase of AAO relative to its negative phase, two anomalous highs develop over the western Pacific in both hemispheres: a huge anticyclone in southeastern Australia and a relatively weak anticyclone in the East China Sea. These teleconnection patterns are examined and compared with previous analyses. Related to the AAO variations, a statistically significant alteration of TC activities is found over the WNP. The difference in the mean TC passage numbers over the East China Sea (120°-140°E, 20°-40°N) between the eight highest-AAO years and the eight lowest-AAO years is as large as 2, equivalent to a 50-100% increase from the climatology. This change is primarily a result of more TCs forming over the eastern Philippine Sea. On the other hand, TC passage numbers slightly decrease over the South China Sea. These changes in TC activity are predominant in August and are consistent with changes in low-level vorticity over the subtropical WNP. The influence of SH circulation variability on large-scale environments and tropical convection in the subtropical NH suggest a possible usage of AAO variation for long-range forecasting of TC activity over the WNP.