1:45 PM - 3:15 PM
[AAS04-P08] Remote Effects of Tropical Cyclones in the Northwest Pacific on the Formation of Atmospheric Rivers
Keywords:Atmospheric rivers, Remote effects, Tropical Cyclones
Atmospheric rivers are thought to play an important role in precipitation activity in the mid-latitudes, as 90% of water vapor transport in the mid-latitudes is attributed to atmospheric rivers (Zhu and Newell 1998, MWR). Recent studies indicated that extreme rainfall events are also linked to atmospheric rivers (e.g., Hirota et al. 2016, MWR). While predicting the formation of atmospheric rivers is important from the perspective of disaster prevention and mitigation, their formation process is not yet fully understood. Recently, a case study (Yoshida and Itoh 2012, JMSJ) shows that tropical cyclones (TCs) may be capable of indirectly generating atmospheric rivers. In this study, we report on the remote effects of tropical cyclones on the formation of atmospheric rivers in the Northwest Pacific Ocean by objectively selecting typhoons for analysis and analyzing the temporal changes in their central pressure and water vapor transport around Kyushu.
Using JRA-55 reanalysis (horizontal 1.25°, 6 hourly), vertically integrated eastward and northward water vapor flux (hereinafter IEF and INF, respectively) were calculated separately for the lower (surface–700 hPa) and upper (700–300 hPa) layers. In order to investigate the remote influence of TCs on atmospheric rivers, we objectively selected 129 TCs that existed only within 10–25°N, and that formed east of 125°E and disappeared west of 125°E. In this study, we mainly focus on the relationship between the selected TCs and the IEF and INF around Kyushu Island (127.5–132.5°E and 30–35°N). The analysis period is 1977–2020.
We found that both the IEF and INF around the Kyushu Island tend to reach a maximum at 48–120 hours after the time when the central pressure of the TC was at its lowest. A composite analysis was conducted for cases with the time lag between 48–120 hours. The results showed that there are statistically significant anomalies in geopotential height, with negative anomalies in the South China Sea, positive anomalies around Kyushu, and negative anomalies around Hokkaido around the time with the TC having its lowest central pressure. The positive anomalies in the upper layer then reduced, while another negative anomalies formed in northern China and developed until around the time with the maximum IEF and INF over the Kyushu Island. The upper-layer IEF over the Kyushu Island increased mainly due to westerly wind anomaly caused by enhanced meridional pressure gradient associated with the negative and positive anomalies at the south and north, respectively. The lower layer INF, on the other hand, increased due to enhanced southerly winds caused by an enhanced zonal pressure gradient in the lower troposphere.
Using JRA-55 reanalysis (horizontal 1.25°, 6 hourly), vertically integrated eastward and northward water vapor flux (hereinafter IEF and INF, respectively) were calculated separately for the lower (surface–700 hPa) and upper (700–300 hPa) layers. In order to investigate the remote influence of TCs on atmospheric rivers, we objectively selected 129 TCs that existed only within 10–25°N, and that formed east of 125°E and disappeared west of 125°E. In this study, we mainly focus on the relationship between the selected TCs and the IEF and INF around Kyushu Island (127.5–132.5°E and 30–35°N). The analysis period is 1977–2020.
We found that both the IEF and INF around the Kyushu Island tend to reach a maximum at 48–120 hours after the time when the central pressure of the TC was at its lowest. A composite analysis was conducted for cases with the time lag between 48–120 hours. The results showed that there are statistically significant anomalies in geopotential height, with negative anomalies in the South China Sea, positive anomalies around Kyushu, and negative anomalies around Hokkaido around the time with the TC having its lowest central pressure. The positive anomalies in the upper layer then reduced, while another negative anomalies formed in northern China and developed until around the time with the maximum IEF and INF over the Kyushu Island. The upper-layer IEF over the Kyushu Island increased mainly due to westerly wind anomaly caused by enhanced meridional pressure gradient associated with the negative and positive anomalies at the south and north, respectively. The lower layer INF, on the other hand, increased due to enhanced southerly winds caused by an enhanced zonal pressure gradient in the lower troposphere.