Circulation Features Associated with the Record-breaking Typhoon Silence in August 2014

Abstract

Climatologically, August is the month with the most tropical cyclone (TC) formation over the western North Pacific (WNP) during the typhoon season. In this study, the reason for abnormal TC activity during August is discussed—especially August 2014, when no TCs formed. The large-scale background of August 2014 is presented, with low-level large-scale easterly anomalies and anticyclonic anomalies dominating over the main TC genesis region, a weak monsoon trough system, and a strong WNP subtropical high (WPSH), leading to significantly reduced low-level convergence, upper-level divergence, and mid-level upward motion. These unfavorable large-scale conditions suppressed convection and cyclogenesis. In August 2014, equatorial waves were inactive within the negative phase of the Madden–Julian Oscillation (MJO), with fewer tropical disturbances. Although the low-level vorticity and convection of those disturbances were partly promoted by the convective envelopes of equatorial waves, the integral evolution of disturbances, as well as the equatorial waves, were suppressed when propagating into the negative MJO phase. Moreover, the upper-level potential vorticity (PV) streamers associated with anticyclonic Rossby wave breaking events imported extratropical cold and dry air into the tropics. The peripheral tropospheric dryness and enhanced vertical wind shear by PV streamer intrusion combined with the negative MJO phase were responsible for the absence of TC formation over the WNP in August 2014.

摘 要

八月是西北太平洋地区热带气旋(TC)生成最为活跃的月份, 但2014年8月份没有TC生成. 从大尺度环流背景分析可知, 该月份季风槽系统偏弱, 对流层低层为反气旋型环流异常和大范围的东风异常, 中层的副热带高压加强, 使得低层辐合和高层辐散的结合减弱, 且中层的垂直运动被抑制. 大尺度环流条件不利于湿对流活动发展和TC的生成. 此外, MJO和赤道波动整体不活跃, 对西太地区8月份热带扰动的发展及TC的生成也有一定的抑制作用. 当热带扰动的演变处于MJO不活跃位相内时, 对流活动和低层涡度的整体发展被抑制. 同时, 赤道外高层的Rossby波破碎和PV入侵过程对热带扰动的发展也有重要的影响, 其带来的干冷空气使得扰动中心附近的水汽偏干, 垂直风切变增加, 使得湿对流活动进一步受到抑制. 受赤道外和赤道地区多尺度系统的综合影响, 2014年8月份没有TC生成.