利用位渦反演的方法探討熱帶氣旋活動對大尺度環流的影響

Abstract

此研究針對2004年6－10月，以濾除颱風和位渦反演的方法，得到原始的大氣環流場和濾除颱風後的背景環流場，分析濾除颱風前後氣候變異量上的差異，了解颱風與大尺度環流之間能量轉換的關係。此外，針對季內振盪活躍期與非活躍期，討論大尺度環流與颱風擾動間的交互作用。 在西北太平洋,低層850百帕颱風路徑頻繁的地區，濾除颱風前後改變的渦度變異量約80%、渦流動能達60%以上，可見颱風擾動影響顯著。而高層風速大，相形之下颱風擾動不明顯，颱風貢獻的渦度變異量僅約20%、渦流動能約10%，皆隨高度增加而減小。天氣尺度下正壓能量轉換的分布情形，在低層，颱風生成初始時提供正的能量轉換，颱風從平均流中獲得能量，其中以−u'v'(∂v/∂x)和u'v'(∂u/∂y)貢獻為主，特別是在20°N以南貢獻皆為正。因為在此濾除颱風前後，動量通量在台灣與菲律賓以東的海面上正值明顯減弱，且背景風場變化為∂v/∂x<0、∂u/∂y<0，因而貢獻正的能量轉換。在高層，颱風生命期末端時提供負的能量轉換，颱風失去能量至平均流中。正壓能量轉換各項貢獻中，−u'v'(∂u/∂y)和−u'v'(∂v/∂x)的貢獻較零散，多呈現相位相反，互相抵銷的情形，而−u'^2(∂u/∂x)和−v'^2(∂v/∂y)颱風貢獻量值大小差異顯著，以這兩項的加成貢獻為主。 在季內尺度，在10°N-20°N和120°E-150°E間正渦度存在60天週期的變化，濾除颱風後渦度有明顯的減小。由波譜分析的結果可知在週期60天時渦度變異度最大，濾除颱風後量值僅為原本的四分之一，可知颱風的擾動會增大西北太平洋地區季內振盪的擾動。在季內振盪活躍時，颱風貢獻在120°E-150°E間有明顯的正的能量轉換，而南海和副高為負的能量轉換；非活躍期時，颱風貢獻僅侷限於日本以南和台灣以東間，呈正負交替的配置。然而，不管在活躍或非活躍期，颱風貢獻的能量轉換皆以−u'v'(∂u/∂y)和−u'^2(∂u/∂x)兩項貢獻為主，以−u'v'(∂u/∂y)相對重要。

This study applies typhoon-filtering technique to remove TC disturbance field from the large-scale field, and then use PV inversion to derive the three-dimensional circulation associated with the TC vortex. The period from June to August 2004 was chosen for study because of the active intraseasonal oscillation and its strong clustering effect on TC. A comparison between the original and TC-removed fields yields an estimation of TC contribution to the large-scale circulation variability and to the energy conversion between eddy and mean flow. Similar approach was also applied during the different phases of intraseasonal oscillation. TC is a vortex with its strong circulation, especially along the TC tracks. In the Western North Pacific (WNP), the TCs contribute about 80% of total vorticity variance and account for 60% of eddy kinetic energy at 850 hPa. However, the contribution of TCs decreases with height and accounts for 20% and 10% of the total vorticity variance and eddy kinetic energy, respectively, in the upper troposphere. The barotropic energy conversion associated with the eddy–mean flow interaction is closely related to the TC activity in the WNP. In synoptic-scale, TCs help convert energy from mean flow to eddy in the lower troposphere in the beginning of its lifetime. u'v'(∂u/∂y) and −u'v'(∂v/∂x) are the dominant terms, both positive to the south of 20∘N. This is because the momentum flux difference between the original and TC-removed field is positive in the east of the Taiwan and Philippines, and the background circulation is characterized by ∂v/∂x<0 and ∂u/∂y<0. In the upper troposphere, TCs help convert energy from eddy to mean flow in the latter part of the lifetime. The −u'v'(∂u/∂y) and −u'v'(∂v/∂x) are small and are in reversed phase. The energy conversion term is thus dominated by −u'^2(∂u/∂x) and −v'^2(∂v/∂y) term. In 2004, large-scale circulation is characterized by intrseasonal oscillation with a period around 60 days period between 10°N-20°N and 120°E-150°E in the original field. Removing TCs significantly reduces the intraseasonal variance. Results of the spectral analysis in the original time series, one spectral peak (exceeding 95% confidence level) are found near 60 days, but drops substantially after the removal of TCs. TCs significantly enlarge the fluctuation of the intraseasonal oscillation. During the ISO active phase, the positive barotropic energy conversion in 120°E-150°E and negative barotropic energy conversion near the South China Sea and subtropical high occurred. During the ISO suppress phase, with alternating positive and negative barotropic energy conversion are limited in the region between south of Japan and east of Taiwan. In both situation, the conversion term is dominated by −u'v'(∂u/∂y) and −u'^2(∂u/∂x) term, especially −u'v'(∂u/∂y) term.