背景流場對颱風冷尾跡之影響

Abstract

本文使用衛星及現場觀測資料探討颱風冷尾跡形成後一種較不常見的冷水潰散過程。2010年9月，凡那比颱風在西北太平洋引發了一個面積約500×100 km2溫降約3oC之冷尾跡，此冷尾跡的西緣受到位於颱風事件前就存在的中尺度氣旋式渦旋與反氣旋式環流所形成的匯流場影響，促使冷水向南平流而形成一道長約300 km寬約20 km的流絲，此窄長流絲之形成與強化機制可藉由匯流之流場形變以及位渦守恆二效應來解釋。另方面，位於流絲行進路徑上的溫度串錨碇觀測顯示在流絲抵達前與抵達後，分別出現21小時及56小時的溫度變動，此點可用都普勒效應來解釋，其中21小時的溫度變動起因於流絲前緣產生的近慣性波，由於波源在移動故當波源接近時錨碇觀測到的波動週期便會短於當地的慣性週期(32小時)，而波源離開後觀測到的週期為56小時(長於慣性週期)波動則可能是由北傳波動激發混合層不穩定現象所造成(衛星海表溫顯示流絲在其側向呈現波形結構以及觀測到之波動週期符合混合層不穩定波頻散關係式便是相關證據)。在颱風生成發展的大洋中，中尺度氣旋式及反氣旋式渦旋同時並存是普遍的現象，渦旋作用造成之背景流場形變，對於颱風冷尾跡事件後的海洋分層回復過程會扮演相當重要的角色。

In September 2010, typhoon Fanapi induced a cold wake of size 500×100 km2 with sea surface temperature cooling of 3oC in the Western North Pacific Ocean. Using satellite and in-situ data, a novel process for cold wake dispersion was investigated. The cold water in the western flank of the wake was advected southward by a confluent flow as a result of the interaction between a preexisting mesoscale cyclonic eddy and an anticyclonic recirculation, forming a cold filament with width and length of 20 km and 300 km, respectively. Both the filamentary intensification as a result of straining filed induced by the confluent flow and the conservation of potential vorticity provide the basis of the development of long and narrow filament. Subsequently, moored temperature measurements in the downstream track of the filament revealed a 21-hour and 56-hour thermal fluctuations before and after the arrival of the filament, respectively. Theoretical analysis suggest that the 21-hour fluctuations are induced by the near-inertial waves emitted from the moving front tongue of the filament, resulting in a shorter period than the local inertial period (32 hours) due to the Doppler shifting. The 56-hour fluctuation is related to the mixed layer instability regarding the presence of lateral billow-like thermal structure observed in satellite SST and the consistency in period to the theoretical dispersion relationship of the instability wave. Cyclonic and anticyclonic eddies are ubiquitous in the open ocean, where the typhoons develop. The strain-induced filament due to the interactions between mesoscale eddies are expected to play a role to re-stratify typhoon-induced cold wake.