對流層上層擾動影響西北太平洋夏季季風研究

Abstract

本文研究副熱帶西北太平洋 (SWNP, 130-150°E; 15-25°N) 夏季季風的肇始過程。該海洋季風肇始主要的環流變化特徵，在於熱帶西北太平洋 (TWNP, 125-140°E; 5-15°N) 海上西風帶的加強，並可見到季風槽由TWNP向東北加深至SWNP。在季風槽加深的同時，西北太平洋副熱帶高壓脊的位置往北移動了約10度，來到日本南端附近。受到高壓脊線移動的影響，於日本、朝鮮半島一帶的鋒面降雨顯著減少。於此，西北太平洋及東亞 (WNP-EA) 地區的季風相位已是更迭。 多數年份的季風肇始發生於七月中、下旬 (normal onset)。季風肇始期間海上對流層上層環流的變化顯著，並有雲及對流系統自SWNP東側移入。該雲簇似與海上對流層上層低壓槽所引發的正渦度環流關係密切。期間旺盛的對流雲區也由TWNP向東北擴展。季風肇始之前，SWNP近似晴空且海表風速減弱，穩定的海表加熱率令海面溫度升高。然而對流層下層依舊為輻散風場所壟罩。季風肇始之後，低層環流場隨即轉為輻合，也使得海表受熱通量及海表溫度驟降。因此認為在七月中旬或下旬，當低對流層大氣具備發展深對流的潛勢時，經由海上對流層上層環流的變化所引發的擾動便可能觸發季風肇始。 另有幾年的季風肇始較早發生(六月底至七月初；early onset)。在這些年的季風肇始之前，SWNP 東側並未見到雲及對流系統的發展 (如normal onset 季風肇始前的情形)。倒是在赤道西太平洋 (新幾內亞東側及東北側) 上的對流系統異常地活躍。於季風肇始期間該對流系統向北及向西北方向移動，移至SWNP 時正值季風的肇始。儘管觸發兩類季風肇始 (normal onset 與 early onset) 的機制看似不同，但二者海表熱通量的變化及低對流層大氣的不穩定情形卻很相似。一般情況下，七月中、下旬季風肇始與海上對流層上層的擾動及向西移動的雲系統密切相關。而若是在更早之前熱帶地區的對流系統過於活躍，則季風肇始可能提早發生。總結認為，深對流雲量在SWNP的驟增並非由局地升高的海表溫度所引發，而是受到外來雲系統的影響。

This study investigates the maritime monsoon onset by identifying the rapid inten-sification of convections at the subtropical western North Pacific (SWNP, 130-150°E; 15-25°N). Relating to the onset, the westerly wind in the tropical western North Pacific (TWNP, 125-140°E; 5-15°N) increases and the monsoon trough (MT) deepens and ex-tends northeastward to the SWNP. Corresponding to the strengthening of the MT, the ridge of the subtropical high-pressure shifts northward by ~10° to the south of Japan, and the rain band at the northern flank of the ridge weakens. The summer monsoon in the western North Pacific-East Asia (WNP-EA) sector shifts to a new monsoon phase corresponding to the SWNP onset. Normally the monsoon onset occurs in mid to late July (normal onset). At the nor-mal onset, clouds move into the SWNP from the east, which coincides with a significant change in the upper tropospheric circulation. It is likely that the moving convective clouds are induced by the westward and southwestward movement of up-per-tropospheric high potential vorticity from the mid-oceanic trough. Correspondingly, there is a rapid northeastward expansion of strong convections from the TWNP. Prior to the onset, the sea surface temperature (SST) over the SWNP increases due to strong surface heating associated with weak winds and clear skies, while winds remain diver-gent. Right after the monsoon onset, winds turn convergent and convections enhance, leading to a rapid decrease of surface heating and SST. It is suggested that west-ward-moving upper-level disturbances might trigger onset of monsoon in mid to late July when low-level atmospheric conditions favor development of deep convections. In some years, the onset of maritime monsoon occurs earlier in late June to early July (early onset). Prior to the early onset, there is no significant development of con-vections east of the SWNP as in the normal onset, and the convection is very active over the equatorial West Pacific (east and northeast of New Guinea). These convections ra-pidly move northward and northeastward to the SWNP, and trigger maritime monsoon onset. Despite the different triggering between the normal onset and the early onset, the surface heating and lower-tropospheric conditions surrounding the two onsets are very similar. It is concluded that the westward moving clouds associated with the up-per-tropospheric disturbance in mid-July play a critical role in triggering monsoon onset in normal years. In some years when tropical convection over the equatorial West Pacific is active above normal in early summer, the onset at the maritime monsoon might occur much earlier. The rapid intensification of convection in the SWNP during the maritime monsoon onset is remotely forced instead of locally forced by high SST as suggested by some studies.