東太平洋間熱帶輻合帶的南北偶極特徵
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Date
2011-12-01
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中華民國氣象學會
Abstract
間熱帶輻合帶（Intertropical Convergence Zone；簡稱ITCZ），是大氣環流中一個很明顯的特徵。在東太平洋地區，由於海氣交互作用及海陸交界海岸線的配置，使得ITCZ偏好出現在北半球。東太平洋赤道北方的ITCZ（簡稱NITCZ）在北半球夏季時移至最北端（20°N），並在春季時回到最低緯度（5°N）；除了北半球的NITCZ，在其南方有另一條對流輻合帶，只在3～4月出現於南半球，稱為東南太平洋間熱帶輻合帶（Southeast Pacific Intertropical Convergence Zone；簡稱SITCZ）。根據降水及低層輻合場的平均來看，在3～4月時NITCZ及SITCZ並存，且呈現雙條ITCZ的現象。本研究以多種獨立觀測資料來分析SITCZ的年際變化，包括降水、海表面溫度（Sea surface temperature；簡稱SST）等。分析結果顯示，SITCZ區域的對流系統，與該地SST的相關，僅侷限在年循環的季節變化上，對於SITCZ的年際變化，該地的SST並非關鍵因素。也就是說，SITCZ強度上的年際變化，和該區域的SST並無顯著關係。相對於此，在其北方的NITCZ強度，卻與SITCZ的年際變化有著明顯的負相關。亦即，若某年的NITCZ較弱，便會產生弱的南北向環流，使得赤道南方SITCZ對流發展有較好的大環境條件，因此SITCZ會較強。相反地，若某年的NITCZ較強，便會產生相反的結果，使SITCZ的年際變化較弱。NITCZ與SITCZ間有著此消彼長，彼消此長的南北偶極相關，是東太平洋ITCZ年際變化上一個重要的特徵，而這樣的相關也可在日尺度的對流分析中被證實。
Over the eastern Pacific Ocean, the Intertropical Convergence Zone (ITCZ) is a prominent feature in the atmospheric general circulation. The ITCZ over the eastern Pacific prefers the Northern Hemisphere, resulting from the ocean-atmosphere interactions and coastal geometries. The northern ITCZ (hereafter NITCZ) reaches the northernmost location (20衹) in boreal summer, and then returns to the lower latitude (5衹) in boreal spring. In addition to this major NITCZ, a band of convergence zone located south of it, the southeast Pacific Intertropical Convergence Zone (hereafter SITCZ), appears in March and April in the Southern Hemisphere. According to the average of rainfall and low-level divergence, NITCZ and SITCZ co-exist in March and April. This study analyzes the interannual variations of the SITCZ based on several independent observational datasets, including precipitation and sea surface temperature (SST) data. Results show that the annual cycle of the local SST over the SITCZ region merely contributes to the seasonal evolution of the convection, but does not contribute to the interannual variation. The strength of the SITCZ does not correlate with its local SST. Nonetheless, the intensity of the NITCZ over the eastern Pacific has a negative correlation with the interannual variation of the SITCZ. In other words, a weak NITCZ produces a weak meridianal circulation which results in a better large-scale condition for convections to grow to its south such that a stronger SITCZ is evident. In contrast, a strong NITCZ leads to a weak SITCZ through an opposite process. This meridional see-saw dipole pattern is an important interannual characteristic of the ITCZs over the eastern Pacific Ocean, and it is found in the daily time scale as well.
Over the eastern Pacific Ocean, the Intertropical Convergence Zone (ITCZ) is a prominent feature in the atmospheric general circulation. The ITCZ over the eastern Pacific prefers the Northern Hemisphere, resulting from the ocean-atmosphere interactions and coastal geometries. The northern ITCZ (hereafter NITCZ) reaches the northernmost location (20衹) in boreal summer, and then returns to the lower latitude (5衹) in boreal spring. In addition to this major NITCZ, a band of convergence zone located south of it, the southeast Pacific Intertropical Convergence Zone (hereafter SITCZ), appears in March and April in the Southern Hemisphere. According to the average of rainfall and low-level divergence, NITCZ and SITCZ co-exist in March and April. This study analyzes the interannual variations of the SITCZ based on several independent observational datasets, including precipitation and sea surface temperature (SST) data. Results show that the annual cycle of the local SST over the SITCZ region merely contributes to the seasonal evolution of the convection, but does not contribute to the interannual variation. The strength of the SITCZ does not correlate with its local SST. Nonetheless, the intensity of the NITCZ over the eastern Pacific has a negative correlation with the interannual variation of the SITCZ. In other words, a weak NITCZ produces a weak meridianal circulation which results in a better large-scale condition for convections to grow to its south such that a stronger SITCZ is evident. In contrast, a strong NITCZ leads to a weak SITCZ through an opposite process. This meridional see-saw dipole pattern is an important interannual characteristic of the ITCZs over the eastern Pacific Ocean, and it is found in the daily time scale as well.