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北印度洋季風與季風低壓-(2)年代際突變之懸案

North Indian Ocean Monsoon and Monsoon Depressions Part Ⅱ: Paradox of Interdecadal Abrupt Change

摘要


1976/1977地球海洋-大氣系統經歷過一次強烈的氣候突變,此事件最先由北太平洋上層海溫改變所揭露,其後北太平洋西風噴流、鮭魚產量以迄於青康藏高原積雪量皆受到該年代際突變的深遠影響。本研究亦據此受到啟發,以北印度洋與南亞季風環流的自然季節爲分析架構,探討北印度洋(孟加拉灣與阿拉伯海)季風低壓生成於1976/1977氣候突變前後的改變。 1959年至1998年的北印度洋季風低壓觀測顯示,1977年後季風低壓年生成個數急遽地減少,自平均每年生成14.9±2.2個減爲每年5.0±2.4個。進一步依據季風低壓的強度及生成季節分類後,各季節無論是強度較強的氣旋風暴或較弱的低壓均呈現減少趨勢,夏、秋兩季尤其明顯:1977年之前,夏季的氣旋風暴平均每季生成0.7±0.8個,1977年後減爲0.4±0.8個;低壓平均每季生成4.2±1.4個,1977年後不再出現。而秋季的氣旋風暴於1977年後自平均每季生成1.8±1.0個減爲1.0±0.8個,低壓則由每季生成2.3±1.4個減爲0.2±0.6個。文中並描述了季風低壓生成突變的時、空間特徵,包括生成頻率季節分佈之顯著改變,及生成區域的變化。 當然,熱帶氣旋長期資料之品質在全球皆有不同程度的可靠性。本文僅針對1976/1977氣候突變前後,北印度洋季風低壓生成記錄是否受到觀測儀器、資料處理等人爲因素影響,從事進一步探討。經過我們僅可能的對原始觀測來源、觀測系統軟、硬體變動進行查證,以及向印度相關學者確認,初步認定,並無任何具體證據否定本文的結果。本文亦列舉數種可能被人爲介入、目前超乎我們查證能力外之因素,提供讀者參考。 在探討環流的相關變化時,除了舉出已知的全球各地氣候系統在同時期發生突變的間接證據,另以較嚴謹的統計方法進行變化點分析(change point analysis),檢驗局地大氣環流之逐年變化,發現若干物理量在1976/1977前後確實產生顯著的突變,中低層大氣尤其明顯。這些遠地與局地的環流突變,相當正面的支持了北印度洋季風低壓生成數量銳減的現象。 在資料可能具不確定性的前提下,本研究結果顯示北印度洋季風低壓反映1976/1977之氣候劇變,並發現特定季節之低壓在1977年後不再出現。雖此變化由何物理機制引起還不清楚,本研究的發現可能隱含有趣的含義,即天氣尺度系統亦可能對氣候突變極爲敏感,並如水圈或冰雪圈一般具有對氣候突變的長遠記憶。

並列摘要


During years of 1976/1977, the global atmosphere-ocean system undergoes a strong abrupt climate change, which starts with the change in the upper-layer temperature of North Pacific Ocean. The North Pacific westerly jets, salmon production, and even Tibetan Plateau snow cover are all subject to profound impacts of this interdecadal abrupt climate change. Motivated by these changes, the current study, using the natural seasons of North Indian Ocean (NIO) and South Asian monsoon as a framework, aims to examine the changes in North Indian Ocean monsoon depression (a generic term for NIO cyclonic disturbances) formation before and after year 1976/1977. The annual NIO monsoon depression formation number as observed from year 1959 to 1998 has dropped abruptly since 1977. It drops from 14.9±2.2 per year to 5.0±2.4 per year. The characteristics of the abrupt change in monsoon depression formation are further manifested when monsoon depressions are classified according to their intensity and formation season. Both stronger 'cyclonic storms' or weaker 'depressions' show decreasing trends. The decreases are particularly significant in summer and autumn. While in average 0.7±0.8 cyclonic storms form each summer before 1977, only 0.4±0.8 form after 1977. Before 1977, in average 4.2±1.4 depressions form each summer, but they no longer appear after 1977. While the average autumn cyclonic storm formation numbers have dropped from 1.8±1.0 (per season) to 1.0±0.8 (per season) since 1977, those of autumn depressions have dropped from 2.3±1.4 (per season) to 0.2±0.6 (per season). The temporal and spatial features of the interdecadal abrupt change of monsoon depression formation are also described, including the significant changes in seasonal variations of formation frequency and formation regions. Of course, the reliability of long-term consistency of tropical cyclone data sets differs in different basins. Focusing on the period right before and after 1976/1977, this study investigates if any artificial influences such as changes in observation platform or data processing have been introduced into NIO monsoon depression observations. As many as possible effors have been made to examine the monsoon depression data quality, including tracing the raw data source, examining when and how the observational network or observation availability might have changed during that period, and consulting relevant Indian scientists. These preliminary examinations show no specific evidences to deny the results of the current study. Several artificial factors that might have led to data uncertainties but cannot be easily confirmed by authors are also listed and briefly discussed for readers' understanding. Changes in both remote and local circulations are investigated. Known abrupt changes in global climate systems during the 1976/1977 period are illustrated, in addition, several local atmospheric variables are shown to indeed exhibit significant abrupt changes during the same period based on the results from a statistical approach 'change point analysis'. The signals in lower to middle atmosphere are particularly strong. These abrupt changes in either remote or local circulations to a certain degree support the decline of monsoon depression formation in 1976/1977. Though there might be uncertainties existing in NIO monsoon depression data sets, this study manifests the responses of NIO monsoon depressions to 1976/1977 abrupt climate change and, in particular, that the weaker 'depressions' in specific seasons no longer appear after 1977. It is not clear yet through what mechanism the interdecadal abrupt climate change results in this abrupt change. This study could imply that synoptic activities might also respond to abrupt climate change sensitively, and that they possess long climate memories just as ocean or ice does.

被引用紀錄


吳宜昭(2006)。亞洲季風區多重尺度的交互作用及其對熱帶氣旋生成的影響〔博士論文,國立臺灣大學〕。華藝線上圖書館。https://doi.org/10.6342%2fNTU.2006.02620

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