利用多重衛星遙測觀測颱風引起的海洋生地化反應

Abstract

海洋浮游植物利用陽光、二氧化碳及營養鹽進行光合作用。因此浮游植物對吸收重要溫室氣體，即二氧化碳，扮演重要的角色。浮游植物行光合作用必須在光線充足的上層海洋(有光層)中進行。在有光層中除了靠近河川出海口地帶的海域，由河川所帶來的營養鹽濃度較高外，大部分遠離陸地的表水海域，營養鹽都是非常缺乏的。因此在大洋中若是要獲得營養鹽要考慮其他外來機制，如大氣的沙塵暴，或是海洋的湧升現象。 長期以來也曾經推測颱風有可能造成大洋中營養鹽的供給，但是因為在廣大洋面颱風期間觀測的困難，因此颱風對初級生產力的貢獻一直無法作定量的分析而被忽略。隨著衛星遙測科學及技術的進步，對於颱風引起的海洋生地化反應有越來越多的研究。但大多數的研究都是個案分析，針對多個颱風有系統分析的研究又集中在大西洋區域，對於西北太平洋這個颱風生成個數最多的區域，在這方面至今仍然沒有系統化的研究。 本研究利用多種先進衛星遙測資料結合海洋初級生產力模式有系統的分析從2000-2006年西北太平洋營養鹽缺乏的開放海域(包含南海)由颱風引起之生地化反應。在颱風經過後兩週內發生藻華現象且不受雲及河流排放干擾的颱風個數共有30個，有8個在南海，22個在太平洋區域。研究發現，在颱風過後葉綠素濃度平均增加22%到6205%，SST平均下降1.35 oC至4.9 oC。並且葉綠素濃度增加與颱風的移動速度呈反比的關係，顯示移動越慢的颱風造成海洋的生物反應越大。當比較兩個不同的海域，在相同的颱風強度下，南海的颱風造成葉綠素增加的比例較西北太平洋大，南海的颱風個案引起的葉綠素濃度增加全都大於100%，增加率介於121%到6205%之間。而太平洋葉綠素濃度平均增加22%到341%。在南海TS(風速介於34-63 knots)強度以上的颱風就可以引發藻華現象，而太平洋區域則需category 1(風速介於64-82 knots)以上的颱風才能引發藻華現象。但由於南海颱風影響面積較小，即使葉綠素濃度增加率較大，颱風過後初級生產力總和仍相對較小。透過初級生產力模式計算颱風過後海洋生物吸收的總碳量的結果，2000-2006年30個颱風造成海洋吸收的碳量為3.20E+16 mg C 。

Contributing roughly half of the biosphere’s net primary production, ocean primary (phytoplankton) production plays a significant role in the earth environmental system. It is important to understand the controls on primary production in the upper ocean, because of the fixation and subsequent sinking of organic particles remove carbon from the surface ocean (the so-called biological pump) by phytoplankton. The distribution of phytoplankton biomass and ocean primary production is defined by the availability of light and nutrient. In coastal area, nutrients mainly come from river runoff. In open oceans, the lack of nutrients limits the ocean primary productivity. The supply of nutrients in open oceans is mostly from the pumping of nutrient-rich deep water to the euphotic zone through various mechanisms, such as upwelling and eddies. For long it has been speculated that typhoon’s strong wind may give rise to significant nutrient pumping, however, due to high observational difficulty, evidences were grossly deficient in quantifying this speculation. Satellite remote sensing provides an opportunity to measure over vast ocean. Thus, it is possible to explore the ocean response to typhoon. In this study we used advanced multiple satellite remote sensing data and an ocean primary production model to observe the biological effects of typhoons. In this research we examined 30 typhoons that passed through the oligotrophic area of the western North Pacific Ocean (including the South China Sea) during the periods between 2000 to 2006. The observed increased in chlorophyll-a concentration after typhoon’s passage reached between 22-6205%. Also, sea surface temperature decreased by about 1.35-4.9 oC after typhoon’s passages. It is also found that much stronger biogeochemical response (i.e., increase in chlorophyll-a concentration) is found in the South China Sea than in the western North Pacific Ocean. It is found that due to the the nutricline in the South China Sea is much shallower than in the western North Pacific ocean. As such, given same typhoon forcing, much more intense response can be induced in the South China Sea than in the western North Pacific. In addition, it was found the the increased in chlorophyll-a concentration is correlated with the typhoon’s transit speed. Finally, the estimated carbon fixation resulting from these 30 typhoons is 3.20E+16 mg C.