利用多重衛星遙測觀測Madden-Julian Oscilltion引起的海洋生地化反應

Abstract

貧瘠海域為不利浮游植物生長的環境，因此在貧瘠且開放的海域中浮游植物需藉由外來供應得到營養鹽，方法分別為ocean mixing與atmospheric deposition，此可使藻華生成，提升海洋初級生產力。 Madden-Julian Oscillation（MJO）生成於印度洋熱帶地區的天氣系統，本身伴隨著強風使海水攪拌，有機會將下層營養鹽抬升。本研究選取2002年至2012年間的八個MJO個案，其中個案2002_B、個案2007、個案2009與個案2012這四個MJO影響過後印度洋有五個區域產生顯著藻華，平均海表葉綠素濃度增加4 ~ 5倍，藻華面積為二十萬平方公里以上，藻華持續時間介於11天到80天。剩餘的個案均無顯著藻華產生。 本研究利用Two Layer Reduced Gravity Model得到海洋26℃等溫線深度（暖水層厚度），藉此了解MJO影響前的海水上層熱力結構，暖水層越薄的海域，海表經過強風的影響，較容易將營養鹽帶到海表，其中顯著藻華生成的個案暖水層厚度均為40公尺以下，此厚度遠薄於其他的個案（60公尺以上）。接著經由Mellor-Yamada 1-D ocean mixed layer model模擬MJO影響過後海水的變化，本研究所有個案中只有顯著藻華生成的個案呈現混和層增厚與海表降溫幅度達2℃以上的現象，此現象顯示這些個案的底層富有營養鹽冷海水確實被帶至海表面。最後採用Vertical Generalized Production Model （VGPM）生產力模式，量化MJO對於海洋的級生產力影響，本研究各個藻華生成個案的初級生產力增加率介於286% ~ 382%，藉由初級生產力計算MJO影響過後海洋的總吸收的碳量為1.14×〖10〗^14 ~ 7.65×〖10〗^15 mg。

Oligotrophic and open ocean is not a favorable environment for phytoplankton’s growth. Phytoplankton grows only in the ocean where there is sufficient external nutrient supply. There are usually two ways to enhance ocean primary production (i.e. phytoplankton growth) and carbon dioxide uptake. The two ways are ocean mixing and atmospheric aerosol deposition. Madden-Julian Oscillation (MJO) originates from the tropical Indian Ocean and make sufficient nutrient in upper ocean by MJO’s strong wind forcing. This study conducts systematic research between 2002 and 2012 over the Indian Ocean using multiple remote sensing observations. We found that during a number of MJO events found between Case 2002_B, Case 2007, Case 2009, and Case 2012, MJO can induce strong biogeochemical responses, as characterized by 4-5 fold increase in chlorophyll-a concentration. It is also found that the bloom can cover a very large area, more than 200,000 〖km〗^2. The bloom duration was usually about 11 days to 80 days. In other cases, they don’t have strong biogeochemical responses after MJO passing. In order to understand the difference between different cases and ocean pre-condition, we conducted series of ocean mixed layer numerical experiment. First, we use a Two Layer Reduced Gravity Model to obtain ocean’s subsurface pre-condition, as characterized by the depth of the 26 ℃ isotherm (D26). Using D26, we quantify the thickness of the subsurface warm layer. In the cases of strong biogeochemical responses, their D26 are less than 40 m, much shallow of the subsurface warm layer, which provides a more inductive condition to enhance vertical mixing and nutrient transport. Next, we use the Mellor-Yamada 1-D ocean mixed layer model to model the change of vertical profile after MJO’s passing. Finally, MJO’s biogeochemical effect on primary production increase is quantified using the Vertical Generalized Production Model (VGPM). In the study, all of inducing bloom cases which primary production reach between 286% ~ 382% and carbon fixation after MJOs passing is between 1.14×〖10〗^14 ~ 7.65×〖10〗^15 mg.