風場數據可靠性及變異性對東海二氧化碳海氣交換通量估算的影響

Abstract

近岸海域位於開放性大洋和陸地的緩衝地帶，其對大氣CO2的吸收與釋放，受到環境與人為活動之影響，其碳通量變化在全球碳循環中扮演相當重要的角色。過往東海CO2通量之研究，受限於時間、經費和人力等因素，其研究區域均不能完整覆蓋東海，各研究之觀測結果是否能代表整個東海？。本研究利用東海pCO2經驗模式(Tseng et al. 2014)並結合衛星遙測表水溫(SST)、葉綠素a (Chl-a)和風速之資料，與現場資料進行評估，佐證經驗模式的可靠性與代表性。進一步進行了衛星風場資料的校正，顯示遙測風速偏高於實測風速(實測風速=0.8×遙測風速)，重新評估了東海之CO2碳量，東海全年平均碳通量約為 -1.1 mol C m-2 y-1，各季節之通量，春(3至5月)、夏(6至8月)、秋(9至11月)、冬(12至2月)分別為 -2.1、-0.3、-0.2、-1.9 mol C m-2 y-1。顯示東海是碳的儲存區(sink)，春季碳之吸收量最大，至夏季因溫度上升導致吸收能力減弱，而在夏末秋初(約在8至10月)，由匯(sink)轉源(source)，秋末又由source轉為sink，至冬季sink增強。相較於前人之結果碳通量少了近一半，顯示風場之空間分布對CO2通量是有影響的，特別是在一些劇烈氣候下，使風速之空間梯度越顯著，對CO2通量影響顯著。

To accurately investigate air-sea CO2 exchange in the coastal waters, especially for the East China Sea (ECS), is challenging because of the environment complexities and diversity of the shelf seas, easily affected by human activities and climate changes. Reliable assessments of air-sea CO2 exchange fluxes in the ECS are additionally limited by inadequately spatiotemporal coverage and shortage of manpower resources. Here, we explore seasonally representative CO2 uptakes by the whole ECS by combining the remote sensing data and field observations. We firstly evaluated the results of Tseng et al. (2014) and further demonstrated the reliability and representativeness of Tseng’s empirical algorithm for computing pCO2 by using remote sensing data including SST, Chlorophyll a (Chl-a) and wind speed. Secondly, we demonstrated the satellite wind speed data are higher than those collected in land weather station (field wind speed = 0.8 × satellite wind speed), in order to re-evaluate the CO2 fluxes in the ECS. The average annual flux between 2003 and 2010 was constrained to -1.1 mol C m-2 y-1 as a net sink of atmospheric CO2 with the seasonal mean fluxes of -2.1 (Mar.-May), -0.3 (June-Aug.), -0.2 (Sep.-Nov.) and -1.9(Dec.-Jan.), respectively. The flux seasonality showed a strong sink in spring and winter, a sink-to-source transition during late summer – mid-fall period and a source-to-sink transition in late fall. Finally, the annual mean CO2 flux estimated in this study was nearly one half of those reported previously, indicating the importance of wind effect regarding spatial variability and reliability of wind field. Especially in some severe weather events, the more spatial gradients of wind speed would make more significant impact on the air-sea exchange flux of CO2 in continental margins.