北太平洋副热带西部模态水年代际变化特征及其机制分析

DECADAL VARIATION OF SUBTROPICAL MODE WATER IN THE NORTH PACIFIC OCEAN AND ITS MECHANISM

采用来自大洋环流模式ECCO2(the estimating the circulation and climate of the ocean,phase II project)的再分析数据对1992-2019年北太平洋副热带西部模态水(subtropical mode water,STMW)的年代际变化特征及机制进行了分析。结果表明:STMW形成体积具有显著的年代际变化,于1992-1997年、2000-2005年和2011-2017年期间为正异常,而于1998-1999年和2006-2010年期间为负异常,由晚冬生成区混合层体积的年代际变化引起。STMW形成厚度和面积均呈现类似的年代际变化。合成分析表明,STMW形成体积正异常期间,黑潮延伸体上游南侧STMW生成区,海表涡动能相对负异常期间减小,同时预先层结相对负异常期间减弱,并伴随着海表高度异常。通过混合层收支分析发现,混合层形成体积年代际变化与海洋预先层结调控的混合层底卷吸作用变化同步且大小相当,而与海气形成率变化无关。增强(减弱)的海洋预先层结通过调控STMW形成区冬季混合层底卷吸过程,阻碍(促进)冬季混合层加深,最终使得STMW形成体积减少(增加)。进一步分析表明,STMW形成体积年代际变化受与太平洋年代际涛动相关的风应力旋度异常的远场调控。

In recent years,many researches have been carried out on the decadal variation mechanism of subtropical mode water(STMW)in the northwest Pacific.However,the specific dynamic process of STMW is still unclear.Reanalysis data from particular simulation Cube92 of the Estimating the Circulation and Climate of the Ocean,Phase II Project(ECCO2)were used to analyze decadal variation of STMW and its mechanism from 1992 to 2019.As one of the results,STMW formation volume shows significant decadal variation,and can be divided into positive abnormal periods(1992-1997,2000-2005,2011-2017)and negative abnormal periods(1998-1999,2006-2010),which can be fully reflected in the late winter mixing layer in the formation region(133°-170°E,30°-36°N).Meanwhile,thickness and area of STMW formation have the similar decadal variations.After composite analysis,we show that during the period of positive anomaly of STMW formation volume,there are remarkable negative anomalous characteristics of ocean pre-stratification and sea surface eddy kinetic energy(EKE)in the south area of KE upstream(132°-147°E,30°-35°N)where STMW thickness or MLD significantly changed and followed by sea level elevation.Moreover,a quantitative analysis of the mixing layer formation volume is mainly conducted in this study and the results reveal that synchronous and sizable decadal variation exists between mixing layer formation volume and mixing layer bottom entrainment,which is regulated by ocean pre-stratification and has no relation with air-sea formation rate variation.Enhanced(weak)pre-stratification dramatically hinders(facilitates)the mixing layer deepening in late winter by adjusting the entrainment process of the winter mixing layer bottom in the STMW formation region,so as to reduce(increase)STMW formation volume,area,and thickness.Subsequently,further analysis showed that the decadal signal is regulated by Pacific decadal oscillation(PDO)related wind stress curl anomaly in the northeast Pacific(130°-162°W,20°-50°N)remotely.