北太平洋副热带模态水盐度的年代际变化

Decadal Variability of North Pacific Subtropical Mode Water Salinity

副热带模态水(Subtropical Mode Water;STMW)在气候变化中起着重要作用。本文利用全球高分辨率数值模拟结果,研究了北太平洋STMW核心层盐度(Core Layer Salinity;CLS)的年代际变化及其物理机制。结果表明,CLS存在显著的年代际变化,其空间分布则与背景流场分布特征有关。侵蚀区CLS滞后生成区CLS约1~2年,这主要是海流平流输运引起的。生成区内,STMW的季节循环一般可分为生成期(12-4月)、隔离期(5-6月)和侵蚀期(7-11月),生成期混合层盐度(Mixed Layer Salinity;MLS)决定着隔离期和侵蚀期的CLS,而MLS年代际变化则主要由同太平洋年代际涛动存在负相关性的海表面淡水通量的变化引起。

The Subtropical Mode Water(STMW) can carry the signal of the upper mid-latitude ocean into the permanent thermocline through subduction, and can be spread to the lower latitude ocean by the background currents. Therefore, STMW play an important role in climate changes and ocean ecosystem. The decadal variations of the North Pacific STMW core layer salinity(CLS) and the responsible physical mechanism are examined base on the global high resolution numerical simulation data(OFES) and various global precipitation data sets. The main results are as follows:The climatological CLS decreases gradually from 28°N to the STMW boundary along longitude with S-N symmetrical zoning. Zonally, the CLS gradually decreases from 120°E to 165°E, yet it increases from 165°E to 180°E. The long-term averaged spatial distributions of CLS and the sea surface height are similar, associated to the background currents. The CLS shows significant annual cycle and decadal variability during 1950-2015. It has minima around 1960, 1977, 2005, while maxima around 1956, 1969, 1996 and 2010. The value of CLS in the erosion region is similar to that in the formation region, with 1~2 years lag, due to the lateral advection related to the ocean currents.Based on the temporal characteristics, the general annual cycle of STMW in the formation region can be classified as three phases:(1) Formation period during December-April;(2) Isolation period during May-June(3) Erosion period during July-November. In the formation region, the CLS during isolation period acquires the latest STMW salinity, determined by the mixed layer salinity(MLS) during formation, and then preserved by the STMW during erosion until the new STMW formed at the next annual cycle. The decadal variations of MLS during formation and the whole year are similar, with a correlation coefficient of 0.91 after 8-yr low-pass filtering. The analysis of MLS budget indicates the decadal variability of MLS in the formation region is mainly induced by the changes of the precipitation, negatively correlated to PDO: during 1977-1998(1996-2015), the PDO is positive(negative), the precipitation in the formation region decreases(increases), leading MLS and CLS to increase(decrease). The entrainment progress arises from the difference of salinity between the MLD and 20 m below the MLD mainly. The absolute value of this difference increased after 1980 s, indicating that the enhanced subsurface stratification is associated with the change of MLS and CLS.