The robust salinity anomaly event during 2015−2017 in the tropical Pacific Ocean
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摘要: 本文利用Argo海水盐度资料、海流同化数据和同期大气再分析数据，探讨热带太平洋盐度趋势变化和相关动力过程。Argo资料显示，2015−2017年热带太平洋出现显著的盐度异常(SAE)，这是改变长期趋势的主要原因，表现为表层显著淡化和次表层咸化特征。这种盐度异常具有明显的区域性特征和垂直结构的差异，体现在热带太平洋北部海区(NTP)和南太平洋辐合区(SPCZ)表层淡化，盐度最大变幅为0.71～0.92，淡化可以达到混合层底；热带太平洋南部海区(STP)次表层咸化，最大变幅为0.46，主要发生在温跃层附近，期间盐度异常沿着等位密面从西向东扩展。平流和挟卷是与SAE密切相关的海洋动力过程，两者在NTP淡化海域有着持续而较为显著的影响，在SPCZ淡化、STP咸化海域后期贡献也较大，其中盐度平流对热带太平洋海区盐度变化起主要贡献。NTP淡化海区表层淡水通量和STP咸化海区密度补偿引起的混合也是SAE的重要影响因素。Abstract: Salinity linear trends and their contributed dynamics processes in the tropical Pacific Ocean were investigated with the Argo salinity and temperature fields, current assimilations and atmospheric reanalysis datasets. The robust salinity anomaly event (SAE) occurred in the tropical Pacific Ocean during 2015−2017, which resulted possibly in the reversing of Argo salinity long-term trends. Such SAE was characterized as distinct regional discrepancies and vertical structures. Significant surface freshening occurred in the northern tropical Pacific (NTP) and Southern Ocean Convergence Zone (SPCZ), whose remarkable freshening maximum could reach 0.71−0.92 and covered the upper mixed layer. Another subsurface salinification in the southern tropical Pacific (STP) with the maximum of 0.46 was found around thermocline layer. Moreover, the salinity anomalies in SAE can expand from west to east along the isopycnal layers. Salinity advection and entrainment were exhibited as the fundamental dynamic processes to the SAE event in the tropical Pacific Ocean, while advection term acted as the major contribution to salinity variability. Both of the dynamic factors played important role on SAE features during the whole event in the NTP and later period in the SPCZ and STP. The surface freshwater flux and subsurface mixing induce by density compensation were also supplements to the SAE events in the NTP freshening and STP salinification respectively.
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图 1 2004−2014年（a，c）和2004−2017年（b，d）热带太平洋表层（a，b）和次表层（100 m）（c，d）的盐度线性变化趋势
图中黑色点覆盖区域表示盐度线性趋势通过95%的显著性检验
Fig. 1 Surface salinity (a, b) and subsurface (100 m) salinity (c, d) linear trends during 2004−2014 (a, c) and 2004−2017 (b, d) in the tropical Pacific Ocean
The regions shaded in black represented the significant area at the 95% confidence level
图 2 热带太平洋上层盐度的线性趋势之差（a），及其显著区两个时段盐度趋势的垂直结构（b）
Fig. 2 The salinity trend differences of the two periods (a) and salinity trends vertical structure of the three regions in the upper tropical Pacific Ocean (b)
图 3 热带太平洋NTP，SPCZ和STP海区的盐度异常低频变化（13个月低通滤波）
Fig. 3 The 13-month lowpass filtered salinity variabilities of three regions (NTP, SPCZ and STP) in the tropical Pacific Ocean
图 4 2004−2014年（a, d）和2004−2017年（b, e）热带太平洋上层的盐度趋势及温跃层深度变化（c, f）
图中黑色点表示盐度线性趋势通过了95%显著性检验
Fig. 4 Vertical structure of the upper salinity trends （a−e） and thermocline depth trends（c, f） during the 2004−2014 (a, d) and 2004−2017 (b, e) periods in 14°N and 8°S sections
The regions significant at the 95% confidence level are hatched in black
图 5 14°N (a−e)和8°S (f−j) 纬度带上2015−2017年盐度异常事件的垂直结构演变
等值线为等位密面，其中黑线分别代表24σθ，22.5σθ和25.3σθ等位密面
Fig. 5 The evolution of salinity anomaly vertical structure during 2015−2017 in 14°N (a−e) and 8°S (f−j) sections
The contour lines indicate potential isopycnal layers. Black lines represent the 24σθ, 22.5σθ and 25.3σθ，respectively
图 6 14°N和8°S纬度带24σθ和25.3σθ等位密面上的盐度异常时空变化
Fig. 6 The salinity anomaly spatiotemporal variability on 24σθ and 25.3σθ potential isopycnal layers respectively in the 14°N and 8°S sections
图 7 NTP淡化海区平流和挟卷过程的贡献（a）、局地（10°～20°N，100°～140°W）表层盐度、淡水通量、Ekman输运速度异常的低频变化（b）
Fig. 7 Salinity advection and entrainment fractions (a), low frequency variability of surface salinity, freshwater flux and Ekman transport velocity in NTP freshening region (10°～20°N，100°～140°W)（b）
图 8 SPCZ淡化海域平流和挟卷的贡献（a），局地（5°～10°S，170°E～180°）表层盐度、淡水通量、Ekman输运速度异常的低频变化（b）
Fig. 8 Salinity advection and entrainment fractions (a), low frequency variability of surface salinity, freshwater flux and Ekman transport velocity in SPCZ freshening region (5°～10°S，170°E～180°) (b)
图 9 咸化海区(STP)平流和挟卷的动力贡献
Fig. 9 The contribution of salinity advection and entrainment terms to the STP salinification SAE
图 10 2015年6–8月STP海区盐度垂向梯度
黑色等值线分别代表混合层、24σθ和25.3σθ等位密面的所在深度；绿线指示了温度垂向梯度；若垂向梯度值大于0，表示随着深度增加分别对应着盐度增加和温度升高
Fig. 10 Salinity vertical gradients in STP from June to August, 2015
The black contours indicate the depth of the mix layer, 24σθ and 25.3σθ isopycnal. Green contours indicate the temperature vertical gradient. If the vertical gradient >0, it means that the salinity and temperature increases with the depth respectively
表 1 热带太平洋3个海区在两个时间段的盐度变化特征
Tab. 1 The salinity variability in the tropical Pacific Ocean three key regions during the two periods
NTP SPCZ STP（100 m） 2004−2014年 2004−2017年 2004−2014年 2004−2017年 2004−2014年 2004−2017年 平均盐度 34.4 34.3 34.9 34.8 36.0 36.1 趋势/psu·（10 a）−1 0.26 −0.08 0.33 0.02 −0.21 0.03 趋势差异*/psu·（10 a）−1 −0.34 −0.31 0.24 盐度变幅** 0.71 0.92 0.46 注：*表示2004−2017年盐度趋势与2004−2014年盐度趋势之差；**表示2004−2017年各海区盐度低频变化的幅度，详见图3。 
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