热带太平洋盐度年际变化对海表温度异常作用比较：1997/1998、2014/2015和2015/2016年El Niño事件
doi: 10.3878/j.issn.1006-9895.1912.19172
Contrasting Salinity Interannual Variations in the Tropical Pacific and Their Effects on Recent El Niño Events: 1997/1998, 2014/2015, and 2015/2016
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摘要: 海洋盐度变化为研究气候变化的机制提供了一个新的视角。本文通过对比1997/1998年、2015/2016年两次强厄尔尼诺（El Niño）事件和2014/2015年特殊El Niño事件，对盐度变化及其影响海表面温度异常（SSTA）的物理过程进行了比较分析。研究表明，El Niño和南方涛动（El Niño–Southern Oscillation, ENSO）发展的强弱与热带西太平洋大范围海表层盐度异常（SSSA）及其向东扩散的差异有明显关联。1997/1998、2015/2016年赤道东太平洋SSTA的增暖，对应两次强El Niño事件，在发生年4月，中西太平洋海域出现了明显的负SSSA，之后东移至日期变更线以西，SSSA引发的混合层深度（MLD）变浅、障碍层厚度（BLT）变厚，导致热带中—西太平洋表层升温增强，促使了赤道中太平洋的早期变暖；2014/2015年弱El Niño事件虽然在发生年4月，位于赤道中西太平洋出现了负SSSA，但没有发展东移，导致BLT的增厚过程减弱，对表层温度的调制作用减弱甚至消失。三次事件对应的盐度变化过程中，水平平流和淡水通量（FWF）引起的表层强迫是影响盐度收支的主要因子，水平平流影响盐度异常的前期变化，触发事件的发生；热带太平洋西部降水引起的FWF负异常的影响最为显著，对ENSO异常信号出现后SSSA的维持起决定性作用。相比较两次强El Niño事件，2014/2015年El Niño对应的早期FWF负异常没有发展和东移，并且之后迅速减弱，导致中西太平洋盐度负趋势减缓，MLD加深，BLT变薄，促使上表层海水冷却，抑制了赤道东太平洋的早期变暖和ENSO发展。研究结果表明，盐度变化与ENSO密切相关，热带中西太平洋海域早期表层盐度变化可能可以作为SSTA的指数。特别地，SSSA在调节SSTA时，不仅影响它的强度，而且可以作为判断ENSO是否发展及其强弱的前兆因子。Abstract: Ocean salinity variation provides a new insight into related ENSO (El Niño–Southern Oscillation) expressed by climate variability. In this study, salinity variations and their related dynamic processes responsible for SSTA (sea surface temperature anomaly) were extensively compared and analyzed considering two strong El Niño events, 1997/1998 and 2015/2016, and one special El Niño, 2014/2015. The study shows that the development of ENSO is significantly associated with the occurrence and eastward diffusion of large-scale SSSA (sea surface salinity anomaly) in the western tropical Pacific. In April 1997 and 2015, corresponding to two strong El Niño events, there was a significant negative SSSA in the western–central Pacific. The anomaly moved eastward to the west of the dateline, which induced a shallower MLD (mixing layer depth), and a thicker BLT (barrier layer thickness), which enhanced the surface warming in the tropical central Pacific and the early warming in the equatorial eastern–central Pacific. Although a negative SSSA occurred in the April 2014/2015 weak event in the equatorial western–central Pacific, it did not develop eastward, resulting in a weakened thickening process of the BLT and a weak modulation effect on surface temperature. For the salinity change process corresponding to three El Niño events, surface advection and surface forcing caused by FWF (freshwater flux) were the major contributors to the salinity budget. Surface advection influenced the former variability of salinity tendency, inducing the occurrence of an ENSO signal. The precipitation in the tropical western Pacific had the most significant negative influence on FWF, which played a decisive role in the SSSA occurrence and ENSO development. Compared with the two strong El Niño events, the early FWF negative anomaly in 2014/2015 did not develop, did not move eastward, and weakened rapidly; this resulted in the slowing down of the negative salinity tendency in the western–central Pacific, deepening of the MLD, thinning of the BLT, and rapid cooling of the surface layer, which inhibited early warming in the equatorial eastern Pacific. The results of this study demonstrate that the salinity change was closely related to ENSO, and early SSS in the tropical western–central Pacific could be used as an index of SSTA. In particular, SSSA not only affects the strength of SSTA in oceans, it can also be used as a precursor to judge the development and strength of ENSO.
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Key words:
- Salinity anomaly /
- Interannual variability /
- Strong El Niño /
- Tropical Pacific
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图 1 El Niño期间赤道（2°S～2°N）平均海表面温度异常（单位：°C）的时间—经度图：（a）1997/1998年；（b）2014/2015年；（c）2015/2016年。黑色虚线表示日期变更线
Figure 1. Longitude–time sections of SST (sea surface temperature) anomalies (units: °C) averaged in the equator (2°S–2°N) during El Niño events for (a) 1997/1998, (b) 2014/2015, and (c) 2015/2016. The black dashed lines represent the location of the international dateline
图 3 1997/1998年（黑色线）、2014/2015年（红色线）、2015/2016年（蓝色线）三次El Niño期间（a）Niño4区、（b）Niño3.4区、（c）Niño3区区域平均海表面盐度异常（单位：psu）的季节变化。黑色水平线为零线，红色虚线为年分界线
Figure 3. Seasonal variability of SSS anomalies during El Niño events for 1997/1998 (black lines), 2014/2015 (red lines), and 2015/2016 (blue lines) averaged over (a) Niño4 box, (b) Niño3.4 box, and (c) Niño3 box. The horizontal black lines represent zero, and the red dashed lines separate the years
图 6 El Niño期间盐度和温度影响下的赤道（2°S～2°N）平均障碍层厚度异常（单位：m）的时间—经度分布：（a）1997/1998年；（b）2014/2015年；（c）2015/2016年。左图表示盐度年际变化和温度年际变化共同作用下的障碍层厚度变化，记为BLT（Tinter, Sinter），中图表示仅盐度年际变化影响的障碍层厚度变化，记为BLT（Tclim, Sinter），右图表示仅受温度年际变化影响的障碍层变化，记为BLT（Tinter, Sclim）。黑实线表示日期变更线
Figure 6. Longitude–time sections of BLT anomalies (units: m) averaged in the equator (2°S–2°N) under the influence of salinity and temperature during El Niño events in (a) 1997/1998, (b) 2014/2015, and (c) 2015/2016. The results of left figures were obtained from interannual temperature and interannual salinity fields [BLT (Tinter, Sinter)], the results of middle figures were obtained from interannual temperature and climatological salinity fields [BLT (Tclim, Sinter)], and the results of right figures were obtained from interannual salinity and climatological temperature fields [BLT (Tinter, Sclim)]. The black lines represent the location of the international dateline
图 8 1997/1998年（黑色线）、2014/2015年（红色线）、2015/2016年（蓝色线）三次El Niño期间Niño4区区域平均（a）混合层盐度趋势及其收支项贡献：（b）表层平流、（c）次表层强迫、（d）表层强迫。黑色水平横线为零线，红色虚线为年分界线
Figure 8. (a) MLS tendency and contribution of MLS budget terms (b) surface advection, (c) subsurface force, and (d) surface force during El Niño events for 1997–1998 (black lines), 2014–2015 (red lines) and 2015–2016 (blue lines) averaged over the Niño4 region. The black line represents zero, and the red dashed lines separate the years
表 1 由温度场和盐度场计算BLT变化的方法，温度和盐度可以被认为是气候变化或年际变化的
Table 1. Methods of BLT (Barrier Layer Thickness) variability calculated by temperature and salinity fields, which can be considered to be interannual or climatologically varying
分解计算BLT 温度和盐度的气候态和年际变化选取情况 BLT (Tclim, Sclim) 气候态温度和气候态盐度 BLT (Tinter, Sinter) 温度的年际变化和盐度的年际变化 BLT (Tinter, Sclim) 温度的年际变化和气候态盐度 BLT (Tclim, Sinter) 气候态温度和盐度的年际变化 
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