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

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 Ni?o）事件和2014/2015年特殊El Ni?o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 Ni?o和南方涛动（El Ni?o–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 Ni?o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 Ni?o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 Ni?o事件，2014/2015年El Ni?o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。

Contrasting Salinity Interannual Variations in the Tropical Pacific and Their Effects on Recent El Niño Events: 1997/1998, 2014/2015, and 2015/2016

Ocean salinity variation provides a new insight into related ENSO (El Ni?o–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 Ni?o events, 1997/1998 and 2015/2016, and one special El Ni?o, 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 Ni?o 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 Ni?o 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 Ni?o 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.