Seasonal variability of salinity budget and water exchange in the northern Indian Ocean from HYCOM assimilation

Based on HYbrid Coordinate Ocean Model (HYCOM) assimilation and observations, we analyzed seasonal variability of the salinity budget in the southeastern Arabian Sea (AS) and the southern part of the Bay of Bengal (BOB), as well as water exchange between the two basins. Results show that fresh water flux cannot explain salinity changes in salinity budget of both regions. Oceanic advection decreases salinity in the southeastern AS during the winter monsoon season and increases salinity in the southern BOB during the summer monsoon season. In winter, the Northeast Monsoon Current (NMC) carries fresher water from the BOB westward into the southern AS; this westward advection is confined to 4°-6°N and the upper 180 m south of the Indian peninsula. Part of the less saline water then turns northward, decreasing salinity in the southeastern AS. In summer, the Southwest Monsoon Current (SMC) advects high-salinity water from the AS eastward into the BOB, increasing salinity along its path. This eastward advection of high-salinity water south of the India Peninsula extends southward to 2°N, and the layer becomes shallower than in winter. In addition to the monsoon current, the salinity difference between the two basins is important for salinity advection.

Water exchange through the Kerama Gap estimated with a 25-year Pacific HYbrid Coordinate Ocean Model

Variations in water exchange through the Kerama Gap （between Okinawa Island and Miyakojima Island） from 1979 to 2003 were estimated with the 0.08° Pacific HYbrid Coordinate Ocean Model （HYCOM）. The model results show that the mean transport through the Kerama Gap （KGT） from the Pacific Ocean to the East China Sea （ECS） was 2.1 Sv, which agrees well with the observed mean KGT （2.0 Sv） for 2009- 2010. Over the time period examined, the monthly KGT varied from -10.9 Sv to 15.8 Sv and had a standard deviation of ＋5.0 Sv. The water mainly enters the ECS via the subsurface layer （300-500 m） along the northeastern slope of the Kerama Gap and mainly flows out of the ECS into the southwest of the Kerama Gap. The seasonal and interannual variations of the KGT and the Kuroshio upstream transport were negatively correlated. The Kuroshio upstream transport was largest in summer and smallest in auamm while the KGT was smallest in summer （1.02 Sv） and largest in spring （2.94 Sv） and autumn （2.44 Sv）. The seasonal and interarmual variations in the Kuroshio downstream （across the PN-line） transport differed significantly from the Kuroshio upstream transport but corresponded well with the KGT and the sum of the transport through the Kerama Gap and the Kuroshio upstream, which indicates that information about variation in the KGT is important for determining variation in the Kuroshio transport along the PN-line.

三种垂向混合方案对HYCOM模拟北太平洋西边界流系的影响研究

基于HYCOM(Hybrid Coordinate Oceanic Circulation Model),以OFES(OGCM for the Earth Simulator)资料为参考,分析了KPP、MY2.5、KT三种不同混合方案对北太平洋西边界流系的模拟结果的影响。结果表明:三种不同混合方案模拟的上层海洋平均流场与OFES资料相似,但在流向和流幅上略有差异,其中KPP方案模拟的流速与OFES资料最为接近,MY2.5方案次之,KT方案与其差别最大。通过代表性断面上流速的对比分析,对模式就北赤道流、棉兰老流、棉兰老潜流、黑潮的模拟效果进行比较,KPP方案模拟的效果同前人的观测和研究最为接近。分析了北赤道流,棉兰老流,棉兰老潜流,黑潮的流量的季节变化特征,其中KPP方案与OFES资料计算的棉兰老流和棉兰老潜流的季节变化特征与前人描述比较一致,表现为春强秋弱。KPP方案和OFES资料的计算结果表明,北赤道流和棉兰老流大致上是同向变化的,而在冬季棉兰老流同黑潮的变化基本上是一致的。

台湾东北黑潮入侵东海陆架强度的变化规律:持续一年的ADCP现场流速观测

本文利用台湾东北陆坡附近海域持续一年的多普勒流速剖面仪(Acoustic Doppler Current Profilers,ADCP)定点深水测流数据提取了观测站点处黑潮跨陆坡入侵强度的时间序列,并与美国海军混合坐标海洋模式(Hybrid Coordinate Ocean Model,HYCOM)分析数据中提取的观测站点处以及台湾东北陆坡东西区段黑潮入侵强度的时间序列进行了对比研究。研究结果显示黑潮在观测站点处的跨陆坡入侵强度存在显著的季节和季节内变化特征,并且与台湾东北黑潮主轴位置的摆动有较好的对应关系,黑潮主轴东(西)移,则黑潮在台湾东北陆坡西段及观测站点处的入侵显著减弱(增强),而在台湾东北陆坡东段的入侵显著增强(减弱)。此外,观测站点处黑潮跨陆坡入侵强度的时间序列还显示出较强的10d和20d左右的短周期信号,其中10d的周期信号对应着东海黑潮斜压不稳定波动的特征周期,而20d的周期信号则体现了台湾东北附近海域局地涡旋的短期变化。

西北太平洋及印度洋三维温盐流数值预报业务系统构建及预报检验

基于HYCOM海洋模式和集合最优插值(En OI)同化方法,开发了一套西北太平洋及印度洋三维温盐流数值预报业务系统(IAPos),2016年5月开始进行业务试验,并对业务试验的结果与原有的三维温盐流数值预报系统(wyl V2)进行对比检验,从检验结果看:在中国近海,IAPos系统的预报效果,较wyl V2有非常显著的改善,但对不同预报变量、不同垂直层、不同区域也有一些差异。整体来说,IAPos系统的预报性能较好。

HYCOM模式中水平分辨率对海温模拟影响的评估

基于HYCOM模式建立两组数值试验:低水平分辨率的全球模型(0.5°)和高分辨率的嵌套试验(嵌套0.1°分辨率的热带太平洋海域),评估水平分辨率对HYCOM模式在热带太平洋海域海温模拟能力的影响。结果表明:(1) HYCOM数值模拟结果除赤道中太平洋的小部分海域误差较大外,基本较好地模拟出热带太平洋海域海表温度的分布特征,并能较好反应西太暖池空间分布的季节变化。(2)提高水平分辨率使得HYCOM对热带太平洋SST的模拟有所改进,有效减少了HYCOM对西太平洋暖池区模拟偏暖的误差,但是提高水平分辨率后使得夏秋季节暖池西缩。(3)通过代表性断面上温度分布的对比分析,两组试验等温线的密集程度均比实测结果稀疏,高分辨率模型对跃层强度的模拟偏弱,这反映模式在提高分辨率的同时要有相应的物理过程(如混合等)与之相匹配。