Recently, the research team led by Research Fellow Hongzhou Xu from the Laboratory of Ocean Circulation Observing and Modeling (LOCOM) published a research paper titled “Dynamics of the summer counter-wind current along south Sri Lanka coast 1: the dominant role of intra-seasonal variability” in the international journal “Journal of Geophysical Research: Oceans”. Based on Argo float and CMEMS reanalysis data, this research verified the existence of abnormal westward south Sri Lanka coastal current (SSLCC) during summer, and revealed the dynamics mechanism driving this counter-wind current. This research finding has important significance for understanding the circulation around Sri Lanka. Dr. Hongyu Xin is the first author of the paper, Research Fellow Weiqiang Wang from South China Sea Institute of Oceanography is the corresponding author, Research Fellow Qiang Xie, Professor Weiqing Han from University of Colorado, Associate Researcher Ke Huang and Research Fellow Kang Xu from South China Sea Institute of Oceanography are the collaborating author.

Climatologically, the SSLCC flows eastward as a part of the southwest monsoon current during summer. However, westward SSLCC occurred lasting more than 20 days in the summer of 2013, 2016, 2017, and 2018 based on reanalysis data, implying significant interannual variability of the SSLCC. The wavelet power spectrum indicates that the SSLCC has significant annual (>240 days), semi-annual (120-240 days), and intra-seasonal (30-105 days) component signal (Fig. 1). The analysis on the summer extreme westward SSLCC further indicates that the intra-seasonal wind associated with atmospheric boreal summer intra-seasonal oscillation (BSISO) is the main factor leading to the westward SSLCC (Fig. 2). Firstly, the northward propagation of the BSISO induces the westerly wind anomaly and positive wind stress curl anomaly along the south and east coast of Sri Lanka, which induces the westward SSLCC. Secondly, driven by equatorial Indian Ocean intra-seasonal wind, the low sea level anomaly associated with upwelling Rossby wave reflected from the west coast of Sumatra Island propagates westward. This propagation significantly influences the cyclonic circulation off the south coast of Sri Lanka, thus facilitating the occurrence of the westward SSLCC.

Figure 1. Wavelet power spectrum of the SSLCC. The white dashed curve marks the cone of influence (COI). The blue contours and red dash line indicate the 95% confidence for a red-noise process. The horizontal dashed black lines mark the 30-, 80-, 105-, 180-, and 360-day periods.

Figure 2. Schematic showing the mechanisms by which the intra-seasonal component modulates the westward SSLCC and circulation around Sri Lanka during summer. ISO, div represent the intra-seasonal oscillation and divergence, respectively.

This research was jointly funded by the National Key R&D Program of China, the National Natural Science Foundation of China, the Guangdong Basic and Applied Basic Research Foundation, the Youth Innovation Promotion Association CAS, the Rising Star Foundation of the South China Sea Institute of Oceanology, the special fund of South China Sea Institute of Oceanology of the Chinese Academy of Sciences. We thank the China-Sri Lanka Joint Center for Education and Research (CSL-CER), Chinese Academy of Science. We deeply cherish the memory of Professor Qiang Xie and thank him for his contribution to this work.

References: Xin Hongyu, Wang Weiqiang*, Xie Qiang, Han Weiqing, Huang Ke, Xu Kang, et al. (2024). Dynamics of the summer counter-wind current along south Sri Lanka coast 1: the dominant role of intra-seasonal variability. Journal of Geophysical Research: Oceans, 129, e2023JC020618.