The onshore intrusion of Kuroshio subsurface water from February to July and a mechanism for the intrusion variation

To examine the variation of Kuroshio subsurface water (KSSW) off the coast of Zhejiang province, China, we carried out six monthly cruises from February to July 2015. Observations show that the KSSW appears in a frequently reported algal bloom area near the coast as early as in April and persists until July. However, the KSSW was absent in February. We set up a fine-resolution (4-km) model, which can reproduce most of the observed features. Model results and cruise data suggest that a nearshore Kuroshio branch current (NKBC) originated from the KSSW northeast of Taiwan can emerge as early as in April, which is responsible for the observed KSSW from April to July. However, in February NKBC turns seaward before reaching this algal bloom area, and thus the KSSW is absent. In addition to the traditional mechanisms of wind/bottom stress and cooling, we suggest a new mechanism of this NKBC variation. In the origin area of the NKBC, the KSSW is more shoreward squeezed and flows more westward (flowing northward in February while northwestward in April) in April than February. The change of flow direction caused different meeting point between the shelf break and the Kuroshio Current. According to topographic beta spiral, different meeting points will induce different upwelling velocity and then contribute to the different intrusion pattern of NKBC. This direction change is one of the essential ingredients which enable NKBC to reach as far as the algal bloom area. Hence, algal blooms in April may be potentially influenced by this NKBC, whose water properties are significantly different to the coastal water.