Variation of the coastal upwelling off South Java and their impact on local fishery resources

There is a vast upwelling area induced by the southeast monsoon in the waters off South Java, making the region an important fishing ground. Climate events can affect the variation of upwelling, but oceanographers have different understandings on the extent to which climate events control upwelling in this area, which leads to a lack of basis for studies on the evaluation and mechanisms of the variability of fishery resources in the region. The correlation between environmental parameters, including surface temperature (SST), chlorophyll- a (Chl- a ) concentration, and climate event indices in South Java from 2003 to 2020 was analyzed. Results show that the Indian Ocean Dipole (IOD) has a greater influence on the interannual variability of upwelling intensity than ENSO. During the IOD, variations in equatorial latitudinal winds excite different types of Kelvin waves that anomalously deepen or shallow the thermocline, which is the main cause of anomalous variations in upwelling, independent of variations in the local wind field. A correlation between the interannual variability in upwelling and the annual catches was revealed, showing that climatic events indirectly affect fishery resources through upwelling effects. During positive IOD/El Niño periods, strong upwelling delivers more nutrients to the surface layer, which favors fish growth and reproduction, resulting in higher annual catches. A negative IOD/La Niña, on the other hand, leads to weaker upwelling and fewer nutrients into the surface waters. Fish tend to move in deeper waters, making traditional fishing methods less efficient and consequently lower annual catches.