Trends and patterns of SST and associated frontal frequency in the Vietnamese upwelling center

Ocean fronts are areas of increased primary productivity susceptible to climate induced ocean warming. Quantifying the spatial and temporal variability of thermal fronts in rapidly warming parts of the ocean is needed. Frontal frequency (FF) in SST imagery was examined in the coastal upwelling region of South-Central Vietnam, a known marine hotspot. Level-1 daily MODIS data between 2003 and 2018 was used to generate level-3 mapped SST products. The Cayula and Cornillon single image edge detection (SIED) algorithm was used to extract fronts from the SST imagery. Two types of fronts were found in the area, both strongly influenced by seasonal monsoons. Upwelling fronts were found along the southeast coast of Vietnam and extending eastward from the upwelling center during the SW monsoon. Shelf fronts, running along the 100 m – 200 m isobath, separate the cold-water current from the north with the open ocean during the NE monsoon. An upward trend of SST was recorded at approximately 0.05 °C/year throughout the 16-year period (p < 0.05), with rates of increase being higher during the Southwest (SW) monsoon period (May-September). The two longest marine heatwaves (MHW) occurred during El Niño years, nine days in 2010 and 16 days in 2016 with highest maximum intensity of 2.95 °C. The results show that FF trend is increasing significantly in region with shelf fronts and that this increase is associated with a positive SST trend. Additionally, El Niño/La Niña is associated with variability of fronts in this small-scale marine hotspot over the last 16 years.