Altimeter Observations of Inner Shelf Water Level Changes in Monsoon-dominated Regions

Coastal water level responds to winds as well as tide. Atmospheric cold fronts cause changes in water level along coastlines in the mid-latitudes, with direction and magnitude depending on the weather system orientation. Coastal regions in tropical regions also respond to seasonal winds, such as Monsoons. The summer Monsoon over East Asian Seas brings rainfall and winds form the south and southeast into the region. The winter Monsoon brings cold, dry conditions, associated with winds from the cold Asian mainland. Winds accompanying the East Asian Monsoons lead to seasonal water level changes in the region.

Eight years (July 2008 to July 2016) of Jason-2 20 Hz data from the L2 AVISO-PISTACH experimental products were processed to within 20 km of the coast at several locations bordering the East Asian Seas. Ten-day interval time series of the closest to shore points for each of the locations show seasonal and intra-seasonal variability. The inner shelf water levels follow the winds with a set-up of water level to the right of the wind direction and a set-down of water level to the left. The southwest Monsoon decreases water levels along the East Asian coast, while the northeast Monsoon increases water levels.

Monsoon associated water level signatures appear in the altimetry data from 9^o N off the Mekong Delta to 26^o N in the Straits of Taiwan. The sea level from the altimeter track north of the Straits of Taiwan off Wenzhou, China shows no evidence of Monsoon induced water levels variability. While rainfall associated with the summer monsoon reaches into Korea and Japan, the water level signature disappears north of the Strait of Taiwan. Water levels are variable during the summer and winter Monsoon periods. Recent investigations have noted that Intra-seasonal Oscillations related to systems moving through a region. The altimeter-derived sea level variability observed during each Monsoon season may be related.