Study of Ocean Climate Variability (1959-2002) in the Eastern Indian Ocean, Java Sea and Sunda Strait Using the HAMburg Shelf Ocean Model

Abstract

The Indonesian waters, located in the tropical area between the Pacific and Indian Oceans, comprise shelf and deep sea areas with their special characteristics. The Java Sea, as one of the shallow waters areas in Indonesia, has an average depth of about 40m, low salinity and high temperature. South of Java is the deep eastern Indian Ocean which is characterized by high salinity and low temperature. Both water masses are interacting and mixing through the Sunda Strait. Although the Sunda Strait is a small passage, its role in influencing the ocean climate variability is quite important and will be discussed as the main topic in this study. The HAMburg Shelf OceanModel (HAMSOM) is used to simulate the long period ocean-climate dynamics in the Java Sea and the eastern Indian Ocean. Numerous simulations and boundary treatments are performed to get results that best agree with the observations. Forcing data from 44 years (1959-2002) NCEP reanalysis are used in the simulation to analyze the variability of current circulation, water mass transport, upwelling south of Java, and the effect of ENSO (in the Pacific Ocean) and DME (in the Indian Ocean) to this area, as well as their interaction. The seasonal variation in the Java Sea is mainly influenced by the monsoon. In January, representing the NW monsoon situation, water from the Southeast China Sea is transported to the Java Sea by (+)2.1Sv and flows out to the eastern part of the Java Sea and the Sunda Strait by (-)1.6Sv and (-)0.5Sv, respectively. In the contrary, in August (representing the SE monsoon situation), the total inflow to the Java Sea from the eastern part of the Java Sea is 1.0Sv and the water flows out to the Southeast China Sea and the Sunda Strait by (-)0.3Sv and (-)0.7Sv respectively. The SST is increased from 28°C during the NW monsoon to 28.5-29°C during the SE monsoon while the SSS is decreased from 32.6psu to 32.2psu. During the whole year, the water mass is transported from the Java Sea to the Indian Ocean through the Sunda Strait with variations of transport between 0.48Sv (minimum) in December and 0.72Sv (maximum) in August/September. The zonal wind during the SE monsoon is the important factor for triggering and propagating the upwelling south of Java. Normally, the upwelling causes the upwards of temperature lower than 24°C (the peak in August) and salinity higher than 34.4psu (the peak in October). The DME and ENSO have influence significantly the variability in the study area. Nevertheless, the strengths of their influence are controlled by the strength of the zonal wind in the eastern Indian Ocean, especially during the SE monsoon. The temperature decreases (increases) during the El Nino (La Nina) years when the zonal wind is stronger (weaker) and reversely the salinity increases (decreases) during the El Nino (La Nina). The strong influence of the DME and ENSO can be identified by the SST/SSS anomaly in the study area when they have the same phase as the strengthening zonal wind. Likewise, the strength of upwelling south of Java depends on the strength of SE wind along the south of Java and its interaction with DME and ENSO events. Under normal conditions, the upwelling causes negative SST and positive SSS anomalies 0.3-0.6°C and 0.2-0.3psu, respectively. In the El Nino (La Nina) years, the anomaly of SST and SSS become lower than -1.0°C and higher than 0.3psu (+1.0°C and -0.3psu), respectively. During the normal years, the variation of SST and SSS anomalies in the Java Sea are about 0.5°C and 0.2psu, respectively. The SST decrease (SSS increase) during the DME and El Nino events because of the transport of water mass with cooler SST (higher SSS) from the eastern Java Sea, while during the La Nina events, the SST increase (SSS decrease) because of the transport of water mass with warmer SST (lower SSS) from the eastern Java Sea. The water mass is still transported through the Sunda Strait from the Java Sea, increased by 0.15-0.2Sv during the El Nino and DME years and decreased by 0.1Sv during the La Nina years.