Transformation and Mixing of Water Masses in Alor Strait in July 2011

Abstract

Alor Strait is the deepest channel in Alor islands after Ombai Strait. Contribution of the strait as one of the secondary exit passages of Indonesian Throughflow (ITF) has not been studied yet. The strait separates Flores Sea and Sawu Sea, and is featured by the existence of high sill within the strait, suggested that turbulence due to interaction between strait flow and bottom topography could drive mixing and then modify the water mass properties. The purpose of this study is to investigate transformation of ITF water mass and turbulent mixing process with Thorpe scale and Richardson number (Ri) profiles. A hydrographic survey has been carried out in July 2011, in which 15 CTD casts were lowered in the strait, together with hull-mounted 75 kHz ADCP and 12 kHz EA500 single beam echosounder. The results show that Alor sill depth is about 300 meters in the main gate. Snapshot estimate of along strait transport volume is 1,07 Sv (1 Sv = 106 m3s-1), feeding Sawu Sea with North Pacific water origin e.g. NPSW in which maximum salinity of NPSW is significantly reduced due to strong mixing, perhaps driven by bottom topography and strait flow which creates turbulence. Percentage of NPSW’s maximum salinity traced in the central part of the strait is 32.1 % but it varies from 2,9 to 21,0% in southern part of Alor Strait (i.e. Sawu Sea). NISW (Northern Indian Subtropical Water) with maximum salinity layer at = 23,5-24,5 is dominant in the southern part of Alor Strait (i.e. Sawu Sea). The existence of NIIW (North Indian Intermediate Water) is also found in the deeper layer of Sawu Sea. The average value of vertical eddy diffussivity (K ) estimate in the thermocline layer and deep layer in northern part and central part of strait channel is within the order of 10-3 m2 s-1. Lower order of K in the thermocline layer and deep layer were found in southern part of the Strait (Sawu Sea), ranging from 10-6 to 10-4 m2 s-1. These indicates that the existence of sills in the northern part and central part of Alor Strait could drive mixing significantly.