Climatology of Surface Winds in the Indonesian Seas Based on Satellite Observations and Reanalysis Data

Climatology of surface winds in the Indonesian seas is investigated based on satellite observations and reanalysis data. The Indonesian seas are routes of the East Asian monsoon and Australian monsoon. The monsoon winds are promising for the development of offshore wind energy in the Indonesian seas. However, it is still a challenge to understand the distributions and variabilities of offshore winds in the Indonesian seas where many islands exist and small seas are connected through the straits. Thus, we analyzed wind measurements acquired by ASCAT onboard MetOP-A and MetOP-B satellites for 2010–2018 and ERA5 reanalysis data for 2003–2018. The ASCAT wind measurements at a spatial resolution of 12.5 km provide observational evidence of surface winds in the island-studded seas. The latest reanalysis data of ERA5 provide hourly wind fields at a spatial resolution of 0.25° and allow to analyze temporal variations from monsoon variations to diurnal variations. The following results are obtained.

We identified monsoon routes in the Indonesian seas consisting of a main route and minor routes. The main monsoon route both for southeast monsoon (Australian monsoon) and northwest monsoon (East Asian monsoon) extends from north of the Australian continent to the South China Sea through the Arafura Sea, Banda Sea, Java Sea, and Karimata Strait. Small seas and straits adjacent to the main route work as the minor routes of the monsoons. During southeast monsoon, the wind blows into the Pacific Ocean from the Arafura Sea through the Halmahera Sea and Molucca Sea. During northwest monsoon, the wind blows into the Java Sea and Banda Sea from the Philippine Islands through the Karimata Strait, Makassar Strait, and Molucca Sea. These monsoon routes are well represented by the major-axis directions of variance ellipses of wind velocities.

Seasonal variations of monsoons in the Indonesian seas were examined. The peak of monthly climatological mean wind in the Arafura Sea is in July and in the South China Sea is in December. The monsoon changes its direction from northwest to southeast in April and from southeast to northwest in November. Along the main monsoon route, the periods of southeast monsoon are longer than the periods of northwest monsoon. In addition, wind speeds during southeast monsoon are greater than those during northwest monsoon. This also happens in the Karimata Strait even though located closer to the South China Sea.

Wind variabilities along the main monsoon routes in the Indonesian seas are induced by the variabilities of SLP. Based on correlation analyses between the major-axis wind component and SLP, we identified two key regions. One is a region fully covering northern Australian continent, and the other is a region covering the South China Sea. The monsoon variabilities in these regions are important for wind variabilities in the Indonesian seas.

Diurnal variations of the wind were also analyzed in this study. Along the main route of monsoon, the variabilities of the monsoon and diurnal wind in Indonesian seas are well-separated by major-and minor-axis wind components, respectively. Land breeze dominates from evening to afternoon on the next day and sea breeze dominates from afternoon to night on the same day. Thus, the wind fields show divergent and convergent patterns along the main monsoon routes during daytime and night-time, respectively. In general, diurnal variations of wind in the Indonesian seas are large in September and small in January.