Stratified Circulation in the Banda Sea and Its Causal Mechanism

This study examines the vertical structure of the Banda Sea circulation, using the HYbrid Coordinate Ocean Model+Navy Coupled Ocean Data Assimilation Global Analysis (GLBa0.08) products. The vorticity distribution shows a four-layer structure in the Banda Sea circulation, stronger in the upper 700 m than in the deeper layers. Circulation during the northwesterly monsoon is cyclonic in the surface layer and anticyclonic in the upper layer, opposite to its counterpart during the southeasterly monsoon. Circulation weakens dramatically in the intermediate layer and is persistently cyclonic in the deep layer. A four-and-a-half-layer quasi-geostrophic model is proposed to identify the processes responsible for the stratified circulation structure. The result indicates that the circulation in the central and eastern Banda Sea is mainly forced by monsoonal winds, while in the western and southern boundaries of the Banda Sea the circulation is dominated by the effect of Coriolis force. The effect of Coriolis force lies in the fact that the Indonesian Throughflow enters the Banda Sea from lower latitudes and exits it at higher latitudes, resulting in a residual potential vorticity flux that drives the circulation in the Banda Sea. As upwelling connects the circulation in different layers, vertical vorticity flux also plays a role in driving the circulation in the thermocline, where Ekman pumping is prominent, and in the deep layer as well, where the Lifamatola Passage overflow-induced upwelling dominates. Plain Language Summary The Banda Sea, known as a mix-master region and confluent reservoir of the Indonesian Throughflow (ITF), mixes waters of North and South Pacific origins, forming the unique warm and fresh Indonesian Seas water and contributing to the Indian Ocean. In the past two decades, the volume transports and variability of the ITF branches were extensively investigated. Until now, fewer studies have focused on the interior circulation in the Banda Sea. To date, the vertical structure of the Banda Sea circulation, from the surface to the bottom, is not addressed yet. As the Banda Sea is subjected to the seasonally reversing monsoonal winds, the seasonal variability of the Banda Sea circulation needs to be ascertained as well. In this study, we depict a four-layer structure of the Banda Sea circulation using HYbrid Coordinate Ocean Model Analysis data, which is stronger in the thermocline than in the deeper layers. The circulation during northwesterly monsoon is opposite to that during southeasterly monsoon in the thermocline, but both are same in the deeper layers. Furthermore, we establish a four-and-a-half-layer quasi-geostrophic model to investigate the processes responsible for the circulation structure and quantify the contributions of the ITF branches, upwelling and monsoonal winds to the Banda Sea circulation.