Frontogenesis and estuarine circulation at the shallow sill of a tropical fjord: Insights from Ambon Bay, eastern Indonesia

Abstract

Surface water flushing of the tropical fjord of Ambon Bay is linked to stratification and frontogenesis that vary with tidal and seasonal cycles. Tidal- and seasonal-based​ deployments of CTD casts and bottom-mounted current meters in 2019 coupled with an analytical model were employed to investigate estuarine circulation (i.e. flood/ebb-mean transport) at the sill of Ambon Bay. Spring tides produce a large (flood/ebb-mean) horizontal salinity gradient, ∂S/∂x, (up to 13 × 10−4‰/m) across the sill, intensifying estuarine circulation. This intensification of ∂S/∂x is driven by strong (flood/ebb-mean) stratification at the inshore sill, associated with buoyant frontogenesis (indicated by densimetric Froude number, Fo2≥ 0.3) during the related strong flood tides. Estuarine circulation is stronger in the easterly monsoon (July) than in the transitional monsoon (October) since buoyant fronts are larger in thickness and horizontal extension along the sill in the easterly monsoon following high freshwater input. The estuarine circulation was intensified by a deep-water renewal process in which deep water from the adjacent open sea is upwelled to the sill by internal waves.Flushing of inner Ambon Bay (IAB, the fjord basin) due to estuarine circulation at the sill only occurs at the surface layer of IAB. Within a spring–neap sequence (∼2 weeks), the net oceanic inflow due to estuarine circulation at the sill replenished 73% and 41% volume of the surface layer of IAB in the easterly monsoon and the transitional season, respectively. The knowledge of flushing of IAB due to net oceanic inflow presented here has application for the mitigation of pollution build-up and for understanding of oceanic nutrient supply in the basin.