Buoyant coastal discharge typically forms a current flowing along the coast in the direction of Kelvin wave propagation (hereinafter referred to as the downstream direction). In this paper the opposite, upstream penetration of buoyancy-driven current is studied using numerical modeling. Previous models of coastal buoyancy-driven currents repeatedly predicted the upstream spreading while in the field this feature was not commonly observed. The mechanism responsible for the propagation of buoyant flow along the coast upstream from its source is identified as follows. In many cases, the boundary conditions applied for buoyant discharge oversimplify the actual dynamics at the mouth blocking landward flow in the lower layer. This generates a strong cyclonic vorticity disturbance with corresponding upstream turning of buoyant flow at the source. This process initiates the upstream spreading of buoyant flow.Alternative boundary condition maintaining constant net transport but allowing baroclinic adjustment of buoyant inflow is formulated and shown to reduce the generation of cyclonic vorticity at the mouth. The upstream propagation is further enhanced by the vertical mixing. The buoyant water forms an anticyclonic bulge at the river or estuary mouth. While spreading around the center of this anticyclone, the fresher water gradually becomes saltier due to vertical mixing/diffusion. As a result, the pool of lightest water does not coincide with the center of the anticyclone (in the sense of integral streamfunction) but tends to occupy the upstream and inshore segment of the bulge where the buoyant water comes first. This sets a new center for the anticyclonic turning at the surface and promotes the upstream shift of the anticyclonic bulge. This process sustains continuous growth of the buoyant plume upstream. It is shown that the upstream ambient current does not produce a similar effect. Instead, buoyant flow periodically sheds anticyclones advected upstream with the mean current. Under certain conditions, upstream spreading is also possible in nature. For example, Beardsley et al. (1985) reported substantial upstream penetration of the Changjiang River discharge in the East China Sea during the period of high runoff.
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