River bulge evolution and dynamics in a non-tidal sea – Daugava River plume in the Gulf of Riga, Baltic Sea

Abstract

Abstract. Satellite remote sensing imagery and numerical modelling were used for the study of river bulge evolution and dynamics in a non-tidal sea, the Gulf of Riga (GoR) in the Baltic Sea. Total suspended matter (TSM) images showed a clearly formed anti-cyclonically rotating river bulge from Daugava River discharge during the studied low wind period. In about 7-8 days the bulge grew up to 20 km in diameter, before being diluted. A high-resolution (horizontal grid step of 125 m) General Estuarine Transport Model (GETM) was used for detailed description of the development of the river plume in the southern GoR over the period when satellite images were acquired. In the model simulation, the bulge growth rate was estimated as r b ∼ t 0.5±0.04 (R 2 = 0.90). Both the model simulation and the satellite images showed that river water was mainly contained in the bulge and there were numerous intrusions at the outer perimeter of the bulge. We performed numerical sensitivity tests with actual bathymetry and measured river runoff without wind forcing (1) having an initial three-dimensional density distribution, and (2) using initially a homogeneous ambient density field. In the first case, the anti-cyclonic bulge did not develop within the course of the model simulation and the coastal current was kept offshore due to ambient densitydriven circulation. In the second case, the river plume developed steadily into an anti-cyclonically recirculating bulge, with r b ∼ t 0.28±0.01 (R 2 = 0.98), and a coastal current. Additional simulations with constant cross-shore and alongshore winds showed a significant effect of the wind in the evolution of the river bulge, even if the wind speed was moderate (3-4 m s −1 ). While previous studies conclude that the midfield bulge region is governed by a balance between centrifugal, Coriolis and pressure gradient terms, our study showed that geostrophic balance is valid for the entire mid-field of the bulge, except during the 1-1.5 rotation period at the beginning of the bulge formation. In addition, while there is discharge into the homogenous GoR in the case of a high inflow Rossby number, the river inflow might split into two jets, with strong mixing zone in-between, in the plume nearfield region.

Origin and fate of dissolved organic matter in four shallow Baltic Sea estuaries

“…The longer a water mass remains within a coastal ecosystem, the higher the proportion of land-derived nutrient consumption by phytoplankton that leads to the production of autochthonous DOM (Dippner et al 2019). When a freshwater bulge is formed the water can remain for or up to 9 days near the coast as shown for the Gulf of Riga (Soosaar et al 2016) or when an eddy develops water in the Vistula estuary remains up to 1 week (Voss et al 2005). This time should allow for a higher degradation of DOP components, while the degradation of DON and DOC is slower, leading to a shift in the ratios over time (Lønborg et al 2009b).…”

Reconstruction of Large-Scale Sea Surface Temperature and Salinity Fields Using Sub-Regional EOF Patterns From Models

“…studies frequently reveal also anticyclonic patterns (Soosaar et al, 2016). Mesoscale variability has rather short spatial scales; the R d values are from a few km to about 7 km (Alenius et al, 2003).…”

“…This causes for example coherent upwelling/downwelling patterns (Lehmann et al, 2012) on the left-hand/right-hand coasts from the direction of weathergenerated wind. Considering also faster heating or cooling of shallow coastal areas compared to the deeper offshore regions (Legrand et al, 2015), and freshwater spreading patterns due to the dynamics of river plumes (Soosaar et al, 2016), there could be significant covariance of sea surface temperature (SST) and sea surface salinity (SSS) in marginal seas over large distances, mainly stretched along the topography isolines and/or coasts (Fu et al, 2011).…”

A method for assessment of the general circulation model quality using K-means clustering algorithm

“…In the gulf-type regions of freshwater 85 influence, like the Gulf of Finland (Maljutenko and Raudsepp, 2019) and the Gulf of Riga (Soosaar et al, 2014), seasonal thermocline coincides with seasonal halocline in spring and summer. During maximum river runoff in spring, the river bulge affects the salinity distribution in the coastal sea (Soosaar et al, 2016;Maljutenko and Raudsepp, 2019).…”