Abstract. When lakes experience surface cooling, the shallow littoral region cools faster than the deep pelagic waters. The lateral density gradient resulting from this differential cooling can trigger a cold downslope density current that intrudes at the base of the mixed layer during stratified conditions. This process is known as a thermal siphon (TS). TSs flush the littoral region and increase water exchange between nearshore and pelagic zones; thus, they may potentially impact the lake ecosystem. Past observations of TSs in lakes are limited to specific cooling events. Here, we focus on the seasonality of TS-induced lateral transport and investigate how seasonally varying forcing conditions control the occurrence and intensity of TSs. This research interprets 1-year-long TS observations from Rotsee (Switzerland), a small wind-sheltered temperate lake with an elongated shallow region. We demonstrate that TSs occur for more than 50 % of the days from late summer to winter and efficiently flush the littoral region within ∼10 h. We further quantify the occurrence, intensity, and timing of TSs over seasonal timescales. The conditions for TS formation become optimal in autumn when the duration of the cooling phase is longer than the time necessary to initiate a TS. The decrease in surface cooling by 1 order of magnitude from summer to winter reduces the lateral transport by a factor of 2. We interpret this transport seasonality with scaling relationships relating the daily averaged cross-shore velocity, unit-width discharge, and flushing timescale to the surface buoyancy flux, mixed-layer depth, and lake bathymetry. The timing and duration of diurnal flushing by TSs relate to daily heating and cooling phases. The longer cooling phase in autumn increases the flushing duration and delays the time of maximal flushing relative to the summer diurnal cycle. Given their scalability, the results reported here can be used to assess the relevance of TSs in other lakes and reservoirs.

[1] A field data set collected under different conditions is analyzed to characterize the spatial arrangement of two large inflows (Ebro and Segre) with distinct physical-chemical characteristics as they join at the upstream end of Ribarroja reservoir in northern Spain. Given the short average residence time of water in the reservoir, the spatial arrangement of the rivers at their confluence and their mixing rates are likely the drivers of the stratification patterns observed near the dam. In winter, inflows have similar densities-Á/ 0 % O(10 À5 )-and their spatial distribution is largely determined by inertial forces, and in particular, by the discharge ratio. Downstream of the confluence, both rivers flow side by side and largely unmixed over long distances. In summer, with Á/ 0 of O(10 À3 ), the flow fields at the confluence are largely controlled by buoyancy forces. Atmospheric forcing during strong wind events and centrifugal forces caused by the meandering shape of the reservoir induce significant tilting of the isotherms, leading to localized high mixing rates. Mixing, in general, though is weak at this time of the year. In fall and early winter, density differences are largely controlled by conductivity differences between the incoming flows. The warmer Ebro water, with larger thermal inertia, flows beneath the colder Segre water. The spatial arrangement of the inflows is largely controlled by the discharge ratio and mixing between sources is strong, likely as a result of mixed water being denser than either of the incoming flows.

Simulations of tracer experiments conducted with a three-dimensional primitive-equation hydrodynamic and transport model are used to understand the processes controlling the rate of mixing between two rivers (Ebro and Segre), with distinct physical and chemical properties, at their confluence, upstream of a meandering reservoir (Ribarroja reservoir). Mixing rates downstream of the confluence are subject to hourly scale oscillations, driven partly by changes in inflow densities and also as a result of turbulent eddies that develop within the shear layer between the confluent rivers and near a dead zone located downstream of the confluence. Even though density contrasts are low-at most O(10 21 ) kg m 23 difference among sources-and almost negligible from a dynamic point of view-compared with inertial forces-they are important for mixing. Mixing rates between the confluent streams under weakly buoyant conditions can be of up to 40% larger than those occurring under neutrally buoyant conditions. The buoyancy effects on mixing rates are interpreted as the result of changes in the contact area available for mixing (distortion of the mixing layer). For strong density contrasts, though, when the contact area between the streams becomes nearly horizontal, larger density differences between streams will lead to weaker mixing rates, as a result of the stabilizing effect of vertical density gradients.

Abstract. When lakes experience surface cooling, the shallow littoral region cools faster than the deep pelagic waters. The lateral density gradient resulting from this differential cooling can trigger a cold downslope density current that intrudes at the base of the mixed layer during stratified conditions. This process is known as thermal siphon (TS). TS flushes the littoral region and increases water exchange between nearshore and pelagic zones, with possible implications on the lake ecosystem. Past observations of TS in lakes are limited to specific cooling events. Here, we focus on the seasonality of the TS-induced lateral transport and investigate how the seasonally varying forcing conditions control the occurrence and intensity of TS. We base our analysis on one year of observations of TS in Rotsee (Switzerland), a small wind-sheltered temperate lake composed of an elongated shallow region. We demonstrate that TS occurs for more than 50 % of the days from late summer to winter and efficiently flushes the littoral region in ~10 hours. We further quantify the seasonal evolution of the occurrence, intensity and timing of TS. The conditions for the formation of TS are optimal in autumn, when the duration of the cooling phase is longer than the initiation timescale of TS. The decrease in surface cooling by one order of magnitude from summer to winter reduces the lateral transport by a factor of two. We interpret this transport seasonality with scaling relationships relating the daily averaged cross-shore velocity, unit-width discharge and flushing timescale to the surface buoyancy flux, mixed layer depth and lake bathymetry. The timing and duration of the diurnal flushing by TS are associated with the duration of the daily heating and cooling phases. The longer cooling phase in autumn increases the flushing duration and delays the time of maximal flushing, compared to the summer period. Our findings based on scaling arguments can be extended to other aquatic systems to assess, at a global scale, the relevance of TS in lakes and reservoirs.