Abstract
Abstract. The summer enhancement of diurnal tidal currents at the shelf break in the southern Weddell Sea is studied using velocity measurements from 29 moorings during the period 1968 to 2014. Kinetic energy associated with diurnal tidal frequencies is largest at the shelf break and decreases rapidly with distance from it. The diurnal tidal energy increases from austral winter to summer by, on average, 50 %. The austral summer enhancement is observed in all deployments. The observations are compared to results from an idealised numerical solution of the properties of coastal trapped waves (CTWs) for a given bathymetry, stratification and an along-slope current. The frequency at which the dispersion curve for mode 1 CTWs displays a maximum (i.e. where the group velocity is zero and resonance is possible) is found within or near the diurnal frequency band, and it is sensitive to the stratification in the upper part of the water column and to the background current. The maximum of the dispersion curve is shifted towards higher frequencies, above the diurnal band, for weak stratification and a strong background current (i.e. austral winter-like conditions) and towards lower frequencies for strong upper-layer stratification and a weak background current (austral summer). The seasonal evolution of hydrography and currents in the region is inferred from available mooring data and conductivity–temperature–depth profiles. Near-resonance of diurnal tidal CTWs during austral summer can explain the observed seasonality in tidal currents.
Highlights
The shelf break region in the southern Weddell Sea (Fig. 1) is an area of great climatic interest
Velocity measurements at 29 moorings located on the continental slope and shelf in the southern Weddell Sea during the period 1968 to 2014 show pronounced diurnal tidal variability
Diurnal tidal currents are strongest at the shelf break and substantially enhanced during austral summer
Summary
The shelf break region in the southern Weddell Sea (Fig. 1) is an area of great climatic interest. Our study expands on previous investigations of tidally generated CTWs at diurnal frequencies in the shelf break region of the southern Weddell Sea. Records from 29 moorings collected over more than 4 decades are used to quantify the strength of the diurnal tidal currents in the area and to describe their temporal and spatial variability. Previous studies – based on a small sub-set of our data records – have suggested that the summertime enhancement is due to changes in the oceanographic ”background”, as it determines the dispersion relation for the CTWs which are responsible for the tidal amplification in the area We investigate this further by using a numerical code (Brink, 2006) to study the sensitivities of the CTW properties and the RF to seasonal changes in hydrography and background current. We investigate the role of divergent bathymetry and the seasonal variability of sea ice cover in the southern Weddell Sea, and we discuss the potential relevance for tidally generated CTWs
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