By combining real-field observations and theoretical predictions, we describe role and relationships among north-propagating internal solitary waves (ISWs) generated by tidal currents in the Messina Strait (Mediterranean Sea), buoyancy deformation, sediment resuspension, and mixing effects. In particular, our results show that the presence of ISWs traveling along the Gioia Basin (north of the Strait) is not strictly related to seasonality. During winter, when the remote observation of ISWs from satellite is particularly rare due to the weak water column stratification, we observe elevation-type ISWs from hydrographic data. This finding reveals a different scenario with respect to the summer one, when the high stratified water column gives rise to depression-type north-propagating ISWs and the subsequent sea surface manifestations, detectable from satellite imagery. Moreover, our beam transmission observations and theoretical predictions of the induced near-bottom horizontal velocity suggest that these elevation-type ISWs induce sediment resuspension over the seafloor, as well as mixing effects as they break on the frontal slope nearby Capo Vaticano.
Antarctic Bottom Water (AABW) stores heat and gases over decades to centuries after contact with the atmosphere during formation on the Antarctic shelf and subsequent flow into the global deep ocean. Dense water from the western Ross Sea, a primary source of AABW, shows changes in water properties and volume over the last few decades. Here we show, using multiple years of moored observations, that the density and speed of the outflow are consistent with a release from the Drygalski Trough controlled by the density in Terra Nova Bay (the "accelerator") and the tidal mixing (the "brake"). We suggest tides create two peaks in density and flow each year at the equinoxes and could cause changes of ~ 30% in the flow and density over the 18.6-year lunar nodal tide. Based on our dynamic model, we find tides can explain much of the decadal variability in the outflow with longer-term changes likely driven by the density in Terra Nova Bay.
Approximately 25% of Antarctic Bottom Water has its origin as dense water exiting the western Ross Sea, but little is known about what controls the release of dense water plumes from the Drygalski Trough. We deployed two moorings on the slope to investigate the water properties of the bottom water exiting the region at Cape Adare. Salinity of the bottom water has increased in 2018 from the previous measurements in 2008-2010, consistent with the observed salinity increase in the Ross Sea. We find High Salinity Shelf Water from the Drygalski Trough contributes to two pulses of dense water at Cape Adare. The timing and magnitude of the pulses is largely explained by an inverse relationship with the tidal velocity in the Ross Sea. We suggest that the diurnal and low frequency tides in the western Ross Sea may control the magnitude and timing of the dense water outflow.
The Mediterranean Sea is one of the first regions where sea surface temperature (SST) increase was linked to greenhouse effects and global warming. Due to its sensitivity to climate variability and its high impact on local and remote climate conditions, much effort has been made to assess the SST variability in the Mediterranean as a whole. However, the Mediterranean is composed of several basins, each of which plays a different role in its conveyor belt's function. This study focuses on the basin of the Tyrrhenian Sea which represents one of the crucial areas for deep mixing of the Mediterranean main water masses. Thirty-seven years (1982-2018) of satellite-derived data were used to investigate the SST variability in relation to large-scale and local forcing mechanisms. A significant warming trend of 0.034 ± 0.004 °C/year was found, which led to an average warming of 1.288 ± 0.129 °C over the considered period. The observed warming presents time-dependent spatial patterns as well as changes in the seasonal cycle. Our results highlight that the Tyrrhenian's individual long-term surface variability has different characteristics than the Mediterranean as a whole and provide insight into the relative influence of large-scale teleconnection patterns and local air-sea interaction on this variability.
Antarctic Bottom Water (AABW) supplies the lower limb of the global overturning circulation and ventilates the abyssal ocean. In recent decades, AABW has warmed, freshened and reduced in volume. Ross Sea Bottom Water (RSBW), the second largest source of AABW, has experienced the largest freshening. Here we use 23 years of summer measurements to document temporal variability in the salinity of the Ross Sea High Salinity Shelf Water (HSSW), a precursor to RSBW. HSSW salinity decreased between 1995 and 2014, consistent with freshening observed between 1958 and 2008. However, HSSW salinity rebounded sharply after 2014, with values in 2018 similar to those observed in the mid-late 1990s. Near-synchronous interannual fluctuations in salinity observed at five locations on the continental shelf suggest that upstream preconditioning and large-scale forcing influence HSSW salinity. The rate, magnitude and duration of the recent salinity increase are unusual in the context of the (sparse) observational record.
