Abstract
Oceanic fronts play a pivotal role in controlling water mass transfer, although little is known about deep frontal structure on appropriate temporal and spatial scales. Here, we present a sequence of calibrated time‐lapse images from a three‐dimensional seismic survey that straddles the Brazil‐Malvinas Confluence—a significant feature of the meridional overturning circulation. Eight vertical transects reveal the evolution of a major front. It is manifest as a discrete planar surface that dips at less than 2° and is traceable to 1.5–2 km depth. Its shape and surface expression are consistent with sloping isopycnal surfaces of the calculated potential density field and with coeval sea surface temperature measurements, respectively. Within the top ∼1 km, where cold fresh water subducts beneath warm salty water, a series of tilted lenses are banked up against the sharply defined front. The largest of these structures is centered at 700 m depth and is cored by cold fresh water. Time‐lapse imagery demonstrates that this tilted lens grows and decays over 9 days. It has a maximum diameter of <34±0.13 km and a maximum height of <750±10 m. Beneath 1 km, where horizontal density gradients are negligible, numerous deforming lenses and filaments on length scales of 10–100 km are being swept toward the advecting front.
Plain Language Summary
Oceanic fronts (i.e., regions of rapid lateral changes in temperature and/or salinity) are key sites of water mass modification, primary productivity, and ocean‐atmosphere exchange. However, fronts occupy a large range of scales (i.e., meters to kilometers and days to years) and present a significant observational challenge. Typically, measurements are restricted to small high‐resolution surveys or to large surveys that have kilometer‐scale gaps between sampling locations. We employ an acoustic imaging technique, which records energy reflected from temperature changes within the water column, to overcome these observational limitations. Vertical cross sections through the ocean are constructed over tens of kilometers that map temperature distribution on 10 m length scales. Critically, this analysis yields frontal observations that span a large range of spatial scales (i.e., 0.1–150 km) over a period of 1 week. This work presents novel time‐lapse observations of frontal structure and behavior. Acoustic images reveal frontal dynamics that are occurring on larger, deeper, and faster scales than previously observed. Our analysis overcomes observational restrictions, revealing new frontal structure and behavior that have significant implications for future studies and ocean dynamics at fronts.
Key Points
Time‐lapse seismic imaging of deep oceanic front with spatial resolution of 5–10 m is presented
Growth and decay of mesoscale tilted lens is visible at depth of 0.5–1 km over 9 days
Rapid horizontal advection of thermohaline structures toward tilted lens is tracked
