Lia Siegelman

Deep-reaching submesoscale fronts generate secondary circulations that transport heat vertically through the ocean interior, yet their seasonal variability remains largely unknown. Here, we combine more than 50,000 sub-kilometer hydrographic profiles collected by 13 instrumented southern elephant seals over six years with satellite-derived mesoscale strain to diagnose balanced submesoscale vertical velocities and the associated vertical heat transport in the Indian sector of the Southern Ocean. We show that both quantities are highly intermittent, extend to at least 500 m depth, and exhibit a pronounced seasonal cycle, with root-mean-square amplitudes nearly doubling between fall and spring. In contrast, the mean vertical heat transport is persistently upward throughout the year and reaches a maximum during winter, exceeding 150 W/m ² below 200 m. These results reveal that the contribution of deep-reaching submesoscale secondary circulations to upper-ocean heat transport is seasonally dependent.

Deep-reaching submesoscale fronts generate ageostrophic secondary circulations that transport heat vertically through the ocean interior, yet their seasonal variability remains largely unknown. Here, we combine more than 75,000 sub-kilometer hydrographic profiles collected by 13 instrumented southern elephant seals over six years with satellite derived mesoscale strain to diagnose submesoscale vertical velocities and vertical heat transport in the Indian sector of the Southern Ocean. We show that both quantities are highly intermittent, extend to at least 500 m depth, and exhibit a pronounced seasonal cycle, with root-mean-square amplitudes nearly doubling between fall and spring. In contrast, the mean vertical heat transport is persistently upward throughout the year and reaches a maximum during winter, exceeding 150 W/m ² below 200 m. These results reveal that the influence of deep-reaching submesoscale secondary circulations on upper ocean heat transport and restratification is strongly seasonally dependent.

Ocean submesoscale fronts with a size of 1–20 km have been extensively documented within the surface mixed layer, but little is known about their existence below it. We analyze a record of 133 598 dives performed by instrumented Southern Elephant Seals from 2014 to 2020 in the Indian sector of the Southern Ocean with a sub-km resolution. Here, we show that submesoscale motions are intensified below the mixed layer and are present year-round from the surface down to depths of at least 500 m. They exhibit a seasonal cycle with an austral spring (Sep–Nov) intensification and a weakening in fall (Mar–May). These deep submesoscales follow the seasonality of surface eddy kinetic energy derived from altimetry, suggesting that they are generated by mesoscale eddies. Since submesoscales are known to be associated with strong vertical velocities, these results suggest that vertical exchanges of heat and nutrient at depth may be more important than commonly assumed.

The Surface Water and Ocean Topography (SWOT) satellite provides a groundbreaking view of sea surface height across a 120 km-wide swath, opening new opportunities to reconstruct vertical velocities (w). Vertical motions play a central role in redistributing properties, influencing climate and ecosystems. Using the effective surface quasigeostrophic framework in the eddy-rich Agulhas Current Retroflection region, we reconstruct w down to 1,000 m. We assess its realism against the 1/60° (~2 km) INALT60 ocean model, finding a spatial correlation of 0.6. SWOT-derived w reveal pronounced high-frequency variability, with events lasting several days to one week during which w variance more than doubles. The Agulhas Retroflection emerges as a hotspot of enhanced w (~300 m day-1), driven by strong horizontal strain. This work provides the first quantitative reconstruction of w from SWOT in the region and highlights the mission’s potential to quantify vertical exchanges linking the ocean surface and interior.

In proximity of the Antarctic ice sheet, oceanic motions with different spatial structures contribute to the transport of warm water onto the continental shelf, fueling submarine ice‐shelf melting. Emerging evidence suggests that km‐sized ocean submesoscale fronts and eddies significantly impact ice‐ocean interactions on timescale of a few hours to a few days, with profound implications for the melting regime. However, ocean models often rely on parameterizations to represent these fine‐scale, high‐frequency processes because of the coarse resolution. This results in uncertainties in heat transfer mechanisms at the ice‐ocean boundary which often leads to conservative estimations of melt rates. Here, we assess the impact of sub‐kilometer model resolution on capturing ocean features at different scales in the Amundsen Sea Embayment, West Antarctica, and their impact on the melting regime of local ice shelves. While the simulations do not show substantial differences in large‐scale ocean dynamics, we find that using a sub‐kilometer resolution is necessary to resolve ocean eddies beneath ice shelves. Furthermore, the ocean kinetic energy using 200 m resolution is approximately three times higher than when using a 1 km resolution. This increase is driven by significantly stronger submesoscale activity, both in the open ocean and beneath the ice shelves, leading to improved model‐data agreement, particularly in capturing high melt rates observed at the grounding line.

The warm and saline Atlantic Water has long been recognized as being subjected to substantial heat loss during its transit towards the polar regions. In particular, the Lofoten Basin, a subpolar sea with energetic eddy activity and strong air-sea interactions, plays a crucial role in the transformation of Atlantic Water. Vertical heat transport at submesoscales (0.1-10 km) in the Lofoten Basin is potentially a key link in the heat transfer to the atmosphere. Here, based on multi-year Seaglider observations augmented by satellite altimeters, radiometers, and high-resolution numerical model results, we evaluate the oceanic vertical heat transport in the Lofoten Basin and demonstrate how geostrophic strain enhances heat transport. The enhancement is found to be associated with submesoscale ageostrophic motions along the mesoscale eddy edges, occurring on spatial scales smaller than 10 km and below the mixed layer depth. These strain-induced submesoscale vertical motions transport heat from the ocean interior to the surface, leading to a 0.4 °C increase in sea surface temperature and the formation of “warm ring” structures in both cyclones and anticyclones. The dominant role of submesoscale heat transport likely represents the primary mechanism for substantial heat loss from Atlantic Water in the Lofoten Basin.

Observations of sea surface height (SSH) from the Surface Water and Ocean Topography (SWOT) satellite have demonstrated remarkable potential for resolving mesoscale and submesoscale ocean features, which are crucial for deriving vertical velocities, a key variable for understanding the transport of heat, carbon, and nutrients between the ocean surface and interior. In the mesoscale energetic region south of Tasmania, we evaluate the accuracy of SWOT small‐scale observations using high‐resolution (8 km) in situ observations of surface tracer fields and vertical temperature. Surface features derived from SWOT align with subsurface horizontal temperature gradients derived from the in situ measurements. These fine scales (100 km) reveal dynamic phenomena, such as a sudden front deflection near the Macquarie Ridge, obscured in conventional low‐resolution products. We then quantify the contributions of these small scales to the currents' energy and shape, compared to the larger mesoscales (100 km) observable with traditional nadir‐looking altimetry. Although larger scales predominantly influence the geostrophic velocity, smaller scales contribute significantly to current stretching and straining, showing up to threefold stronger strain and tenfold stronger vorticity than larger scales in a few hotspots. Finally, we investigate the contribution of these small scales to the vertical velocities down to 1,000 m that are reconstructed using effective surface Quasi‐Geostrophic theory. SWOT‐derived exhibit twice the amplitude compared to nadir altimetry, underscoring SWOT's capacity to resolve energetic meso‐ and submesoscale ocean dynamics. These results highlight the need to fully harness SWOT's high‐resolution data in gridded products, as current smoothing limits the retention of valuable small‐scale information.

Greenland's fjords are the gateways through which the Greenland Ice Sheet (GrIS) interacts with the open ocean. These fjords are projected to undergo significant change as glaciers retreat with implications ranging from impacts to local fisheries and infrastructure to global sea level rise. Icebergs discharged by marine‐terminating glaciers in these fjords progressively melt on their way out of the fjord. Studies have shown that the cooling/freshening of the water column by icebergs contributes to the fjord overturning circulation. However, the majority of these studies are focused on summer conditions when data exist, and little is known about the impact and melting of icebergs year‐round. Here, we use a realistic regional configuration of a numerical model (MITgcm) of Sermilik fjord in southeast Greenland, from 2015 to 2017, to show that iceberg melting occurs year‐round, is focused in the upper 100 m of the water column, and largely occurs in the mélange region near the glacier. Comparison with an identical run without icebergs shows that upwelling induced by subsurface iceberg melt drives a warming (°C) of the surface layer in spring/winter in the entire fjord. This mélange upwelling supports a year‐round overturning circulation. Overall, our findings emphasize that icebergs must be included in simulations of fjords in order to correctly simulate water properties and circulation, including upwelling and heat flux. Representing these dynamics accurately is critical for understanding the impacts of icebergs on glacial melt rates and the interactions between glacial fjords, the GrIS, and the open ocean.