Effects of Balanced Motions and Unbalanced Internal Waves on Steric Height in the Mid-Latitude Ocean

The baroclinic component of the sea surface height, referred to as steric height, is governed by geostrophically balanced motions and unbalanced internal waves, and thus is an essential indicator of ocean interior dynamics. Using yearlong measurements from a mooring array, we assess the distribution of upper-ocean steric height across frequencies and spatial scales of O (1-20 km) in the northeast Atlantic. Temporal decomposition indicates that the two largest contributors to steric height variance are large-scale atmospheric forcing (32.8%) and mesoscale eddies (34.1%), followed by submesoscale motions (15.2%), semidiurnal internal tides (8%), super-tidal variability (6.1%) and near-inertial motions (3.8%). Structure function diagnostics further reveal the seasonality and scale dependence of steric height variance. In winter, steric height is dominated by balanced motions across all resolved scales, whereas in summer, unbalanced internal waves become the leading-order contributor to steric height at scales of O (1 km). Steric height is the sea surface height component associated with changes in water-column density, and is typically contributed by ocean dynamic processes across a wide range of scales, from the large-scale ocean circulation to the small-scale wave motion. In this study, the effects of balanced motions (e.g., eddies and ocean fronts) and unbalanced wave motions (e.g., internal waves) on steric height are quantified based on yearlong moored observations at a mid-latitude ocean site of the northeast Atlantic. Overall, balanced motions and unbalanced wave motions account for approximately 67% and 33% of the upper-ocean steric height variance, respectively. Steric height variance also show notable seasonal variations and scale dependence. At spatial scales of O (10 km), the steric height is predominately determined by balanced motions throughout the year. By contrast, at spatial scales of O (1 km), unbalanced wave motions are the major contributor to steric height in summer whereas balanced motions still dominate in winter. Together, our findings provide insights for the exploration of next-generation high-resolution altimetry data and highlight the non-negligible role of unbalanced wave motions in forming an energy sink for the balanced flow. The distribution of steric height variance across frequencies and spatial scales of O (1-20 km) is revealed by yearlong mooring measurements Balanced motions dominate the upper-ocean steric height variance, and account for similar to 67% of the total variance Internal waves become increasingly important in summer, and are able to dominate over balanced motions at spatial scales of O (1 km)