Global submesoscale diagnosis using alongtrack satellite altimetry

Abstract

Abstract. The ocean’s sea surface height (SSH) field is a complex mix of motions in geostrophic balance and unbalanced motions including high-frequency tides, internal tides and internal gravity waves. Barotropic tides are well estimated for altimetric SSH in the open ocean, but the SSH signals of internal tides remains. The transition scale, Lt, at which these unbalanced ageostrophic motions dominate balanced geostrophic motions, is estimated for the first-time using satellite altimetry. Lt is critical to define the spatial scales above which surface geostrophic currents can be inferred from SSH gradients. We use a statistical approach based on the analysis of 1 Hz altimetric SSH wavenumber spectra to obtain four geophysical parameters that vary regionally and seasonally: the background error, the spectral slope in the mesoscale range, a second spectral slope at smaller scales, and Lt. The mesoscale slope and error levels are similar to previous studies based on satellite altimetry. The break in the wavenumber spectra to a flatter spectral slope can only be estimated in mid-latitude regions where the signal exceeds the altimetric noise level. Small values of Lt are observed in regions of energetic mesoscale activity, while larger values are observed towards low latitudes and regions of lower mesoscale activity. These results are consistent with recent analyses of in situ observations and high-resolution models. Limitations of our results and implications for reprocessed nadir and future swath altimetric missions are discussed.

Refining the Resolution of DUACS Along-Track Level-3 Sea Level Altimetry Products

“…While only a few years ago the high noise level in altimeter measurements prevented us from being able to access these wavelengths, progress in altimeter measurement now leads us to revisit the analysis of the SLA spectral content. Additionally, recent work from [70,73] proposed a refined methodology for a more accurate SLA PSD analysis. Here, we adapt the methodology to analyze the spectral content of the S3A measurements.…”

“…The mean SLA PDS over the years 2017 and 2018 and for the winter (February-March-April; FMA) and summer (August-September-October; ASO) seasons have been analyzed. The spectral decomposition used is similar to the one proposed by [73]. It consists of first estimating the noise level-represented by the flat signal at the shorter wavelengths (<10 km)-then the slope characterizing the geostrophic or mesoscale signal-usually well-defined at wavelengths > ~150 km-and a second slope expected to be representative of the unbalanced motion visible at the small mesoscale.…”

“…However, ref. [73] showed that part of this signal can be recovered with conventional 1 Hz measurements. In this case, a long temporal period is used in order to reduce the errors during the estimation.…”

Estimating the Oceanic Kinetic Energy Cascade from Along-Track Altimetry

“…2a and 2b). T is estimated from the JASON-3 along-track data with a previously published method 28 and is averaged over the same 5 • × 5 • grid on which we estimate the scale KE flux. It provides a lower bound for the scales at which the respective gradients of η can be used to derive geostrophic flows.…”

“…2c and 2d). There are also large regional differences, with lower values in regions with strong balanced flows (large-scale currents and surface eddy path) and higher values in regions with weak balanced flows (weak eddy activity) 28 . In particular in summer, in regions of weak mesoscale activity at scales of 70 km and smaller, the measured absolute dynamic topography is not dominated by balanced flows and Π est gives spuriously enhanced values.…”