Seasonality of the Sub‐Mesoscale to Mesoscale Sea Surface Variability From Multi‐Year Satellite Altimetry

Abstract

Sea surface slope (SSS) varies in response to a range of physical processes: tides, geostrophic flows, surface and internal waves, etc. We present the sea surface variation in the form of the SSS variability using 30 years of heterogeneous satellite altimetry measurements. We apply band‐pass filters to the along‐track SSS, and derive the mean and seasonal (annual and semi‐annual) components of SSS variability in multiple wavelength sub‐bands from 10 to 1,000 km. We show that the seasonal components are generally small (<10% in amplitude) compared to the mean variability. Through correlation analysis, we show evidence that SSS variability with wavelengths less than 30 km is dominated by wave height noise. At sub‐mesoscale to mesoscale (30–100 km) wavelengths, we identify high variability over western boundary currents and regions of rough topography. In this band, the high‐latitude Northern Hemisphere and the south Indian Ocean are associated with large annual cycles. The variability is higher in local wintertime except for a few regions, for example, the Bay of Bengal, which shows high variability in the boreal spring and fall. Through power spectral density analysis of the seasonal SSS variability, we find that the energy differences between local winter and summer are stronger at smaller scales (<100 km). The Ka‐band radar interferometry instrument on the Surface Water and Ocean Topography satellite mission will allow observation of ocean surface activities down to ∼20 km at submonthly time scales, but wave‐related errors (sea state bias, aliasing, wind‐driven activities, etc.) will still be a major challenge.

Accuracy and Resolution of SWOT Altimetry: Foundation Seamounts

“…The short wavelength (<16 km) SSS variability in the region is relatively low (e.g., SSS variability of ∼2 μrad in the observation of radar altimeter along‐track profiles (1 μrad = 10 −6 which corresponds to ∼1 mGal) (Yu et al., 2023). The uncertainty in the mean slope is between 0.5 and 2 μrad depending on the number of repeat altimeter profiles.…”

“…This results in a relatively uniform gravity to topography ratio at wavelengths less than about 60 km. Within the areas of the 1‐day SWOT coverage there are approximately 30 closely‐spaced volcanic edifices having heights ranging from 1,500 to 3,500 m. These produce gravity peaks with very large amplitudes (20–120 mGal) and short wavelengths (Figure 2 and Figure S1 in Supporting Information ). The accuracy of the shipboard gravity is better than 1 mGal so the signal to noise ratio exceeds 100. The short wavelength (<16 km) SSS variability in the region is relatively low (e.g., SSS variability of ∼2 μrad in the observation of radar altimeter along‐track profiles (1 μrad = 10 −6 which corresponds to ∼1 mGal) (Yu et al., 2023). The uncertainty in the mean slope is between 0.5 and 2 μrad depending on the number of repeat altimeter profiles.…”