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

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. Plain Language Summary: Many ocean processes have signatures in sea surface slope (SSS) in the 30–100 km wavelength scales, yet SSS in this band is neither well observed nor well studied globally. This paper focuses on seasonal variations of the SSS on length scales from 30 to 100 km, using satellite data collated over the past 30 years. We find high variability over strong currents and regions of rough ocean bottom topography. This variability is related to surface gravity waves and the density changes in the sub‐surface of the ocean. The seasonal changes are small compared to the mean spatial variations in SSS. Key Points: We identify high sub‐mesoscale sea surface slope (SSS) variability over western boundary currents and regions of rough topographySeasonal variations are small (<10% in amplitude) compared to the mean variability on length scales from 30 to 100 kmSSS variation is related to the significant wave height and thermocline depth [ABSTRACT FROM AUTHOR]