An upcoming space mission aims to capture high-resolution satellite measurements of Earth’s global surface waters. After launching in 2020, the Surface Water and Ocean Topography (SWOT) satellite will measure ocean surface height at a resolution of about 15 kilometers, up to 10 times finer than current altimeters. These data will then be used to infer fine-scale ocean dynamics and improve global ocean circulation models.
To interpret SWOT sea surface height data correctly, a clear understanding of their relationship with fine-scale flow must be established. In a new study, Rocha et al. demonstrate that scientists will need to consider seasonal variations in upper ocean internal wave dynamics when working with SWOT data.
The researchers focused on a portion of the Kuroshio Current, which flows northward along the western side of the North Pacific Ocean. After it passes Japan, the Kuroshio Current turns eastward, and its warm waters take a meandering path through the chilly Pacific, forming the Kuroshio Extension. This region is known for its major seasonal changes in upper -ocean water stratification.
The authors used two global computer simulations to examine seasonal changes of features known as inertia-gravity waves in the Kuroshio Extension. Inertia-gravity waves are internal waves with wavelengths long enough to be affected by Earth’s rotation.
The simulations revealed that fine-scale inertia-gravity waves in the Kuroshio Extension undergo a strong seasonal cycle. Inertia-gravity waves with wavelengths between 10 and 100 kilometers peak in the late summer and early fall when the upper ocean is highly stratified.
An important observation is that the inertia-gravity wave cycle is out of phase with the Kuroshio Extension’s seasonal cycle of macroturbulence. Although inertia-gravity waves have a bigger impact on sea surface height variability and horizontal flow in late summer and early fall, macroturbulence dominates in late winter and early spring.
These findings have important implications for the interpretation of SWOT sea surface height data. However, the authors caution that patterns observed in the Kuroshio Extension may not apply to other ocean regions. They plan to explore the variability of fine-scale upper ocean seasonality of different ocean regions in a future study. (Geophysical Research Letters, doi:10.1002/2016GL071349, 2016)
—Sarah Stanley, Freelance Writer