  Since the advent of satellite altimetry, our perception of the oceanic
circulation has brought into focus the pervasiveness of mesoscale eddies that
have typical scales of tens to hundreds of kilometers [5], are the ocean's
analogue of weather systems, and are often thought of as the peak of the
ocean's kinetic energy (KE) wavenumber spectrum [7, 19, 23]. Yet, our
understanding of the ocean's spatial scales has been derived mostly from
Fourier analysis in small representative regions (e.g. [16, 14, 4]), typically
a few hundred kilometers in size, that cannot capture the vast dynamic range at
planetary scales. Here, we present the first truly global wavenumber spectrum
of the oceanic circulation from satellite data and high-resolution re-analysis
data, using a coarse-graining method to analyze scales much larger than what
had been possible before. Spectra spanning over three orders of magnitude in
length-scale reveal the Antarctic Circumpolar Current (ACC) as the spectral
peak of the global extra-tropical ocean, at $\approx 10 \times 10^3~$km. We
also find a previously unobserved power-law scaling over scales larger than
$10^3~$km. A smaller spectral peak exists at $\approx 300~$km associated with
the mesoscales, which, due to their wider spread in wavenumber space, account
for more than $50\%$ of the resolved surface KE globally. Length-scales that
are twice as large (up to \(10^3\)~km) exhibit a characteristic lag time of
\(\approx40~\)days in their seasonal cycle, such that in both hemispheres KE at
$100~$km peaks in late spring while KE at $10^3~$km peaks in late summer. The
spectrum presented here affords us a new window for understanding the
multiscale general oceanic circulation within Earth's climate system, including
the largest planetary scales.
