The Seasonal Cycle of Significant Wave Height in the Ocean: Local vs
Remote Forcing
Abstract
Significant wave height (SWH) stems from a combination of locally
generated “wind-sea” and remotely generated “swell” waves. In the
Northern and Southern Hemispheres, wave heights typically undergo a
sinusoidal annual cycle, with larger SWH in winter in response to
seasonal changes in high-latitude storm patterns that generate
equatorward propagating swell. However, some locations deviate from this
hemispheric-scale seasonal pattern in SWH. For example, in the
California coastal region, local wind events occur in boreal spring and
summer, leading to a wind speed (WSP) annual cycle with a distinct
maximum in boreal spring and a corresponding local response in SWH. Here
ocean regions with a WSP annual cycle reaching a maximum in late spring,
summer, or early fall are designated as seasonal wind anomaly regions
(SWARs). Intra-annual variability of surface gravity waves is analyzed
globally using two decades of satellite-derived SWH and WSP data. The
phasing of the WSP annual cycle is used as a metric to identify SWARs.
Global maps of probability of swell based on wave age confirm that
during the spring and summer months, locally forced waves are more
statistically more likely in SWARs than in surrounding regions. The
magnitude of the deviation in the SWH annual cycle is determined by the
exposure to swell and characteristics of the wave field within the
region. Local winds have a more identifiable impact on Northern
Hemisphere SWARs than on Southern Hemisphere SWARs due to the larger
seasonality of Northern Hemisphere winds.