Waves generated by tropical cyclones can have devastating effects on coastal regions. However, the role of ocean currents in modifying wave amplitudes, wavelengths, and directions is commonly overlooked in wave forecasts, despite the fact that these interactions can lead to extreme wave conditions. Here, we use satellite observations and wave modeling to quantify the effects of ocean currents on the surface waves generated during a tropical cyclone event in the Arabian Sea. As a case study, this paper documents beams of wave heights originating from the eyewall of a tropical cyclone caused by current-induced refraction. Alternating regions of high and low wave heights in the model simulations are consistent with observations and extend for thousands of kilometers all the way to 100 m isobath. Our results highlight the importance of accounting for wave refraction by currents in order to accurately predict the impact of tropical cyclone generated waves on coastal regions.

Sea surface winds off the California coast are characterized by high wind events that occur in spring and summer. In June, a well-defined wind event region is formed off the five major capes, extending ~300km offshore. In the present work, a satellite wind product is used to study the spatial variability of these wind events. High-speed and long-duration events primarily occur off Cape Mendocino, whereas low-speed and short-duration events are more uniformly distributed over the wind event region. Coastal buoy observations show an anti-correlation between wind speed and sea surface temperature (SST) during wind events: a decrease in wind speed accompanies an increase in SST before the start of events, and an increase in wind speed accompanies a decrease in SST after the start of events. Different SST cooling patterns are observed within different categories of wind events: (1) High-speed events lead to more SST cooling compared to low-speed events. (2) Long-duration events lead to longer SST cooling times compared to short-duration events. SST cooling is observed both at nearshore buoy locations and at locations far from the coast. The magnitude of cooling is about 1°C nearshore and 0.3°C offshore. A case study of upper-ocean responses from mooring observations suggests that a combination of enhanced wind-driven mixing and Ekman pumping processes may explain SST cooling nearshore during wind events.

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.

While waves propagating over a uniform current are simply subject to a Doppler shift of their phase, inhomogeneous flows can modify the wavenumber, direction, and amplitude of the waves, having the potential to largely modulate the surface wave field. Even though the theoretical basis for such interactions is well established, comprehensive observations and modeling of wave–current interactions are mostly limited to either tidal or large–scale currents and a lot remains unknown about how waves and currents interact when both fields are highly variable, such as near ocean fronts and eddies. In the present work, the extent to which the surface wave field off the California coast is modulated by the California Current System is investigated. Optimized currents and winds from a state estimate of the California Current System are used to force the wave model WaveWatch III in order to quantify the relative importance of local winds and currents in modulating the surface wave variability in this region. As satellite altimeters evolve towards resolving finer scales, knowing the wave field with precision may help the interpretation of sea surface height measurements at high wavenumbers and frequencies, which has particular relevance for the planning of the Surface Water and Ocean Topography (SWOT) mission.