Ocean dynamics at the submesoscale play a key role in mediating upper-ocean energy dissipation and dispersion of tracers. Observations of ocean currents from synoptic mesoscale surveys at submesoscale resolution (250 m–100 km) from a novel airborne instrument (MASS DoppVis) reveal that the kinetic energy spectrum in the California Current System is nearly continuous from 100 km to sub-kilometer scales, with a k-2 spectral slope. Although there is not a transition in the kinetic energy spectral slope, there is a transition in the dynamics to non-linear interactions at scales of O(1 km). Barotropic kinetic energy transfer across spatial scales is enabled by interactions between the rotational and divergent components of the flow field at the submesoscale. Kinetic energy flux is intermittent but can be large, particularly at submesoscale fronts. Kinetic energy is transferred both downscale and upscale from 1 km in the observations of a cold filament.

Over nine years of hourly surface current data from high-frequency radar (HFR) off the US West Coast are analyzed using a Bayesian least-squares fit for tidal components. The spatial resolution and geographic extent of HFR data allow us to assess the spatial structure of the non-phase-locked component of the tide. In the frequency domain, the record length and sampling rate allow resolution of discrete tidal lines corresponding to well-known constituents and the near-tidal broadband elevated continuum resulting from amplitude and phase modulation of the tides, known as cusps. The FES2014 tide model is used to remove the barotropic component of tidal surface currents in order to evaluate its contribution to the phase-locked variance and spatial structure. The mean time scale of modulation is 243 days for the M$_2$ constituent and 181 days for S$_2$, with overlap in their range of values. These constituents’ modulated amplitudes are significantly correlated in several regions, suggesting shared forcing mechanisms. Within the frequency band M$_2$ $\pm$ 5 cycles per year, an average of 48\% of energy is not at the phase-locked frequency. When we remove the barotropic model, this increases to 64\%. In both cases there is substantial regional variability. This indicates that a large fraction of tidal energy is not easily predicted (e.g. for satellite altimeter applications). The spatial autocorrelation of the non-phase-locked variance fraction drops to zero by 150 km, comparable to the width of the swath of the recently launched Surface Water and Ocean Topography (SWOT) altimeter.

The Canada Basin has exhibited a significant trend toward a fresher surface layer and thus a more stratified upper ocean over the past three decades. Here, we explore the extent to which the Community Earth System Model (CESM) accurately simulates the observed surface freshening and seasonal processes that contribute to the freshening. We examine 30 simulations from CESM1 (used in the IPCC AR5), 3 simulations from CESM2 (IPCC AR6), and ocean observations from 1975 and 2006-2012. In contrast to the observations, the models simulate salinity profiles that show relatively little variation between 1975 and 2012. We demonstrate that this bias can be partly attributed to the model’s tendency to mix freshwater too deep, creating a surface layer that is saltier than observed. The results provide insight for climate model improvement that could have wide-reaching implications because upper-ocean stratification influences the vertical transport of heat and nutrients.

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.

The seasonal halocline impacts the exchange of heat, energy, and nutrients between the surface and the deeper ocean, and it is changing in response to Arctic sea ice melt over the past several decades. Here, we assess seasonal halocline formation in 1975 and 2006-2012 by comparing daily, May to September, below-ice salinity profiles collected in the Canada Basin. We evaluate differences between the two time periods using a one-dimensional (1D) bulk model to quantify differences in freshwater input and vertical mixing. The 1D model metrics indicate that two separate factors contribute similarly to stronger stratification in 2006-2012 than in 1975: (1) larger surface freshwater input and (2) less vertical mixing of that freshwater. The first factor is mainly important in August-September, consistent with a longer melt season in recent years. The second factor is mainly important from June until mid-August, when similar levels of freshwater input in 1975 and 2006-2012 are mixed over a different depth range, resulting in different stratification. These results imply that decadal changes to ice-ocean dynamics, in addition to freshwater input, significantly contribute to the stronger seasonal stratification in 2006-2012 than in 1975. The findings highlight the need for near-surface process studies to elucidate the roles of lateral processes and ice-ocean momentum exchange on vertical mixing.

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.