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Air-sea interactions have been found to substantially affect and drive marine extreme events. Such extreme events comprise, among others, highly anomalous conditions in ocean temperature, pH, and oxygen content - all of which are crucial parameters directly impacting marine ecosystem. Nevertheless, our understanding of the role of such events in the marine environment remains limited. In addition, the extent to which the interplay between atmospheric and oceanic forcings impacts the spatial and temporal scales of extreme events and affects the marine ecosystem and ocean biogeochemistry remains largely unknown. Given these complex interactions between the atmosphere, the ocean, and marine biogeochemistry, we developed a coupled regional high-resolution Earth System Model (ROMSOC). ROMSOC comprises the latest officially released GPU-accelerated Consortium for Small-Scale Modeling (COSMO) version as the atmospheric model, coupled to the Regional Oceanic Modeling System (ROMS). ROMS in turn includes the Biogeochemical Elemental Cycling (BEC) model that describes the functioning of the lower trophic ecosystem in the ocean and the associated biogeochemical cycle. Our current model setup includes thermodynamical coupling and will be extended further to include mechanical coupling between the atmosphere and the ocean. Here, we present first simulations of our coupled model system for the California Current System (CalCS) at the US west coast at kilometer-scale resolution. We will test the hypothesis if the strong mesoscale coupling of the atmosphere and the ocean as represented in our model impacts the spatial and temporal scales of marine heatwaves and can potentially act to shorten their duration.

The 2015-2016 El Nino contributed to large anomalies across the California Current System (CalCS), but these anomalies ceased unexpectedly in late 2015. Here, we use a suite of three hindcast simulations with the Regional Oceanic Modeling System (ROMS) to assess the responsible mechanisms for this development. We find that the early build-up was primarily driven by the early onset of this event in the tropical Pacific, driving anomalies in the CalCS through the propagation of coastally trapped waves. In contrast, the abrupt end in the central CalCS was caused by the unusual onset of upwelling favorable winds in the fall of 2015, which offset the continuing remote forcing through the coastal waveguide. Nevertheless, low nutrient anomalies persisted, causing anomalously low phytoplankton abundance in the upwelling season of 2016. This is a recurring pattern for all El Nino events over the last 37 years, suggesting predictive skill on seasonal timescales.

Many aspects of the coupling between the ocean and atmosphere at the mesoscale (on the order of 20–100 km) remain unknown. While recent observations from the Southern Ocean revealed that circular fronts associated with oceanic mesoscale eddies leave a distinct imprint on the overlying wind, cloud coverage, and rain, the mechanisms responsible for explaining these atmospheric changes are not well established. Here the atmospheric response above mesoscale ocean eddies is investigated utilizing a newly developed coupled atmosphere–ocean regional model [Consortium for Small-Scale Modeling–Regional Ocean Modelling System (COSMO-ROMS)] configured at a horizontal resolution of ~10 km for the South Atlantic and run for a 3-month period during austral winter of 2004. The model-simulated changes in surface wind, cloud fraction, and rain above the oceanic eddies are very consistent with the relationships inferred from satellite observations for the same region and time. From diagnosing the model’s momentum balance, it is shown that the atmospheric imprint of the oceanic eddies are driven by the modification of vertical mixing in the atmospheric boundary layer, rather than secondary flows driven by horizontal pressure gradients. This is largely due to the very limited ability of the atmosphere to adjust its temperature over the time scale it takes for an air parcel to pass over these mesoscale oceanic features. This results in locally enhanced vertical gradients between the ocean surface and the overlying air and thus a rapid change in turbulent mixing in the atmospheric boundary layer and an associated change in the vertical momentum flux.

Much of the observed interannual variability in the physical and biogeochemical state of the California Current System (CalCS) is associated with El Nino Southern Oscillation. Yet it is unclear whether this is primarily a result of atmospheric teleconnections forcing the ocean locally through changes in wind and fluxes of heat and freshwater, or whether this is a consequence of oceanic interior processes that transport tropical variability through, e.g., coastally trapped waves to the region. Here we investigate the relative contribution of these two mechanisms in the CalCS using a novel setup of the Regional Oceanic Modeling System coupled to a biogeochemical/ecological model. We conducted a hindcast simulation over the period 1979–2013 and contrast the results with those from sensitivity simulations with climatological atmospheric boundary conditions either for the U.S. West Coast or the rest of the Pacific. We find that remote forcing dominates the variability of the physical state in the nearshore region of the CalCS, explaining up to 80% of monthly mean sea-surface height and temperature variability. In contrast, local processes tend to drive variations in the biogeochemical/ecological state, particularly along central and northern California, explaining up to 50% of the observed surface variability. Most of the remote forcing is a consequence of coastally trapped waves that travel northward at speeds of approximately 230 km d−1, and thereby alter sea-level height, thermocline structure, and upwelling along California. Biogeochemically active tracers respond to this remote forcing as well, especially at depth, but are more strongly modulated by local atmospheric forcing, especially variations in upwelling-favorable winds.

Owing to the turbulent nature of the ocean, mesoscale eddies are omnipresent. The impact of these transitory and approximately circular sea surface temperature fronts on the overlying atmosphere is not well known. Stationary fronts such as the Gulf Stream have been reported to lead to pronounced atmospheric changes1, 2. However, the impact of transient ocean eddies on the atmosphere has not been determined systematically, except on winds and to some extent clouds3, 4, 5, 6. Here, we examine the atmospheric conditions associated with over 600,000 individual eddies in the Southern Ocean, using satellite data. We show that ocean eddies locally affect near-surface wind, cloud properties and rainfall. The observed pattern of atmospheric change is consistent with a mechanism in which sea surface temperature anomalies associated with the oceanic eddies modify turbulence in the atmospheric boundary layer. In the case of cyclonic eddies, this modification triggers a slackening of near-surface winds, a decline in cloud fraction and water content, and a reduction in rainfall. We conclude that transient mesoscale ocean structures can significantly affect much larger atmospheric low-pressure systems that swiftly pass by at the latitudes investigated.

The zonal surface torque budget associated with the tropical wind stress anomalies during El Nino/Southern Oscillation is analyzed. Mountain and surface stress torques over South America are found to play a prominent role. Local momentum change is negligible for 6 month averages allowing the balance among regional contributions to the torque anomalies to be compared. During El Nino, eastward torque anomalies over the central equatorial Pacific are largely compensated by westward anomalies elsewhere in the equatorial band, notably over South America. Torque anomalies over South America and the Pacific in latitude bands north and south of the equator are both westward and are not compensated within the band, implying an export of eastward momentum to higher latitudes.

We analyze the linearized version of an analytical model, which combines linear ocean dynamics with a simple version of the Bjerknes hypothesis for El Nino. The ocean is represented by linear shallow water equations on an equatorial beta-plane. It is driven by zonal wind stress, which is assumed to have a fixed spatial form. Stress amplitude is set to be proportional to the thermocline displacement at the eastern boundary. It is shown that, for physically plausible parameter values, the model system can sustain growing Oscillations. Both growth rate and period scale directly with the time that an oceanic Kelvin wave needs to crow the basin. They are quite sensitive to the coupling parameter between thermocline displacement and wind stress, and the zonal location and meridional width of the wind. The most important parameter determining this behavior of the system is the coupling constant. For strong coupling the system exhibits exponential growth without oscillation. As the coupling is decreased ...

[1] In late boreal spring, especially May, a strong relationship exists in observations among precipitation anomalies over equatorial South America and the Atlantic intertropical convergence zone (ITCZ), and eastern equatorial Pacific and central equatorial Atlantic sea surface temperature anomalies (SSTA). A chain of correlations of equatorial Pacific SSTA, western equatorial Atlantic wind stress (WEA), equatorial Atlantic SSTA, sea surface height, and precipitation supports a causal chain in which El Nino/Southern Oscillation (ENSO) induces WEA stress anomalies, which in turn affect Atlantic equatorial ocean dynamics. These correlations show strong seasonality, apparently arising within the atmospheric links of the chain. This pathway and the influence of equatorial Atlantic SSTA on South American rainfall in May appear independent of that of the northern tropical Atlantic. Brazil's Nordeste is affected by the northern tropical Atlantic. The equatorial influence lies further to the north over the eastern Amazon and the Guiana Highlands.