Matthias Münnich

A Study of Self-excited Oscillations of the Tropical Ocean–Atmosphere System. Part II: Nonlinear Cases

Abstract 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 ...

Entrainment, Rayleigh Friction, and Boundary Layer Winds over the Tropical Pacific

Abstract Winds over the tropical Pacific are interpreted using mixed-layer theory. The theory—which posits that the surface winds can be derived in terms of a force balance among surface drag, pressure gradients, Coriolis forces, and the vertical mixing of momentum into the boundary layer (entrainment)—is very successful in predicting the seasonal climatology of the surface winds. The model is also used as a basis for interpreting previous results. In particular the model illustrates why studies that model the momentum flux divergence as a Rayleigh damping find optimal damping coefficients that are anisotropic. A linear variant of the model, which also incorporates entrainment but neglects the quadratic relation between the wind speed and the surface stress, is also found to predict the surface winds skillfully. In addition to improving the representation of the winds, it leads to realistic representations of the divergence of the vector wind. If the key parameters of the model (the entrainment rate and t...

Remote versus local influence of ENSO on the California Current System

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.

Southern Ocean eddy phenomenology

Mesoscale eddies are ubiquitous features in the Southern Ocean, yet their phenomenology is not well quantified. To tackle this task, we use satellite observations of sea level anomalies and sea surface temperature (SST) as well as in situ temperature and salinity measurements from profiling floats. Over the period 1997–2010, we identified over a million mesoscale eddy instances and were able to track about 105 of them over 1 month or more. The Antarctic Circumpolar Current (ACC), the boundary current systems, and the regions where they interact are hot spots of eddy presence, representing also the birth places and graveyards of most eddies. These hot spots contrast strongly to areas shallower than about 2000 m, where mesoscale eddies are essentially absent, likely due to topographical steering. Anticyclones tend to dominate the southern subtropical gyres, and cyclones the northern flank of the ACC. Major causes of regional polarity dominance are larger formation numbers and lifespans, with a contribution of differential propagation pathways of long-lived eddies. Areas of dominance of one polarity are generally congruent with the same polarity being longer-lived, bigger, of larger amplitude, and more intense. Eddies extend down to at least 2000 m. In the ACC, eddies show near surface temperature and salinity maxima, whereas eddies in the subtropical areas generally have deeper anomaly maxima, presumably inherited from their origin in the boundary currents. The temperature and salinity signatures of the average eddy suggest that their tracer anomalies are a result of both trapping in the eddy core and stirring.

Sea-ice transport driving Southern Ocean salinity and its recent trends

Recent salinity changes in the Southern Ocean are among the most prominent signals of climate change in the global ocean, yet their underlying causes have not been firmly established. Here we propose that trends in the northward transport of Antarctic sea ice are a major contributor to these changes. Using satellite observations supplemented by sea-ice reconstructions, we estimate that wind-driven northward freshwater transport by sea ice increased by 20 ± 10 per cent between 1982 and 2008. The strongest and most robust increase occurred in the Pacific sector, coinciding with the largest observed salinity changes. We estimate that the additional freshwater for the entire northern sea-ice edge entails a freshening rate of −0.02 ± 0.01 grams per kilogram per decade in the surface and intermediate waters of the open ocean, similar to the observed freshening. The enhanced rejection of salt near the coast of Antarctica associated with stronger sea-ice export counteracts the freshening of both continental shelf and newly formed bottom waters due to increases in glacial meltwater. Although the data sources underlying our results have substantial uncertainties, regional analyses and independent data from an atmospheric reanalysis support our conclusions. Our finding that northward sea-ice freshwater transport is also a key determinant of the mean salinity distribution in the Southern Ocean further underpins the importance of the sea-ice-induced freshwater flux. Through its influence on the density structure of the ocean, this process has critical consequences for the global climate by affecting the exchange of heat, carbon and nutrients between the deep ocean and surface waters.

The influence of bottom topography on internal seiches in stratified media

Abstract Standing internal waves, so-called seiches, are ubiquitous in reservoirs and lakes. Although the stratification in such basin is often continuous, the modeling of seiches has been confined mostly to two-layer models. Such models are unable to give reliable insights into the vertical structure of the seiches, which might be crucial for the understanding of vertical mixing in natural water basins. To obtain this kind of information a two-dimensional computer model has been developed, which takes both continuous stratification and bottom topography into account. The results of this model are presented. The computed seiche modes reveal that (1) several large-scale modes can exist with similar eigenfrequencies, (2) the modes have a tendency to develop narrow jets and (3) only the lowest modes are strongly influenced by the bottom topography.

Atmospheric Response to Mesoscale Sea Surface Temperature Anomalies: Assessment of Mechanisms and Coupling Strength in a High-Resolution Coupled Model over the South Atlantic

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.

Local atmospheric forcing driving an unexpected California Current System response during the 2015–2016 El Niño

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.

Climatic modulation of recent trends in ocean acidification in the California Current System

We reconstruct the evolution of ocean acidification in theCalifornia Current System (CalCS) from 1979 through 2012 using hindcast simulations with an eddy-resolving ocean biogeochemicalmodel forcedwith observation-based variations of wind andfluxes of heat and freshwater.We find that domain-wide pH and arag W in the top 60mof thewater columndecreased significantly over these three decades by about−0.02 decade and−0.12 decade, respectively. In the nearshore areas of northernCalifornia andOregon, ocean acidification is reconstructed to have progressedmuchmore rapidly, with rates up to 30%higher than the domain-wide trends. Furthermore, ocean acidification penetrated substantially into the thermocline, causing a significant domain-wide shoaling of the aragonite saturation depth of on average−33m decade and up to−50m decade in the nearshore area of northernCalifornia. This resulted in a coast-wide increase in nearly undersaturatedwaters and the appearance of waters with 1 arag W < , leading to a substantial reduction of habitat suitability. Averaged over thewhole domain, themain driver of these trends is the oceanic uptake of anthropogenic CO2 from the atmosphere. However, recent changes in the climatic forcing have substantiallymodulated these trends regionally. This is particularly evident in the nearshore regions, where the total trends in pH are up to 50% larger and trends in arag W and in the aragonite saturation depth are even twice to three times larger than the purely atmospheric CO2-driven trends. This modulation in the nearshore regions is a result of the recentmarked increase in alongshorewind stress, which brought elevated levels of dissolved inorganic carbon to the surface via upwelling. Our results demonstrate that changes in the climatic forcing need to be taken into consideration in future projections of the progression of ocean acidification in coastal upwelling regions.

Mesoscale atmosphere ocean coupling enhances the transfer of wind energy into the ocean

Although it is well established that the large-scale wind drives much of the worlds ocean circulation, the contribution of the wind energy input at mesoscales (10–200 km) remains poorly known. Here we use regional simulations with a coupled high-resolution atmosphere–ocean model of the South Atlantic, to show that mesoscale ocean features and, in particular, eddies can be energized by their thermodynamic interactions with the atmosphere. Owing to their sea-surface temperature anomalies affecting the wind field above them, the oceanic eddies in the presence of a large-scale wind gradient provide a mesoscale conduit for the transfer of energy into the ocean. Our simulations show that this pathway is responsible for up to 10% of the kinetic energy of the oceanic mesoscale eddy field in the South Atlantic. The conditions for this pathway to inject energy directly into the mesoscale prevail over much of the Southern Ocean north of the Polar Front.