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Ocean acidification is punctuated by episodic extremes of low pH and saturation state with regard to aragonite (omega(A)). Here, we use a hindcast simulation from 1984 to 2019 with a high-resolution regional ocean model (ROMS-BEC) to identify and track ocean acidification extremes (OAX) in the northeast Pacific and the California current system (CCS). In the first step, we identify all grid-cells whose pH and omega(A) are simultaneously below their first percentile over the analysis period (1984-2019). In the second step, we aggregate all neighboring cells with extreme conditions into three-dimensional time evolving events, permitting us to track them in a Lagrangian manner over their lifetime. We detect more than 22 thousand events that occur at least once in the upper 100 m during their lifetime, with broad distributions in terms of size, duration, volume, and intensity, and with 26% of them harboring corrosive conditions (omega(A) < 1). By clustering the OAXs, we find three types of extremes in the CCS. Near the coast, intense, shallow, and short-lasting OAXs dominate, caused by strong upwelling. A second type consists of large and long-lasting OAX events that are associated with westward propagating cyclonic eddies. They account for only 3% of all extremes, but are the most severe events. The third type is small extremes at depth arising from pycnocline heave. OAXs potentially have deleterious effects on marine life. Marine calcifiers, such as pteropods, might be especially impacted by the long-lasting events with corrosive conditions., Journal of Geophysical Research: Oceans, 127 (5), ISSN:0148-0227, ISSN:2169-9275

Oceanic mesoscale eddies constitute ephemeral hotspots for marine life and are pivotal for the lateral transport of nutrients and organic matter. Here, we use a high-resolution coupled physical-biogeochemical model to study the processes sustaining biological production and export in long-living cyclonic (CE) and anticyclonic (AE) eddies of the northern Canary Upwelling System (CanUS). We track the eddies for 18 months as they propagate offshore, and study their composite properties in time in a Lagrangian manner. Our model shows that long-living CEs sustain their production with the nitrogen that they initially trap in the nearshore nutrient-rich waters and keep isolated in their cores. The vertical input of nitrate from below tends to be comparatively small, and is mostly driven by mixing. In contrast, AEs tend to start with low nutrient concentrations in their core as they do not trap coastal waters, but have elevated concentrations at their periphery. In AEs, stirring is responsible for both the building up of the positive nitrate anomaly at depth and the enhanced lateral input of organic nitrogen in the near-surface. Compared to CEs, the input of nitrate into the euphotic zone by vertical mixing is substantially more important. Though regenerated production dominates in both types of eddies, new production is higher than the regional average in CE cores and at the rim of AEs, partially compensating for the intense losses due to sinking. Both cyclonic trapping and transport and anticyclonic stirring shape the regional pattern of organic matter and nutrients in the northern CanUS., Journal of Geophysical Research: Oceans, 127 (2), ISSN:0148-0227, ISSN:2169-9275

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.

ISSN:0148-0227 ISSN:2169-9275

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.

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.

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.

[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.

Ocean deoxygenation has led to a measurable shoaling of subsurface hypoxic waters ([O-2] < 60 mmol m(-3)) in the tropical Pacific and elsewhere. This shoaling compresses the vertical habitat of heterotrophic organisms between the well oxygenated surface and the hypoxic interface. Superimposed on this long-term trend, natural variability can cause shorter-term, extreme habitat compression events, which we term Transient Habitat Reduction Extreme Events (THREEs). Here, we investigate THREEs in the Eastern Pacific (EP) between 1979 and 2016 using output from a high-resolution hindcast simulation with the coupled physical-biogeochemical ocean model ROMS-BEC. We identify THREEs on the basis of the hypoxic interface being shallower than the 99th percentile of its variability and requiring a minimum duration of five days. We find that THREEs compress the aerobic habitat by up to 50%-70% (locally exceeding 80%) in the subtropical and tropical EP and reach horizontal scales exceeding 300 km in the tropical EP. There, THREEs are pre-conditioned by La Nina conditions and occur primarily in boreal winter and spring. In the subtropical EP, THREEs are associated with mesoscale eddies and occur independently of the season. 71% (32%) of all THREEs are characterized by unusually high acidity (cold) conditions (i.e., below the first percentile), thus constituting compound events. The strong habitat compression during THREEs is bound to lead to substantial changes in water column biogeochemistry, in food-web interactions, and in ecosystem structure. Yet, so far, very little is known about the impact of these transient phenomena., Journal of Geophysical Research: Oceans, 127 (6), ISSN:0148-0227, ISSN:2169-9275