ALBERT

Description: For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017.

Description: The Observing Air–Sea Interactions Strategy (OASIS) is a new United Nations Decade of Ocean Science for Sustainable Development programme working to develop a practical, integrated approach for observing air–sea interactions globally for improved Earth system (including ecosystem) forecasts, CO2 uptake assessments called for by the Paris Agreement, and invaluable surface ocean information for decision makers. Our “Theory of Change” relies upon leveraged multi-disciplinary activities, partnerships, and capacity strengthening. Recommendations from 〉40 OceanObs’19 community papers and a series of workshops have been consolidated into three interlinked Grand Ideas for creating #1: a globally distributed network of mobile air–sea observing platforms built around an expanded array of long-term time-series stations; #2: a satellite network, with high spatial and temporal resolution, optimized for measuring air–sea fluxes; and #3: improved representation of air–sea coupling in a hierarchy of Earth system models. OASIS activities are organized across five Theme Teams: (1) Observing Network Design & Model Improvement; (2) Partnership & Capacity Strengthening; (3) UN Decade OASIS Actions; (4) Best Practices & Interoperability Experiments; and (5) Findable–Accessible–Interoperable–Reusable (FAIR) models, data, and OASIS products. Stakeholders, including researchers, are actively recruited to participate in Theme Teams to help promote a predicted, safe, clean, healthy, resilient, and productive ocean.

Description: State of the climate in 2019

Description: In the equatorial Atlantic Ocean, meridional velocity variability exhibits a pronounced peak on intraseasonal timescales whereas zonal velocity dominantly varies on seasonal to interannual timescales. We focus on the intraseasonal meridional velocity variability away from the near-surface layer, its source regions and its pathways into the deep ocean. This deep intraseasonal velocity variability plays a key role in equatorial dynamics as it is an important energy source for the deep equatorial circulation. The results are based on the output of a high-resolution ocean model revealing intraseasonal energy levels along the equator at all depths that are in good agreement with shipboard and moored velocity data. Spectral analyses reveal a pronounced signal of intraseasonal Yanai waves with westward phase velocities and zonal wavelengths longer than 450 km. Different sources and characteristics of these Yanai waves are identified: near the surface between 40°W and 10°W low-baroclinic-mode Yanai waves with periods of around 30 days are exited. These waves have a strong seasonal cycle with a maximum in August. High-frequency Yanai waves (10–20-day period) are excited at the surface east of 10°W. In the region between the North Brazil Current and the Equatorial Undercurrent high-baroclinic-mode Yanai waves with periods between 30 and 40 days are generated. Yanai waves with longer periods (40-80 days) are shed from the Deep Western Boundary Current. The Yanai wave energy is carried along beams east- and downward thus explaining differences in strength, structure and periodicity of the meridional intraseasonal variability in the equatorial Atlantic Ocean.

Description: Marine heatwaves along the coast ofWestern Australia, referred to as Ningaloo Niño, have had dramatic impacts on the ecosystem in the recent decade. A number of local and remote forcing mechanisms have been put forward, however little is known about the depth structure of such temperature extremes. Utilizing an eddy-active global Ocean General Circulation Model, Ningaloo Niño and the corresponding cold Ningaloo Niña events are investigated between 1958-2016, with focus on their depth structure. The relative roles of buoyancy and wind forcing are inferred from sensitivity experiments. Composites reveal a strong symmetry between cold and warm events in their vertical structure and associated large-scale spatial patterns. Temperature anomalies are largest at the surface, where buoyancy forcing is dominant and extend down to 300m depth (or deeper), with wind forcing being the main driver. Large-scale subsurface anomalies arise from a vertical modulation of the thermocline, extending from the western Pacific into the tropical eastern Indian Ocean. The strongest Ningaloo Niños in 2000 and 2011 are unprecedented compound events, where long-lasting high temperatures are accompanied by extreme freshening, which emerges in association with La Niñas, more common and persistent during the negative phase of the Interdecadal Pacific Oscillation. It is shown that Ningaloo Niños during La Nina phases have a distinctively deeper reach and are associated with a strengthening of the Leeuwin Current, while events during El Niño are limited to the surface layer temperatures, likely driven by local atmosphere-ocean feedbacks, without a clear imprint on salinity and velocity.

Description: The Antarctic Slope Front (ASF) is a fundamental feature of the subpolar Southern Ocean that is still poorly observed. In this study we build a statistical climatology of the temperature and salinity fields of the upper 380 m of the Antarctic margin. We use a comprehensive compilation of observational datasets including the profiles gathered by instrumented marine mammals. The mapping method consists first of a decomposition in vertical modes of the combined temperature and salinity profiles. Then the resulting principal components are optimally interpolated on a regular grid and the monthly climatological profiles are reconstructed, providing a physically plausible representation of the ocean. The ASF is located with a contour method and a gradient method applied on the temperature field, two complementary approaches that provide a complete view of the ASF structure. The front extends from the Amundsen Sea to the eastern Weddell Sea and closely tracks the continental shelf break. It is associated with a sharp temperature gradient that is stronger in winter and weaker in summer. The emergence of the front in the Amundsen and Bellingshausen sectors appears to be seasonally variable (slightly more westward in winter than in summer). Investigation of the density gradients across the shelf break indicates a winter slowdown of the baroclinic component of the Antarctic Slope Current at the near surface, in contrast with the seasonal variability of the temperature gradient.

Description: Diatoms often dominate phytoplankton in temperate, polar and upwelling regions. Decreases in silicate availability or silicon to nitrogen (Si:N) ratios may induce silicon limitation in diatoms and lower their proportion within phytoplankton communities. The effects of such changes on the nutritional quality of phytoplankton are not well understood. To examine how changing Si:N ratios affect plankton nutritional value, we applied a range of Si:N ratios on a natural plankton community and manipulated grazing pressure to assess top-down effects of copepod selective grazing. Diatom proportion in phytoplankton increased with increasing Si:N ratios and so did phytoplankton nutritional quality in terms of major fatty acid concentrations, such as polyunsaturated fatty acids, docosahexaenoic (DHA) and eicosapentaenoic (EPA) acids. However, stoichiometric quality (carbon to nitrogen and carbon to phosphorus ratios), DHA:EPA and omega 3:6 (omega 3:omega 6) ratios declined with increasing Si:N ratios, suggesting that proportions between essential compounds in copepod diet may be more favorable in lowered Si:N ratios. Copepods had a negative effect on DHA contents, DHA:EPA and omega 3:omega 6 ratios, indicating possible selective grazing on more nutritious plankton. Our findings show that declining silicate concentrations can affect stoichiometric and biochemical quality of phytoplankton, which copepods can also moderate by selective grazing.

Description: Temperature and dehydration stress are two major co-occurring environmental stressors threatening the physiology, biochemistry, and ecology of insects. As such, understanding adaptive responses to desiccation stress is critical for predicting climate change impacts, particularly its influence on insect invasions. Here, we assessed water balance and desiccation resistance of the invasive Tuta absoluta (Meyrick, 1917) (Lepidoptera: Gelechiidae), and infer how eco-physiology shapes its niche. We measured basal body water and lipid content, water loss rates (WLRs), and desiccation resistance in larvae (second to fourth instars) and adults. Body -water, -lipid, and WLRs significantly varied across life stages. Second instars recorded the lowest while fourth instars exhibited the highest body water and lipid content. Adult body water and lipid content were higher than second and third instars and lower than fourth instars while proportion of body water and lipid contents were highest in adults and second larval instars respectively. Water loss rates were significantly highest in fourth-instar larvae compared to other life stages, but differences among stages were less apparent at longer exposure durations (48 h). Desiccation resistance assays showed that second instars had greatest mortality while fourth-instar larvae and adults were the most desiccation tolerant. Our results show that T. absoluta fourth-instar larvae and adults are the most resilient developmental stages and potentially contribute most to the invasion success of the pest in arid environments. Incorporation of these species-specific eco-physiological traits in predictive models can help refine invasive species potential spread under changing climates.

Description: There is a controversy about the nature of multidecadal climate variability in the North Atlantic (NA) region, concerning the roles of ocean circulation and atmosphere–ocean coupling. Here we describe NA multidecadal variability from a version of the Kiel Climate Model, in which both subpolar gyre (SPG)–Atlantic meridional overturning circulation (AMOC) coupling and atmosphere–ocean coupling are essential. The oceanic barotropic and meridional overturning streamfunctions and the sea level pressure are jointly analyzed to derive the leading mode of Atlantic sector variability. This mode accounting for 23.7% of the total combined variance is oscillatory with an irregular periodicity of 25–50 years and an e-folding time of about a decade. SPG and AMOC mutually influence each other and together provide the delayed negative feedback necessary for maintaining the oscillation. An anomalously strong SPG, for example, drives higher surface salinity and density in the NA’s sinking region. In response, oceanic deep convection and AMOC intensify, which, with a time delay of about a decade, reduces SPG strength by enhancing upper-ocean heat content. The weaker gyre leads to lower surface salinity and density in the sinking region, which reduces deep convection and eventually AMOC strength. There is a positive ocean–atmosphere feedback between the sea surface temperature and low-level atmospheric circulation over the southern Greenland area, with related wind stress changes reinforcing SPG changes, thereby maintaining the (damped) multidecadal oscillation against dissipation. Stochastic surface heat flux forcing associated with the North Atlantic Oscillation drives the eigenmode.