Dukhovskoy, D. S., Yashayaev, I., Proshutinsky, A., Bamber, J. L., Bashmachnikov, I. L., Chassignet, E. P., et al. (2019). Role of Greenland Freshwater Anomaly in the Recent Freshening of the Subpolar North Atlantic. J. Geophys. Res. Oceans, 124(5), 3333–3360.
Abstract: The cumulative Greenland freshwater flux anomaly has exceeded 5000 km3 since the 1990s. The volume of this surplus fresh water is expected to cause substantial freshening in the North Atlantic. Analysis of hydrographic observations in the subpolar seas reveal freshening signals in the 2010s. The sources of this freshening are yet to be determined. In this study, the relationship between the surplus Greenland freshwater flux and this freshening is tested by analyzing the propagation of the Greenland freshwater anomaly and its impact on salinity in the subpolar North Atlantic based on observational data and numerical experiments with and without the Greenland runoff. A passive tracer is continuously released during the simulations at freshwater sources along the coast of Greenland to track the Greenland freshwater anomaly. Tracer budget analysis shows that 44% of the volume of the Greenland freshwater anomaly is retained in the subpolar North Atlantic by the end of the simulation. This volume is sufficient to cause strong freshening in the subpolar seas if it stays in the upper 50�100 m. However, in the model the anomaly is mixed down to several hundred meters of the water column resulting in smaller magnitudes of freshening compared to the observations. Therefore, the simulations suggest that the accelerated Greenland melting would not be sufficient to cause the observed freshening in the subpolar seas and other sources of fresh water have contributed to the freshening. Impacts on salinity in the subpolar seas of the freshwater transport through Fram Strait and precipitation are discussed.
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Misra, V., & Bhardwaj, A. (2020). The impact of varying seasonal lengths of the rainy seasons of India on its teleconnections with tropical sea surface temperatures. Atmos Sci Lett, 21(3), 9658–9689.
Abstract: We present in this paper the interannual variability of seasonal temperature and rainfall in the Indian meteorological subdivisions (IMS) for boreal winter and summer seasons that take in to account the varying length of the seasons. Our study reveals that accounting for the variations in the length of the seasons produces stronger teleconnections between the seasonal anomalies of surface temperature and rainfall over India with corresponding sea surface temperature anomalies of the tropical Oceans (especially over the northern Indian and the equatorial Pacific Oceans) compared to the same teleconnections from fixed length seasons over the IMS. It should be noted that the IMS show significant spatial heterogeneity in these teleconnections.
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Rahaman, H., Srinivasu, U., Panickal, S., Durgadoo, J. V., Griffies, S. M., Ravichandran, M., et al. (2020). An assessment of the Indian Ocean mean state and seasonal cycle in a suite of interannual CORE-II simulations. Ocean Modelling, 145.
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Timko, P. G., Arbic, B. K., Hyder, P., Richman, J. G., Zamudio, L., O'Dea, E., et al. (2019). Assessment of shelf sea tides and tidal mixing fronts in a global ocean model. Ocean Modelling, 136, 66–84.
Abstract: Tidal mixing fronts, which represent boundaries between stratified and tidally mixed waters, are locations of enhanced biological activity. They occur in summer shelf seas when, in the presence of strong tidal currents, mixing due to bottom friction balances buoyancy production due to seasonal heat flux. In this paper we examine the occurrence and fidelity of tidal mixing fronts in shelf seas generated within a global 3-dimensional simulation of the HYbrid Coordinate Ocean Model (HYCOM) that is simultaneously forced by atmospheric fields and the astronomical tidal potential. We perform a first order assessment of shelf sea tides in global HYCOM through comparison of sea surface temperature, sea surface tidal elevations, and tidal currents with observations. HYCOM was tuned to minimize errors in M2 sea surface heights in deep water. Over the global coastal and shelf seas (depths <200 m) the area-weighted root mean square error of the M2 sea surface amplitude in HYCOM represents 35% of the 50 cm root mean squared M2 sea surface amplitude when compared to satellite constrained models TPXO8 and FES2014. HYCOM and the altimeter constrained tidal models TPXO8 and FES2014 exhibit similar skill in reproducing barotropic tidal currents estimated from in-situ current meter observations. Through comparison of a global HYCOM simulation with tidal forcing to a global HYCOM simulation with no tides, and also to previous regional studies of tidal mixing fronts in shelf seas, we demonstrate that HYCOM with embedded tides exhibits quite high skill in reproducing known tidal mixing fronts in shelf seas. Our results indicate that the amount of variability in the location of the tidal mixing fronts in HYCOM, estimated using the Simpson-Hunter parameter, is consistent with previous studies when the differences in the net downward heat flux, on a global scale, are taken into account. We also provide evidence of tidal mixing fronts on the North West Australian Shelf for which we have been unable to find references in the existing scientific literature.
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Jeon, C. - H., Buijsman, M. C., Wallcraft, A. J., Shriver, J. F., Arbic, B. K., Richman, J. G., et al. (2019). Improving surface tidal accuracy through two-way nesting in a global ocean model. Ocean Modelling, 137, 98–113.
Abstract: In global ocean simulations, forward (non-data-assimilative) tide models generally feature large sea-surface-height errors near Hudson Strait in the North Atlantic Ocean with respect to altimetry-constrained tidal solutions. These errors may be associated with tidal resonances that are not well resolved by the complex coastal-shelf bathymetry in low-resolution simulations. An online two-way nesting framework has been implemented to improve global surface tides in the HYbrid Coordinate Ocean Model (HYCOM). In this framework, a high-resolution child domain, covering Hudson Strait, is coupled with a relatively low-resolution parent domain for computational efficiency. Data such as barotropic pressure and velocity are exchanged between the child and parent domains with the external coupler OASIS3-MCT. The developed nesting framework is validated with semi-idealized basin-scale model simulations. The M2 sea-surface heights show very good accuracy in the one-way and two-way nesting simulations in Hudson Strait, where large tidal elevations are observed. In addition, the mass and tidal energy flux are not adversely impacted at the nesting boundaries in the semi-idealized simulations. In a next step, the nesting framework is applied to a realistic global tide simulation. In this simulation, the resolution of the child domain (1/75°) is three times as fine as that of the parent domain (1/25°). The M2 sea-surface-height root-mean-square errors with tide gauge data and the altimetry-constrained global FES2014 and TPXO9-atlas tidal solutions are evaluated for the nesting and no-nesting solutions. The better resolved coastal bathymetry and the finer grid in the child domain improve the local tides in Hudson Strait and Bay, and the back-effect of the coastal tides induces an improvement of the barotropic tides in the open ocean of the Atlantic.
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Jeon, C. - H., Buijsman, M. C., Wallcraft, A. J., Shriver, J. F., Arbic, B. K., Richman, J. G., et al. Improving surface tidal accuracy through two-way nesting in a global ocean model. Ocean Modelling, .
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Kelly, T. B., Goericke, R., Kahru, M., Song, H., & Stukel, M. R. (2018). CCE II: Spatial and interannual variability in export efficiency and the biological pump in an eastern boundary current upwelling system with substantial lateral advection. Deep Sea Research Part I: Oceanographic Research Papers, 140, 14–25.
Abstract: Estimating interannual variability in carbon export is a key goal of many marine biogeochemical studies. However, due to variations in export mechanisms between regions, generalized models used to estimate global patterns in export often fail when used for intra-regional analysis. We present here a region-specific model of export production for the California Current Ecosystem (CCE) parameterized using intensive Lagrangian process studies conducted during El Niño-Southern Oscillation (ENSO) warm and neutral phases by the CCE Long-Term Ecological Research (LTER) program. We find that, contrary to expectations from prominent global algorithms, export efficiency (e-ratio = export / primary productivity) is positively correlated with temperature and negatively correlated with net primary productivity (NPP). We attribute these results to the substantial horizontal advection found within the region, and verify this assumption by using a Lagrangian particle tracking model to estimate water mass age. We further suggest that sinking particles in the CCE are comprised of a recently-produced, rapidly-sinking component (likely mesozooplankton fecal pellets) and a longer-lived, slowly-sinking component that is likely advected long distances prior to export. We determine a new algorithm for estimating particle export in the CCE from NPP (Export = 0.08 · NPP + 72 mg C m-2 d-1). We apply this algorithm to a two-decade long time series of NPP in the CCE to estimate spatial and interannual variability across multiple ENSO phases. Reduced export during the warm anomaly of 2014-2015 and El Niño 2015-2016 resulted primarily from decreased export in the coastal upwelling region of the CCE; the oligotrophic offshore region exhibited comparatively low seasonal and interannual variability in flux. The model resolves intra-regional patterns of in situ export measurements, and provides a valuable contrast to global export models.
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Kelly, T. B., Goericke, R., Kahru, M., Song, H., & Stukel, M. R. (2018). CCE II: Spatial and interannual variability in export efficiency and the biological pump in an eastern boundary current upwelling system with substantial lateral advection. Deep Sea Research Part I: Oceanographic Research Papers, 140, 14–25.
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Morrow, R. M., Ohman, M. D., Goericke, R., Kelly, T. B., Stephens, B. M., & Stukel, M. R. (2018). CCE V: Primary production, mesozooplankton grazing, and the biological pump in the California Current Ecosystem: Variability and response to El Niño. Deep Sea Research Part I: Oceanographic Research Papers, 140, 52–62.
Abstract: Predicting marine carbon sequestration in a changing climate requires mechanistic understanding of the processes controlling sinking particle flux under different climatic conditions. The recent occurrence of a warm anomaly (2014-2015) followed by an El Nino (2015-2016) in the southern sector of the California Current System presented an opportunity to analyze changes in the biological carbon pump in response to altered climate forcing. We compare primary production, mesozooplankton grazing, and carbon export from the euphotic zone during quasi-Lagrangian experiments conducted in contrasting conditions: two cruises during warm years – one during the warm anomaly in 2014 and one toward the end of El Nino 2016 – and three cruises during El Ninoneutral years. Results showed no substantial differences in the relationships between vertical carbon export and its presumed drivers (primary production, mesozooplankton grazing) between warm and neutral years. Mesozooplankton fecal pellet enumeration and phaeopigment measurements both showed that fecal pellets were the dominant contributor to export in productive upwelling regions. In more oligotrophic regions, fluxes were dominated by amorphous marine snow with negligible pigment content. We found no evidence for a significant shift in the relationship between mesozooplankton grazing rate and chlorophyll concentration. However, massspecific grazing rates were lower at low-to-moderate chlorophyll concentrations during warm years relative to neutral years. We also detected a significant difference in the relationship between phytoplankton primary production and photosynthetically active radiation between years: at similar irradiance and nutrient concentrations, productivity decreased during the warm events. Whether these changes resulted from species composition changes remains to be determined. Overall, our results suggest that the processes driving export remain similar during different climate conditions, but that species compositional changes or other structural changes require further attention.
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Morrow, R. M., Ohman, M. D., Goericke, R., Kelly, T. B., Stephens, B. M., & Stukel, M. R. (2018). CCE V: Primary production, mesozooplankton grazing, and the biological pump in the California Current Ecosystem: Variability and response to El Niño. Deep Sea Research Part I: Oceanographic Research Papers, 140, 52–62.
Abstract: Predicting marine carbon sequestration in a changing climate requires mechanistic understanding of the processes controlling sinking particle flux under different climatic conditions. The recent occurrence of a warm anomaly (2014-2015) followed by an El Nino (2015-2016) in the southern sector of the California Current System presented an opportunity to analyze changes in the biological carbon pump in response to altered climate forcing. We compare primary production, mesozooplankton grazing, and carbon export from the euphotic zone during quasi-Lagrangian experiments conducted in contrasting conditions: two cruises during warm years – one during the warm anomaly in 2014 and one toward the end of El Nino 2016 – and three cruises during El Ninoneutral years. Results showed no substantial differences in the relationships between vertical carbon export and its presumed drivers (primary production, mesozooplankton grazing) between warm and neutral years. Mesozooplankton fecal pellet enumeration and phaeopigment measurements both showed that fecal pellets were the dominant contributor to export in productive upwelling regions. In more oligotrophic regions, fluxes were dominated by amorphous marine snow with negligible pigment content. We found no evidence for a significant shift in the relationship between mesozooplankton grazing rate and chlorophyll concentration. However, massspecific grazing rates were lower at low-to-moderate chlorophyll concentrations during warm years relative to neutral years. We also detected a significant difference in the relationship between phytoplankton primary production and photosynthetically active radiation between years: at similar irradiance and nutrient concentrations, productivity decreased during the warm events. Whether these changes resulted from species composition changes remains to be determined. Overall, our results suggest that the processes driving export remain similar during different climate conditions, but that species compositional changes or other structural changes require further attention.
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