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An T. Nguyen, Ronald Kwok, and Dimitris Menemenlis

Abstract

A coupled ocean and sea ice model is used to investigate dense water (DW) formation in the Chukchi and Bering shelves and the pathways by which this water feeds the upper halocline. Two 1992–2008 data-constrained solutions at 9- and 4-km horizontal grid spacing show that 1) winter sea ice growth results in brine rejection and DW formation; 2) the DW flows primarily down Barrow and Central–Herald Canyons in the form of bottom-trapped, intermittent currents to depths of 50–150 m from the late winter to late summer seasons; and 3) eddies with diameters ~ 30 km carry the cold DW from the shelf break into the Canada Basin interior at depths of 50–150 m. The 4-km data-constrained solution does not show eddy transport across the Chukchi Shelf at shallow depths; instead, advection of DW downstream of polynya regions is driven by a strong (~0.1 m s−1) mean current on the Chukchi Shelf. Upper halocline water (UHW) formation rate was obtained from two methods: one is based on satellite data and on a simple parameterized approach, and the other is computed from the authors’ model solution. The two methods yield 5740 ±1420 km3 yr−1 and 4190–4860 ±1440 km3 yr−1, respectively. These rates imply a halocline replenishment period of 10–21 yr. Passive tracers also show that water with highest density forms in the Gulf of Anadyr and along the eastern Siberian coast immediately north of the Bering Strait. These results provide a coherent picture of the seasonal development of UHW at high spatial and temporal resolutions and serve as a guide for improving understanding of water-mass formation in the western Arctic Ocean.

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Ayan H. Chaudhuri, Rui M. Ponte, and An T. Nguyen

Abstract

The uncertainties related to atmospheric fields in the Arctic Ocean from commonly used and recently available reanalysis products are investigated. Fields from the 1) ECMWF Interim Re-Analysis (ERA-Interim), 2) Common Ocean–Ice Reference Experiment version 2 (CORE2), 3) Japanese 25-yr Reanalysis Project (JRA-25), 4) NCEP–NCAR reanalysis, 5) NCEP Climate Forecast System Reanalysis (CFSR), and 6) Modern-Era Retrospective Analysis for Research and Applications (MERRA) are evaluated against satellite-derived and in situ observations for zonal and meridional winds, precipitation, specific humidity, surface air temperature, and downwelling longwave and shortwave radiation fluxes. Comparison to reference observations shows that for variables such as air temperature and humidity, all reanalysis products have similar solutions. However, other variables such as winds, precipitation, and radiation show large spreads. The magnitude of uncertainties in all fields is large when compared to the signal. Biases in Arctic cloud parameterizations and predicted temperature and humidity profiles in reanalyses as discussed in other studies are likely common sources of error that affect surface downwelling radiation, air temperature, humidity, and precipitation.

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An T. Nguyen, Patrick Heimbach, Vikram V. Garg, Victor Ocaña, Craig Lee, and Luc Rainville

Abstract

The lack of continuous spatial and temporal sampling of hydrographic measurements in large parts of the Arctic Ocean remains a major obstacle for quantifying mean state and variability of the Arctic Ocean circulation. This shortcoming motivates an assessment of the utility of Argo-type floats, the challenges of deploying such floats due to the presence of sea ice, and the implications of extended times of no surfacing on hydrographic inferences. Within the framework of an Arctic coupled ocean–sea ice state estimate that is constrained to available satellite and in situ observations, we establish metrics for quantifying the usefulness of such floats. The likelihood of float surfacing strongly correlates with the annual sea ice minimum cover. Within the float lifetime of 4–5 years, surfacing frequency ranges from 10–100 days in seasonally sea ice–covered regions to 1–3 years in multiyear sea ice–covered regions. The longer the float drifts under ice without surfacing, the larger the uncertainty in its position, which translates into larger uncertainties in hydrographic measurements. Below the mixed layer, especially in the western Arctic, normalized errors remain below 1, suggesting that measurements along a path whose only known positions are the beginning and end points can help constrain numerical models and reduce hydrographic uncertainties. The error assessment presented is a first step in the development of quantitative methods for guiding the design of observing networks. These results can and should be used to inform a float network design with suggested locations of float deployment and associated expected hydrographic uncertainties.

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