Vol 2, No 2 (2017)

Table of Contents

Articles

by Xiaoming Zhai
64 Views, 109 PDF Downloads
The annual cycle of surface eddy kinetic energy (EKE) and its influence on eddy momentum fluxes are investigated using an updated record of satellite altimeter data. It is found that there is a phase difference between the annual cycles of EKE in the western boundary current regions and EKE in the interior of the subtropical gyres, suggesting that different mechanisms may be at work in different parts of the subtropical gyres. The annual cycles of EKE averaged in the two hemispheres are found to be of similar magnitude but in opposite phase. As a result, the globally-averaged EKE shows little seasonal variability. The longer record of altimeter data used in this study has brought out a clearer and simpler picture of eddy momentum fluxes in the Gulf Stream and Kuroshio Extension. Considerable seasonal variations in eddy momentum fluxes are found in the western boundary current regions, which potentially play an important role in modulating the strength of the western boundary currents and their associated recirculation gyres on the seasonal time scale.
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Articles

by Tao Xie, Li Zhao, William Perrie, He Fang
30 Views, 41 PDF Downloads

Climate change, increasing activities in areas like offshore oil and gas exploration, marine transport, eco-tourism, in additional to the usual activities of northerners resident are leading to reductions in sea ice. Therefore, there is an urgent need for improvement in the sea ice detection in polar areas. Starting from the mechanism of electromagnetic scattering, based on an empirical dielectric constant model, we apply EM multi-reflection and transmission formulas for coefficients between the air-ice interface and sea water-ice interface to develop a model for estimating the capability of detection of sea ice and ice thickness based on a pulse radar system, synthetic aperture radar (SAR). Although the dielectric constant of sea ice is less than that of sea water, this model can provide a rational methodology as the normalized radar cross section (NRCS) of sea ice is larger than that of sea water due to multiple reflections. The numerical simulations of this model showed that the convergence rate is rapid. With 3 or 4 reflections and transmissions (depending on temperature, salinity, and dielectric constants of sea ice and water), truncation errors can be satisfied using theoretical considerations and practical applications. The model is applied to estimate the capability of SAR to discriminate ice from water. The numerical results suggested that the model ability to measure ice thickness decreases with increasing radar incident angles and increases with increasing radar pulse width. Reflection and transmission coefficients decrease monotonically with ice thickness and are saturated for ice thicknesses above a certain critical value which depends on SAR incidence angle, frequency and dielectric constants of sea ice. The capability to detect ice thickness for given different bands of pulse radar widths can be estimated with this model.

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Articles

by Guang Jun Zhang, Mingcheng Wang
56 Views, 43 PDF Downloads

How high convective clouds can go is of great importance to climate. Cloud ice and liquid water that detrain near the top of convective cores are important for the formation of anvil clouds and thus impact cloud radiative forcing and the Earth’s radiation budget. This study uses CloudSat observations to evaluate convective cloud top heights in the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM5). Results show that convective cloud top heights in the tropics are much lower than observed by CloudSat, by more than 2 km on average. Temperature and moisture anomalies from climatological means are composited for convective clouds of different heights for both observations and model simulation. It is found that convective environment is warmer and moister, and the anomalies are larger for clouds of higher tops. For a given convective cloud top height, the corresponding atmosphere in CAM5 is more convectively unstable than what the CloudSat observations indicate, suggesting that there is too much entrainment into convective clouds in the model.

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Articles

by Yi Sui, Jinyu Sheng, Kyoko Ohashi, Yongsheng Wu
12 Views, 19 PDF Downloads
A nested-grid ocean circulation modelling system is used in this study to examine the circulation of surface waters over the Scotian Shelf and its adjacent waters. The modelling system consists of a coarse-resolution (1/12°) barotropic storm surge (outer) model covering the northwest Atlantic Ocean, and a fine-resolution (1/16°) baroclinic (inner) model covering the Gulf of St. Lawrence, Scotian Shelf, and Gulf of Maine. The external model forcing includes tidal forcing, atmospheric forcing, surface heat fluxes, freshwater discharge, and large-scale currents specified at model open boundaries. The three-dimensional model currents are used to track trajectories of particles using a Lagrangian particle-tracking model. The simulated particle movements and distributions are used to examine the dispersion, retention, and hydrodynamic connectivity of surface waters over the study region. The near-surface dispersion is relatively high over western Cabot Strait, the inner Scotian Shelf, and the shelf break of the Scotian Shelf, while relatively low in Northumberland Strait. A process study is conducted to examine the physical processes affecting the surface dispersion, including tidal forcing and local wind forcing. The model results show that the tidal currents significantly influence the dispersion of surface waters in the Bay of Fundy.
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Articles

by David B. Fissel, Yuehua Lin, Alison Scoon, Jose Lim, Leslie Brown, Ryan Clouston
24 Views, 14 PDF Downloads

The Nass River discharges into Nass Bay and Iceberg Bay, which are adjoining tidal inlets located within the northern inland waters of British Columbia, Canada. After the Skeena River, the Nass River is the second longest river within northern British Columbia, which discharges directly into Canadian waters of the Pacific Ocean. It is also supports one of the most productive salmon fisheries in northern British Columbia. The Nass River discharges into the eastern end of Nass Bay. Nass Bay, in turn feeds into Portland Canal and the fresh surface waters then flows westward to the Pacific Ocean via Dixon Entrance. The tides in Northern British Columbia are very large with a tidal height range of just over 7 m. Nass Bay is a shallow inlet of less than 10 km in length with typical water depths of than 10 m or less. The existing knowledge of oceanographic processes in Nass and Iceberg Bays was rudimentary until three years ago, when the first modern oceanographic measurements were obtained. In this study, the seasonal and tidal variability of the lateral extent of the Nass River surface plume is mapped from analyses of Landsat satellite data spanning the period from 2008 to 2015. A high resolution coupled three dimensional (3D) hydrodynamic model was developed and implemented, within the widely used and accepted Delft3D modeling framework, which was forced and validated using recent 2013-2016 in-situ oceanographic measurements. The combined satellite and numerical modeling methods are used to study the physical oceanographic and sediment transport regime of Nass and Iceberg Bays and the adjoining waters of Portland Inlet and Observatory Inlet. The ocean circulation of Nass and Iceberg Bays was found to be dominated by tidal currents, and by the highly seasonal and variable Nass River freshwater discharges. Complex lateral spatial patterns in the tidal currents occur due to the opening of the southwestern side of Nass Bay onto the deeper adjoining waters of Iceberg Bay. Surface winds are limited to a secondary role in the circulation variability. The sediment dynamics of the Nass Bay system features a very prominent surface sediment plume present from the time of freshet in mid-spring through to large rainfall runoff events in the fall. The time-varying turbidity distribution and transport paths of the Nass River sediment discharges in the study area were characterized using the model results combined with an analysis of several high-resolution multi-year Landsat satellite data sets.

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Articles

by Youyu Lu, Jiaxing Li, Ji Lei
13 Views, 8 PDF Downloads

The model simulated meso-scale eddies in the Northeast Pacific Ocean, using two models with nominal horizontal resolutions of 1/12° and 1/36° in latitude/longitude (grid spacing of 7.5 km and 2.5 km), respectively, are presented. Compared with the 1/12° model, the 1/36° model obtains (1) similar variance and wavenumber spectra of  the sea level anomaly and water temperature anomaly, and (2) increases in the level of the domain-averaged total kinetic energy, eddy kinetic energy (EKE), and variance of horizontal gradient of water temperature. In the interior basin of the southern region, both models show stronger eddy frontal activities, represented by EKE, temperature and its horizontal gradient, in summer and fall than in winter and spring. The challenge of evaluating the realism of high-resolution ocean models with conventional satellite remote sensing observations is discussed.

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