Lagrangian modelling of frazil ice in the ocean

A new modelling framework using Lagrangian particle tracking has been developed to assess dynamic and thermodynamic effects of underwater frazil ice. This frazil-ice model treats a Lagrangian particle as a bulk cluster of many frazil crystals, and calculates the thermodynamic growth of each particle and the corresponding budget of latent heat and fresh water. The effective density and viscosity of sea water depend on the mass fraction of underwater frazil ice, and hence affect ocean convection. An idealized experiment using our model successfully reproduces the formation of underwater frazil ice and its transition to grease ice at the surface. Because underwater frazil ice does not reduce the atmosphere/ocean heat exchange, surface heat flux and net sea-ice production in the experiment with frazil ice are relatively high compared with the experiment where surface cooling directly leads to columnar growth of a solid ice cover which effectively insulates the heat flux. These results suggest that large-scale sea-ice models which do not take account of the effects of frazil ice might underestimate atmosphere/ocean heat exchange, particularly at times of active new ice formation

Sea-Ice Production in Antarctic Coastal Polynyas Estimated From AMSR2 Data and Its Validation Using AMSR-E and SSM/I-SSMIS Data

Antarctic coastal polynyas are very high sea-ice production areas. The resultant large amount of brine rejection leads to the formation of dense water. The dense water forms Antarctic bottom water, which is the densest water in the global overturning circulation and a key player in climate change as a significant sink for heat and carbon dioxide. In this study, an algorithm was developed that uses Advanced Microwave Scanning Radiometer 2 (AMSR2) data (2012-present) to detect polynya area and estimates thin ice thickness by a method similar to that used to develop the algorithm for Advanced Microwave Scanning Radiometer for EOS (AMSR-E) data. Landfast sea-ice areas were also detected using AMSR2 data. Ice production in the polynyas was estimated by a heat flux calculation using AMSR2 sea-ice data. In four major polynyas, AMSR2 ice production was compared with AMSR-E (2003-2011) ice production through comparison of them with Special Sensor Microwave Imager (SSM/I) and Special Sensor Microwave Imager/Sounder (SSMIS) ice production. The comparison confirmed that the ice production from AMSR-E/2 data, which have higher spatial resolution than SSM/I-SSMIS data, can be used to analyze time series covering more than 10 years. For example, maps of annual ice production based on AMSR-E/2 data revealed detailed changes of the Mertz Polynya, where the ice production decreased significantly after the Mertz Glacier Tongue calving in 2010. Continuous monitoring of the coastal polynyas by the AMSR series sensors is essential for climate-change-related analyses in the Antarctic Ocean

Two-layer model of wind-driven circulation in the Antarctic Ocean

In this study, we investigate the wind-driven circulation in the Antarctic Ocean using a primitive two-layer model with realistic topography. A prominent feature of steady circulation driven by the annual mean wind stress is a clockwise(cyclonic) circulation in the lower layer at the Weddell Basin and the Australia Antarctic Basin. In particular, the circulation pattern in the Australia Antarctic Basin agrees with the observations. In these basins, negative vorticity input from the wind stress is transmitted to the lower layer through the diffusion term(Gent and McWilliams term) and causes prominent cyclonic gyres within closed geostrophic contours of f/H(f: Coriolis parameter, H: water depth). The model result forced by the seasonal wind stress shows that variations of the Antarctic Coastal Current are explained by wind stress variations along the coast. The transport of this current is determined by the integration of onshore Ekman transport along the coast. It is also shown that this Antarctic Coastal Current can be a part of the western boundary current in the Weddell Sea. On a time scale of 10 to 100 days, the variation of the upper layer thickness coincides with the sea level variation at Syowa Station. This variation might be attributed to coastal trapped waves driven by the alongshore wind stress

In-situ ice and meteorological observations in the southern Sea of Okhotsk in 2001 winter: ice structure, snow on ice, surface temperature, and optical environments

The 2001-ice season in the Sea of Okhotsk was characterized by extraordinarily developed ice extent. During the period February 17 to 21, we conducted in-situ observations in the southern ice area with the icebreaker \u27Soya\u27. In this paper, we show the observational results, concerning the core sampling of about 1.3 m-thick ice, snow sampling, surface temperature, and solar radiation under clear sky conditions. It is shown that (1) the sampled ice core is composed entirely of granular ice, (2) the 30 cm-thick snow pack overlying sea ice is composed mainly of depth hoar and significant vertical gradients of δ^O and temperature are found within the snow, probably associated with the depth hoar formation, (3) surface temperatures as low as about -30°C are detected on snow-covered sea ice floes at nighttime under almost clear and light breeze conditions, (4) from the estimation of the turbidity coefficient, the atmosphere over the ice-covered area is considered to be significantly clean. Furthermore, the NOAA/AVHRR satellite data are used to estimate the surface temperature distribution in the southern Sea of Okhotsk. It is shown that these data are useful for discriminating thick ice floes

The variability of the East Sakhalin Current induced by winds over the continental shelf and slope

Long-term current measurements of the East Sakhalin Current (ESC) in the Sea of Okhotsk are analyzed using the technique of empirical orthogonal functions (EOFs) in the frequency domain. The first and second EOFs at subtidal frequencies represent motions over the continental shelf and slope, respectively, corresponding to the variability of the two cores of the intense ESC. The first EOF can be explained by the first-mode coastal trapped wave (CTW). The structure of the second EOF is similar to that of the second-mode CTW to the first approximation. According to the distribution of the cross-spectra between EOFs and the wind stress over the whole area of the Sea of Okhotsk, the first EOF is correlated with the alongshore component of the wind stress over the northern and western shelves. The distribution of the phase of the wind stress, which is correlated with the first EOF, indicates that a resonance between the CTW and wind stress drives the motion represented by the first EOF at lower frequencies. At higher frequencies the phase of the wind stress correlated with the first EOF is almost uniform in space, being consistent with the greater speed of phase propagation of the EOF compared with that for the free CTW at these frequencies. The second EOF is correlated with the wind stress curl in the central part of the Sea of Okhotsk. The motion by the second EOF is confined over the slope at lower frequencies and becomes large over the shelf at higher frequencies. This change in the structure of the second EOF is consistent with the results of the numerical experiment of the flow induced by the offshore forcing by Chapman and Brink (1987). The phase of the wind stress curl which is correlated with the second EOF changes clearly in space at some frequencies, suggesting that the motion represented by the second EOF propagates along the isobath with the coast to the right. The wind stress curl contains the wavenumber resonant with the lowest two or three modes of CTWs

Retrieval of Wintertime Sea Ice Production in Arctic Polynyas Using Thermal Infrared and Passive Microwave Remote Sensing Data

Precise knowledge of wintertime sea ice production in Arctic polynyas is not only required to enhance our understanding of atmosphere‐sea ice‐ocean interactions but also to verify frequently utilized climate and ocean models. Here, a high‐resolution (2‐km) Moderate Resolution Imaging Spectroradiometer (MODIS) thermal infrared satellite data set featuring spatial and temporal characteristics of 17 Arctic polynya regions for the winter seasons 2002/2003 to 2017/2018 is directly compared to an akin low‐resolution Advanced Microwave Scanning Radiometer‐EOS (AMSR‐E) passive microwave data set for 2002/2003 to 2010/2011. The MODIS data set is purely based on a 1‐D energy‐balance model, where thin‐ice thicknesses (≤ 20 cm) are directly derived from ice‐surface temperature swath data and European Centre for Medium‐Range Weather Forecasts Re‐Analysis‐Interim atmospheric reanalysis data on a quasi‐daily basis. Thin‐ice thicknesses in the AMSR‐E data set are derived empirically. Important polynya properties such as areal extent and potential thermodynamic ice production can be estimated from both pan‐Arctic data sets. Although independently derived, our results show that both data sets feature quite similar spatial and temporal variations of polynya area (POLA) and ice production (IP), which suggests a high reliability. The average POLA (average accumulated IP) for all Arctic polynyas combined derived from both MODIS and AMSR‐E are 1.99×105 km2 (1.34×103 km3) and 2.29×105 km2 (1.31×103 km3), respectively. Narrow polynyas in areas such as the Canadian Arctic Archipelago are notably better resolved by MODIS. Analysis of 16 winter seasons provides an evaluation of long‐term trends in POLA and IP, revealing the significant increase of ice formation in polynyas along the Siberian coast