Geographical inhomogeneity and temporal variability of mixing property and driving mechanism in the Arctic Ocean

Upper ocean mixing plays a key role in the atmosphere-ocean heat transfer and sea ice extent and thickness via modulating the upper ocean temperatures in the Arctic Ocean.Observations of diffusivities in the Arctic that directly indicate the ocean mixing properties are sparse.Therefore,the spatiotemporal pattern and magnitude of diapycnal diffusivities and kinetic energy dissipation rates in the upper Arctic Ocean are important for atmosphere-ocean heat transfers and sea ice changes.These were first estimated from the Ice-Tethered Profilers dataset(2005–2019)using a strain-based fine-scale parameterization.The resultant mixing properties showed signifi cant geographical inhomogeneity and temporal variability.Diapycnal diff usivities and dissipation rates in the Atlantic sector of the Arctic Ocean were stronger than those on the Pacific side.Mixing in the Atlantic sector increased significantly during the observation period;whereas in the Pacific sector,it weakened before 2011 and then strengthened.Potential impact factors include wind,sea ice,near inertial waves,and stratifi cation,while their relative contributions vary between the two sectors of the Arctic Ocean.In the Atlantic sector,turbulent mixing dominated,while in the Pacific sector,turbulent mixing was inhibited by strong stratification prior to 2011,and is able to overcome the stratifi cation gradually after 2014.The vertical turbulent heat fl ux constantly increased in the Atlantic sector year by year,while it decreased in the Pacific sector post 2010.The estimated heat flux variability induced by enhanced turbulent mixing is expected to continue to diminish sea ice in the near future.

Internal solitary wave generation by the tidal flows beneath ice keel in the Arctic Ocean

A series of non-hydrostatic,non-linear numerical simulations were carried out to investigate the generation and evolution of internal solitary waves(ISWs)through the interaction of a barotropic tidal current with an ice keel in the Arctic Ocean.During the interaction process,the internal surge was generated at first,and then the wave gradually steepened due to non-linearity during its propagation away from the ice keel.The internal surge eventually disintegrated into multi-modal and rank-ordered ISW packets with the largest having an amplitude of O(10)m.Sensitivity experiments demonstrated that the ISWs’amplitudes and energy were proportional to the varying ice keel depths and barotropic tidal fl ow amplitudes,but were insensitive to the changing ice keel widths.Typical ISWs can enhance the turbulent dissipation rate of O(10^(-6))W/kg along their propagation path.Further,heat entrainment induced by the wave-ice interaction can reach O(10)MJ/m per tidal cycle.This study reveals a particular ISW generation mechanism and process in the polar ice environment,which could be important in impacting the energy transfer and heat balance in the Arctic Ocean.

Measurements of sea ice thickness and its subice morphology analysis using ice-penetration radar in the Arctic Ocean

Based on radar penetrating measurements and analysis of sea ice in the Arctic Ocean, The potential of radar wave to measure sea ice thickness and map the morphology of the underside of sea ice is investigated. The results indicate that the radar wave can penetrate Arctic summer sea ice of over 6 meters thick; and the propagation velocity of the radar wave in sea ice is in the range of 0.142 m·ns -1 to 0.154 m·ns -1 . The radar images display the roughness and micro-relief variation of sea ice bottom surface. These features are closely related to sea ice types, which show that radar survey may be used to identify and classify ice types. Since radar images can simultaneously display the linear profile features of both the upper surface and the underside of sea ice, we use these images to quantify their actual linear length discrepancy. A new length factor is suggested in relation to the actual linear length discrepancy in linear profiles of sea ice, which may be useful in further study of the area difference between the upper surface and bottom surface of sea ice.

Observation of atmospheric methane in the Arctic Ocean up to 87° north

Although the Arctic methane reservoir is large,the emission of methane from the Arctic Ocean into the atmosphere remains poorly constrained.Continuous ship-borne measurements of atmospheric methane near the surface ocean were carried out during two cruises to investigate methane emission from the Arctic Ocean up to the latitude of 87°N.Three-day air mass back trajectories along the cruise tracks indicated that the surface Arctic Ocean could be a potentially important source of methane to the atmosphere.Rapid bursts in methane concentration occurred mainly along the ocean frontal area,suggesting that frontal upwelling in the upper layer of the Arctic Ocean might contribute to methane emissions into the atmosphere.

Composition of algal pigments in surface freshen layer after ice melt in the central Arctic

Seasonal meltwater input creates a thin freshen layer in surface seawater under ice, which largely shifts the algae assemblages. Our recent observation of photosynthetic pigments in the high Arctic showed that ice bottom and 5 m of seawater under ice contained relatively high concentration of fucoxanthin, while chlorophyll b and lutein were the major diagnostic pigments in ice-water interface and 0 m of seawater under ice. Additionally, a notable change of dominant phytoplankton occurred in the top 5 m of seawater under ice, from chlorophytes-dominated at surface to diatoms-dominated at 5 m depth, which might attribute to the sharp salinity gradient （salinity from 12.5 to 28.1） in the surface seawater under ice. Our results imply that phytoplankton community in surface layer under ice would become more chlorophytes in the future warming Arctic Ocean.

Using MAN and Coastal AERONET Measurements to Assess the Suitability of MODIS C6.1 Aerosol Optical Depth for Monitoring Changes from Increased Arctic Shipping

Collocated data of the moderate resolution imaging spectroradiometer (MO-DIS) Collection 6.1 aerosol optical depths (AOD) at 3 km × 3 km north of 59.9°N over ocean were assessed at 550 nm by aerosol robotic network (AERONET) data from coastal sites and marine aerosol network (MAN) data from vessels during June to October 2006 to 2018. Typically, MODIS AOD was higher at low and lower at high values than the AERONET AOD. Discrepancies were largest for sites where the Earth’s surface around the site is very heterogeneous (Canadian Archipelago, coast of Greenland). Due to the higher likelihood for sea-ice, MAN and MODIS AOD differed stronger west of Greenland and over the Beaufort Sea than at location in the Greenland and Norwegian Seas and Atlantic. MODIS AOD well captured the inter-seasonal variability found in the AERONET AOD data (R = 0.933). At all sites, MO-DIS and AERONET AOD agreement improved as time progressed in the shipping season, hinting at errors in sea-ice vs. open water classification. Overall 75.3% of the MODIS AOD data fell within the limits of the error envelops of the AERONET/MAN AOD data with MAN ranging between 87.5% and 100%. Changes in both MODIS and AERONET mean AOD between two periods of same length (2006-2011, 2013-2018) were explainable by changes in emissions for all sites.

Geographical distribution of general aerobic heterotrophic bacteria in surficial sediments from the Chukchi Sea and Canadian Basin

This paper determined the abundance of General Aerobic heterotrophic Bacteria （GAB） in surficial sediments from the Chukchi Sea and the Canadian basin by using MPN and discussed their geographical distribution. The result shows that the determination percentages of the GAB were high, even till 100 percentage. The abundance range and averages of GAB for 4℃and 25℃ were from 4.00 × 10^2 to 2.40× 10^6 , 1.71×10^6 ind.·g^-1 （wet sample ） and from 2.40 ×10^5 to 2.40×10^7 , 1.10×10^7 ind. ·g^-1 （wet sample ） respectively. Not only the abundance range but also the averages of GAB in 25℃ were higher than that in 4℃. The abundance of GAB in sediments show a tendency that it is roughly greater in the lower latitudinal area than in the higher latitudinal area. The abundance of GAB increased from east to west as for the longitudinal distribution. With the water depth increasing, the abundance of GAB at 4℃ decreased, but GBA at 25℃ is not changed obviously with water depth. It seems that warmer circumstantial temperature is more suitable for some GAB.

Evaluation of the relationships and uncertainties of airborne and ground-based sea ice surface temperature measurements against remotely sensed temperature records

Sea ice surface temperature(IST)is an important indicator of environmental changes in the Arctic Ocean.In this study,the relative performance of four mainstream IST records,i.e.airborne IST,infrared radiometer measured IST(IR IST),longwave radiation derived IST(LWR IST),and snow and ice mass balance array buoy derived IST(Buoy IST),were evaluated against the MODIS IST product.Bias,standard deviation(STD),and root mean square error(RMSE)were used to evaluate the data quality.Results revealed that airborne IST had the best accuracy,which was 0.21 K colder than MODIS IST,with STD of 1.46 K and RMSE of 1.47 K.Ground-based ISTs were biased with each other but all warmer than the MODIS IST.The IR IST had the best overall accuracy(bias=0.55 K;STD=1.52 K;RMSE=1.61 K),while the LWR IST was the noisiest measurement with the largest outlier data percent.Besides,co-located IR and LWR ISTs were more consistent than any type of evaluated IST against MODIS IST(correlation coefficient=0.99).Airborne and IR ISTs are thus the premier choice for monitoring the rapidly changing Arctic sea ice,together with satellite observations.