Chinese National Arctic Research Expedition Research Articles

RETRACTION: Rapid Acidification of the Arctic Chukchi Sea Waters Driven by Anthropogenic Forcing and Biological Carbon Recycling

RETRACTIOND. Qi, Y. Wu, L. Chen, W.‐J. Cai, Z. Ouyang, Y. Zhang, L. G. Anderson, R. A. Feely, Y. Zhuang, H. Lin, R. Lei, H. Bi. (2022). Rapid Acidification of the Arctic Chukchi Sea Waters Driven by Anthropogenic Forcing and Biological Carbon Recycling. Geophysical Research Letters, 49, e2021GL097246. https://doi.org/10.1029/2021GL097246.The above article, published online on 21 February 2022 in Wiley Online Library (wileyonlinelibrary.com), has been retracted by agreement between the journal Editor‐in‐Chief, Harihar Rajaram; American Geophysical Union; and John Wiley & Sons LLC. The retraction has been agreed as it was discovered that the 2019 pCO2 data collected on the 10th Chinese National Arctic Research Expedition and analyzed along with other data in this paper was used without proper permission. AGU Publications and Wiley evaluated all information submitted by the authors and scientific agencies and carefully considered Committee on Publication Ethics (COPE) guidelines before making this decision. The authors have revised their statistical analyses by excluding the 2019 Chinese National Arctic Research Expedition data and have stated they intend to submit the revised manuscript to the journal. D. Qi acknowledges that the data ownership is disputed, and therefore accepts the retraction. L. Chen and W.‐J. Cai disagree with the retraction. Z. Ouyang, Y. Zhang, L. G. Anderson, R. A. Feely, Y. Zhuang, H. Lin, R. Lei, and H. Bi did not respond to our notice of retraction.

Anisotropic Characterization of the Chukchi Boardland Based on Ocean-Bottom Seismic Experiment during N11-CHINARE

Abstract The current research focus at Chukchi Boardland (CB) revolves around sediment stratification and crustal structure, but investigations into deep stress fields and mantle dynamics are limited. This article presents a study on the anisotropic characteristics of the CB. Shear-wave splitting measurements were conducted using the transverse energy minimization at six stations recovered from the 11th Chinese National Arctic Research Expedition. The observation period for these six stations ranged from 2 August 2020 to 8 September 2020. The results demonstrate significant anisotropy within the CB, with the fast shear-wave polarization direction ranging from N60°E to N70°E. The time delays between fast and slow shear waves were found to be ∼0.7 s. By comparing the anisotropy observed at the CB with that at land stations in Arctic Alaska, this study suggested that the genesis of anisotropy beneath the CB was related to the formation of the Amerasian basin. The tectonic processes of rifting during basin evolution and midocean ridge spreading led to the development of anisotropy in the lithosphere beneath the CB during expansion.

Potential Contributions of Ammonia‐Oxidizing Microorganisms to the Distributions of Nitrous Oxide in the Northern Bering Sea

AbstractOceanic N2O is a major source of atmospheric N2O gas and is involved in global warming and ozone depletion. It is thought to be mainly produced by nitrification, denitrification and nitrifier denitrification processes mediated by ammonia‐oxidizing bacteria, ammonia‐oxidizing archaea (AOA) and denitrifying bacteria. The Bering Sea, especially its continental shelf area, is considered a typical source of atmospheric N2O. During the 7th Chinese National Arctic Research Expedition (CHINARE2016), the distributions of N2O and ammonia‐oxidizing microorganisms (AOOs) in the Bering Sea continental shelf and abyssal basin water were investigated. At a depth of 50∼900 m within the abyssal basin, the in‐situ ammonia oxidation process, particularly performed by AOA, exhibits considerable potential for the formation of supersaturated N2O. Meanwhile, beneath the oxygen minimum zone (depth range is approximately 800∼1,000 m), supersaturated N2O is primarily driven by mixing processes, while the ammonia oxidation mediated by AOA also contributes to a certain extent. In addition, the N2O distribution characteristic exhibits a substantial disparity between the southern and northern Bering Sea shelves, with the former characterized as a mild sink and the latter as a weak source. The water column of the Bering Sea demonstrates a considerable potential for generating supersaturated N2O through ammonia oxidation, as corroborated by the current study.

Modeled variations in the inherent optical properties of summer Arctic ice and their effects on the radiation budget: a case based on ice cores from 2008 to 2016

Abstract. Variations in Arctic sea ice are apparent not only in its extent and thickness but also in its internal properties under global warming. The microstructure of summer Arctic sea ice changes due to varying external forces, ice age, and extended melting seasons, which affect its optical properties. Sea ice cores sampled in the Pacific sector of the Arctic obtained by the Chinese National Arctic Research Expedition (CHINARE) during the summers of 2008 to 2016 were used to estimate the variations in the microstructures and inherent optical properties (IOPs) of ice and determine the radiation budget of sea ice based on a radiative transfer model. The variations in the volume fraction of gas bubbles (Va) of the ice top layer were not significant, and the Va of the ice interior layer was significant. Compared with 2008, the mean Va of interior ice in 2016 decreased by 9.1 %. Meanwhile, the volume fraction of brine pockets increased clearly during 2008–2016. The changing microstructure resulted in the scattering coefficient of the interior ice decreasing by 38.4 % from 2008 to 2016, while no clear variations can be seen in the scattering coefficient of the ice top layer. These estimated ice IOPs fell within the range of other observations. Furthermore, we found that variations in interior ice were significantly related to the interannual changes in ice ages. At the Arctic basin scale, the changing IOPs of interior ice greatly changed the amount of solar radiation transmitted to the upper ocean even when a constant ice thickness is assumed, especially for thin ice in marginal zones, implying the presence of different sea ice bottom melt processes. These findings revealed the important role of the changing microstructure and IOPs of ice in affecting the radiation transfer of Arctic sea ice.

Hydrological characteristics of the Bering Sea in the summer of 2019

Based on the data of three CTD sections in the northern, northeastern and western Bering Sea of 2019 voyage of Chinese National Arctic Research Expedition (CHINARE), this paper analyzes and studies the hydrological characteristics of the water mass distribution, layered structure, and cline characteristics in different sea areas of the Bering Sea. The results indicate that the hydrological characteristics of the Bering Sea in the summer of 2019 are different from those in the past and that the water mass is warming in many locations. The maximum water temperature reaches 11.13 °C, and the maximum thickness of the warm water is about 32 m. The water mass composition and characteristics of the north-northeast-west sections are significantly different: the BL section has the highest salinity, while the BS section has the lowest salinity, and both the lowest temperature and the largest temperature variation appear in the BL section. The stratification characteristics in all sea areas are noteworthy. In the deep-water seas, there are three types of water masses: upper water (BSW), middle water (BIW) and deep water (BDW) from top to bottom, while two main water masses appear in the shelf waters with the Alaska Coastal Water (ACW) overlies the Bering Sea Shelf Water (BSW). Along the Bering Sea Slope Current (BSC), the water mass is essentially steady. Statically unstable hydrological inversion structure appears near the bottom of the three stations at the northern end of the BL section.

Characteristics of seafloor ambient noise under different sea ice concentrations in the northern Chukchi Sea: Results from the N11 CHINARE ocean‐bottom seismic experiment

AbstractSeafloor ambient noise in the Arctic Ocean is related to sea ice. The characteristics of low‐frequency seafloor ambient noise (<10 Hz) in the northern Chukchi Sea have rarely been reported. Here, we investigated the seafloor ambient noise recorded using ocean‐bottom seismometers in the northern Chukchi Sea under four different sea ice‐concentration periods from 3 to 23 August 2020, in the 11th Chinese National Arctic Research Expedition. The causes and mechanisms of the changes in seafloor ambient noise that correspond to the variation in sea ice concentration were discussed. The energy of infragravity waves and primary microseisms is weak compared with other oceans. Combined with the analysis of land station, we argue that the variations in sea ice extent have little effect on the energy of the primary microseisms and infragravity waves. The power of secondary microseisms is growth with the loss of sea ice and is influenced by storms. We find that a high concentration of sea ice can impede the process of storm‐sea surface interaction, which in turn affects the power of microseisms inspired by storm.

Distribution and Driving Mechanism of N2O in Sea Ice and Its Underlying Seawater during Arctic Melt Season

Nitrous oxide (N2O) is the third most important greenhouse gas in the atmosphere, and the ocean is an important source of N2O. As the Arctic Ocean is strongly affected by global warming, rapid ice melting can have a significant impact on the N2O pattern in the Arctic environment. To better understand this impact, N2O concentration in ice core and underlying seawater (USW) was measured during the seventh Chinese National Arctic Research Expedition (CHINARE2016). The results showed that the average N2O concentration in first-year ice (FYI) was 4.5 ± 1.0 nmol kg−1, and that in multi-year ice (MYI) was 4.8 ± 1.9 nmol kg−1. Under the influence of exchange among atmosphere-sea ice-seawater systems, brine dynamics and possible N2O generation processes at the bottom of sea ice, the FYI showed higher N2O concentrations at the bottom and surface, while lower N2O concentrations were seen inside sea ice. Due to the melting of sea ice and biogeochemical processes, USW presented as the sink of N2O, and the saturation varied from 47.2% to 102.2%. However, the observed N2O concentrations in USW were higher than that of T-N2OUSW due to the sea–air exchange, diffusion process, possible N2O generation mechanism, and the influence of precipitation, and a more detailed mechanism is needed to understand this process in the Arctic Ocean.

Under-ice target echo time delay estimation using [formula omitted]-norm based matched filter

In the ice-covered underwater environment of the Arctic area, impulsive noise is one of the most challenging problems for target echo time-delay estimation (TDE) and severely degrades the performance of some so-called traditional TDE algorithms. In this work, we propose a TDE approach called “ℓp-norm-based correlation,” which is explored for use in active sonar systems in an impulsive environment. The ℓp-norm-based optimization is presented here to improve the robustness, and we introduce the Toeplitz matrix to formulate the received signal. The different columns of the desired matrix represent the various time-delay replicas of the transmitted signal, and the global minimizer can be found by rewriting the optimization problem. Thus, all global minimizers are efficiently solved simultaneously. The proposed TDE technique can enhance time-delay resolution and provide reliable estimation results compared with conventional matched filtering and other competing approaches. Furthermore, we show that the performance of the resulting method can be verified by the results of numerical simulation and by comparison with the experimental data collected from the 11th Chinese National Arctic Research Expedition.

Spatial variability of summertime aragonite saturation states and its influencing factor in the Bering Sea

The Bering sea is susceptible to ocean acidification driven by both human activities (anthropogenic CO2) and distinctive natural processes. To assess the situation of ocean acidification, we investigated the spatial variability of aragonite saturation states (ΩAr) in July 2010 during the 4th Chinese National Arctic Research Expedition (CHINARE). The surface waters were all oversaturated with respect to aragonite (ΩAr > 1) due to high biological removal, and ΩAr ranged from 1.43 to 3.17. The relatively low ΩAr values were found in the western Bering Strait and eastern nearshore region of the Bering Sea Shelf, which were associated with the upwelling and riverine input, respectively. In the subsurface, the ΩAr decreased to generally low saturation states and were observed to be strongly undersaturated (ΩAr < 1) in the bottom waters with a lowest value of 0.45, which might be caused by remineralization. However, unlike prior studies, the low ΩAr values in the shallow nearshore region were still above the saturation horizon throughout the water column, which were probably counteracted by high local primary production. In the context of climate change and increasing anthropogenic CO2 absorption, the suppression and undersaturation of ΩAr in the Bering Sea are not only attributed to the natural processes but also the accumulation of anthropogenic CO2.

Characteristics of low-level temperature inversions over the Arctic Ocean during the CHINARE 2018 campaign in summer

Across the Arctic Ocean, the semi-permanent sea-ice layer insulates the atmosphere from the Arctic Ocean, thereby influencing the lower boundary conditions of the atmosphere in this region. The presence of a temperature inversion predominantly characterizes the Arctic lower troposphere throughout the year. The 9th Chinese National Arctic Research Expedition took place in the Pacific Sectors of the Arctic Ocean from July to September 2018, gathering high-resolution radiosonde data to provide a detailed structure of the low-level temperature inversion (LLI) over the summer months. Statistical analyses showed that LLIs occurred most frequently below 500m and were generally weaker and shallower than those occurring during the winter months. Low-level inversions are influenced by both local meteorology and large scale synoptic conditions. The continual presence of low-level clouds or fog tends to lift the inversion layer at the local scale, transferring it from a surface-based inversion (SBI) to an elevated inversion (EI). By contrast, SBIs were found to dominate over regions of melting sea-ice. Regimes, where the cloud top penetrated the inversion base were more frequently observed than cloud capped by inversion regimes. Inversions tended to be deeper, stronger, and with a lower base during cloud top penetrating inversion base regimes. At the synoptical scale, the intense poleward intrusions of warm air brought moist air to the top of low-level inversions, creating a robust thermal stratification between layers and promoting extensive and frequency occurrences of low cloud/fog. Moreover, the polar high contributed to establishing a multilayer inversion structure at relatively high altitudes by subsidence. These findings contribute to an improving understanding of low-level vertical temperature structures and their influence on a rapidly warming Arctic.

Characterization of sea ice and its snow cover in the Arctic Pacific sector during the summer of 2016

A comprehensive analysis of sea ice and its snow cover during the summer in the Arctic Pacific sector was conducted using the observations recorded during the 7th Chinese National Arctic Research Expedition (CHIANRE-2016) and the satellite-derived parameters of the melt pond fraction (MPF) and snow grain size (SGS) from MODIS data. The results show that there were many low-concentration ice areas in the south of 78°N, while the ice concentration and thickness increased significantly with the latitude above the north of 78°N during CHIANRE-2016. The average MPF presented a trend of increasing in June and then decreasing in early September for 2016. The average snow depth on sea ice increased with latitude in the Arctic Pacific sector. We found a widely developed depth hoar layer in the snow stratigraphic profiles. The average SGS generally increased from June to early August and then decreased from August to September in 2016, and two valley values appeared during this period due to snowfall incidents.

High‐resolution distribution pattern of surface water nitrous oxide along a cruise track from the Okhotsk Sea to the western Arctic Ocean

AbstractNitrous oxide in the surface water was measured using an automatic underway system, along with measurements of surface water pCO2, dissolved oxygen, salinity, temperature, and sea ice coverage, along a cruise track through the Bering Sea and Chukchi Sea to the Canadian Basin during the 7th Chinese National Arctic Research Expedition. The results show that, overall, the regions along the cruise track are net sources of N2O to the atmosphere. Several N2O oversaturation maxima were observed along the cruise track, with an absolute maximum of approximately 60%. According to the hydrographic setting and the distribution patterns of pCO2, dissolved oxygen, and sea ice coverage in the study area, it was concluded that the N2O oversaturation maxima may result from hydrographic processes, such as mixing of different water masses, upwelling or convection, and possible production beneath the sea ice. Additionally, the lowest value of approximately 90% may result from dilution related to the melting of sea ice. An evaluation of the air–sea flux along the cruise track shows that the continental shelf and upwelling region are N2O sources, whereas the study area in the open Arctic Ocean does not show obvious source or sink characteristics.

Feasibility study of miniature near-infrared spectrometer for the measurement of solar irradiance within Arctic snow-cover sea ice

The extremely low temperature, high humidity and limited power supply pose considerable challenges when using spectrometers within the Arctic sea ice. The feasibility of using a miniature low-power near-infrared spectrometer module to measure solar radiation in Arctic sea ice environments was investigated in this study. Temperature and integration time dependences of the spectrometer module were examined over the entire target operating range of −50°C to 30°C, well below the specified operating range of this spectrometer. Using these observations, a dark output prediction model was developed to represent dark output as a function of temperature and integration time. Temperature-induced biases in the saturation output and linear operating range of the spectrometer were also determined. Temperature and integration time dependences of the signal output were evaluated. Two signal output correction models were developed and compared, to convert the signal output at any temperature within the operating temperature range and integration time to that measured at the reference temperature and integration time. The overall performance of the spectrometer was evaluated by integrating it into a refined fiber optic spectrometry system and measuring solar irradiance distribution in the ice cover with thickness of 1.85 m in the Arctic during the 9th Chinese National Arctic Research Expedition. The general shape of the measured solar irradiance above the snow surface agreed well with that measured by other commercial oceanographic spectroradiometers. The measured optical properties of the sea ice were generally comparable to those of similar ice measured using other instruments. This approach provides a general framework for assessing the feasibility of using spectrometers for applications in cold environments.

The role of bias correction on subseasonal prediction of Arctic sea ice during summer 2018

Subseasonal Arctic sea ice prediction is highly needed for practical services including icebreakers and commercial ships, while limited by the capability of climate models. A bias correction methodology in this study was proposed and performed on raw products from two climate models, the First Institute Oceanography Earth System Model (FIOESM) and the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS), to improve 60 days predictions for Arctic sea ice. Both models were initialized on July 1, August 1, and September 1 in 2018. A 60-day forecast was conducted as a part of the official sea ice service, especially for the ninth Chinese National Arctic Research Expedition (CHINARE) and the China Ocean Shipping (Group) Company (COSCO) Northeast Passage voyages during the summer of 2018. The results indicated that raw products from FIOESM underestimated sea ice concentration (SIC) overall, with a mean bias of SIC up to 30%. Bias correction resulted in a 27% improvement in the Root Mean Square Error (RMSE) of SIC and a 10% improvement in the Integrated Ice Edge Error (IIEE) of sea ice edge (SIE). For the CFS, the SIE overestimation in the marginal ice zone was the dominant features of raw products. Bias correction provided a 7% reduction in the RMSE of SIC and a 17% reduction in the IIEE of SIE. In terms of sea ice extent, FIOESM projected a reasonable minimum time and amount in mid-September; however, CFS failed to project both. Additional comparison with subseasonal to seasonal (S2S) models suggested that the bias correction methodology used in this study was more effective when predictions had larger biases.

Under-ice ambient noise in the Arctic Ocean: observations at the long-term ice station

Under-ice ambient noise in the Arctic Ocean is studied using the data recorded by autonomous hydrophones at the long-term ice station during the 9th Chinese National Arctic Research Expedition. Time-frequency analysis of two 7-s-long ice-induced noise samples shows that both ice collision and ice breaking noise have many outliers in the time-domain (impulsive characteristic) and abundant frequency components in the frequency-domain. Ice collision noise lasts for several seconds while the duration of ice breaking noise is much shorter (i.e., less than tens of milliseconds). Gaussian distribution and symmetric alpha stable (sαs) distribution are used in this paper to fit the impulsive under-ice noise. The sαs distribution can achieve better performance as it can track the heavy tails of impulsive noise while Gaussian distribution fails. This paper also analyzes the meteorological variables during the under-ice noise observation experiment and deduces that the impulsive ambient noise was caused by the combined force of high wind speed and increasing atmosphere temperature on the ice canopy. The Pearson correlation coefficients between long-term power spectral density variations of under-ice ambient noise and meteorological variables are also studied in this paper.

Investigation of Compression and Reconstruction Methods for Solar Radiation Spectra above, within, and below the Sea Ice in Polar Environments

AbstractTo reduce the amount of data transmitted, while retaining essential information on the original signal after reconstruction, this study investigated three different compression and reconstruction methods for the solar irradiance signals at different levels of the Arctic sea ice. To achieve this, characteristics of solar irradiance at different depths of the Arctic sea ice were analyzed; the solar irradiance profile was measured using a fiber optic spectrometer system at several ice packs from 11 to 25 August 2018, during the ninth Chinese National Arctic Research Expedition. Based on these measurements, three potential methods—fast Fourier transform (FFT), discrete wavelet transform (DWT), and Prony’s method—were selected and applied for the compression and reconstruction of a group of 12 measurements from one profile. The results indicated that the FFT method was generally superior to the DWT method considering the corrected reconstruction rate Rc, under the same compression ratio C. The FFT method had the highest Rc for signals with higher roughness, whereas Prony’s method was more suitable for smoother signals. To ensure that Rc was at least 80% and C was as large as possible, the compression and reconstruction method proposed in this study included the FFT method (C = 6.4) for the signals rougher than 1.5 and Prony’s method employing the matrix pencil method (C = 8) for the smoother signals. The proposed method was successfully verified using an independent group of solar irradiance measurements from another profile in the Arctic sea ice.

Comparisons of passive microwave remote sensing sea ice concentrations with ship-based visual observations during the CHINARE Arctic summer cruises of 2010–2018

In order to apply satellite data to guiding navigation in the Arctic more effectively, the sea ice concentrations (SIC) derived from passive microwave (PM) products were compared with ship-based visual observations (OBS) collected during the Chinese National Arctic Research Expeditions (CHINARE). A total of 3 667 observations were collected in the Arctic summers of 2010, 2012, 2014, 2016, and 2018. PM SIC were derived from the NASA-Team (NT), Bootstrap (BT) and Climate Data Record (CDR) algorithms based on the SSMIS sensor, as well as the BT, enhanced NASA-Team (NT2) and ARTIST Sea Ice (ASI) algorithms based on AMSR-E/AMSR-2 sensors. The daily arithmetic average of PM SIC values and the daily weighted average of OBS SIC values were used for the comparisons. The correlation coefficients (CC), biases and root mean square deviations (RMSD) between PM SIC and OBS SIC were compared in terms of the overall trend, and under mild/normal/severe ice conditions. Using the OBS data, the influences of floe size and ice thickness on the SIC retrieval of different PM products were evaluated by calculating the daily weighted average of floe size code and ice thickness. Our results show that CC values range from 0.89 (AMSR-E/AMSR-2 NT2) to 0.95 (SSMIS NT), biases range from −3.96% (SSMIS NT) to 12.05% (AMSR-E/AMSR-2 NT2), and RMSD values range from 10.81% (SSMIS NT) to 20.15% (AMSR-E/AMSR-2 NT2). Floe size has a significant influence on the SIC retrievals of the PM products, and most of the PM products tend to underestimate SIC under smaller floe size conditions and overestimate SIC under larger floe size conditions. Ice thickness thicker than 30 cm does not have a significant influence on the SIC retrieval of PM products. Overall, the best (worst) agreement occurs between OBS SIC and SSMIS NT (AMSR-E/AMSR-2 NT2) SIC in the Arctic summer.

An improved least mean square/fourth direct adaptive equalizer for under-water acoustic communications in the Arctic

An improved least mean square/fourth direct adaptive equalizer (LMS/F-DAE) is proposed in this paper for underwater acoustic communication in the Arctic. It is able to process complex-valued baseband signals and has better equalization performance than LMS. Considering the sparsity feature of equalizer tap coefficients, an adaptive norm (AN) is incorporated into the cost function which is utilized as a sparse regularization. The norm constraint changes adaptively according to the amplitude of each coefficient. For small-scale coefficients, the sparse constraint exists to accelerate the convergence speed. For large-scale coefficients, it disappears to ensure smaller equalization error. The performance of the proposed AN-LMS/F-DAE is verified by the experimental data from the 9th Chinese National Arctic Research Expedition. The results show that compared with the standard LMS/F-DAE, AN-LMS/F-DAE can promote the sparse level of the equalizer and achieve better performance.

Vertical distribution of nutrient tracers in the western Arctic Ocean and its relationship to water structure and biogeochemical processes

During the 3rd Chinese National Arctic Research Expedition cruise in the summer of 2008, nutrients (NO3, NO2, SiO32 , and PO43 ) and dissolved oxygen were measured in the western Arctic Ocean, to derive the vertical distribution of nutrient tracers and its relationship to water structure and biogeochemical processes. The nutrient data show that surface waters had the lowest NO3/PO43 (mean of 0.5) and SiO32 /PO3 (mean of 2.8) values in the water column, suggesting an excess of phosphate. Winter Bering Shelf water (wBSW) had high Si* (16.7 µmol/L; Si*=[Si(OH)4]-[NO3]) with negative N* (−11.7 µmol/L; N*=[PO43 ]-16[PO43 ]+3.5 µmol/L) in the water column, indicating nitrate deficiency. The warm Atlantic layer had positive N* (0.8 µmol/L) and negative Si* (−5.4 µmol/L) compared with Pacific source water. The vertical distribution of nutrients indicates that wBSW can be characterized by N* minimum and Si* maximum. In contrast, minima of Si* and SiO32 /PO43 below 200 m indicate the distribution of Atlantic warm water.

Comparison of summer Arctic sea ice surface temperatures from in situ and MODIS measurements

Ship-borne infrared radiometric measurements conducted during the Chinese National Arctic Research Expedition (CHINARE) in 2008, 2010, 2012, 2014, 2016 and 2017 were used for in situ validation studies of the Moderate Resolution Imaging Spectroradiometer (MODIS) sea ice surface temperature (IST) product. Observations of sea ice were made using a KT19.85 radiometer mounted on the Chinese icebreaker Xuelong between July and September over six years. The MODIS-derived ISTs from the satellites, Terra and Aqua, both show close correspondence with ISTs derived from radiometer spot measurements averaged over areas of 4 km×4 km, spanning the temperature range of 262–280 K with a ±1.7 K (Aqua) and ±1.6 K (Terra) variation. The consistency of the results over each year indicates that MODIS provides a suitable platform for remotely deriving surface temperature data when the sky is clear. Investigation into factors that cause the MODIS IST bias (defined as the difference between MODIS and KT19.85 ISTs) shows that large positive bias is caused by increased coverage of leads and melt ponds, while large negative bias mostly arises from undetected clouds. Thin vapor fog forming over Arctic sea ice may explain the cold bias when cloud cover is below 20%.