Rapidly shrinking Arctic sea ice has had significant impacts on the Arctic Ocean and many outer Arctic regions. It is therefore urgently needed to reliably estimate Arctic sea-ice thickness (SIT) by combined use of available observation and numerical modeling. Here, for the first time, we assimilate the latest CryoSat-2 summer SIT data into a coupled ice-ocean model. In particular, an Incremental Analysis Update scheme is implemented to overcome the discontinuity brought by assimilating biweekly SIT and daily sea ice concentration data. Along with an improvement in sea ice volume, our SIT estimates have smaller errors than that without SIT assimilation in areas where the sea ice is roughest and experiences strong deformation, e.g., around the Fram Strait and Greenland. This study suggests that the newly-developed CryoSat-2 SIT product, when assimilated properly with our approach, has great potential for Arctic sea ice simulation and prediction.

Given the role played by the historical and extensive coverage of sea ice concentration (SIC) observations in reconstructing the long-term variability of Antarctic sea ice, and the limited attention given to model-dependent parameters in current sea ice data assimilation studies, this study focuses on enhancing the performance of the Data Assimilation System for the Southern Ocean (DASSO) in assimilating SIC through optimizing the localization and observation error estimate, and two assimilation experiments were conducted from 1979 to 2018. By comparing the results with the sea ice extent of the Southern Ocean and the sea ice thickness in the Weddell Sea, it becomes evident that the experiment with optimizations outperforms that without optimizations due to achieving more reasonable error estimates. Investigating uncertainties of the SIV anomaly modeling reveals the nonnegligible role played by the sea ice-ocean interaction during the SIC assimilation, implying the necessity of assimilating more oceanic and sea-ice observations.

A document by Xiaoran Dong. Click on the document to view its contents.

Sea fog strongly affects human activities in the Southern Ocean (SO), but little is known about the factors controlling its occurrence and properties. Using multi-year ship-based observations and state-of-the-art atmospheric reanalysis data, we summarize two methods to identify sea fog episodes on a climatologic scale and estimate the sea fog occurrence in SO from 1979 to 2019. The first method considers the sea-air temperature difference and atmospheric humidity, while the second explores the similarity between fog and low-level clouds. Both methods revealed three centers of sea fog occurrence in the SO. Further analysis shows that the downward surface sensible heat flux due to the southward heat advection in the lower troposphere determines the monthly climatology and linear trends of sea fog occurrence. Precipitation suppresses summer sea fog over the entire SO and partly determines the latter’s response to climate signals on an interannual scale associated with the Southern Annular Mode.

Mesoscale eddies play an important role in both momentum and heat balances in the Southern Ocean. Previous studies have documented an increasing intensity of the Southern Ocean eddy field during recent decades; however, it is still unclear whether the mesoscale eddies with different lifetimes have different temporal variations. Using satellite altimeter observations from 1993 to 2020, we find that the increasing trend in the intensity of eddies is dominated by long-lived eddies (with lifetimes ≥ 90 days), whose amplitude has increased at a rate of ~2.8% per decade; the increase is concentrated downstream of topography. In contrast, short-lived eddies (with lifetimes < 90 days) do not appear to have a significant trend in their amplitudes since the early 1990s. An energy conversion analysis indicates that the increased baroclinic instabilities of the mean flows associated with topography are responsible for the amplitude increase of the long-lived eddies.

Most of the classic wind-driven circulation theories based on the Sverdrup balance have neglected the profound influence of eddy mixing on the large-scale distribution of the potential vorticity (PV), thus failing to explain some prominent features of the observed circulation. In this study, using a series of numerical experiments based on the MITgcm, we diagnose the PV balance to quantify the effect of eddy mixing on the subtropical gyre. Four grid-spacings of 1, 1/3.2, 1/10, and 1/32 degrees are selected to compare the structure of the upper-ocean circulation. In the 1° grid case, the structure of the thermocline is as predicted by the Sverdrup dynamics, with its maximum depth located in the subtropical interior where the wind stress curl is strongest. With increasing resolution, however, this maximum depth is displaced toward the subtropical front, which more closely resembles the observed thermocline. From 1° to 1/32°, the enhanced eddy mixing tends to homogenize the macroscopic PV in the subtropical gyre and reduces the latitudinal PV range to about 25% of the non-eddy solution; and the region where the Sverdrup balance holds is relegated to isolated patches, with its area reduced by about 60%. Furthermore, sensitivity experiments show that the observed thermocline structure is well reproduced in eddy-resolving runs, indicating that the PV mixing provides a better explanation of the subtropical circulation than the Sverdrup dynamics. Our results suggest that the Sverdrup relationship should be treated carefully in the eddy-rich region, even in the subtropical interior.