Heat transport across the Antarctic Slope Front controlled by cross-slope salinity gradients.

The Antarctic Slope Front (ASF) is a strong gradient in water mass properties close to the Antarctic margins, separating warm water from the Antarctic ice sheet. Heat transport across the ASF is important to Earth's climate, as it influences melting of ice shelves, the formation of bottom water, and thus the global meridional overturning circulation. Previous studies based on relatively low-resolution global models have reported contradictory findings regarding the impact of additional meltwater on heat transport toward the Antarctic continental shelf: It remains unclear whether meltwater enhances shoreward heat transport, leading to a positive feedback, or further isolates the continental shelf from the open ocean. In this study, heat transport across the ASF is investigated using eddy- and tide-resolving, process-oriented simulations. It is found that freshening of the fresh coastal waters leads to increased shoreward heat flux, which implies a positive feedback in a warming climate: Increased meltwater will increase shoreward heat transport, causing further melt of ice shelves.

Observed Antarctic sea ice expansion reproduced in a climate model after correcting biases in sea ice drift velocity.

The Antarctic sea ice area expanded significantly during 1979-2015. This is at odds with state-of-the-art climate models, which typically simulate a receding Antarctic sea ice cover in response to increasing greenhouse forcing. Here, we investigate the hypothesis that this discrepancy between models and observations occurs due to simulation biases in the sea ice drift velocity. As a control we use the Community Earth System Model (CESM) Large Ensemble, which has 40 realizations of past and future climate change that all undergo Antarctic sea ice retreat during recent decades. We modify CESM to replace the simulated sea ice velocity field with a satellite-derived estimate of the observed sea ice motion, and we simulate 3 realizations of recent climate change. We find that the Antarctic sea ice expands in all 3 of these realizations, with the simulated spatial structure of the expansion bearing resemblance to observations. The results suggest that the reason CESM has failed to capture the observed Antarctic sea ice expansion is due to simulation biases in the sea ice drift velocity, implying that an improved representation of sea ice motion is crucial for more accurate sea ice projections.

Modeling the breakup of tabular icebergs.

Nearly half of the freshwater flux from the Antarctic Ice Sheet into the Southern Ocean occurs in the form of large tabular icebergs that calve off the continent's ice shelves. However, because of difficulties in adequately simulating their breakup, large Antarctic icebergs to date have either not been represented in models or represented but with no breakup scheme such that they consistently survive too long and travel too far compared with observations. Here, we introduce a representation of iceberg fracturing using a breakup scheme based on the "footloose mechanism." We optimize the parameters of this breakup scheme by forcing the iceberg model with an ocean state estimate and comparing the modeled iceberg trajectories and areas with the Antarctic Iceberg Tracking Database. We show that including large icebergs and a representation of their breakup substantially affects the iceberg meltwater distribution, with implications for the circulation and stratification of the Southern Ocean.

Observational determination of albedo decrease caused by vanishing Arctic sea ice.

The decline of Arctic sea ice has been documented in over 30 y of satellite passive microwave observations. The resulting darkening of the Arctic and its amplification of global warming was hypothesized almost 50 y ago but has yet to be verified with direct observations. This study uses satellite radiation budget measurements along with satellite microwave sea ice data to document the Arctic-wide decrease in planetary albedo and its amplifying effect on the warming. The analysis reveals a striking relationship between planetary albedo and sea ice cover, quantities inferred from two independent satellite instruments. We find that the Arctic planetary albedo has decreased from 0.52 to 0.48 between 1979 and 2011, corresponding to an additional 6.4 ± 0.9 W/m(2) of solar energy input into the Arctic Ocean region since 1979. Averaged over the globe, this albedo decrease corresponds to a forcing that is 25% as large as that due to the change in CO2 during this period, considerably larger than expectations from models and other less direct recent estimates. Changes in cloudiness appear to play a negligible role in observed Arctic darkening, thus reducing the possibility of Arctic cloud albedo feedbacks mitigating future Arctic warming.

The magnitude and duration of Late Ordovician-Early Silurian glaciation.

Understanding ancient climate changes is hampered by the inability to disentangle trends in ocean temperature from trends in continental ice volume. We used carbonate "clumped" isotope paleothermometry to constrain ocean temperatures, and thereby estimate ice volumes, through the Late Ordovician-Early Silurian glaciation. We find tropical ocean temperatures of 32° to 37°C except for short-lived cooling by ~5°C during the final Ordovician stage. Evidence for ice sheets spans much of the study interval, but the cooling pulse coincided with a glacial maximum during which ice volumes likely equaled or exceeded those of the last (Pleistocene) glacial maximum. This cooling also coincided with a large perturbation of the carbon cycle and the Late Ordovician mass extinction.