National Aeronautics and Space Administration

Sea Level Rise Seminar
Speaker: Evelyn Powell (Columbia/LDEO)
Title: Sea-level modeling: from resolving tribal land disputes to inferring Earth's viscoelastic structure

This is an on-line, virtual presentation only. Please consult with event host Patrick Alexander for connection details.

Abstract:
Our ability to understand past sea-level changes as well as predict polar ice sheet contributions to future sea-level rise depends on our ability to accurately model the Earth’s response to surface mass loading. This deformational, gravitational and rotational response to ice sheet and ocean changes termed Glacial Isostatic Adjustment (GIA) strongly depends on the assumed mantle rheology. In this talk, I will discuss GIA modeling in two regions of complex Earth structure. For the first case study, I will use sea-level modeling to estimate the emergence of an island from Canada’s waters in order to corroborate an Indigenous people’s land claim. In the second case study, I will focus on Antarctica -- a region with limited observational constraints. I will demonstrate that although local geodetic observations such as GNSS may provide invaluable constraints on uncertain mantle viscosity parameters, traditional modeling of crustal deformation and sea level change does not accurately capture the response of the 3-D viscoelastic mantle.

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GISS Lunch Seminar
Speaker: Bruno Tremblay (McGill)
Title: The role of ocean heat transport on rapid sea ice declines in the Community Earth System Model Large Ensemble

This is hybrid presentation. Please consult with event host Lettie Roach for connection details if you wish to virtually attend.

Abstract:
Many global climate models simulates rapid sea ice declines in the Arctic Ocean when the pack ice transitions from a perennial to a seasonal ice cover. Most of these events are linked with anomalous northward Ocean Heat Transport (OHT). Using the Community Earth System Model Large Ensemble (CESM-LE), we find that the pathway by which the ocean heat enters the Arctic (Barents Sea Opening (BSO), Fram Strait or Bering Strait) is key to this link with the rapid declines. The interaction between OHT and sea ice happens mainly over continental shelves where the BSO and Bering Strait OHT are strongly correlated with basal melt, ice-ocean heat flux, absorbed short wave radiation in the ocean and ice growth/melt. The Bering Strait OHT is linked with more rapid declines than the BSO OHT, presumably because of the broader Eurasian shelf. No clear link is found between rapid declines and the Fram Strait OHT: Atlantic water entering the Arctic through Fram Strait circulate around the Arctic Ocean at depth with little vertical heat transfer with the surface waters. In total, 64 of the 79 rapid simulated declines in CESM-LE are linked with anomalous OHT. When the September Sea Ice Extent (SIE) before the rapid decline is located only over deep basins in the central Arctic, we observe a decrease in basal melt during the decline. We hypothesize that this is due to an enhanced stratification that reduces heat transfer between the ocean and the ice. The ice-atmosphere heat flux anomalies are more strongly correlated with ocean heat flux anomalies. Our results suggest that OHT are causing rapid sea ice declines mostly when the SIE is large enough to cover the continental shelves and that the atmosphere is the main driver when the initial SIE is located only over the deep basins.

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GISS Lunch Seminar
Speaker: Xiyue Sally Zhang (Johns Hopkins Univ)
Title: Global impacts of observed Southern Ocean surface temperature variability

This is an on-line, virtual presentation only. Please consult with event host Clara Orbe for connection details.

Abstract:
Despite global warming, the Southern Ocean (SO) surface cooled from 1979 to recent years. This SO cooling phase was preceded by a warming phase from 1950 to 1978 that warms faster than the global mean surface temperature trend. The global impact of the SO cooling has been investigated recently by a series of GCM simulations where the SO surface temperature anomalies are nudged to observed evolution. The most significant response to observed SO cooling is found in the tropical South Atlantic, where increased clouds and strengthened trade winds cool the sea surface, partially offsetting the radiatively forced warming trend. When we boost the signal by simulating the earlier SO warming phase, a significant response in the tropical eastern Pacific emerges. While the response in the tropical Pacific is weaker than expected, the efficiency of this teleconnection is dependent on the strength of the subtropical low cloud feedback. When we repeat the same experiment with a model with stronger low cloud feedback, we find a more robust global response, including Southeast Pacific cooling, equatorial Pacific drying, Aleutian Low weakening, North Pacific warming, and Antarctic sea-ice expansion. Our results suggest that SO surface temperature variability can contribute to observed tropical Pacific variability, which offers new insights into the model-observation discrepancy in the recent climate trends.

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