National Aeronautics and Space Administration

Sea Level Rise Seminar
Speaker: Indrani Das (LDEO)
Title: Basal Processes Underneath the West Antarctic Ice Sheet and Ice Shelves and Implications for Stability

Abstract: Changes in ice sheets and glaciers have raised global sea levels in the past and will continue to affect them in the future. The West Antarctic Ice sheet (WAIS) is of particular importance because most of it is situated on a retrograde bedrock slope below sea level and may undergo rapid collapse. Thwaites Glacier, a large and rapidly-changing glacier located in the Amundsen Sea side of WAIS is perhaps the most likely candidate to trigger a collapse. Ice shelves provide back-stress to the grounded ice and control its loss. However, in the Amundsen Sea region, the ice shelves experience the largest ocean-induced basal melt, further complicating how Thwaites Glacier will respond to a future warming scenario. On the other side of the WAIS (the Ross Sea side), large ice streams drain onto the Ross Ice Shelf and have their own dynamic characteristics. Kamb Ice Stream has stopped and Whillans Ice Stream is slowing down. These dynamic processes change the flux of ice flowing onto the large Ross Ice Shelf. In this talk, I will use airborne ice-penetrating radar and satellite data to study the basal processes such as sliding and basal melt underneath Thwaites Glacier and quantify ocean-induced basal melt rates underneath the Ross Ice Shelf to understand processes that could be impacting the stability of WAIS. I will also elaborate on ice shelf modeling efforts to constrain basal boundary conditions using these observations, particularly under Thwaites Glacier to improve simulations of basal processes and estimates of WAIS contributions to sea level rise.

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GISS Lunch Seminar
Speaker: Zhonghai Jin (NASA GISS)
Title: Can we neglect LW scattering in GCM?

Abstract: The potential importance of longwave (LW) cloud scattering has been recognized but the actual estimate of this effect on thermal radiation varies greatly among different studies. We use a rigorous radiative transfer algorithm to explicitly consider LW multiple-scattering and apply the GCM to quantify the impact of cloud LW scattering on thermal radiation fluxes. Our study shows that the cloud scattering effect on downward thermal radiation at the surface is concentrated in the infrared atmospheric window spectrum (800-1250 cm-1). The scattering effect on the outgoing longwave radiation (OLR) is also present in the window region over low clouds but it is mainly in the far-infrared spectrum (300-600 cm-1) over high clouds. For clouds with small to moderate optical depth (t < 10), the scattering effect on thermal fluxes shows large variation with the cloud t and has a maximum at an optical depth of ~3. For opaque clouds, the scattering effect approaches an asymptote and is smaller and less important. The 2-stream radiative transfer scheme could have an error over 10% with an RMS error around 3.5%-4.0% in the calculated LW flux. This algorithm error of the 2-stream approximation could readily exceed the no-scattering error in LW flux. Therefore, not much can be gained from explicitly including the time-consuming computation of multiple scattering in a LW radiative transfer scheme using 2-stream approximation. However, the calculation error rapidly decreases as stream number increases and the RMS error in LW flux using the 4-stream scheme is under 0.3%, an accuracy sufficient for most climate studies. We implement the 4-stream algorithm in the GISS GCM and find that, when cloud LW scattering is included, the global annual mean OLR is reduced by 2.7 W/m2, and the downward surface flux and the net atmospheric absorption are increased by 1.6 W/m2 and 1.8 W/m2, respectively. Using one year of ISCCP clouds and running the standalone radiative transfer, the estimated LW scattering effects are similar. Overall, the effect of neglecting scattering on the thermal fluxes is comparable to the reported clear sky radiative effect of doubling CO2.

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GISS Lunch Seminar
Speaker: David Lindo-Atichati (CUNY)
Title: What are Eddy fluxes? Biogeochemical Feedbacks from and to the Ocean

Abstract: Marine physics influence biogeochemical systems from microscales to planetary scales. Ocean eddies are ubiquitous and highly energetic rotating features of ocean circulation. Their influence on biological and biogeochemical processes stems not only from advective transport but also from the generation of variations in the environment that affect biological and chemical rates. Eddies modulate the marine environment by vertically distributing and laterally stirring water parcels, chemicals, and planktonic organisms. The ephemeral nature of eddies makes it difficult to elucidate the overlaying mechanism of physical-biological-biogeochemical feedbacks. Therefore, the use of multidisciplinary approaches involving sampling, remote sensing, and modeling is necessary. All three aspects are woven through this presentation in an attempt to: 1) bridge long-standing scientific controversies on the signature of eddies on larval-fish distribution, 2) shed light on the transport and fate of underwater hydrocarbon plumes and surface UV filters in estuaries, and 3) build a paradigm-shift in marine biophysics; quantifying the relationship of eddy activity at the length scale of biological community aggregations, where the collective behavior and motion of marine animal might also be relevant to the large scale driven motion of eddies.

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