Sep
26
Mon
Department Lecture Series
PAOC Colloquium – Christopher Wolfe (Stony Brook) @ 54-915 and https://mit.zoom.us/j/93231196106
Sep 26 @ 12:00 pm – 1:00 pm

Effect of Zonal Asymmetry on Global Climate and Tropical Variability in an Idealized Coupled Climate Model

In a series of pioneering studies, Marshall and colleagues examined the effect of oceanic continental boundaries on global climate using idealized coupled configurations of the MIT general circulation model. They showed that a climate lacking oceanic zonal boundaries (an aquaplanet configuration referred to as “Aqua”) correctly represents the tropical overturning circulation (i.e., the Hadley circulation) and the partition of atmospheric and oceanic heat transport in the topics but oceanic heat transport in the extra-tropics is weak, leading to the formation of large polar caps of sea ice. Adding a simple pole-to-pole ridge of land to create the “Ridge” configuration allows gyres to contribute to poleward heat transport in the extra-tropics, leading to a retreat of the polar sea ice caps.

In this talk, we revisit these foundational results using the Community Earth System Model (CESM)—a “CMIP-class” coupled climate model—and with a focus on tropical circulation and variability. These CESM configurations have higher resolution and more complete atmospheric physics than previous idealized coupled climate models as well as numerous model-dependent differences. These differences lead to features of the climate that are previously unreported or different from those reported. One of the most dramatic features of our Aqua configuration is the presence of a global equatorial cold belt driven by equatorial upwelling. The cold belt leads to subsidence rather than lofting of air over the equator and an atmospheric overturning circulation in the opposite sense as the standard Hadley circulation within a few degrees of the equator. In contrast, our Ridge configuration has an oceanic warm pool and eastern cold tongue instead of a global cold belt and the reverse Hadley circulation seen in Aqua is suppressed. Instead, a Walker circulation connects the warm pole and cold tongue, reinforcing the mean state in a Bjerknes feedback. Consistently, Ridge supports an ENSO-like mode of interannual variability characterized intermittent weakening of the Walker circulation and warming of the eastern tropical ocean. Surprisingly, Aqua also supports an interannual mode of tropical variability characterized by warming and cooling of the cold belt coupled to fluctuations in the strength of the trade winds. In both cases, the tropical interannual variability is driven by a classical Bjerknes feedback between sea surface temperature and wind anomalies.

About this series:

The PAOC Colloquium is a weekly interdisciplinary seminar series that brings together the whole PAOC community. Seminar topics include all research concerning the physics, chemistry, and biology of the atmospheres, oceans and climate, but also talks about e.g. societal impacts of climatic processes. The seminars take place on Monday from 12-1pm. Contact paoc-colloquium-comm@mit.edu for more information and Zoom password.

Seminars
PAOC Colloquium
Sep 26 @ 4:00 pm – 5:00 pm
Sep
27
Tue
Department Lecture Series
Geophysics Seminar – Mike Floyd (MIT) @ Room 54-209
Sep 27 @ 10:00 am – 11:00 am

Title: Slip deficit accumulation, earthquake rupture and seismicity on the North Anatolian Fault in Turkiye… and beyond?

I present results from GNSS observations over the last three decades which show variations in the rate of slip deficit accumulation along the North Anatolian Fault around the Marmara Sea. I compare this to catalogs and studies of background seismicity, and explore the relationship between this and strain accumulation that may lead to an eventual rupture using a recent earthquake on the extension of the North Anatolian Fault into the Aegean Sea as an example. The North Anatolian Fault has a well-recorded history of earthquakes along its entire length during the 20th and early 21st century, which enables us to study further the potential relationship between geodetically determined strain accumulation, background seismicity and the eventual extent of coseismic rupture, plus associated phenomena such as creep and repeating earthquakes. Such relationships may be globally applicable, for example to the San Andreas Fault system in California where there have been relatively few instrumentally recorded earthquakes in the last century and those that have occurred have often not been on the most major known faults. I also present evidence that the strain accumulation rate may change over the seismic cycle as the effects of afterslip and post-earthquake stress relaxation diminish and reestablish the pre-earthquake deformation pattern. Finally I discuss possible future studies to address these questions, as additional data constraints on fault behavior may improve probabilistic seismic hazard analyses.

Department Lecture Series
Planetary Lunch Seminar (PLS) – Sarah Millholland (MIT) @ 54-517 and https://mit.zoom.us/j/96525774689
Sep 27 @ 12:30 pm – 1:30 pm

Title: Systems of Short-Period Exoplanets: Regularities and Anomalies

Abstract: Exoplanetary systems containing multiple close-in, sub-Neptune-sized planets are ubiquitous, representing a dominant outcome of planet formation. These systems exhibit a striking degree of architectural regularity and uniformity, such as nearly circular and coplanar orbits, regular orbital spacings, and intra-system correlations in planetary masses and radii. Beyond this first-order structure, however, these systems also display mysterious anomalies that require explanations. This talk will address three “architectural anomalies” from both an observational and theoretical perspective. I will propose that planet-planet and planet-star dynamical interactions can explain several of the anomalies. I will close with the discovery of a new anomaly that is as-yet unexplained.

About this Series: The MIT Planetary Lunch Seminar [PLS] is a weekly seminar series organized within the EAPS department. Colloquia topics span the range of research interests of the department’s planetary sciences research program. The seminars take place on Tuesdays from 12:30–1:30 pm, unless otherwise noted (term-time only). Speakers include members of the MIT community and visitors. Talks are intended to appeal to graduate students, postdocs, research scientists, and faculty with a background in planetary science. To be added to the planetary email listserv, please contact planetary-org@mit.edu. In general, contact: planetary-org@mit.edu.

Sep
28
Wed
Department Lecture Series
Sack Lunch Seminar (SLS) – Leo Middleton (WHOI) @ 54-915 and https://mit.zoom.us/j/99953804618
Sep 28 @ 12:00 pm – 1:00 pm

Title: Un-mixing the ocean: Double diffusion and turbulence in polar oceans.

Abstract:
Double diffusion refers to a variety of turbulent processes in which potential energy is released into kinetic energy, made possible by the difference in molecular diffusivities between salinity and temperature in the ocean. We present a new method for estimating the kinetic energy dissipation rates forced by double-diffusive convection using timescale temperature and salinity data. The method estimates the up-gradient diapycnal buoyancy flux associated with double diffusion, which is hypothesized to balance the dissipation rate. To calculate the temperature and salinity gradients on small scales we apply a canonical scaling for compensated thermohaline variance (or ‘spice’) on sub-measurement scales with a fixed buoyancy gradient. Our predicted dissipation rates compare favorably with microstructure measurements collected in the Chukchi Sea and beneath George VI Ice Shelf.

About this series: The Atmosphere, Ocean and Climate Sack Lunch Seminar Series is an informal seminar series within PAOC that focuses on more specialized topics than the PAOC Colloquium. Seminar topics include all research concerning the science of atmosphere, ocean and climate. The seminars usually take place on Wednesdays from 12-1pm. The presentations are either given by an invited speaker or by a member of PAOC and can focus on new research or discussion of a paper of particular interest. Contact: sacklunch-committee@mit.edu

Sep
30
Fri
Department Lecture Series
COG3 Seminar – Jacqueline Reber @ 54-915 and https://mit.zoom.us/j/94285020791
Sep 30 @ 10:00 am – 11:00 am

Between End-Members: The Impact of Semi-Brittle Rheology on Deformation Patterns and Dynamics

Abstract:
Constraining the rheology of the lithosphere is of fundamental importance for understanding plate tectonics as well as earthquake generation. This task, however, is exceedingly difficult as a variety of deformation mechanisms contribute to the integrated strength of the lithosphere. Rocks at high-pressure and high-temperature conditions flow viscously by a number of deformation mechanisms whereas at low-pressure and low-temperature conditions rocks crack, fracture, lose cohesion and slide frictionally. At intermediate crustal depth, deformation is hence achieved by a complex spatial and temporal interplay between “viscous” and “brittle” processes. The interaction between these end-member cases where viscous flow cannot accommodate all the imposed displacement and abundant pervasive fracturing occurs leads to “semi-brittle” deformation. Semi-brittle deformation links the time scales associated with fracturing and earthquakes with the time scales associated with a flowing viscous crust leading to a mixture of stick-slip and creep. The co-occurrence of brittle and viscous deformation in rocks can be observed in the field over many length scales and in various lithologies.

A field examples of semi-brittle deformation will serve as starting points to investigate how forces are distributed between the brittle and viscous phases, how deformation localizes, and how the two phases impact the deformation dynamics. By combining field observations with physical and numerical experiments, deformation evolution can be observed and documented, length scales from micro to the macro scale can be investigated, and resulting deformation dynamics do not need to be preassigned, but can emerge from the material interactions. By employing various experimental materials, we can quantify the impact of the strength contrast between the brittle and viscous phases, the distribution between the phases, as well as the role of fracture formation and geometries on slip dynamics. The results show that semi-brittle rheology significantly impacts deformation dynamics, deformation localization, and the force distribution within the different material phases. All these aspects have a direct impact on the stability of fault zones and how deformation will manifest itself.

About this series:
The Chemical Oceanography, Geology, Geochemistry, and Geobiology Seminar [COG3] is a student-run seminar series. Topics include chemical oceanography, geology, geochemistry, and geobiology. Contact cog3_seminar_organizers@mit.edu for more information and Zoom password.

Oct
3
Mon
Department Lecture Series
PAOC Colloquium – Morgan Raven (UCSB) @ 54-915 and https://mit.zoom.us/j/93231196106
Oct 3 @ 12:00 pm – 1:00 pm

About this series:

The PAOC Colloquium is a weekly interdisciplinary seminar series that brings together the whole PAOC community. Seminar topics include all research concerning the physics, chemistry, and biology of the atmospheres, oceans and climate, but also talks about e.g. societal impacts of climatic processes. The seminars take place on Monday from 12-1pm. Contact paoc-colloquium-comm@mit.edu for more information and Zoom password.

Seminars
PAOC Colloquium
Oct 3 @ 4:00 pm – 5:00 pm