Published work

2022
Sarah Weidman, Ned Kleiner, and Zhiming Kuang. 8/9/2022. “A rotation procedure to improve seasonally varying Empirical Orthogonal Function bases for MJO indices.” Geophysical Research Letters., 49, 15. PDFAbstract
Various indices have been defined to characterize the phase and amplitude of the Madden-Julian oscillation (MJO). One widely used index is the Outgoing Longwave Radiation (OLR) based MJO index (OMI), which is calculated using the spatial pattern of 30-96-day eastward-filtered OLR. The EOFs used to calculate the OMI in observations are prone to degeneracy and exhibit oscillations on the order of 10-20 days, despite initial filtering of the OLR. We propose a simple modification to the OMI that involves aligning the EOFs between neighboring days while retaining the spatial pattern described by the EOFs. This rotation method is implemented as a postprocessing step of the current OMI calculation and cleanly removes the spurious oscillations and degeneracy issues seen in the standard method. A similar rotation procedure can be implemented in calculations of other MJO indices.
Sarah Weidman. 7/2022. “Software for Empirical Orthogonal Function rotation procedure for MJO indices”. github
2021
Sarah Weidman, Thomas L. Delworth, Sarah B. Kapnick, and William F. Cooke. 8/2021. “The Alaskan Summer 2019 Extreme Heat Event: The Role of Anthropogenic Forcing, and Projections of the Increasing Risk of Occurrence.” Earth's Future, 9. Publisher's VersionAbstract
Extreme heat occurred over Alaska in June–July 2019, posing risks to infrastructure, ecosystem, and human health. It is vital to improve our understanding of the causes of such events and the extent to which anthropogenic forcing may alter their likelihood and magnitude. Here, we use multiple large ensembles of climate models, comprising thousands of simulated years, to investigate these issues. Our results suggest that the presence of anthropogenic radiative forcing increased the likelihood of the 2019 extreme heat event by as much as 6%. Further we show the rate of occurrence of such an extreme heat event is likely to substantially increase in the future with increasing levels of atmospheric greenhouse gases. While uncertainty in projected climate risk from model choice leads to a broad range of future extreme heat event probabilities, some models project that with rapidly increasing levels of greenhouse gases the likelihood of such events would exceed 75% by 2090.
alaskan_heat_weidman_2021.pdf
Sarah Weidman. 5/14/2021. “Understanding changes in precipitation with climate change over wet and dry land.” Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences. MIT DspaceAbstract
Observed precipitation increased moderately in both wet and dry regions over the past 60 years, contradicting the wet-gets-wetter, dry-gets-drier (WWDD) mechanism. This study uses the perturbation atmospheric energy budget to understand the projected increase in precipitation over dry and wet land with climate change. Using global climate model output, the change in each term in the perturbation energy budget was calculated as a regional average over wet and dry tropical land. Increases in precipitation over wet and dry tropical land are primarily driven by increases in radiative loss, which are moderated by increases in sensible heat flux. Although the dry static energy flux divergence has a strong spatial pattern, cancellation between the horizontal and vertical advection terms, as well as cancellation between the dynamic and thermodynamic components of vertical advection, result in relatively small contributions from the dry static energy flux divergence towards the change in precipitation in the regional average. Only when wet and dry regions are allowed to change seasonally is WWDD given, suggesting that the movement of wet and dry regions is a significant factor in opposing the WWDD mechanism over land.
weidman-sb-eaps-2021-thesis.pdf