IRI@AGU: Linking Ocean Temperatures and Sahel Climate

Catherine Pomposi, PhD student in the Department of Earth and Environmental Sciences

This post is the second in a series of Q&As with scientists from the International Research Institute for Climate and Society who will be presenting their work at the annual meeting of the American Geophysical Union in San Francisco December 9 to 13.

During the 1970s and 80s, the western Sahel suffered from severe and prolonged drought. In the past couple of decades, the Sahel has benefited from more rain on average, though there is still significant variation between years, with annual precipitation ranging from 300 to 700 mm. Given the reliance on rainfall for agriculture in this region, researchers would like to better understand what atmospheric and ocean processes lead to drought in this region, especially in the context of the future and how a changing and more variable global climate may affect the area. Most rainfall occurs in the monsoon season of July – September, so the amount of rain carries the additional weight of sustaining populations during the dry season.

Proposed reasons for the observed 20^th century variability in the area include the effects of global ocean temperatures and resulting feedbacks in the climate system. IRI’s Alessandra Giannini has shown that sea surface temperatures in the Indian Ocean and North Atlantic Ocean play a significant role in the amount of rainfall the Sahel receives. Catherine Pomposi, a PhD student in the Department of Earth and Environmental Sciences, works with Giannini and another advisor, Yochanan Kushnir of the Lamont Ocean and Climate Physics Division, to further understand influence of atmospheric and oceanic conditions on the Sahel climate. Read the Q&A with Pomposi below to learn more and visit her poster and presentation if you’re going to AGU. ­

In basic terms, what do you do in your research?

My research aims to answer questions surrounding Sahel rainfall variability on different timescales, from decadal to interannual. I am trying to better understand what causes rainfall in the area, and how precipitation mechanisms are driven by the underlying climate system.

To examine changes in precipitation, my research uses a moisture budget framework. The moisture budget equation relates surface changes in precipitation and evaporation to the overall humidity and wind convergence in an atmospheric column. By using this equation, we can better understand whether precipitation changes are more influenced by changes in the atmospheric circulation or changes in the humidity of the air. Furthermore, this technique allows for analysis of precipitation variability in terms of seasonal-scale processes and shorter term, sub-seasonal climate events (or eddies).

How does this work contribute to better understanding of the Sahel climate?

Past studies have shown that global ocean temperatures influence precipitation variability in the Sahel, and the use of the moisture budget equation has been used to understand precipitation processes elsewhere in the world, such as the southeastern United States. In our current work, we are using a model driven by sea surface temperatures to better understand changes in precipitation, particularly during the 20^th century drought, in terms of the seasonal and sub-seasonal atmospheric circulation patterns. We also break up the seasonal term into the mean wind field and specific humidity to see which may have a greater influence on Sahel precipitation.

Have you had any initial findings on the moisture budgets?

Our initial analysis of the moisture budgets shows us that, in general, the seasonal flow contributes a larger proportion of the precipitation changes than do the sub-seasonal eddies. We are also finding that changes in precipitation due to variations in the seasonal dynamics are likely more affected by the wind field than by increase in specific humidity.

How might your research eventually be used by people in the region?

While precipitation in the region is known to be influenced by sea surface temperatures, understanding the mechanisms linking these two variables is far from complete. Since our results are based on sea surface temperatures however, we can relate what we’re learning back to oceanic conditions, with the hope that new understanding of the dynamics can improve projections and forecasts and inform decision making in the region.

What are the next steps in your research?

Now that we’ve validated the model we want to use and begun using the moisture budget, the next steps for my research include looking more closely at the differences in the moisture budget between wet and dry decades in the 20^th century. We are hoping that this may tell us more specifically about how the differences are tied to ocean temperatures. Eventually, I would also like to apply this research more directly with various stakeholders in the Sahel.

You are also giving a presentation about your work with the New York Academy of Sciences Afterschool STEM Mentoring Fellowship Program. What was that experience like?

For ten weeks, another fellow and I co-taught an afterschool earth science program to 7^th and 8^th graders at a charter school in New York City. We focused on natural disasters and started each program with an introductory science lesson based on the day’s topic (e.g., hurricanes or volcanoes). We then used a hands-on, inquiry-based activity to reinforce the concepts. The semester culminated with a scientific poster that the class presented on Hurricane Sandy, including elements such as why it was a unique storm, how it formed, impacts on communities, and observations from students about their experience of the hurricane.

One of the most rewarding experiences I’ve found with the STEM Mentoring program has been the opportunity to work with children and get them excited about science. Many of the students we’ve worked with start with the idea that a scientist is always in a lab working with chemicals. Using the curriculum we developed, we were able to demonstrate different methods that scientists use in their research, such as understanding various weather or climate events using computer simulations. The students became more excited about science as they were able to complete interactive experiments, and it allowed them to recognize that scientists come in many forms. I was continuously impressed by the engagement of our students and was excited that when the semester ended many expressed an interest in continuing to study science and perhaps pursue it as a career path.