A Model Student

The first paper a scientist publishes is always noteworthy, but such an achievement done during the whirlwind ICTP Postgraduate Diploma Programme is both rare and impressive. Former Diploma student Mostafa Hamouda worked with ICTP climatologist Fred Kucharski and did just that, exploring the theory behind climate models. His hard work recently resulted in a paper published in February in the journal Climate Dynamics ( https://doi.org/10.1007/s00382-018-4169-4).

Diploma students have two intense semesters of classes followed by a summer spent writing their thesis, and it’s hard to do publishable work in such a short period of time. Hamouda was a aware of this time crunch, so he started familiarizing himself with the climate models used at ICTP months before it was time to write his thesis. “I knew I wanted to work with Fred,” Hamouda explains, “so I started early.” Hamouda also was able to stay at ICTP after his Diploma graduation in August 2017 for a couple months to continue work before starting his PhD program at the University of Milan, to do further analyses.

“I had this idea for project in the back of my head for several years,” Kucharski said. “I had the initial idea, but Hamouda did all of his own experiments and came up with new ideas.” It’s research that is a bit outside of popular research areas in climate, Kucharski explains. “The models that climatologists use today are very complex. The aim is to combine separate models of water, ice, atmosphere, etc, to create full earth models, but these models still have terrible biases built in. This project looks at fundamental climate dynamics, going back to look at reasons for the biases.”

Their work starts with the fact that there is sometimes a patch of relatively warm sea surface temperature on the equator in the Atlantic Ocean. This temperature anomaly can affect rainfall in the tropics thousands of kilometers away, in what’s known as a teleconnection, and Hamouda and Kucharski's paper investigates the mechanisms behind it. The resulting precipitation patterns are often complex, beyond the original simple theoretical explanations for how far-off conditions can affect precipitation. This paper revisits these precipitation patterns, still using a simple model, but keeping enough complexity to model them.

The pair analyzed three mechanisms using a model developed at ICTP, the SPEEDY global circulation model. While it turns out all three contribute, it was one known as Ekman pumping that had the biggest effect contributing to precipitation patterns. Atmospheric teleconnections between regions with warmer surface water and far-away regions are well explored in older theoretical papers. The key is the atmospheric boundary layer, a layer right next to the surface of the earth, where air is forced to flow out from the high-pressure and into low-pressure systems. This changes how air moves vertically, which is one of the main ingredients for rainfall increases or suppression. Vertical motion in turn causes cyclonic and anti-cyclonic air patterns around the globe. How this motion is impacted by Ekman pumping and changes remote rainfall patterns is the main focus of the paper.

One of the other mechanisms behind the teleconnection is a modification of the Walker Circulation, which is a pattern of east-west circulations of the atmosphere along the equator. The air over the warm water anomaly in the Atlantic rises and  disturbs the normal currents of the Walker Circulation, which cover huge swaths of the globe and can carry air over to the distant tropics.  What goes up must come down, and so air sinks in some other regions, decreasing rainfall.

Including these, and many other details, meant that the computer simulations of this teleconnection matched the observed rainfall patterns. “We were nicely surprised that we found something,” said Hamouda. “Now we can look at other questions, for instance, we examined a sea surface temperature anomaly over the Atlantic, but would a similar anomaly in the Indian Ocean have a similar effect?”

Hamouda is now working on another subject for his PhD, studying extreme precipitation events over Europe, specifically the Alps, using a different climate model. Both he and Kucharski would one day like to continue exploring the questions they examined in this paper. “I would love to continue with this work,” Kucharski said. “We started with a fairly simplified setup; it would be interesting to add complexity.

Whatever happens in the future, Kucharski and Hamouda both enjoyed the whirlwind project, and were proud to present it as a poster at the 2018 European Geophysical Society meeting in Vienna. “It was a very positive experience,” Kucharski said, one that may turn into a collaboration after Hamouda finishes his PhD and returns to his position as a teaching assistant at Cairo University in Egypt. “I really appreciate how incredibly supportive Fred was,” Hamouda says. With any luck, this first collaboration is only the start.

---- Kelsey Calhoun