CliSAP successfully finished in 2018. Climate research continues in the Cluster of Excellence "CLICCS".

Will the climate phenomenon El Niño mean a harsh winter for Hamburg?

23.11.2015

We’ve already seen the first signs: El Niño, a climate phenomenon that can set off extreme weather events around the globe, is due to come calling this winter. Every two to seven years, it turns the entire Pacific on its head: the warm surface waters of the Pacific no longer flow from east to west, but from west to east – i.e., from Southeast Asia toward South America. The atmosphere is also affected: because the air circulation changes, there’s no more precipitation over Southeast Asia. At the same time, South America can expect to see warmer coastal waters and heavy rainfall.

El Niño has a great influence in the Pacific region. What does that mean for the Hamburg Winter 2015?

In my Working Group at the Cluster of Excellence for climate research, our goal is to be able to better predict such El Niño events. The more accurate we are, the better the countries affected can prepare for the often devastating impacts.

But making prognoses for up to the next six months is extremely difficult. With the weather forecast (for the next 14 days) and the climate prognosis (for the next few decades), we already have two established tools at our disposal. Yet they’re based on completely different approaches. What’s more: the seasonal prognosis falls exactly in the gap between the two.

With the help of a special trick, we can bridge that gap: we have adapted a tried and trusted climate simulation so that we can constantly supply it with new, real weather data. That’s no mean feat, since the models aren’t actually designed to accommodate new data once they’re already running. In our method we “feed” the simulation the latest readings on the atmosphere, ocean and sea ice every month, and then determine the probable “general weather situation” for the next half-year.

But how reliable are the results? To answer that question, we used our model to “back-forecast” the past 35 years; from that time on, there is sufficient data to allow us to check.

We entered the recorded weather data for each month, then used the model to prepare a prognosis for the next six months – and did so for the entire 35 years. In the next step, we compared the results with the actual recorded data for the respective timeframe. What we found: the method works; in most cases, we were able to accurately “predict” past El Niño events.

According to our model, we should also expect a strong El Niño this year. But what does that mean for the winter weather in Hamburg? To find out, we’re currently exploring how El Niño affects Europe. Fundamentally speaking, it can have a cooling effect – though initially only a difference of ca. 0.1 degree Celsius. But even such a small change in temperature can produce extremely cold winters. So should we expect a long, frosty winter? Not necessarily, because the weather here is especially chaotic and unpredictable. Whereas the weather in the Pacific is almost entirely determined by El Niño, here it depends on a broad range of factors. El Niño can be a major one – but doesn’t necessarily have to be. The undisputed number one on the list is the wrestling match between the Azores High and Icelandic Low: roughly one out of two winters in Hamburg is shaped by which of the two comes out on top.

Johanna Baehr is a Professor at the Institute of Oceanography and an expert on seasonal prognoses.