How the Indian Ocean is linked to Europe's early-winter rain and storms

October 28, 2026 dialog How the Indian Ocean is linked to Europe's early-winter rain and storms Lisa Lock Scientific Editor Robert Egan Senior Editor When we think about Europe's winter weather, the Indian Ocean is probably not the first place we look. We might picture Atlantic storms, cold air arriving from the north or rain sweeping across the Mediterranean. In my research, however, I look at how these familiar weather systems fit into a much larger climate system. My colleagues and I asked whether an ocean thousands of kilometers away could leave a detectable signature on Europe's early winter. The answer matters because rain is more than a monthly total. For a farmer, a reservoir manager or someone planning work outdoors, the spacing of wet days can matter too. The same amount of water arriving in a few episodes or over a prolonged wet spell can create very different practical challenges. In our study published in Geophysical Research Letters, I wanted to explore the connection between a distant ocean and both rainfall patterns and the storms that help shape them. A tropical starting point Our starting point was the Indian Ocean Dipole (IOD), a pattern of temperature differences across the tropical Indian Ocean. During its positive phase, the western side is warmer than usual relative to the eastern side. The opposite arrangement characterizes its negative phase. This pattern typically reaches its peak in boreal autumn. The atmosphere responds to more than the temperature of the sea directly beneath a particular storm. Tropical ocean temperatures influence where air rises and where rainfall releases heat into the atmosphere. That heating can disturb atmospheric circulation, with effects extending far beyond the tropics. Scientists call such distant connections teleconnections. I find the term useful because it reminds us that the atmosphere links regions that look separate on a map. A teleconnection does not mean that water evaporating from the Indian Ocean simply travels to Europe and falls as rain. The connection can work through changes in the circulation itself. Looking beyond the winter average We analyzed data for the period 1979–2024, examining atmospheric pressure, several rainfall measures and cyclone tracks across the North Atlantic, Europe and the Mediterranean. Because the Indian Ocean Dipole and El Niño can occur together, we statistically accounted for their linear relationship to examine the IOD-related signal separately. The clearest connection appeared in December. Positive IOD conditions were associated with a north–south contrast in rainfall, including wetter conditions over parts of the northern Atlantic and drier conditions farther south, including the western-central Mediterranean. The differences involved rainfall totals, the frequency of wet days and the length of wet spells. We also found reduced cyclone activity over the eastern Atlantic and western Mediterranean. The broader signal weakened later in winter. For me, this timing is an important part of the story. We often talk about "winter weather" as though December, January and February were interchangeable. Combining them into one average is useful, but it can hide a connection that is strongest during just one part of the season. What changes in the atmosphere? We also examined the atmospheric conditions in which storms develop. The cyclone changes aligned with shifts in baroclinicity, linked mainly to vertical wind shear and north–south temperature gradients. These terms describe part of the environment that supports storm growth. Extratropical storms can draw energy from contrasts between warm and cold air. Winds also change with height, and that vertical structure helps determine how disturbances develop. To understand a change in storm activity, I therefore need to look beyond the paths of individual storms and examine the conditions around them. A clue for prediction, rather than a weather schedule I would not interpret our findings as a promise that a particular Indian Ocean Dipole event will produce a particular December in Europe. Statistical relationships describe tendencies across many years. They do not remove the atmosphere's variability or the influence of other climate drivers. Nor did we demonstrate an improvement in forecasting skill. That requires testing forecast systems and checking whether including a source of information produces more reliable predictions. What interests me is the possibility that a better understanding of these connections could help us ask more precise forecasting questions: Which regions, which rainfall characteristics and which part of winter might carry a useful signal? When I look at a weather map of Europe, I still see the Atlantic storms and Mediterranean low-pressure systems. Our research gives me another reason to look farther afield as well. Understanding early winter may require paying attention to what happened in the tropical Indian Ocean during autumn. This story is part of Science X Dialog, where researchers can report findings from their published research articles. Visit this page for information about Science X Dialog and how to participate. Publication details M. Reale et al., Response of Early Winter Precipitation and Storm Activity in the North Atlantic–European–Mediterranean Region to Indian Ocean SST Variability, Geophysical Research Letters (2025). DOI: 10.1029/2025gl116732. Journal information: Geophysical Research Letters Key concepts Cyclonic StormsMarco Reale is a climate and ocean scientist at the National Institute of Oceanography and Applied Geophysics–OGS in Trieste, Italy. His research focuses on atmosphere–ocean interactions, regional climate modeling, Mediterranean climate and extratropical cyclones. He is the corresponding author of the study discussed here.