New Hurricane Polo Study Finds Climate Change Strengthened Storm Alongside El Niño

A study released Thursday, October 15, finds that human-caused climate change and El Niño both strengthened the conditions that fuelled Hurricane Polo last month. Its conclusions are more specific than saying either factor simply “caused” the storm.

The World Weather Attribution analysis estimates that the regional potential-intensity conditions became about 23 times more likely because of human-induced warming. It estimates a roughly fourteenfold increase associated with the current El Niño compared with neutral conditions. Potential intensity is a measure combining ocean and atmospheric information to estimate the maximum winds a hurricane could support; it is not a direct measurement of every gust.

The researchers also report an important limitation: their models could not adequately reproduce observed rainfall trends in the region, so they could not quantify climate change’s contribution to Polo’s rainfall. That uncertainty belongs alongside the wind-related findings when explaining what the new research establishes.

Today’s news concerns a September storm

The Associated Press reported on the study Thursday, describing it as joint work by World Weather Attribution and Climate Central. AP reported an estimated climate-change contribution of around 6 mph to the storm’s peak strength, compared with about 5 mph associated with El Niño. It also described Polo as the Pacific’s longest-lasting Category 5 storm.

The publication date matters: the research is new today, while the hurricane itself occurred in September. This article explains the retrospective findings and their scientific context. It is not a current Polo forecast or a new landfall warning.

NASA documented an unusually favourable environment

In its September 25 account of Polo’s intensification, NASA described several ingredients working together: weak wind shear, abundant moisture, atmospheric instability and unusually warm water. NASA reported a 90-knot increase in maximum sustained winds within 24 hours by September 22, when Polo reached Category 5 strength.

The agency’s ocean-temperature analysis showed the storm moving over waters as warm as 32°C, with some areas around 2–3°C above the reference average for September 23. Its map used a 2003–2014 comparison period. That baseline is different from a preindustrial comparison, so the numbers should not be treated as interchangeable estimates of human-caused warming.

NASA also discussed heat below the surface. Hurricanes can bring cooler water upward, reducing the energy available to them. In Polo’s case, scientists pointed to considerable warmth deeper in the ocean and a limited cool-water wake. These observations describe the storm’s physical environment; the new attribution study asks a further question about why that environment was so favourable.

Contemporaneous advisories show how the storm evolved

The National Hurricane Center’s September 24 forecast discussion estimated Polo’s maximum sustained winds at 140 knots and described it as a Category 5 hurricane. Satellite imagery showed a distinct eye surrounded by a symmetrical ring of deep convection. The forecasters also noted outer rainbands spreading onto southwestern Mexico while the core remained offshore.

A separate September 25 discussion again assigned 140-knot winds after changes in the eyewall and cloud structure. These dated records help explain why descriptions of Polo’s category can differ between reports written at different times.

An advisory is an operational assessment made with the information available at that moment. It is useful evidence of the storm’s observed development, while a later climate analysis addresses the statistical and physical influences behind its environment. Keeping the two forms of evidence separate avoids confusing a forecast update with a retrospective attribution finding.

How attribution research separates different questions

World Weather Attribution’s methods explanation starts with defining the event: the geographical area, duration and variable being studied. Researchers then examine observations to assess how common comparable conditions are and whether their frequency or intensity has changed.

The modelling stage compares the present climate with a hypothetical climate without human-caused warming. Results from observations and models are brought together to estimate changes in likelihood or intensity. Confidence intervals and agreement between the different lines of evidence help indicate how tightly those estimates are constrained.

This framework makes the event definition central to interpreting a headline number. A result about a regional environmental measure is not automatically a result about landfall location, total financial losses or the rainfall recorded at an individual address. WWA also explains that some studies cannot support a numerical attribution statement for every variable.

The practical reading rule is to ask what was measured before comparing percentages or probability ratios. Different measures can answer useful questions about the same hurricane without being competing estimates of one identical quantity.

El Niño is part of a changing ocean–atmosphere system

NOAA describes El Niño as the warm phase of the El Niño–Southern Oscillation, a coupled pattern involving Pacific temperatures and atmospheric circulation. It occurs on average every two to seven years and typically lasts nine to twelve months. Its effects depend on both the warming water and the atmosphere’s response.

As trade winds weaken, the distribution of heat and tropical rainfall changes. That can alter weather far beyond the equatorial Pacific. NOAA notes that El Niño can favour hurricane activity in the central and eastern Pacific while hindering formation in the Atlantic. These are broad influences rather than a promise about the path or intensity of a particular storm.

This helps explain why a natural climate pattern and human-caused warming can be considered together. Identifying El Niño does not resolve every question about an exceptionally warm ocean or an individual hurricane. The attribution analysis examines how different influences alter the environment, rather than assuming that finding one influence excludes all others.

A busy season and a powerful storm are different measures

The Climate Prediction Center’s eastern Pacific hurricane-season background distinguishes storm counts from accumulated cyclone energy, or ACE. The latter combines the strength and duration of named storms and hurricanes to describe overall seasonal activity.

This is useful context for reports about an active Pacific season. One short-lived powerful hurricane and several longer-lasting storms can contribute differently to a season’s total. A tally of Category 5 hurricanes therefore does not describe every aspect of activity, and a seasonal statistic does not tell an individual community what conditions it will experience.

The distinction also keeps retrospective comparisons meaningful. Before calling a season or a storm a record, reporting needs to specify the basin, time period and measure involved. Maximum wind speed, time spent at a particular category and total seasonal energy describe related but different features. A precise label lets readers compare like with like instead of treating every record claim as equivalent.

Why a hurricane’s category cannot describe every hazard

The Saffir–Simpson Hurricane Wind Scale runs from Category 1 to Category 5 and is based on maximum sustained wind speed. The National Hurricane Center explicitly says it does not include storm surge, rainfall flooding or tornadoes. Category 5 begins at sustained winds of 157 mph, or 137 knots.

This means a reduction in category describes a change in the wind classification. It does not establish that every other hazard has declined by the same amount. Nor does it explain how exposed buildings, roads or communities are to the storm.

For readers looking back at Polo, the wind category is an important part of the chronology, but it cannot stand in for a complete account of impacts. The same distinction applies when interpreting future advisories: a category label answers a specific wind question. Local rainfall, coastal flooding and other warnings provide additional information that the five-category scale was never designed to contain.

Flooding depends on more than maximum wind

The hurricane centre’s hazards guidance explains that rainfall totals depend on storm speed and size as well as local geography. Slow-moving systems can produce prolonged rain, while mountainous terrain can enhance precipitation. Flooding can continue after the main storm has passed.

It also distinguishes storm surge from rainfall flooding. Surge is an abnormal rise in coastal water driven by the storm, while storm tide includes the astronomical tide. Strong winds, waves, rip currents and tornadoes add further hazards. The effects can extend beyond the place where the centre crosses the coast.

These distinctions explain why research about environmental conditions supporting strong winds should not be used as a direct substitute for research about rainfall or flooding. Both questions matter, but they require appropriate measurements and methods. A limitation in one part of an analysis does not turn another variable into a reliable proxy for everything the storm did.

How to read the probability figures

The two likelihood ratios should not be multiplied together to manufacture a combined estimate. They describe comparisons with different reference conditions. Without a joint calculation, multiplying them would introduce an extra assumption about how the influences relate. Similarly, “23 times more likely” is not a 23-fold increase in wind speed. Probability and intensity are different quantities, so replacing one with the other would change the meaning of the result. The distinction is essential when turning a scientific estimate into an understandable headline.

What to take from the new findings

The useful contribution of today’s study is its attempt to separate influences that are often compressed into a single explanation. Readers can assess the work by following the chain from the event definition to the observations, model comparisons and stated uncertainties.

That approach supports careful reporting of the results without treating every figure as a forecast. A probability estimate does not give a date for the next hurricane. A wind estimate does not assign a percentage of an individual household’s losses to climate change. And identifying uncertainty is part of describing the evidence accurately, rather than a reason to omit the study.

For related reporting, explore Smashology’s News coverage and our account of the Nepal–Tibet flash floods.