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Fri 22 September 16:06 GMT
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    Richard Dixon3 weeks ago

    Idalia made landfall as a major hurricane this week in a season where we’ve been anticipating what might become of a Battle Royale between the anomalously warm seas in the Atlantic and an ongoing El Niño in the Pacific. A reminder: we’re not even climatologically half way through the season yet…

    Taking a look at the sea surface temperatures in the Gulf of Mexico, it’s little surprise we saw a strong landfall: in the week leading up to Idalia, the sea in its path was the warmest it’s been in since 1982 (when this particular sea temperature dataset started). The chart here shows the average sea temperature for the period of Aug 23-29th for every year for the past 40 years in the red box shown that broadly straddles the region through which Idalia passed and developed. Naturally other factors will always influence how a storm develops, but you don’t typically get the heavy-hitters of hurricanes without the warmer seas.

    And now for the rest of the season…

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    Ruth Petrie4 weeks ago

    Lahaina, located on the northwest coast of the Hawaiian island of Maui, experienced a devasting wildfire in August 2023. Several factors likely contributed to the severity of this wildfire including enhancement of strong easterly winds by Hurricane Dora, Katabatic winds flowing from the West Maui Mountains, located to the east of Lahaina, and drought conditions.

    The precipitation rate for all of Hawaii has been extracted from the NCEP Reanalysis data between 1980 and 2023 and is shown in the graph. The precipitation rate for February 2023 was the highest recorded since 1980 at nearly three times the average. Precipitation rates throughout the spring were above average but by June it is below average, with July being extremely dry in the bottom 5% of years.

    The February and spring rainfall likely contributed to a growth of vegetation. During dry seasons an increase in fuel availability contributes to the wildfire risk. Precipitation rates over Hawaii have been well below normal for June and July leading to drought conditions across Hawaii. According to the US Drought Monitor the most severe droughts over Hawaii in South and West Maui (where Lahaina is located). It is likely that the drought conditions combined with an abundance of fuel from the wet February and spring were contributory factors in the severity of the Lahaina wildfire.

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    Richard Dixon1 month ago

    The Copernicus Climate Change Service produces monthly seasonal forecasts from many different weather forecasting centres to help us understand possible future conditions around the globe.

    The chart below shows the expected rainfall through the months of September to November from five different seasonal forecast models. Greens show rainier conditions, brown shows drier conditions. The rainier conditions can be indicative of increased tropical cyclone activity. This year we have an El Nino – that usually weakens hurricane activity – but a warm Atlantic, that can increase activity. So, any indications of what is come in such a difficult year to forecast are always useful.

    It’s interesting to note the green, wetter region across the Tropical Atlantic. However,this region exists more towards the central / eastern Atlantic, which may be indicative of a busier hurricane season here, but this anomalous wetness is reduced towards the eastern seaboard of the US – although rainfall is still expected to be slightly above average here. All to play for with the three key months ahead – but will we be spared hurricane landfalls with a busy tropical season staying over the sea?

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Hurricanes Hub

Hurricanes Hub

2023: A hurricane ‘Battle Royale’

As ever, we’ve barely had time to take a breather after the 1/1 and 1/4 renewals and the hurricane season is nearly with us again. This year sees a particularly rare combination of conditions as we approach the key months of August, September, and October: the presence of an El Nino and the potential for some warm seas in the Atlantic. Can we make head or tail of what’s to come by looking at what’s gone before; and what other data can we use to help us understand how the season might pan out?

History: The “Truth”, but with limitations

In framing the forthcoming season, we’re going to talk about two competing factors here:

  • The temperature of the “Main Development Region” in the Atlantic, an area of sea over which a large proportion of US landfalling storms will have to pass before hitting the mainland.
  • The El Nino Southern Oscillation, or ENSO: where warm (El Nino) or cold (La Nina) waters in central Pacific influence global wind patterns that can suppress or enhance hurricane activity, respectively.

The chart below takes historical data from 1950 to 2019 and shows, for each season, the percentage difference compared to the mean of Atlantic Basin hurricanes (left) and landfalling hurricanes (right) plotted on the chart according to August-to-October ENSO index (x-axis) and the temperature of the Main Development Region (y-axis) relative to the mean: we take the mean here as 1981-2011. Seasons with above-average basin count or landfall count are in red, and below-average are in blue.

What we notice, for historical data, is how the basin count is above average as seas get warmer and a hint that maybe El Nino causes fewer storms: but the latter statement is a pretty sketchy one based on the data.  

For landfall counts, it’s just a lot of noise. Landfalling events are much rarer than storms in the basin, and historical data doesn’t really give us any hints towards how warm the Atlantic is or how strong an El Nino or La Nina is might influence landfalling storm risk. There’s aren’t enough years to help us out there – and indeed not enough landfalls to try and make some sensible conclusions.

Climate Models: Imperfect, but useful extensions of history

Now, we could just shrug our shoulders and give up here. But there are climate models – especially those that are run in an “hindcast ensemble” mode where multiple versions of historical years are re-simulated using similar (but not identical) sea surface temperatures and greenhouse gases to previous years.

At Inigo we’ve been investing time into understanding how we can best use climate model data to glean as much information as possible to help us make underwriting and risk management calls.  It is vital that we understand how tropical cyclone risk might be shifting where historical data might not inform us satisfactorily. The two main topics of investigation are:

   1. A world of seemingly increasingly warm seas
   2. Season-to-season “natural variability” like ENSO and how this can impact short-term pricing.

We can plot the same chart as above, but here, for every year from 1951 to 2011, we’ve got 100 complete re-runs of each year, with slightly different atmospheric and sea surface conditions. So: around 6000 years rather than 60 or so years of history.

What emerges from this is again, a little noisy – but we can see some patterns emerging more from the data when compared to the earlier “historical data” plot, with regards how Atlantic MDR sea temperatures and the ENSO phase might dovetail:

Again, we are showing the percentage increase or decrease over the average in basin count (left) or landfalling storm count (right) for various combinations of MDR sea temperature and ENSO index. 

The key thing to note here is the chart of both basin activity and landfall activity goes from negative to positive from the bottom-right to the top-left: i.e., the warmer the sea and more the Pacific Ocean is in a La Nina phase, the more there is activity in the basin and commensurately more chance of a season with higher landfall activity. This landfalling trend was not evident from the historical data. Whilst climate models are anything but perfect, here they provide some helpful indicators as to how the risk might shift that you simply cannot see easily from a short historical dataset.

There is a storyline here that points clearly to increased risk – both in the basin and at landfall – as seas warm, but there is still a catch here in that when a strong El Nino is present and the sea is warm (the top-right of the diagram), activity can still be reduced.

2023: An unprecedented ‘Battle Royale’

But how does this year sit in the context of all this data? Just to anchor us historically, we’ve shown the first diagram again, but we’ve annotated a few years on the chart: the busy basin years of 2010 and 2005 – a year to bring out a cold sweat in anyone who worked through it.

The question mark in the top-right hand corner here represents how seasonal forecasts might be calling this season: very warm in the MDR, of the order 2010 warmth, but with the potential for a strong El Nino. We are in uncharted territory here by way of historical data – and indeed most of the climate model data we’ve used. 

The nearest analogue one could argue was 2015: comfortably above-average warmth in the Atlantic Basin but with a strong El Nino. This produced an average basin season, with no landfalls. Even if we look back the climate model re-simulations of 1951-2011 in the 2nd chart, our dataset falls short of what we might be seeing this season, but there are hints from this more extensive dataset that even for warm MDR years, a strong El Nino could nullify that warmth’s impact.

So go on, pick a side: I dare you! Is it a busy season because of the warm Atlantic or a damp squib thanks to the suffocating wind shear of an El Nino? Let the battle commence!

Atlantic Tropical Storm Count


Sea Surface Temperature Data

Monthly Historical Tropical Cyclone Probability


Richard Dixon
Richard Dixon
Jeongmin Han
Jeongmin Han
Ruth Petrie
Ruth Petrie


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