Guest post: How Arctic warming could have steered Hurricane Florence towards the US

§ Blocking weather patterns

As their name suggests, a high-pressure “block” parks in one location for days or weeks, deflecting other weather systems around it. In summer, they are typically associated with bringing lengthy heatwaves and, in winter, with extended periods of cold.

But they can also affect the path of hurricanes, sometimes steering them on unusual tracks. Both Florence and Hurricane Sandy in 2012 were steered by blocking highs that propelled them westward at abnormally far northern latitudes rather than on a more typical curved path northward along the west Atlantic then toward Europe.

The science of blocking highs – what causes them to form and break up, and whether they are becoming more or less numerous – is widely debated and investigated.

Numerical weather prediction models often struggle to predict them and most climate models underestimate how often they occur. Measurements of the real world tell us that they are occurring more frequently in the North Atlantic during summer months.

However, it is tricky to tease out the cause – or causes – of blocking events. The trend of blocking highs is consistent with our predictions in a world with rapid Arctic warming. This tends to weaken the strength of the jet stream – a band of fast-flowing westerly winds high up in the atmosphere – because of a smaller temperature difference between the Arctic and the mid-latitudes.

A weaker jet stream tends to take a more “wavy” path. As the jet stream bulges, forming large northward excursions (called “ridges”), blocking highs can form at either side.

Added to this, we have been observing much warmer than normal ocean temperatures off the east coast of North America in recent years. This warm water also helps form stronger ridges and, thus, makes blocking highs more likely. A similar pattern was seen in the “Ridiculously Resilient Ridge”, a huge high-pressure weather system over the northeastern Pacific Ocean that caused California’s recent multi-year drought.

Arctic warming is not the only factor that favours a wavier jet stream; new research suggests that the influx of freshwater from the faster melt of Greenland’s ice sheet, melting sea ice and stronger river flow into the Arctic Ocean may be creating a “cold pool” in the North Atlantic.  

This pool is believed to be impeding the flow of warm, salty Gulf Stream waters – which flow northwards across the Atlantic Ocean from the Gulf of Mexico up towards Europe – like a backed-up sewer pipe. This also contributes to the abnormally warm temperatures in the western North Atlantic and may be fueling stronger ridges, causing an Arctic warming “double whammy”.

§ Stuck storms

Not only may Arctic warming be helping to steer Florence on an unprecedented path as compared with previous storms with a similar formation history, she is also carrying more moisture – owing to enhanced evaporation from a warmer ocean – and travelling more slowly.

Her loitering over the Carolinas causes persistent, flooding rains, much like the situation when Hurricane Harvey got stuck near Houston, Texas, last summer. Research has since shown that climate change tripled the chances of the record rain that descended on Texas.

These more persistent weather patterns have been echoed across the northern hemisphere during 2018 – from prolonged cold spells and parades of coastal storms during winter, to heatwaves and droughts this summer – and now into hurricane season.

Several new studies have found evidence for more persistent summer weather patterns over northern hemisphere continents, along with a mechanism to explain this behaviour. A 2017 paper in Scientific Reports, for example, described a “fingerprint” of the conditions that lead to this stagnation, which basically consists of a “double peak” in the north-south temperature gradient.

Before human-caused warming, if you travelled from south to north on a continent, you would generally expect to experience a gradual cooling. With the “fingerprint” pattern, after starting out experiencing cooling, you then encounter abnormal warming along the northern belt of the continent, followed by cooling again. This double peak in the temperature tends to drive the splitting of the jet stream, with a branch flowing west to east along the north coast and another branch through the middle of the North America continent.

Between these branches the steering currents are very weak, so weather systems become stagnant – they get stuck, leading to prolonged heatwaves, droughts and rainy periods.

Florence stumbled into one of these weak-flow areas as it neared the coast, leading to prolonged heavy rains fuelled by overheated Atlantic water and evaporation. This is a similar pattern to Hurricane Sandy in 2012, where the storm stalled over the Caribbean with severe rainfall and resulting damage.

§ Declining snow cover

So, what causes this double peak in the temperature gradient? My hypothesis is that the answer lies in the ever-earlier loss of the spring snow cover in the high latitudes. This early melt allows the soil underneath to dry out and warm up much earlier in a belt along the high latitudes of the continents – yet another connection to rapid Arctic warming. Ongoing research will clarify the influence of this factor.

In short, much of the extreme weather that we have endured in recent years may be boosted by the warming, melting Arctic. Slowing these changes by reducing our carbon emissions will help soften the blow of climate change for generations to come.