Hurricane Season Causes: Climate Change Versus Natural Variability

This is part one in a two part series that will examine the causes of hurricanes, including climate change, and their impacts.

It’s Hurricane Preparedness Week in the US. This hurricane season, which officially begins June 1, is predicted to be the most active season since 2005. Some might chalk it up to climate change, but the reasons are a little more complex than that.

To refresh your memory, in 2005 there were so many named storms that meteorologists exhausted the Roman alphabet and switched to Greek. This year, most reputable sources are predicting 15-17 named storms and 2-4 major hurricanes that make landfall. Not enough storms to make it to Alpha, but still more than the 11 named storms and 2 major hurricanes on average.

So what made 2005 such an intensely active year and what makes this year likely to follow suit? The factors that affect hurricane development are much like a wedding cake. Starting at the top, there are local, immediate factors that drive hurricane formation. Those in turn are driven by longer-term, more spatially distant factors.

Immediate and Local Conditions

At the top of the cake are immediate, local conditions. The two most important to hurricane formation and strength are sea surface temperatures (SST) and vertical wind shear. SST affects both atmospheric stability and the amount of water that can be evaporated, which feeds hurricanes. Vertical wind shear is the difference in wind speeds and directions at different altitudes.  Strong wind shear essentially “tears apart” storms while weak wind shear is ideal for storm development.

SST in the main development region (MDR) where most hurricanes form in April was warmer than usual by 1.46 degrees Celsius, which makes it the warmest month ever recorded. (The previous holder of this distinction was March 2010.) Wind shear is still seasonably high. The record warm ocean is likely to reduce wind shear over time, though, in addition to increasing atmospheric humidity.

The Next Layer Down

The local conditions are driven by other underlying factors. There are two particularly important sea level pressure oscillations that affect SST in the MDR. These oscillations are called the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO).

These are measured by a comparison of sea level pressure between the Azores and Iceland. If the difference in pressure between these two areas is small (or negative in the meteorological world), then there’s less wind blowing across the Atlantic. This means there’s less mixing of warm surface waters with the cold water underneath, which means SST are warmer overall.

From December through February, the AO and NAO were the most negative they’ve been since 1950. This is in part why there’s now record SST in the MDR.

In addition, there’s a longer term oscillation that affects SST: the Atlantic Multidecadal Oscillation (AMO). The AMO affects SST in one of two ways: it makes them warmer or colder. Each of these phases last for roughly 40 years, which you can see in the graphic below. Right now, the AMO is in a warm phase, which again lends itself to an increase in intense hurricanes.

Remote Factors

Finally, there are remote factors which make up the bottom layer of the cake. The most widely understood one is El Nino-Southern Oscillation (ENSO). During a positive phase, commonly known as El Nino, the eastern Pacific Ocean warms. This warming increases vertical wind shear across the MDR. The opposite is true of the negative ENSO phase, called La Nina. Guess what? There’s an increasing probability that La Nina conditions could form this summer, though at this point average conditions are the slight more likely as the graphic below shows.

Basically, all the signs out there point to this hurricane season having the potential be incredibly active. Whether forecasts are correct remains to be seen.

Where Does Climate Change Fit In?

That’s a big question for climate scientists. In one season, it’s hard to say. In the longer-term, well, that’s also tough. Hurricanes and climate change is considered an “active area of research” because there isn’t a strong scientific consensus yet. Some studies have shown that climate change has led to less frequent but more intense storms. Others have shown that the warming of our planet hasn’t just warmed the land but the oceans as well. As mentioned above, warmer SST generally mean more intense hurricanes.

However, there are problems with linking climate change and hurricanes from the current data available. For starters, observations have become much more complete and sophisticated since the data record for hurricanes begins. It’s possible that many hurricanes were missed in the early part of the 20th century due to a limited observation network as well.

In addition, there are also questions about how hurricanes will be affected. Intensity, wind speed, and precipitation are the main components of hurricanes. Each one might be affected differently by climate change.

A February 2010 study in Nature Geoscience that united formerly divided climate scientists gets at this. The study’s model runs showed with greater than 66% confidence that hurricanes in all ocean basins would have stronger wind speeds and more precipitation due to climate change. However, the study’s conclusions on hurricane intensity were less strong, with only a 50% or greater chance that there will be more intense hurricanes but fewer overall hurricanes.

This is where it all comes back to the link between anthropogenic climate change and natural variability. There are many factors that play into hurricane genesis and intensity. Climate change due to increased carbon emissions will likely have some impact on hurricanes. However, the timescale of that effect and the strength is still very open-ended and fiercely debated by climate scientists. And natural variability like the factors mentioned above will also continue to affect hurricanes. In other words, no one component of the climate system can be fully responsible for any given climate event.

Photo Credit: Flickr