Friday, February 18, 2022

Cyclone Trends

I'm in the middle of a busy season and unable to post much, but some readers may be interested in a bit of historical analysis that I did the other day in response to a Twitter query about North Pacific low pressure systems.  Are North Pacific winter cyclones becoming stronger?

There are probably dozens of ways to look at this question, but I used the ERA5 reanalysis to add up the number of hours below several MSLP thresholds at each location, by year, from 1950-2021.  I then calculated a linear trend for the number of hours, for example:

This shows that much of the basin has seen an increase in time spent below 960mb, which is a threshold that represents a moderately strong extratropical cyclone for this part of the world.  Gray areas on the map have less than 10 years in which the MSLP dropped below 960mb, so the sample size is much too small to draw a trend.

However, the trends since 1979 are mixed, showing that there hasn't been a robust basin-wide trend in the past 40 years.

For a basin-total view, here's a look at the annual area-time fraction below 960mb, 950mb, and 940mb.  To understand this metric, consider that if the entire area were below 960mb for 50% of the year, the annual area-time fraction would be 50%.  Alternatively, if 50% of the area were below 960mb for 50% of the year, the fraction would be 25%.  The actual values are mostly less than 0.1% for 960mb, and much smaller for lower thresholds.

Clearly ERA5 shows a significant increase in the frequency of strong storms in 1977, which coincides with the well-known change into the positive PDO phase in late 1976 (sometimes known as the great Pacific climate shift).  Since then, it appears there has not been a significant trend, but interestingly the last two years had a relatively high number of 940mb and 950mb storms.

Looking north to the Arctic, and using a higher MSLP threshold of 980mb, it's interesting to see that the same contrast emerges between the trends starting in 1950 versus 1979.  Click to enlarge:

This deserves more investigation: did Arctic cyclones also respond to the PDO shift?  A basin-wide Arctic analysis is hampered by the fact that storms are typically far stronger on the European side of the basin, so I'll have to think about the best way to deal with that.  But in the meantime, a subregion of the Arctic on the Alaska side shows generally higher frequencies since 1979 - see below.

Note that it's distinctly possible that the ERA5 reanalysis has a discontinuity in 1979, because the 1950-1978 data is a "back extension" that does not benefit from satellite data; perhaps that influences the intensity of cyclones in the model, although I suspect it's unlikely to explain the differences seen here.


  1. There is a large decadal variability on this side of the hemisphere - any trend is dependent on where one starts. This (irregular) variability goes on as far back as one can see (200 years) - there are no long-term significant changes. The reanalysis is bland on intense cyclones - most (but not all) major systems are there (after 1880) - but they are typically too weak.

  2. Thanks for the comment. I’d be interested in any key references that illustrate your points. I have no doubt that the extended reanalysis products are quite deficient pre-1950, but how about ERA5 1950-1978?