Tuesday, January 12, 2021

Happy New Climate Decade, CoCoRaHS!


Average annual precipitation (inches) from 1991 to 2020. Source: GRIDMET.

A New Normal for Weather and Climate: New Decade? Didn't that start last year? Yes, but every ten years the climate world updates our “climate normals.” A "climate normal" is an average using the most recent three decades as a frame of reference. They are updated on years ending in "1" rather than "0." The climate normals we have grown accustom to were for 1981-2010, but this year we’ll kick out the 1980’s and officially usher in the 2010’s. You’ve probably heard the phase “new normal” a lot recently. In weather and climate, we quite literally have new normals.

In honor of the new decade, and all your efforts, I thought we could take a look at how our normals changed in the last decade. The official National Centers for Environmental Information Normals won't be out until later this year. In the meantime, we can get a preview using a dataset called GRIDMET. As is the case with a number of spatial representations of weather data, CoCoRaHS data were used to help create this product. The map shown above is GRIDMET's best estimate of our new precipitation normals across the Continental US. Again, this is using 1991-2020 data. 

You may have heard Nolan mention that CoCoRaHS stations with a long enough record will be included in the computation of the new official normals. This is a big deal. The observations used to compute normals are the backbone of our understanding of climate across the US. More and more is being done with satellites and computer models, and while we fully embrace these advances, they’re not possible without great validation from ground-truth, or surface weather observations. That’s you! One million times over, thank you for all you do.

How have things changed? 

While wet and dry climate extremes tend to balance each other out over time, updating our climate normals can come with significant changes. Not all decades look alike. Sometimes the changes you see in normals are most influenced by what you’ve experienced recently. For instance, the Ohio River Valley has been wetter in the last 10 years than any previous decade on record. In others, it may be more about the data you’re leaving behind. In my home state of Colorado, we are ousting the 80’s, in which we had six consecutive years of precipitation above our long-term average. 

The US as a whole experienced a wet decade. The last eight consecutive years were wetter than our country’s long-term average. 2019 was a record year with a Contiguous US (CONUS) average of 34.82” of precipitation. 1991-2020 climate normals east of the Mississippi River will be almost exclusively wetter than 1981-2010 normals.  

Change in average annual precipitation (inches): 1991-2020 - 1981-2010. Source: GRIDMET.

However, one might argue the rich got richer and the poor got poorer. For example, the desert southwest, which is already the most arid part of CONUS, became even drier. This pattern is hinted at by computer models that show us the implications of a warming climate. A warmer atmosphere can hold more moisture, which can create bigger storms. However, arid zones, like the southwest, may expand as air circulation patterns change.

We also know there are many factors creating natural variations in climate, like the El Niño Southern Oscillation. Typically, the Pacific Northwest is wetter than normal during La Niña years and drier than normal during El Niño years. Conversely, California receives more significant rain events during El Niño. The changes to our normals would seem to suggest that the last 10 years were La Niña-dominant. They were. Not only did we see more La Niña than El Niño the last 10 years, we actually experienced the reverse in the 1980s. 

All things considered, we know each 10 year period is going to give us something new. But when we consider differences in the climate system between the 1980s and 2010s, we can make some sense of the patterns we're seeing.

Take care,

Peter Goble

CoCoRaHS Headquarters