Temperature Trend Follow-Up

While the topic is still fresh in my mind, I did some follow-up on Wednesday's question about whether the ERA5 temperature trends are realistic for Southeast Alaska.  Specifically, I wanted to compare the ~9km resolution ERA5-Land data to the ~30km ERA5 reanalysis.

According to ECMWF, ERA5-Land "adds value to ERA5 surface fields and provides users with a more accurate dataset for surface applications.  The impact can be particularly important over complex terrain, where accurate orography is very important."

This sounds promising for Southeast Alaska.  Here's a visual comparison of the 1957-2021 linear trend in 2m temperature from the two sources: ERA5 on top (the data I looked at on Wednesday) and ERA5-Land below.

Clearly the ERA5-Land trend is smaller overall in this region, and it does not reach such high values in some of the areas of high terrain.  Also, the ERA5-Land trend values are obviously much lower over the coastal islands of Southeast Alaska.

Here's the elevation versus trend analysis for all grid cells within the Panhandle climate divisions: ERA5 on top, ERA5-Land below.

The highest trends are significantly lower for ERA5-Land than for ERA5, and the lowest elevation grid cells have trends that are quite consistent with the results from the climate divisions (~0.16°C/decade) and from Sitka and Juneau (0.14 and 0.26°C/decade).  Contrast this with the area-average ERA5 trends of 0.44-0.55°C/decade for the Panhandle climate divisions, as presented in the new paper by Ballinger et al.

In my view, these ERA5-Land results strengthen the argument that the coarse-resolution ERA5 data produces unrealistic trends for Southeast Alaska as a whole, and especially if we're interested in the low elevations where most people live.  Given the obvious shortcoming of the coarse ERA5 data, and the likelihood that ERA5-Land still isn't quite right, I think we should accept the NCEI climate division trends as more representative of reality, especially for inhabited locations; and arguably then inhabited Southeast Alaska may have the smallest warming trend of anywhere in Alaska.

 

Here's a comparison of the ERA5 vs ERA5-Land linear trends for each climate division: the ERA5-Land trends are slightly higher for the west and interior, but much lower for the Northeast Gulf and Panhandle divisions.

Division	ERA5	ERA5-Land
North Slope	0.55	0.55
West Coast	0.37	0.39
Central Interior	0.35	0.36
Northeast Interior	0.38	0.41
Southeast Interior	0.28	0.32
Cook Inlet	0.31	0.30
Bristol Bay	0.34	0.32
Northwest Gulf	0.26	0.25
Northeast Gulf	0.37	0.31
North Panhandle	0.55	0.41
Central Panhandle	0.49	0.32
South Panhandle	0.49	0.32
Aleutians	0.20	0.19

And finally, a statewide comparison in map form: