US Warmhole

Not all places on earth are experiencing global warming at the same rate. Let’s consider the U.S., the “lower 48 states.” Taking data from NOAA for the 344 climate divisions in this region, and computing the linear trend rate for each, we can see differences between different parts of the USA, with red dots for warming and blue for cooling, larger dots faster and smaller dots more slowly:

The most notable feature is that in the southeastern US there’s a region which hasn’t warmed nearly as much as the rest of the country. There are even places which have shown slight cooling, most notably in Alabama and Mississippi.

The part of the U.S. that has warmed far less (if at all) has been dubbed the “US warming hole.” I’ll call it the “warmhole” (just because I like the name). It covers a larger area than just Alabama and Mississippi. A number of papers over the last several years have investigated why this might be.

Meehl et al. (2012, J. Climate, 25, 6394) begin their abstract thus:

A linear trend calculated for observed annual mean surface air temperatures over the United States for the second-half of the twentieth century shows a slight cooling over the southeastern part of the country, the so-called warming hole, while temperatures over the rest of the country rose significantly.

The impression is that there is a small warming rate, especially during the second half of the 20th century. I’m not so sure that’s actually the case. Let’s look at the southeast, as defined by NOAA; here’s the data for mean temperature (note: I’ve converted from °F to °C, and these are temperature anomalies):

The blue line is a linear regression to all the data; its slope is a mere 0.004 °C/yr. But the red line shows a 2-piece fit, separating the data pre- and post-1958. The time was chosen by changepoint analysis. This fit is strongly statistically significant, and that’s taking both autocorrelation and the multiple testing problem into account. In my opinion, this means we shouldn’t think of “not warming since 1950” as much as we should consider the cause of the sudden drop in 1958, one which wan’t a one-year event but a lasting shift.

Some of the investigation centers around the seasonal pattern of the warmhole, and the trends in daily high temperature vs. daily low temperature. We can look at the data for daily mean, daily high, and daily low, both for the year-round average and for the four seasons (winter is D-J-F, spring M-A-M, summer J-J-A, autumn S-O-N). We’ve already seen the daily mean temperature year-round; here is the same for different seasons:

The most likely shift time is 1958 during winter and spring, 1961 in summer, and 1950 during autumn. However, the spring and autumn shift times don’t achieve statistical significance (at 95% confidence), not even close, while winter and summer do.

Most notable is the size of the apparent shift. For summer it’s a sudden drop of about -0.63°C, and the estimated size is about the same for spring (-0.56°C) and autumn (-0.58°C), but they don’t reach statistical significance due to the higher variance of those seasons’ temperature. But for winter, the drop in 1958 is a whopping -2.01°C, about three times as large as for any other season.

Looking at daily high temperature, the story is much the same. Here’s the data for annual means:

and here’s the breakdown by season:

The annual shift time estimate is 1958, as it is for winter and spring but for summer it’s again 1961, with autumn happening in 1947. Again, the spring and autumn shifts are not statistically significant (not even close); for summer it just makes 95% confidence, and for winter the significance is beyond doubt.

And, as for mean temperature, high temperature shows a much larger drop in winter (-2.02°C), with summer more modest (-0.76°C).

The overnight low also shows much the same behavior. Here’s the annual mean:

and here are the seasonal changes:

Yet again, significant in winter and summer, not in spring or fall. The winter shift is 1958, the summer shift is 1961, and winter drops a lot (-2.01°C) while summer drops far less (-0.52°C).

What I haven’t done is look in greater detail at the geographic distribution of changes; I simply took the “southeast” as defined by NOAA. But I do think that other authors haven’t considered the possibility that the “warmhole” is a consequence, not of a trend change, but of a sudden shift. In fact, not only has the research I’ve seen emphasize trend changes rather than value changes, some have focused on how 10-year trends change over time, but my study suggests that 10 years is such a brief time span that the natural fluctuations of 10-year trends are too large to provide maximum information.

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Wherefore the warmhole? There seem to be many ideas.

Meehl et al. conclude that the warmhole is likely related to SST (sea surface temperature) anomalies in the Pacific and Atlantic oceans, particularly the tropical Pacific. Its seasonal differences may be due to different phenomena related to the formation of more persistent “ridges” in the west and “troughs” in the east, which increases advection of cold air from the northeast to the southeast region during winter, but in summer primarily causes greater precipitation and cloudiness.

Yu et al. (2014, Scientific Reports, 4, 6929) focus on high temperature during summer, attributing the warmhole to the aerosol indirect effect and precipitable water vapor. In summer, short-wave cloud forcing dominates, partly offset by the greenhouse effect of increased water vapor, while long-wave cloud forcing “can warm both winter T_max and T_min.”

Mascioli et al. (2017, Environmental Res. Lett., 12, 034008) consider a wider geographic area and its diverse history. They emphasize the role of the North Atlantic Oscillation (NAO) and Pacific Decadal Oscillation (PDO), but also emphasize the complexity of the situation, that the warmole “reflects both anthropogenic aerosol forcing and internal climate variability, but the dominant drivers vary by season, region, and time period.”

Most recently, Partridge et al. (2018, GRL, doi: 10.1002/2017GL076463) define the geographic limits of the warmhole more precisely, and (largely in agreement with what I see) point to a regime shift in 1958 where annual maximum (T_max) and minimum (T_min) temperatures decreased by 0.46°C and 0.83°C respectively.” They also identify slightly different regions for winter/spring effect as opposed to summer/autumn, and suggest that the two seasons’ behaviors have different causes. They point to the Meridional Circulation Index (MCI), the AMO, and the PDO as strongly related to winter temperatures but don’t explain the summer temperature pattern. What I find most plausible is their “evidence that the jet-stream exhibited a shift in the late 1950’s coincident with the start of the warming hole, resulting in a greater tendency of northerly winds to bring cool air to the southern U.S.

I don’t see that any consensus has yet emerged, particularly about the causes, so I expect there’s still more to be learned about this. But I definitely credit Partridge et al. for identifying the regime shift in 1958, and for delineating the location of the warmhole more precisely, including the differences in its location during different seasons. In any case, this is science in action. Different researchers propose different ideas, and until we have tested competing theories strongly enough we’ll have to wait for real confidence in any explanation of the details of how and why U.S. temperature history shows such interesting differences in different regions.

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