With apologies to Paul Revere, this post is on the lookout for cooler weather with an eye on both the Land and the Sea. While you will hear a lot about 2020 temperatures matching 2016 as the highest ever, that spin ignores how fast the cooling has set in. The UAH data analyzed below shows that warming from the last El Nino is now fully dissipated after all regions headed down, now reversing slightly.
UAH has updated their tlt (temperatures in lower troposphere) dataset for May. Previously I have done posts on their reading of ocean air temps as a prelude to updated records from HADSST3. This month also has a separate graph of land air temps because the comparisons and contrasts are interesting as we contemplate possible cooling in coming months and years.
Note: UAH has shifted their baseline from 1981-2010 to 1991-2020 beginning with January 2021. In the charts below, the trends and fluctuations remain the same but the anomaly values change with the baseline reference shift.
Presently sea surface temperatures (SST) are the best available indicator of heat content gained or lost from earth’s climate system. Enthalpy is the thermodynamic term for total heat content in a system, and humidity differences in air parcels affect enthalpy. Measuring water temperature directly avoids distorted impressions from air measurements. In addition, ocean covers 71% of the planet surface and thus dominates surface temperature estimates. Eventually we will likely have reliable means of recording water temperatures at depth.
Recently, Dr. Ole Humlum reported from his research that air temperatures lag 2-3 months behind changes in SST. He also observed that changes in CO2 atmospheric concentrations lag behind SST by 11-12 months. This latter point is addressed in a previous post Who to Blame for Rising CO2?
HadSST3 belatedly reported March along with the April updates, so hopefully May will appear later in June. For comparison we can look at lower troposphere temperatures (TLT) from UAHv6 which are now posted for May. The temperature record is derived from microwave sounding units (MSU) on board satellites like the one pictured above. Recently there was a change in UAH processing of satellite drift corrections, including dropping one platform which can no longer be corrected. The graphs below are taken from the new and current dataset.
The UAH dataset includes temperature results for air above the oceans, and thus should be most comparable to the SSTs. There is the additional feature that ocean air temps avoid Urban Heat Islands (UHI). The graph below shows monthly anomalies for ocean temps since January 2015.
Note 2020 was warmed mainly by a spike in February in all regions, and secondarily by an October spike in NH alone. End of 2020 November and December ocean temps plummeted in NH and the Tropics. In January SH dropped sharply, pulling the Global anomaly down despite an upward bump in NH. A further drop in March brought new lows for this period. April stayed cool, and now in May SH and the Tropics warmed to converge on the same anomaly as NH, ~0.07 All regions are showing temps comparable to to 2015 prior to the 2016 El Nino event.
Land Air Temperatures Tracking Downward in Seesaw Pattern
We sometimes overlook that in climate temperature records, while the oceans are measured directly with SSTs, land temps are measured only indirectly. The land temperature records at surface stations sample air temps at 2 meters above ground. UAH gives tlt anomalies for air over land separately from ocean air temps. The graph updated for May is below.
Here we have fresh evidence of the greater volatility of the Land temperatures, along with an extraordinary departure by SH land. Land temps are dominated by NH with a 2020 spike in February, followed by cooling down to July. Then NH land warmed with a second spike in November. Note the mid-year spikes in SH winter months. In December all of that was wiped out. Then January showed a sharp drop in SH, but a rise in NH more than offset, pulling the Global anomaly upward. Then in February to April NH and the Tropics cooled further, pulling down the Global anomaly, despite slight SH land warming. In May all regions warmed pulling up the Global anomaly from its lowest value since 2015. All regions are roughly comparable to early 2015, prior to the 2016 El Nino.
The Bigger Picture UAH Global Since 1995
The chart shows monthly anomalies starting 01/1995 to present. The average anomaly is 0.04, since this period is the same as the new baseline, lacking only the first 4 years. 1995 was chosen as an ENSO neutral year. The graph shows the 1998 El Nino after which the mean resumed, and again after the smaller 2010 event. The 2016 El Nino matched 1998 peak and in addition NH after effects lasted longer, followed by the NH warming 2019-20, with temps now returning again to the mean.
TLTs include mixing above the oceans and probably some influence from nearby more volatile land temps. Clearly NH and Global land temps have been dropping in a seesaw pattern, more than 1C lower than the 2016 peak. Since the ocean has 1000 times the heat capacity as the atmosphere, that cooling is a significant driving force. TLT measures started the recent cooling later than SSTs from HadSST3, but are now showing the same pattern. It seems obvious that despite the three El Ninos, their warming has not persisted, and without them it would probably have cooled since 1995. Of course, the future has not yet been written.