With apologies to Paul Revere, this post is on the lookout for cooler weather with an eye on both the Land and the Sea. UAH has updated their tlt (temperatures in lower troposphere) dataset for December. Previously I have done posts on their reading of ocean air temps as a prelude to updated records from HADSST3. This month I will add a separate graph of land air temps because the comparisons and contrasts are interesting as we contemplate possible cooling in coming months and years.
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?
The December update to HadSST3 will appear later this month, but in the meantime we can look at lower troposphere temperatures (TLT) from UAHv6 which are already posted for December. The temperature record is derived from microwave sounding units (MSU) on board satellites like the one pictured above.
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.
The anomalies over the entire ocean dropped to the same value, 0.12C in August (Tropics were 0.13C). Warming in previous months was erased, and September added very little warming back. In October and November NH and the Tropics rose, joined by SH last month., In December 2018 all regions cooled resulting in a global drop of nearly 0.1C.
Taking a longer view, we can look at the record since 1995, that year being an ENSO neutral year and thus a reasonable starting point for considering the past two decades. On that basis we can see the plateau in ocean temps is persisting. Global ocean temps are the lowest December since 2014. It also appears that the NH Autumn upward bump is over and temps will likely trend downward.
Land Air Temperatures Plunged in September, then Rose in October
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 record air temps at 2 meters above ground. UAH gives tlt anomalies for air over land separately from ocean air temps. The graph updated for December is below.
The greater volatility of the Land temperatures is evident, and also the dominance of NH, which has twice as much land area as SH. Note how global peaks mirror NH peaks. In December air over Tropics fell sharply, SH slightly, while the NH land surfaces rose, pulling up the Global anomaly for the month. Despite the warming, air temps over land were the lowest December since 2013 both Globally and for the Tropics. And all regions are cooler than December 2015 when the El Nino was starting in earnest.
TLTs include mixing above the oceans and probably some influence from nearby more volatile land temps. It is striking to now see NH and Global land temps dropping rapidly. 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.