Republished from Dr. Roy Spencer's weblog
November 7, 2019 by Roy W. Spencer, PhD
A disciple of our UAH knowledge set named "JJ" on the worldwide low troposphere (LT) international temperature has despatched me an electronic mail asking me what could possibly be thought of a false characteristic of the entire world. dataset.
The characteristic is simpler to see for those who draw the month-to-month international time collection of Land temperature anomalies minus the ocean (hereinafter "L-O"). The end result appears to point out a rise of about zero.16 levels. C in Might 1998.
Fig. 1. Distinction between temperature anomalies of land and oceans of the UAH within the decrease troposphere (LT) between January 1979 and August 2019, displaying an obvious enhance within the distinction occurred in Might 1998. The dotted strains point out the common values earlier than and after that. date, whereas the curve corresponds to a polynomial match of the fifth order to the info.
The yr 1998 is essential for our knowledge set because it was at the moment that the primary Superior Microwave Sounding Unit (NOAA-15) (AMSU) was put on-line, which initiated the transition from the outdated microwave survey items (MSUs), the latter of which was a part of the NOAA. -14 satellites).
AMSU didn’t have precisely the identical channel frequency choice because the MSU, so the nominal layers of the detected ambiance had been barely totally different. Most significantly, the AMSU channel 5 has a weighting operate that detects the floor and the decrease troposphere a bit greater than the MSU channel 2. If this reality is just not taken into consideration, the higher floor sensitivity of the AMSU would end in larger floor temperatures and decrease temperatures. the ocean (after an total common intercalibration between MSU and AMSU). [The reason why is that these channel frequencies are not sensitive to changes in sea surface temperature, because the microwave emissivity decreases as SST increases. The effect is small, but measurable.]
However as it’s scanners crossing the nadir, every angle of view in relation to the native vertical nonetheless measures a barely totally different layer, which permits us to adapt the AMSU and MSU measurements. After we developed model 6 of the dataset, we discovered that the speculation of oxygen absorption from 50 to 60 GHz used to search out the angle of view of AMSU5 which finest corresponds to MSU2, the ensuing temperature anomalies on the earth had been nonetheless too heat in comparison with the oceans. This meant that we needed to make an empirical (data-dependent) slightly than theoretical correspondence of the AMSU and MSU view angles.
The best way during which we now have evaluated the correspondence between MSU and AMSU is the best way temperature anomaly fashions transfer from one ocean to a different: we requested that there be little perceptible change on this mannequin. Earlier than our optimized pairing, terrestrial anomalies had been considerably hotter than ocean anomalies when the options crossed the shoreline. Nevertheless, after optimization in our model 6 knowledge set, right here is the LT anomaly map for final month (October 2019), which exhibits no proof of land-ocean artifacts.
Fig. 2. LT temperature anomalies in October 2019 in comparison with the 1981-2010 common annual cycle. Notice that the anomalies have a clean transition between land and ocean, as could possibly be anticipated for tropospheric temperatures in deep layers (however not essentially floor temperatures).
However, such changes are by no means good. The query due to this fact stays: is there an irregular change within the temperature distinction L-O in 1998?
Proof that the change of L-O in 1998 is actual
Some proof means that the Might 1998 L-O temperature enhance is actual.
First, if the impact was because of the introduction of AMSU in 1998, it could have occurred in August and never in Might (three months). As well as, the impact ought to have been progressive, since almost four years after August 1998, the LT dataset consists of half MSU (NOAA-14) and half AMSU (NOAA-15), after which it turns into 100% AMSU.
However a extra essential piece of proof is the impact of El Nino and La Nina on L-O. In El Nino, ocean air plenty are hotter than terrestrial air plenty (particularly within the tropics), so L-O tends to be extra damaging. Till the El Niño Tremendous 1997-1998, there was a interval of robust El Niño exercise, following which a change of exercise passed off. (That is in all probability additionally what brought about the "pause" in international warming after this El Nino occasion.)
I’ve statistically regressed the L-O values of FIG. 1 in contrast with present averages over three months of the multivariate ENSO index (MEI) and have eliminated this estimate of the affect of ENSO knowledge. The ensuing ENSO adjusted time collection is illustrated in FIG. three.
Fig. three. As in FIG. 1, however with the common affect of El Nino and La Nina (ENSO) subtracted. Notice that the proof of a "break" in 1998 is far decrease.
Notice that the rise in the course of 1998 is far much less apparent, and the fifth order polynomial match to the info is extra fluid with a extra gradual transition of L-O over the 41-year satellite tv for pc document.
However this isn’t the one factor taking place throughout this era that impacts L-O values. Two main volcanic eruptions (El Chichon in early 1982 and particularly Pinatubo in mid-1991) brought about extra cooling on the land than the ocean, leading to a brief enhance in damaging L-O values. Since these occasions usually are not as simply correlated with an index as MEI is with ENSO, I merely deleted the 1982-1983 and 1992-1993 knowledge from Determine three and reproduces the leads to Determine four.
Fig. four. As in FIG. three, however with the info influenced by the primary El Chichon and Pinatubo volcanoes eliminated.
We now see that the fifth order polynomial match to the info is kind of near the linear pattern (dashed grey line), suggesting that the 1998 development in LO was actual and that it was primarily associated to a change of exercise of the ENSO earlier than. after the 1997-1998 Tremendous El Nino, and with the foremost volcanic eruptions of 1982 and 1991, contributing to the seemingly fallacious characteristic.
The upward pattern that continues to be in L-O is just the truth that terrestrial air plenty are warming sooner than the ocean, as can be anticipated for any warming pattern, whether or not pure or man-made.
There stays what could possibly be a parasitic characteristic in 1980-1981 in Determine four, which might in all probability be associated to our advert hoc correction for MSU channel three drift throughout this era. This, nonetheless, ought to have little affect on land and ocean traits, as evidenced by the adjustment of the pattern line (dashed grey line) of Fig. 2. four.