A follower of our UAH international decrease tropospheric temperature (LT) dataset named “JJ” emailed me asking about what may well be regarded as a spurious function within the dataset.
The function is most simply observed if you happen to plot the per thirty days international time collection of Land-minus-Ocean (hereafter “L-O”) temperature anomalies. The outcome turns out to turn a step-up of about zero.16 deg. C in Might of 1998.
The yr 1998 is essential for our dataset as a result of this is when the primary (NOAA-15) Complicated Microwave Sounding Unit (AMSU) got here on-line, which initiated the transition from the older Microwave Sounding Devices (MSU, the remaining of which was once at the NOAA-14 satellite tv for pc).
AMSU didn’t have precisely the similar channel frequency variety because the MSU, so the nominal layers of the ambience sensed had been rather other. Most significantly, the AMSU channel five has a weighting serve as that senses rather extra of the skin and decrease troposphere than MSU channel 2. If one didn’t account for this truth, the AMSU’s higher floor sensitivity would produce upper temperatures over land and decrease temperatures over the sea (after a global-average intercalibration between MSU and AMSU was once carried out). [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 since those are through-nadir scanners, every view perspective relative to the native vertical measures a rather other layer anyway, which permits us to compare the AMSU and MSU measurements. After we advanced Model 6 of the dataset, we discovered that the 50-60 GHz oxygen absorption idea used to search out the view perspective from AMSU5 that easiest suits MSU2, the ensuing temperature anomalies over land had been nonetheless too heat relative to the oceans. This supposed that we needed to carry out an empirical (data-dependent) somewhat than theoretical matching of the AMSU and MSU view angles.
The way in which we gauged the fit between MSU and AMSU is how the temperature anomaly patterns transition throughout coastlines: we required that there must be little discernible exchange in that trend. Earlier than our optimized matching, the land anomalies had been noticeable hotter than the sea anomalies as options crossed coastlines. However after optimization in our Model 6 dataset, right here’s the LT anomaly map for remaining month (October 2019), which presentations no proof for land-vs-ocean artifacts.
However, changes like those are by no means very best. So, the query stays: Is there a spurious exchange within the L-O temperature distinction going on in 1998?
Proof that the L-O exchange in 1998 is actual
There are a couple of traces of proof that counsel the Might 1998 step-up in L-O temperatures is actual.
First, if the impact was once because of the advent of AMSU in 1998, it will have happened in August, now not in Might (Three months earler). Additionally, the impact must were slow since for just about four years after August 1998 the LT dataset is part MSU (NOAA-14) and part AMSU (NOAA-15), and then it becaume 100% AMSU.
However a extra essential piece of proof is the impact of El Nino and L. a. Nina on L-O. All through El Nino, the sea airmasses heat greater than the land airmasses (particularly within the tropics), in order that L-O has a tendency to be extra unfavorable. Up till the 1997-98 tremendous El Nino a length of higher El Nino process existed, and then a shift to extra L. a. Nina process happened. (That is almost definitely additionally what precipitated the prolonged international warming ‘hiatus’ after that El Nino tournament.)
I statistically regressed the L-O values in Fig. 1 in opposition to Three-month operating averages of the Multivariate ENSO Index (MEI), and got rid of that estimate of the ENSO affect from the knowledge. The ensuing ENSO-adjusted time collection in proven in Fig. Three.
Notice the step-up in mid-1998 is way much less obvious, and the fifth order polynomial are compatible to the knowledge is smoother with a extra slow transition in L-O over the 41-year satellite tv for pc document.
However that’s now not the one factor occurring throughout this era that is affecting the L-O values. There have been two main volcanic eruptions (El Chichon in early 1982, and particularly Pinatubo in mid-1991) that precipitated extra cooling over land than ocean, inflicting briefly enhanced unfavorable values in L-O. Since those occasions aren’t as simply correlated with an index like MEI is with ENSO, I merely got rid of the knowledge from 1982-83 and 1992-93 in Fig. Three and replotted the ends up in Fig. four.
Now we see that the fifth order polynomial are compatible to the knowledge comes relatively with reference to the linear pattern (dashed grey line), which implies that the step-up in 1998 in L-O was once actual, and comparable most commonly to a metamorphosis in ENSO process prior to as opposed to after the 1997-98 tremendous El Nino, and with the key volcanic eruptions in 1982 and 1991 contributing to the reputedly spurious function.
The remainder upward pattern in L-O is just the land airmasses warming quicker than the sea, as could be anticipated for any warming pattern, whether or not herbal or human-caused.
There stays what may well be a spurious function throughout 1980-81 in Fig. four, which might in all probability be associated with our advert hoc correction for MSU channel Three float throughout that point. This, alternatively, must have little affect at the land and ocean traits as evidenced via the craze line are compatible (dashed grey line) in Fig. four.