Home / Weather / At the 1998 Obvious Step-Up in UAH Land-minus-Ocean Decrease Tropospheric Temperatures « Roy Spencer, PhD

At the 1998 Obvious Step-Up in UAH Land-minus-Ocean Decrease Tropospheric Temperatures « Roy Spencer, PhD

A follower of our UAH international decrease tropospheric temperature (LT) dataset named “JJ” emailed me asking about what could be thought to be a spurious characteristic within the dataset.

The characteristic is most simply noticed in the event you plot the per month international time sequence of Land-minus-Ocean (hereafter “L-O”) temperature anomalies. The end result turns out to turn a step-up of about zero.16 deg. C in Would possibly of 1998.

Fig. 1. Distinction between the UAH decrease tropospheric (LT) land and ocean temperature anomalies between January 1979 and August 2019, appearing an obvious step-up within the distinction going on in Would possibly 1998. The dashed strains display the common values ahead of and after that date, whilst the curve is a fifth order polynomial are compatible to the knowledge.

The 12 months 1998 is essential for our dataset as a result of this is when the primary (NOAA-15) Complex Microwave Sounding Unit (AMSU) got here on-line, which initiated the transition from the older Microwave Sounding Devices (MSU, the closing 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 relatively other. Most significantly, the AMSU channel five has a weighting serve as that senses quite extra of the outside and decrease troposphere than MSU channel 2. If one didn’t account for this truth, the AMSU’s larger 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 relatively other layer anyway, which permits us to check the AMSU and MSU measurements. Once 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 highest 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) quite than theoretical matching of the AMSU and MSU view angles.

The best way 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 alternate in that development. 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 closing month (October 2019), which presentations no proof for land-vs-ocean artifacts.

Fig. 2. October 2019 LT temperature anomalies relative to the 1981-2010 common annual cycle. Be aware the anomalies have a easy transition between land and ocean, as could be anticipated for deep-layer tropospheric temperatures (however now not essentially floor temperatures).

However, changes like those are by no means very best. So, the query stays: Is there a spurious alternate within the L-O temperature distinction going on in 1998?

Proof that the L-O alternate in 1998 is actual

There are a couple of strains of proof that counsel the Would possibly 1998 step-up in L-O temperatures is actual.

First, if the impact was once because of the advent of AMSU in 1998, it might have came about in August, now not in Would possibly (Three months earler). Additionally, the impact must had been slow since for nearly 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 vital piece of proof is the impact of El Nino and L. a. Nina on L-O. All over 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 detrimental. Up till the 1997-98 tremendous El Nino a length of larger El Nino process existed, and then a shift to extra L. a. Nina process came about. (That is most definitely additionally what prompted the prolonged international warming ‘hiatus’ after that El Nino tournament.)

I statistically regressed the L-O values in Fig. 1 towards 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 sequence in proven in Fig. Three.

Fig. Three. As in Fig. 1, however with the common affect of El Nino and L. a. Nina (ENSO) subtracted out. Be aware the proof for a “destroy” in 1998 is way weaker.

Be aware the step-up in mid-1998 is way much less glaring, 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 report.

However that’s now not the one factor occurring right through this era that is affecting the L-O values. There have been two primary volcanic eruptions (El Chichon in early 1982, and particularly Pinatubo in mid-1991) that prompted extra cooling over land than ocean, inflicting quickly enhanced detrimental 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.

Fig. four. As in Fig. Three, however with the knowledge influenced via primary volcanoes El Chichon and Pinatubo got rid of.

Now we see that the fifth order polynomial are compatible to the knowledge comes rather on the subject of the linear pattern (dashed grey line), which means that the step-up in 1998 in L-O was once actual, and comparable most commonly to a metamorphosis in ENSO process ahead of 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 characteristic.

The remainder upward pattern in L-O is solely the land airmasses warming sooner than the sea, as could be anticipated for any warming pattern, whether or not herbal or human-caused.

There stays what could be a spurious characteristic right through 1980-81 in Fig. four, which might in all probability be associated with our advert hoc correction for MSU channel Three glide right through that point. This, then again, must have little affect at the land and ocean tendencies as evidenced via the craze line are compatible (dashed grey line) in Fig. four.

A follower of our UAH international decrease tropospheric temperature (LT) dataset named “JJ” emailed me asking about what could be thought to be a spurious characteristic within the dataset.

The characteristic is most simply noticed in the event you plot the per month international time sequence of Land-minus-Ocean (hereafter “L-O”) temperature anomalies. The end result turns out to turn a step-up of about zero.16 deg. C in Would possibly of 1998.

Fig. 1. Distinction between the UAH decrease tropospheric (LT) land and ocean temperature anomalies between January 1979 and August 2019, appearing an obvious step-up within the distinction going on in Would possibly 1998. The dashed strains display the common values ahead of and after that date, whilst the curve is a fifth order polynomial are compatible to the knowledge.

The 12 months 1998 is essential for our dataset as a result of this is when the primary (NOAA-15) Complex Microwave Sounding Unit (AMSU) got here on-line, which initiated the transition from the older Microwave Sounding Devices (MSU, the closing 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 relatively other. Most significantly, the AMSU channel five has a weighting serve as that senses quite extra of the outside and decrease troposphere than MSU channel 2. If one didn’t account for this truth, the AMSU’s larger 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 relatively other layer anyway, which permits us to check the AMSU and MSU measurements. Once 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 highest 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) quite than theoretical matching of the AMSU and MSU view angles.

The best way 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 alternate in that development. 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 closing month (October 2019), which presentations no proof for land-vs-ocean artifacts.

Fig. 2. October 2019 LT temperature anomalies relative to the 1981-2010 common annual cycle. Be aware the anomalies have a easy transition between land and ocean, as could be anticipated for deep-layer tropospheric temperatures (however now not essentially floor temperatures).

However, changes like those are by no means very best. So, the query stays: Is there a spurious alternate within the L-O temperature distinction going on in 1998?

Proof that the L-O alternate in 1998 is actual

There are a couple of strains of proof that counsel the Would possibly 1998 step-up in L-O temperatures is actual.

First, if the impact was once because of the advent of AMSU in 1998, it might have came about in August, now not in Would possibly (Three months earler). Additionally, the impact must had been slow since for nearly 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 vital piece of proof is the impact of El Nino and L. a. Nina on L-O. All over 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 detrimental. Up till the 1997-98 tremendous El Nino a length of larger El Nino process existed, and then a shift to extra L. a. Nina process came about. (That is most definitely additionally what prompted the prolonged international warming ‘hiatus’ after that El Nino tournament.)

I statistically regressed the L-O values in Fig. 1 towards 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 sequence in proven in Fig. Three.

Fig. Three. As in Fig. 1, however with the common affect of El Nino and L. a. Nina (ENSO) subtracted out. Be aware the proof for a “destroy” in 1998 is way weaker.

Be aware the step-up in mid-1998 is way much less glaring, 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 report.

However that’s now not the one factor occurring right through this era that is affecting the L-O values. There have been two primary volcanic eruptions (El Chichon in early 1982, and particularly Pinatubo in mid-1991) that prompted extra cooling over land than ocean, inflicting quickly enhanced detrimental 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.

Fig. four. As in Fig. Three, however with the knowledge influenced via primary volcanoes El Chichon and Pinatubo got rid of.

Now we see that the fifth order polynomial are compatible to the knowledge comes rather on the subject of the linear pattern (dashed grey line), which means that the step-up in 1998 in L-O was once actual, and comparable most commonly to a metamorphosis in ENSO process ahead of 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 characteristic.

The remainder upward pattern in L-O is solely the land airmasses warming sooner than the sea, as could be anticipated for any warming pattern, whether or not herbal or human-caused.

There stays what could be a spurious characteristic right through 1980-81 in Fig. four, which might in all probability be associated with our advert hoc correction for MSU channel Three glide right through that point. This, then again, must have little affect at the land and ocean tendencies as evidenced via the craze line are compatible (dashed grey line) in Fig. four.

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