## 2 Data
### Matchups with Satellite SSTs  
The matchups of \(T_{IS}\) (with all iQuam QLs) with clear-sky \(T_{SAT}\) have been accumulated during ACSPO reprocessings (RANs) of multiple LEO satellites [18; 19; 20]. Table 1 shows the periods covered by each MDS and the types of the satellites' orbits. Altogether, these MDSs cover 42 yr+ period since September 1981. The orbits of the heritage NOAA satellites from NOAA-07 to NOAA-19 have been drifting during their missions, and their LEXT varied within wide ranges [21].  
The ACSPO data files and corresponding matchups report two satellite SSTs: 'Subskin', produced with a global regression and highly sensitive to the temperature of the upper \(\sim\)10 \(\upmu\)m'skin' layer of the ocean, and 'Depth', produced with a piecewise regression and representing a closer proxy of the \(T_{IS}\)[22]. In this study, we use matchups with the 'Subskin' SST, due to its higher sensitivity to the diurnal signal. (Note, however, that one should not expect full consistency between the 'Subskin' \(T_{SAT}\) and 'bulk' \(T_{IS}\).) As discussed in Section 4, the availability of multiple MDSs, especially in the two recent decades, greatly  
\begin{table}
\begin{tabular}{c c c} \hline
**Satellite, Instrument** & \multicolumn{2}{c}{**Covered Period**} & **Orbit** \\  & **(DD.MM.YYYY)** & **Orbit** \\ \hline NOAA-07 AVHRR/2 (GAC) & 09.01.1981–02.02.1985 & Variable, p.m. \\ NOAA-09 AVHRR/2 (GAC) & 01.31.1985–11.07.1988 & Variable, p.m. \\ NOAA-11 AVHRR/2 (GAC) & 11.08.1988–09.13.1994 & Variable, p.m. \\ NOAA-12 AVHRR/2 (GAC) & 09.16.1991–12.14.1998 & Variable, p.m. \\ NOAA-14 AVHRR/2 (GAC) & 01.19.1995–10.19.2001 & Variable, a.m. \\ NOAA-15 AVHRR/3 (GAC) & 11.01.1998–present & Variable, a.m./p.m. \\ NOAA-16 AVHRR/3 (GAC) & 10.26.200–09.17.2007 & Variable, p.m. \\ NOAA-17 AVHRR/3 (GAC) & 07.10.2002–03.08.2010 & Variable, a.m. \\ NOAA-18 AVHRR/3 (GAC) & 06.06.2005–present & Variable, p.m./a.m. \\ NOAA-19 AVHRR/3 (GAC) & 02.22.2009–present & Variable, p.m./a.m. \\ Terra MODIS & 02.25.2000–present & Stable, 10.30 a.m. \\ Aqua MODIS & 07.04.2002–present & Stable, 1:30 p.m. \\ Metop-A AVHRR/3 (FRAC) & 12.01.2006–11.25.2021 & Stable, 9:30 a.m. \\ Metop-B AVHRR/3 (FRAC) & 10.19.2012–present & Stable, 9:30 a.m. \\ Metop-C AVHRR/3 (FRAC) & 12.04.2018–present & Stable, 9:30 a.m. \\ S-NPP VIRS & 12.01.2012–present & Stable, 1:30 p.m. \\ NOAA-20 VIIRPS & 01.05.2018–present & Stable, 1:30 p.m. \\ \hline \end{tabular}
\end{table}
Table 1: Satellites and instruments represented in each MDS and local equator crossing times.  
Figure 1: (**a**) Monthly number of (1) DTM \(T_{IS}\)’s and (2) DTM \(T_{IS}\)’s matched with satellite \(T_{SAT}\)’s. (**b**) (1) Monthly fraction of \(T_{IS}\)’s matched with \(T_{SAT}\)’s (%) and (2) Number of satellites whose MDS are available during each month.  
facilitates the validation of in situ QC algorithms. Moreover, the data of multiple satellites with different LEXTs allow validation of the diurnal signal in the \(T_{IS}\)'s.  
Note that the ACSPO MDSs report 'one-to-many' matchups, i.e., each \(T_{IS}\) is matched with all clear-sky \(T_{SAT}\)'s found in its neighborhood, individually. In order to reduce the number of matchups to a manageable size, we created a single matchup for each \(T_{IS}\) by averaging all clear-sky L2P 'Subskin' SSTs within \(\pm\)0.5 h and \(\pm\)10 km of each \(T_{IS}\). Figure 1a shows the time series of monthly numbers of DTM \(T_{IS}\)'s (with all QLs) and monthly numbers of \(T_{IS}\)'s matched with \(T_{SAT}\)'s in at least one MDS. Figure 1b demonstrates a close consistency between the fraction of matched \(T_{IS}\)'s in the total monthly numbers of \(T_{IS}\) measurements (curve 1) and the number of satellites' MDSs available for each month (curve 2). The fraction of matched \(T_{IS}\)'s increased from -3% in the 1990s to -27% in the 2020s due to the increased number of available satellites (from 1 to -10) and their corresponding MDSs.