## 2 Data
### L4 Analyses  
The current \(i\)Quam QC employs two \(T_{L4}\)'s. From September 1981-present, the NOAA Optimal Interpolation SST (OISST) [23; 24] is used. Since 1 Sep 1991, the second \(T_{L4}\) is obtained from the Canadian Meteorological Center (CMC) SST (the 0.2\({}^{\circ}\) version [25] before 1 January 2016 and the 0.1\({}^{\circ}\) version [26] after this date). The \(i\)Quam RC evaluates \(T_{IS}\)'s using a single OISST PGE before 1 September 1991 and has the option to use OISST or CMC PGEs, or their combination, after this date. In this study, we use the CMC SST as a reference since 1 September 1991. Our prior analyses suggest that it is the optimal choice for this period (note that the CMC is also employed as the first guess in ACSPO during this period [18; 19; 20]). Additional analyses were performed in this study to select the reference L4 SST before September 1991 from three global products currently available: OISST [23; 24], Ocean SST and Sea Ice Reprocessed (OSTIA-RAN) [27; 28] and the Climate Change Initiative (CCI) [29; 30].  
Figure 2 shows time series of monthly medians of \(\Delta T_{L4}\), \(M(\Delta T_{L4})\), and corresponding robust standard deviations (RSD), \(S(\Delta T_{L4})=1.4826\times M(\,|\,\Delta T_{L4}-M(\Delta T_{L4})\,|\,)\), for the \(T_{L4}\)'s obtained from the three analyses.  
The statistics are for the nighttime \(T_{IS}\)'s measured between 11 p.m. and 6 a.m. local solar time, without applying any QC. The deviations from CCI exhibit the smallest on average, but most variable in time medians, and the largest RSDs. The medians with respect to OSTIA are larger but more stable in time, with the smallest and least variable RSDs. Note that during this early period, comparisons of DTM \(T_{IS}\)'s with \(T_{SAT}\)'s are based on a small number of matchups, which is oftentimes insufficient for reliable training of the satellite SST algorithms. More efficient training requires adding less accurate but much  
Figure 2: Time series of monthly (**a**) medians and (**b**) RSDs of nighttime DTM SST deltas from three L4 SSTs for a period from September 1981–1991: (black) OSTIA, (blue) OISST, and (red) CCI. The numbers in the plots show the temporal means and SDs calculated over the whole 10-year period.  
more numerous matchups with ship SSTs (at least, for the NOAA-07/09) [19]. Figure 3 re-plots time series from Figure 2 but using \(T_{IS}\)'s from both ships and DTMs. As in Figure 2, the deviations from OSTIA appear lowest in terms of both average median and average RSD, and most stable and consistent in time.  
Another factor in selecting L4 for use in \(i\)Quam QC is users' interest in in situ SSTs over lakes and in coastal areas. The availability of the OISST, OSTIA, and CCI analyses in these two domains is illustrated in Figure 4 using a representative example of May 1985.  
The CCI does not report temperatures over lakes. The OISST reports only Great Lakes and its coastal SSTs exhibit significantly fewer details compared with OSTIA (likely due to the 0.25\({}^{\circ}\) OISST vs. 0.05\({}^{\circ}\) OSTIA resolution).  
Summarizing analyses in this section, OSTIA-RAN SST was selected as a reference for the period from September 1981-August 1991.  
Figure 4: Monthly composite SST maps for May 1985 for (**a**) CCI, (**b**) OISST, and (**c**) OSTIA.  
Figure 3: Same as in Figure 2 but for combined ship and DTM \(\Delta T_{L4}\)’s.