# Sea Surface Temperature in Global Analyses: Gains from the Copernicus Imaging Microwave Radiometer
## 3 Results
### Reconstruction in Dynamical Regions  
Figure 11 shows the spatial power spectra of the SST fields from the January analysis using the data with realistic errors for the Aghulas Current and Gulf Stream regions. These were generated by calculating a fast Fourier transform along the row (constant-latitude) direction of the data in the region. Rows containing any land or sea-ice cells were discarded. The mean transform was found across all the remaining rows and the power was calculated from this in the normal way. The frequency scale was added using a grid cell spacing of 0.05\({}^{\circ}\) and the mean latitude of the contributing rows. The resulting spectra are plotted as a ratio to the \"truth\" power spectra in these regions and so perfect replication of the truth field in these plots would give a ratio of 1:1 across the whole spectrum. In both cases, there was a marked improvement in the agreement with the original \"truth\" dataset by the addition of the CIMR data, as shown by the blue lines being closer to the 1:1 ratio across almost all spatial frequencies than the red lines. The structure inherent in the SST fields in these regions is shown in Figure 12. This shows a map of the mean magnitude of the gradient of the SST field in the second half of January. Figure 13 shows the difference from these fields for the reconstructions using the input data with realistic errors. In many instances, these difference maps show pairs of red and blue areas that would be expected from a spatial offset of a feature in the SST field. The area of these regions was greatly reduced by the addition of \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **Configuration** & **Global** & \begin{tabular}{c} **Aghulas** \\ **Current** \\ \end{tabular} & \
\begin{tabular}{c} **Gulf** \\ **Stream** \\ \end{tabular} & **Maritime** &
\begin{tabular}{c} **Southern** \\ **Ocean** \\ \end{tabular} & **Coast** \\ \hline \multirow{4}{*}{Jan} & Perfect IR-only & 0.310 & 0.532 & 1.286 & 0.394 & 0.369 & 0.641 \\  & Perfect IR + CIMR & 0.175 & 0.312 & 0.358 & 0.212 & 0.132 & 0.608 \\  & Realistic IR-only & 0.369 & 0.587 & 1.289 & 0.462 & 0.396 & 0.667 \\  & Realistic IR + CIMR & 0.210 & 0.337 & 0.386 & 0.233 & 0.169 & 0.622 \\ \hline \multirow{2}{*}{July} & Perfect IR-only & 0.338 & 0.325 & 0.410 & 0.236 & 0.493 & 0.981 \\  & Perfect IR + CIMR & 0.225 & 0.159 & 0.258 & 0.159 & 0.078 & 0.969 \\  & Realistic IR-only & 0.395 & 0.384 & 0.506 & 0.335 & 0.496 & 0.995 \\  & Realistic IR + CIMR & 0.249 & 0.199 & 0.284 & 0.200 & 0.130 & 0.969 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Root-mean-square error in different regions for the various experimental configurations. Figure 11: Comparison of the power spectra of the SST field in January from IR-only analysis (red) and combined IR and MW analysis (blue) where both contain realistic input errors. The data are plotted as a ratio to the “truth” SST power spectrum at each wavelength: (**a**) Aghulas Current region and (**b**) Gulf Stream region. CIMR data. The fraction of cells where the magnitude of the difference was greater than 0.02 \({}^{\circ}\)C-km\({}^{-1}\) decreased from 6.6 % to 3.1% for the Aghulas Current and from 30.2% to 9.8% for the Gulf Stream. In some locations, there were larger patches of difference, e.g., just left of centre in Figure 13a,c and the bottom left of Figure 13b,d. Here, in the IR-only case, the interpolation scheme had produced structures that were not present in the \"truth\" dataset. These were artefacts resulting from interpolation of data that were too sparse. This limitation of optimal interpolation in regions with strong SST gradients was noted in the context of an AVHRR-only reconstruction by [PERSON] et al. (2018) and discussed more generally in [PERSON] et al. (2018). With the sparser coverage of the IR-only case, the analysis only had intermittent input observations of these structures, i.e., the analysis got to \"see\" the evolving structures on some days and reconstructed the features only at their locations on those days.