## 3 Results
### Prediction of Arctic and Antarctic SIT
Figure 7: Weekly maps of predicted thin SIT (**a**,**d**), the corresponding ground truth SIT\({}_{SMOS}\) (**b**,**e**), and deviations with respect to SIT\({}_{SMOS}\) (**c**,**f**), over the Arctic sea from 15–21 October and from 12–18 November 2020, respectively. Areas with SIT\({}_{SMOS}>0.6\) m were filtered in the predictions. SIT\({}_{CS2 SMOS}\) products are expected to be lower. Most part of the elevated value of the total MAE (0.237 m) is made up by the high deviations of for SIT between 0.5 m and 1.5 m (\(\sim\)0.25 -0.30 m, with a MAE of 0.275 m in Table 2). This may occur due to the fact that, after summer melt and at the beginning of the freeze-up period, mainly thick multi-year ice remained together with thin newly-formed ice. Instead, only a few values of sea ice in the intermediate thickness range can be provided for model training during the observed period. Therefore, the intermediate range may be underrepresented in the resulting predictions. In addition, the ground truth values are based on T\({}_{B}\) and altimetry observations using an optimal interpolation scheme. This may introduce some artifacts at intermediate SIT ranges, resulting in distortions in the training that cannot be adequately conceptualized by the NN model. In Figure 9, the predicted weekly composite based on the full-range SIT model is compared to the corresponding SIT\({}_{CS2 SMOS}\) map in the period from 22-28 October 2020. The over- and underestimations may be due to the large footprint of the antenna, which smoothes out the observations, reducing the small-scale variability. Underestimations (indicated in blue) can be attributed to areas of more heterogeneous multi-year ice. Most of the overestimated values (indicated in red) are located in the Beaufort Sea and in areas with high contrasts between newly-formed thin ice and thicker ice. Hereby, predicted values are within the intermediate SIT range, in which the model performance is also less accurate. Highly overestimated values are located to the north of the Baffin Bay around North-Western Greenland. This anomaly may also be due to land-sea contamination or to most recently formed ice, which is not yet captured in the predicted weekly composite. Figure 10 shows the predicted SIT map over the Antarctic sea from 15 October to 14 November 2020 (monthly composite), obtained after applying the full-range SIT model to Antarctic data. The massive ice shelves (e.g., Ronne and Ross) located in the Ross and Weddell sea were excluded from the predictions. It is important to note that a disseminated product of SIT at the Antarctic scale was at the time of the study not available for comparison. Therefore, the model cannot be validated in the same way as for the Arctic. SIT has an important impact on the melting trend of sea ice and it can be a good proxy of the upcoming SIE distribution. Antarctic SIE is already decreasing after having passed  
Figure 8: Evaluation of training and prediction of the full-range SIT model. (**a**) Validation and training error determined after each epoch converge close to each other; early stopping occurred around 353 iterations and the model resulted in a MAE of 0.237 m; (**b**) prediction error after application of the model to the entire Arctic dataset; (**c**) distribution of the MAE with SIT; (**d**) relation between predicted and ground truth SIT\({}_{CS2 SMOS}\); Good generalization is obtained for thin ice (SIT < 0.5 m) and thicker ice (SIT > 1.5 m). its annual sea ice maximum around mid-September. Thus, the spatial patterns of the distribution of SIT can be compared to those of future SIE, assuming that areas consisting of mainly thin ice are supposed to melt first. Because Antarctic sea ice melts and refreezes almost completely during a course of a year, it is mainly composed by thinner first-year ice. This is in agreement with the model predictions, which result in an average SIT of 0.67 m for the Antarctic, in comparison with 1.25 m obtained for the Arctic.