Spatial and temporal variability of the freezing level in Patagonia's atmosphere

Abstract. The height of the 0 °C isotherm (H0), which commonly signals the freezing level, denotes the lowest altitude within the atmosphere where the air temperature reaches 0 °C. This can be used as an indicator of the transition between rain and snow, making it useful for monitoring and visualizing the height of freezing temperatures in the atmosphere. We study the spatial and temporal variability of H0 across Patagonia (41–54° S) for the 1959–2021 period using reanalysis data from ERA5. Our results indicate that the average isotherm in Patagonia is 1691 m above sea level (m a.s.l.). The spatial distribution of the annual mean field highlights the contrast in the region, with an average maximum of 2658 m a.s.l. in the north and minimum of 913 m a.s.l. in the south. Regarding seasonal variability in the region, H0 ranges from 575 m a.s.l. (winter) to 3346 m a.s.l. (summer). Further, the significant trends calculated over the period show positive values in the whole area. This indicates an upward annual trend in the H0, between 8.8 and 36.5 m per decade from 1959–2021, with the higher value observed in northwestern Patagonia. These upward trends are stronger during summer (8–61 m per decade). Empirical orthogonal function (EOF) analysis was performed on H0 anomalies. The first EOF mode of H0 variability accounts for 84 % of the total variance, depicting a monopole structure centred in the northwestern area. This mode exhibits a strong and significant correlation with the spatial average H0 anomaly field (r=0.85), the Southern Annular Mode (SAM; r= 0.58), temperature at 850 hPa in the Drake Passage (r=0.56), and sea surface temperature off the western coast of Patagonia (r=0.66), underscoring the significant role of these factors in influencing the vertical temperature profile within the region. The spatial distribution of the second (8 %) and third (4.4 %) EOF modes depicts a dipole pattern, offering additional insights into the processes influencing the 0 °C isotherm, especially on the western slope of Patagonia.