Spatial and temporal variability of free tropospheric freezing level in Patagonia

Abstract. The free tropospheric height of the 0 °C isotherm (H0), commonly referred to as the freezing level, denotes the lowest altitude within a specific location’s atmosphere where the air temperature reaches 0 °C. This serves as an indicator for the transition between rain and snow, making it useful for monitoring and visualizing the altitude of freezing temperatures in the atmosphere. We study the spatial and temporal variability of H0 across Patagonia (41°–54° S) for the 1950–2021 period. Our results highlight the contrast around the Andes, manifested in lower/higher H0 on the western/eastern side, indicating the orographical influence on temperature vertical profiles on both sides of mountains. Our results indicate that the spatial mean value of the isotherm field in the region is 1691 meters above sea level (m.a.s.l). In terms of seasonal variability, H0 ranges from 585 m.a.s.l (winter) to 3480 m.a.s.l (summer). Moreover, the significant trends calculated over the period only show positive values in the area. This indicates an upward trend in the isotherm, averaging an increase of 8 to 61 meters per decade from 1959–2021, where the higher value is over northwestern Patagonia. Empirical orthogonal function (EOF) analysis was performed on isotherm anomalies. The first empirical orthogonal function (EOF) mode of H0 variability accounts for 84 % of the total variance, depicting a monopole structure centered in the northwest area. This mode exhibits a positive correlation with the spatial average H0 anomalies field, the Southern Annular Mode (SAM), temperature at 850 hPa in the Drake Passage, and sea surface temperature off the western coast of Patagonia; underscoring the significant role of these factors in influencing the vertical temperature profile within the region.