Recent changes in freezing level heights in the Tropics with implications for the deglacierization of high mountain regions

The height of the freezing level in the tropical atmosphere (the free air 0°C isotherm) has increased across most of the region, particularly in the outer Tropics. In the tropical Andes, south of the Equator, high elevation surface temperatures and upper air data show a similar trend in temperature, of ∼0.1°C/decade over the last 50 years. Meteorological observations at 5680 m on the summit of the Quelccaya Ice Cap, the largest ice mass in the Tropics, indicate that daily maximum temperatures often exceed 0°C from October–May, and rise well above freezing for much of the year around the ice cap margin at 5200 m. This is consistent with observations of a rise in the percolation facies (an indicator of surface melting) in recent decades, and other observations of marginal recession, showing that the ice cap is rapidly losing mass. Similar conditions are likely to be affecting other high elevation ice caps and glaciers in Ecuador, Perú and Bolivia, with important implications for water supplies in the region. Over the Tropics as a whole, freezing level height (FLH) is closely related to mean SSTs, with inter‐annual variations in FLH controlled by the phase of ENSO variability. More extensive monitoring of climatic conditions at high elevations in the mountains of the Tropics is urgently needed.