Variability of residence time in the Tropical Tropopause Layer during Northern Hemisphere winter

Abstract

For the first time the long-term interannual and spatial variability of residence time (τ) is presented for the TTL between 360K and 400 K potential temperature (∼14 to 18 km altitude). The analysis is based on a Lagrangian approach using offline calculated diabatic heating rates as vertical velocities, covering Northern Hemisphere (NH) winters from 1962-2004. The residence time τLCP-400K, being the duration time of air parcels in the layer between the Lagrangian Cold Point (LCP) and 400K, varies spatially and is longer (>50days) over the maritime continent as the LCP is lowest there (<370K). Comparing three theta layers within the TTL reveals the vertical dependence of τ. We derive a mean duration time of 34 days for 360-380 K (lower TTL), 38 days for 380-400 K (upper TTL) and 70 days for 360-400 K theta layers for the 1962-2001 period. A case analysis reveals that T is positively skewed for 360-380 K and 380-400 K during La Niña and El Niño Southern Oscillation (ENSO) neutral years. For these cases, ∼60% of air parcels travel from 360 K to 380 K within 25 days. There is large interannual variability for T varying up to ±20% from the long-term mean, with strongest variability seen in the lower part of the TTL. Statistical analysis reveals a significant anti-correlation between the residence time and the extratropical and subtropical wave driving in the lowermost stratosphere.