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 360 K 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–400 K, being the duration time of air parcels in the layer between the Lagrangian Cold Point (LCP) and 400 K, varies spatially and is longer (>50 days) over the maritime continent as the LCP is lowest there (<370 K). 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 τ 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 τ 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.