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For the first time the long-term interannual and spatial variability ofresidence time (τ) is presented for the TTL between 360 K and 400 Kpotential temperature (~14 to 18 km altitude). The analysis is based on a Lagrangianapproach using offline calculated diabatic heating rates as verticalvelocities, covering Northern Hemisphere (NH) winters from 1962–2004.  The residence time τ<sub>LCP–400 K</sub>, being the duration time of air parcels inthe layer between the Lagrangian Cold Point (LCP) and 400 K, variesspatially and is longer (>50 days) over the maritime continent as the LCPis lowest there (<370 K). Comparing three theta layers withinthe TTL reveals the vertical dependence of τ. We derive a mean durationtime of 34 days for 360–380 K (lower TTL), 38 days for 380–400 K(upper TTL) and 70 days for 360–400 K thetalayers for the 1962–2001 period. A case analysis reveals that τ ispositively skewed for 360–380 K and 380–400 K during La Niña and ElNiño 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 ofthe TTL. Statistical analysis reveals a significantanti-correlation between the residence time and the extratropical andsubtropical wave driving in the lowermost stratosphere

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