Variability of residence time in the Tropical Tropopause Layer during Northern Hemisphere winter
1Leibniz-Institute for Marine Sciences at Kiel University (IFM-GEOMAR), Kiel, Germany
2Environment Canada, Toronto, Canada
3Alfred Wegener Institute for Polar and Marine Research, Potsdam, Germany
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 theta (~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 varies spatially. τ, analysed for the Lagrangian Cold Point (LCP), displays a longer duration time of air parcels between LCP and 400 K over the maritime continent (>50 days), as the LCP tropopause has a minimum over the maritime continent (<370 K theta). 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. The interannual variability is influenced by extratropical and subtropical wave driving. Statistical analysis reveals a significant anti-correlation between the residence time and the extratropical and subtropical wave driving in the lowermost stratosphere.