The Transit-Time Distribution from the Northern Hemisphere Midlatitude Surface

Abstract

he distribution of transit times from the Northern Hemisphere (NH) midlatitude surface is a fundamental property of tropospheric transport. Here, the authors present an analysis of the transit-time distribution (TTD) since air last contacted the NH midlatitude surface, as simulated by the NASA Global Modeling Initiative Chemistry Transport Model. Throughout the troposphere, the TTD is characterized by young modes and long tails. This results in mean transit times or ?mean ages? Γ that are significantly larger than their corresponding modal transit times or ?modal ages? τmode, especially in the NH, where Γ ≈ 0.5 yr, while τmode < 20 days. In addition, the shape of the TTD changes throughout the troposphere as the ratio of the spectral width ??the second temporal moment of the TTD?to the mean age decreases sharply in the NH from ~2.5 at NH high latitudes to ~0.7 in the Southern Hemisphere (SH). Decreases in ?/Γ in the SH reflect a narrowing of the TTD relative to its mean and physically correspond to changes in the contributions of fast transport paths relative to slow eddy-diffusive recirculations. It is shown that fast transport paths control the patterns and seasonal cycles of idealized 5- and 50-day loss tracers in the Arctic and the tropics, respectively. The relationship between different TTD time scales and the idealized loss tracers, therefore, is conditional on the shape of the TTD.