A case study of excessive subtropical transport in the stratosphere of a data assimilation system

Assessments of transport show that results derived from assimilated (or analyzed) winds exhibit significantly larger mixing and entrainment rates compared to those derived from general circulation model (GCM) winds, where the GCM transport is somewhat closer (statistically) to values inferred from observations. This discrepancy presents a challenge to our ability to understand and model the global distributions of long-lived trace gases. We use the Goddard Earth Observation System's Finite Volume Data Assimilation System to explore this issue by examining how the data assimilation process alters the dynamics of the underlying GCM and how this leads to greater lower stratospheric mixing and transport in the subtropics. We show that the excessive subtropical transport is related to the proliferation of eddy features in the subtropics, and we examine various possibilities that may cause this. These include the generation of upward propagating features, equatorward propagating features, and meridionally confined features in the process of data assimilation. In particular, it is argued that unstable regions forced directly by the analysis increments play an important role in generating the excess subtropical transport.