Climate drift in a coupled land-atmosphere model

A coupled land-atmosphere climate model is examined for evidence of climate drift in the land surface state variable of soil moisture. The drift is ch aracterized as pathological error growth in two different ways. First is th e systematic error that is evident over seasonal timescales, dominated by t he error modes with the largest saturated amplitude: systematic drift. Seco nd is the fast-growing modes that are present in the first few days after e ither initialization or a data assimilation increment: incremental drift. W hen the drifts are robust across many ensemble members and from year to yea r, they suggest a source of drift internal to the coupled system. This sour ce may be due to problems in either component model or in the coupling betw een them. Evidence is presented for both systematic and incremental drift. The relationship between the two types of drift at any given point is shown to be an indication of the type and strength of feedbacks within the coupl ed system. Methods for elucidating potential sources of the drift are propo sed.