Large-scale circulation and stratocumulus variability

Abstract. This study aims to understand the relationship between large-scale circulation and stratocumulus variability. We use reanalysis and satellite data to identify circulation patterns that couple with estimated inversion strength (EIS) and low-cloud cover (LCC) in stratocumulus areas. The results update the current understanding in two aspects: the limited direct influence of the tropical thermodynamic framework on stratocumulus, and the different responses of EIS and LCC to large-scale circulation.

Extratropical dynamics control EIS variability. From synoptic to interseasonal timescales (after deseasonalization), synoptic-scale Rossby ridges located directly over stratocumulus enhance stability throughout the tropospheric column and thereby increase EIS. On interannual timescales, planetary-scale Rossby waves coupled with a negative PDO-like (Pacific Decadal Oscillation-like) sea surface temperature pattern increase EIS. In contrast, LCC responds to circulation patterns similar to those associated with EIS, but with a systematic upstream (west and poleward) shift. This shift suggests a direct response of LCC to circulation through enhanced pressure gradients, which increase cold advection and offset the drying effect of Rossby ridges via stronger winds. The upstream Rossby ridges associated with increased LCC often overlap with the subtropical highs, which has led to the previous emphasis on thermodynamic processes that strengthen subtropical highs by enhancing the descending branch of the Hadley cell.