Rossby modes leading stratocumulus and lower-tropospheric stability

Stratocumulus decks (Sc) are semi-persistent over the eastern subtropical oceans. They are hypothesized to stay there because of cooler ocean and subsidence warming, which together create strong lower-tropospheric stability. It is therefore naturally assumed that the variability of Sc is controlled by subsidence and local SST as well. However, our study finds this assumption is not supported by observations. In this study, we decompose the circulation coupled with lower-tropospheric stability (represented by estimated inversion strength, EIS) and Sc (represented by low-cloud cover, LCC), respectively, from synoptic to interannual timescales. The signals show that local subsidence doesn't dominate EIS variability. Instead, EIS variability is controlled by extratropical Rossby waves. SST only influences EIS on long timescales. More interestingly, LCC is associated with circulation patterns similar to those of EIS but shifted about 10 degrees upstream. Local SST cooling appears to be no more important than upstream warming for Sc. Since Klein et al. (1995), studies have observed that the conditions on the Lagrangian trajectory of Sc are important for its growth. Our results are consistent with them and further emphasize the importance of the upstream Rossby ridge across timescales. It suggests that an upstream warming with a relatively unchanged local Sc condition can also cause an Sc increase.