Formation of rain layers in the Warm Pool and their feedbacks to atmospheric convection in an idealized modeling framework

Colorado State University, Fort Collins CO

Investigators

Abstract

Over the warm tropical ocean, the supply of moist air sets up convective systems which manifest themselves in rainfall as a pattern of wet and dry spells, each lasting 30 to 60 days and propagating slowly Eastward. Within this so-called Madden-Julian Oscillation (MJO), the understanding of ocean feedback processes to atmospheric convection remains incomplete, and hinders its representation in climate and forecast models. The MJO is sensitive to upper ocean stability, because a stable surface ocean does not mix as readily or deeply, which makes it easier to generate larger temperature anomalies within a shallow surface layer through solar heating or evaporative cooling. This effect has been studied more with warm surface layers, but thin layers of relatively fresh water often formed by rainfall can also enhance stability of the surface layer. Because these layers tend to be small and ephemeral, they have not been studied as much. This project will use a regional atmospheric model coupled with a simplified model of the upper ocean to explore the impacts of rain layers on atmospheric convection. In addition to better understanding MJO mechanisms, the project will introduce a new model for investigating air-sea interactions, and novel analyses of ocean feedbacks to atmospheric convection. The project will also enable collaboration between atmospheric and oceanographic scientists, support for female scientists, and graduate education. Studies during the past decade have advanced our understanding of the importance of diurnal warm layers (DWLs) for the onset of the MJO, how MJO convection and wind anomalies interact with the upper ocean heat content, and have documented the evolution of large- scale sea surface salinity (SSS) patterns shaped by MJO rainfall. Less clear, however, is how the formation of transient ocean surface freshwater lenses, or rain layers (RLs), may temper ocean mixing, marine surface fluxes, and MJO convection. RLs are typically more buoyant and colder than Warm Pool surface waters, producing a cold patch on the ocean surface. Because RLs are shallow (0.5 - 5 m) and short-lived (< 1 day), they are likely undersampled by in situ observing systems. Direct observations of RLs in the Warm Pool are sparse, and efforts to document their observed spatial and temporal characteristics are in their infancy. The frequency, intensity, and duration of RLs, and their effects on atmospheric convection throughout the MJO lifecycle are largely unknown. A collection of rain layers generated by widespread convection would reduce surface latent and sensible heat fluxes via SST cooling, but would also generate a fine-scale network of sharp SST gradients, which have been linked to convective initiation. This study will use the Regional Atmospheric Modeling System (RAMS) coupled to many columns of a 1-dimensional KPP ocean mixed layer model 1) to study the formation of upper ocean stable layers by surface heating and freshening, including their frequency, spatial variability, and duration, and their regulation of upper ocean stability and mixing, and 2) to study how rain layers may interact with convection during the MJO suppressed-to-active transition period. Model-generated stable layer statistics will be compared to previously published ocean observations collected during the DYNAMO field campaign. Ocean feedbacks to convection will be assessed through a progression of diagnostics designed to link variations in upper ocean stability to surface fluxes and the organization of tropical convection. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.