The Upscaling of Tropical Pacific Ocean Rain Layers to Convection and Modes of Pacific Climate Variability

Colorado State University, Fort Collins CO

Investigators

Abstract

Air-sea interactions are the key to the weather and climate of the tropics, as energy and moisture extracted from the ocean fuels the deep convective clouds that in turn produce the large-scale atmospheric circulation and drive the hydrological cycle. The critical dependence of weather and climate on heat and moisture fluxes from the ocean means that any restriction on those fluxes can have an outsized influence on the convective clouds and their large-scale effects. Previous work by the Principal Investigators (PIs) has shown that rainwater falling on the ocean can form a "cold lid", also called a rain layer, that restricts air-sea fluxes. Fresh rainwater is less dense than salty sea water, thus rain falling on the ocean stays at the surface and forms a shallow stable layer typically less than a meter thick and slightly colder than the underlying ocean. The lower temperature of the cold lid can inhibit the onset of convection, which in turn can allow more sunlight to reach the ocean, most of which passes through the lid to warm the ocean below it. But when convection develops it can generate strong surface winds that break up the lid and expose the warmer ocean, thus intensifying convection. Work performed here explores the implications of this delay-then-intensify effect for the aggregation of individual convective clouds into broad areas of organized convection, and for the development of El Nino events. In both cases the work focuses on the Madden-Julian Oscillation (MJO), a broad area of organized convection that forms in the Indian Ocean and propagates slowly eastward across the Pacific. The MJO generates westerly wind bursts (WWBs), periods of strong west-to-east surface winds along the equator, which promote the development of El Nino events. Thus the intensification of MJO convection through the cold lid mechanism could affect El Nino by intensifying WWBs. The research is carried out through comparison of model simulations in which cold lid effects are either active or disabled, some involving a regional atmospheric model (the Weather Research and Forecasting model) coupled to a regional ocean model with very high vertical resolution near the surface so that rain layers can be simulated. Other simulations use the Community Earth System Model (CESM), a global model which simulates El Nino events but lacks the fine resolution needed to explicitly represent rain layers and the cold lid mechanism. Instead the PIs implement a new ocean surface flux parameterization which indirectly captures the flux restriction caused by the cold lid. Some simulations are performed with higher levels of carbon dioxide to investigate the effects of the cold lid in a warmer climate. The work is of societal as well as scientific interest given the substantial weather and climate impacts of the MJO and El Nino, which include changes in the frequency of landfalling hurricanes and atmospheric rivers in the US. Models used for weather prediction and climate projections typically produce MJO events which are too weak and propagate too fast, thus the delay and intensification produced by the cold lid mechanisms could be a missing ingredient in these models. The project has educational value as it provides support and training to a graduate student. The PIs also serve as mentors to summer interns at the CSU Research Experiences for Undergraduates (REU) site. 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.