Friday, April 13, 2018, 3pm
Location: Planetary Hall Room 212

David Straus
Department of Atmospheric, Oceanic, and Earth Sciences
George Mason University

Diabatic Heating and the Tropical Forcing of Mid-Latitudes on Intra-Seasonal Time Scales: Observations and Modeling

Abstract

    The tropical large-scale diabatic heating and associated circulation fields in the earth’s atmosphere are characterized by coherent oscillations on both inter-annual time scales and intra-seasonal time scales. These oscillations exert a remote influence on the circulation patterns and weather regimes in mid-latitudes, and so are of interest in the context of mid-latitude forecasting beyond the deterministic limit of predictability of about two weeks.
    In this talk I will focus on the coherent intra-seasonal heating associated with the Madden-Julian Oscillation (MJO), which dominates tropical variability on time scales of 30 - 60 days. Observed MJO episodes have a range of phase speeds, which is something we must grapple with to understand the response in in higher latitudes.
    I will first review the basic mechanism by which the tropical heating affects the upper-level divergent circulation and produces a source for Rossby waves which propagate into mid-latitudes. In order to understand the response to this source, even in the linear approximation, we need to take into account the interference of wave trains originating from different tropical locations at different times, that is a time-dependent Green’s function. In the presence of non-linearities, this is a formidable problem, to which we apply a full atmosphere-ocean coupled climate model. I will introduce an added heating technique which our group has developed and applied to a climate model to extract the mid-latitude signal that is, at least in principle, predictable. The response to more slowly evolving MJO episodes will be contrasted to those of more rapidly evolving MJO episodes. Implications for mid-latitude predictability will be discussed.