School of Earth Sciences - Research Publications

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Type: Journal Article × Author: Lane, Todd × Author: King, Malcolm ×

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    5-day-wave interactions with tropical precipitation in CMIP5 models
    King, MJ ; Wheeler, MC ; Lane, TP (American Meteorological Society, 2016-12)
    The 5-day Rossby-Haurwitz wave is unlike other large-scale wave modes that interact with tropical rainfall in that associated rainfall presents as a modulation of localized areas of rainfall instead of propagating with the wave. This form of wave-modulated convective organization in climate models has received little attention. This study investigates the simulation of interactions between the 5-day wave and tropical convection in 30 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) and compares these with the interaction diagnosed from ERA-Interim and TRMM precipitation data. Models simulate the dry dynamics of the 5-day wave well, with realistic coherences between upper- and lower-tropospheric winds, as well as magnitudes and geographic distribution of wave wind anomalies being close to observations. The models consistently display significant coherences between 5-day-wave zonal winds and precipitation but perform less well at simulating the spatial distribution and magnitude of precipitation anomalies. For example, a third of the models do not reproduce significant observed anomalies near the Andes, and the best-performing model simulates only 38% of the observed variance over the tropical Andes and 24% of the observed variance over the Gulf of Guinea. Models with higher resolution perform better in simulating the magnitude of the Andean rainfall anomalies, but there is no similar relationship over the Gulf of Guinea. The evidence therefore suggests that the simulated interaction is mostly one way only, with the wave dynamics forcing the precipitation variations on the 5-day time scale.
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    Association of convection with the 5-Day Rossby-Haurwitz wave
    King, MJ ; Wheeler, MC ; Lane, TP (Americal Meteorological Society, 2015-09)
    The seasonality, regionality, and nature of the association between tropical convection and the 5-day wavenumber-1 Rossby-Haurwitz wave are examined. Spectral coherences between daily outgoing longwave radiation (OLR), a proxy for convection, and 850-hPa zonal wind over the period January 1979- February 2013 are compared for different seasons and for phases of El Niño-Southern Oscillation (ENSO) and the quasi-biennial oscillation (QBO). Increased coherence, indicating a stronger association, occurs in boreal spring and autumn, with slightly reduced coherence in boreal summer and significantly reduced coherence in boreal winter. The regionality of the association is examined using lagged-regression techniques. Significant local signals in tropical convection are found over West Africa, the tropical Andes, the eastern Pacific Ocean, and the Marshall Islands. The relative phasing between the 5-day wave wind and OLR signals is in quadrature inAfrica and theMarshall Islands, in phase with easterlies over theAndes, and out of phase with easterlies over the eastern Pacific. Frequency spectra of precipitation averaged over the identified local regions reveal spectral peaks in the 4-6-day range. The phasing between the large-scale wind and local convection signals suggests that the 5-day wave is actively modulating the convection around the Americas.