Intraseasonal variations in west Africa and east Pacific easterly wave energy budgets
Intraseasonal variations in west Africa and east Pacific easterly wave energy budgets Ghassan Alaka and Adam Rydbeck Eric D. Maloney Colorado State University Sponsors: NSF Climate and Large‐Scale Dynamics Program NOAA MAPP Program Easterly Waves: Animation ~3000 km ~10 m/s Courtesy of CIMSS, Total Precipitable Water Introduction • Previous studies have documented substantial ISV in easterly wave (EW) activity in the east Pacific and west Africa that may affect tropical cyclogenesis frequency (e.g. Molinari et al. 1997; Maloney and Hartmann 2001; Maloney and Shaman 2008; Matthews 2008; Leroux and Hall 2009; Janicot et al. 2010; Ventrice et al. 2011; Alaka and Maloney 2012) • We will examine intraseasonal variability in perturbation kinetic energy (PKE) and perturbation available potential energy (PAPE) budgets in ERA‐I as they affect EWs to provide insight into this modulation of EWs • Most important modulating terms in energy budget differ by region 3 NH Summer Impacts of the MJO in the East Pacific (Opposite Phases) Easterly/ Convectively Suppressed Maloney and Esbensen 2007 Westerly/ Convectively Enhanced Precip: mm day-1 20‐100 day anomalies TC Modulation in the East Pacific by the MJO Westerly phases Maloney and Hartmann 2000 Easterly phases ERA‐I PKE for Intraseasonal Easterly and Westerly Phases (June‐October) Westerly phase PKE 1 2 u v2 2 Vertically averaged to 200 hPa. Bar=10‐day running mean Prime=Deviation from mean Easterly phase TCs removed using method of Aiyyer et al. (2012) Units: m2 s‐2 Rydbeck and Maloney 2014 PKE and PAPE Budgets • PKE budget (e.g. Serra et al. 2010): • PAPE budget: • Budgets computed during lifecycles of easterly waves defined using an EOF‐based EW index. Budgets examined separately during ISO westerly and easterly periods 7 Westerly vs. Easterly Events Budgets Rydbeck and Maloney 2014 Westerly phase Easterly phase Q1T / T Units: 10‐5 (m2 s‐2) s‐1 • Barotropic conversion and conversion from PAPE (diabatic heating dominated) are enhanced during westerly events. • Defined shift of axis of barotropic conversion 8 Vertical Profile of Barotropic Conversion Westerly phase Barotropic conversion shows prominent increases in the middle troposphere during westerly phases Easterly phase Mainly due to: u uv y Rydbeck and Maloney 2014 9 550 hPa Winds and Meridional Shear Westerly phase Easterlies at mid levels weaken and negative meridional shear expands along a southeast‐northwest axis during westerly phases Easterly phase Negative shear confined to the south in a zonal band during easterly phases Recall: 10‐6 s‐1 u uv y Rydbeck and Maloney 201410 Q1 and d/dp profiles Westerly phase Easterly phase Top‐Heavy Q1 profile during westerly phase is consistent with vorticity generation in the middle tropopshere. Heating profile during easterly phase is weaker and more bottom‐heavy. d/dp is shown as the difference from neutral periods. Rydbeck and Maloney 2014 11 Significant Spectral Peak in PKE in West African Monsoon Region 700 hPa PKE anomalies • • Composites relative to 30‐90 day 700 hPa PKE index (box). Significant 30‐90 Day PKE anomalies propagate eastward, consistent with Alaka and Maloney 2008. Alaka and Maloney 2014 Correlation of Local 30‐90 Day PKE Index and RMM1 • • • Overall, about 10% of PKE variance explained by MJO In strong MJO years, this variance explained increases to 25%. Currently conducting some experiments with WRF that removes the MJO influence at boundaries to assess changes in PKE variance in the domain. Alaka and Maloney 2014 Composite PKE/PAPE Budget Terms R T / p • • • Q1T / T East African baroclinic energy conversions enhanced in advance of periods of strong west African PKE. Consistent with eastward AEJ extension into east Africa on advance of strong PKE events (e.g. see also Alaka and Maloney 2012) Alaka and Diabatic heating does not support PAPE growth downstream Maloney 2014 Diabatic Heating Opposes Wave Growth Alaka and Maloney 2014 • Q1T / T 0 It appears that dry air from Sahara is advected south in warm sector of waves, suppressing convection, causing destruction of PAPE due to negative covariance of Q’ and T’. Conclusions • East Pacific: – Periods of enhanced intraseasonal EW activity accompanied by enhanced barotropic conversion and diabatic generation of PAPE. – Mid troposphere meridional shear variations extremely important for enhanced barotropic conversion, associated with top‐heavy heating profile • West Africa – Enhanced baroclinic energy conversions in east Africa precede periods of enhanced intraseasonal EW activity – Barotropic conversion important downstream for supporting periods of enhanced AEW activity – Diabatic heating anomalies destroy PAPE downstream in west Africa, opposing wave growth 16 Thanks
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