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
Easterly Waves: Animation
~3000 km
~10 m/s
Courtesy of CIMSS, Total Precipitable Water
• 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)
Convectively Suppressed
Maloney and Esbensen 2007
Convectively Enhanced
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
1 2
u  v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
Westerly vs. Easterly Events Budgets
Rydbeck and Maloney 2014
Westerly phase
Easterly phase
 Q1T  / 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:
Rydbeck and Maloney 2014
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
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
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
 Q1T  / 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
 Q1T  / 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