Coherent Potential Vorticity Maxima and Their Relationship to Extreme Summer Rainfall in the Australian and North African Tropics

2017 ◽  
Vol 66 (4) ◽  
pp. 424-456 ◽  
Author(s):  
Lam Hoang ◽  
Michael Reeder ◽  
Gareth Berry ◽  
Juliane Schwendike
Keyword(s):  
Potential Vorticity ◽  
Summer Rainfall ◽  
North African

scholarly journals Coherent Potential Vorticity Maxima and Their Relationship to Extreme Summer Rainfall in the Australian and North African Tropics

2016 ◽  
Vol 66 (4) ◽  
pp. 424
Author(s):  
Lam P. Hoang ◽  
Michael J. Reeder ◽  
Gareth. J. Berry ◽  
Juliane Schwendike
Keyword(s):  
Potential Vorticity ◽  
West African Monsoon ◽  
Lower Troposphere ◽  
Extreme Rainfall ◽  
Summer Rainfall ◽  
Horizontal Wind ◽  
Synoptic Scale ◽  
North African ◽  
Eastern Australia ◽  
The Tropics

Extreme rainfall in the tropics is frequently linked with coherent synoptic-scale potential vorticity (PV) disturbances. Here, an objective technique is used to identify coherent synoptic-scale cyclonic PV maxima with a focus on those that occur during summer over the African and Australian tropics. These two regions are chosen for comparison because of their geographical and climatological similarities. In particular, in both regions oceans lie equatorward and extensive deserts lie pole-ward, a juxtaposition that produces a reversal in the mean north-south temperature gradient and, through thermal wind, a low level easterly jet.In general, in the lower troposphere there are more coherent PV maxima in the tropics in the summer hemisphere than the winter hemisphere. These coherent PV maxima generally move with the background flow in the lower troposphere. The largest meridional flux of coherent PV maxima lies along eastern Australia with about half of the coherent PV maxima generated through the filamentaton and eventual isolation of midlatitude PV. In contrast, in the north African tropics, coherent PV maxima are generated mostly in the tropics and move westward through the west African monsoon region.Composites based on the extreme rainfall days for two regions are broadly similar with large, statistically significant PV maxima to the east of the maximum positive rainfall anomalies. The vertical structures of the PV fields in the two regions reveal a cyclonic PV maximum in the mid-troposphere collocated with the maximum of diabatic heating. The composite horizontal wind structures in the Australian tropics show structures similar to mesoscale convective systems (MCSs), whereas in the African tropics, they are similar to easterly waves.

scholarly journals North African vegetation-precipitation feedback in early and mid-Holocene climate simulations with CCSM3-DGVM

2014 ◽  
Vol 10 (3) ◽  
pp. 2055-2086 ◽  
Author(s):  
R. Rachmayani ◽  
M. Prange ◽  
M. Schulz
Keyword(s):  
Regional Scale ◽  
West African Monsoon ◽  
Coupled Model ◽  
Summer Precipitation ◽  
Summer Rainfall ◽  
Precipitation Anomaly ◽  
Multiple Equilibrium ◽  
North African ◽  
African Region ◽  
Sahel Region

Abstract. The present study analyses the sign, strength and working mechanism of the vegetation-precipitation feedback over North Africa in middle (6 ka BP) and early Holocene (9 ka BP) simulations using the comprehensive coupled climate-vegetation model CCSM3-DGVM. The coupled model simulates enhanced summer rainfall and a northward migration of the West African monsoon trough along with an expansion of the vegetation cover for the early and middle Holocene compared to pre-industrial. It is shown that dynamic vegetation enhances the orbitally triggered summer precipitation anomaly by approximately 20% in the Sahara/Sahel region (10° N–25° N, 20° W–30° E) in both the early and mid-Holocene experiments compared to their fixed-vegetation counterparts. The primary vegetation-rainfall feedback identified here operates through surface latent heat flux anomalies by canopy evaporation and transpiration and their effect on the mid-tropospheric African Easterly Jet, whereas the effects of vegetation changes on surface albedo and local water recycling play a negligible role. Even though CCSM3-DGVM simulates a positive vegetation-precipitation feedback in the North African region, this feedback is not strong enough to produce multiple equilibrium climate-ecosystem states on a regional scale.

scholarly journals North African vegetation–precipitation feedback in early and mid-Holocene climate simulations with CCSM3-DGVM

2015 ◽  
Vol 11 (2) ◽  
pp. 175-185 ◽  
Author(s):  
R. Rachmayani ◽  
M. Prange ◽  
M. Schulz
Keyword(s):  
Regional Scale ◽  
Summer Rainfall ◽  
Precipitation Anomaly ◽  
Multiple Equilibrium ◽  
North African ◽  
Vegetation Model ◽  
African Region ◽  
Dynamic Global Vegetation Model ◽  
Climate System Model ◽  
Sahel Region

Abstract. The present study analyses the sign, strength, and working mechanism of the vegetation–precipitation feedback over North Africa in middle (6 ka BP) and early Holocene (9 ka BP) simulations using the comprehensive coupled climate–vegetation model CCSM3-DGVM (Community Climate System Model version 3 and a dynamic global vegetation model). The coupled model simulates enhanced summer rainfall and a northward migration of the West African monsoon trough along with an expansion of the vegetation cover for the early and middle Holocene compared to the pre-industrial period. It is shown that dynamic vegetation enhances the orbitally triggered summer precipitation anomaly by approximately 20% in the Sahara–Sahel region (10–25° N, 20° W–30° E) in both the early and mid-Holocene experiments compared to their fixed-vegetation counterparts. The primary vegetation–rainfall feedback identified here operates through surface latent heat flux anomalies by canopy evaporation and transpiration and their effect on the mid-tropospheric African easterly jet, whereas the effects of vegetation changes on surface albedo and local water recycling play a negligible role. Even though CCSM3-DGVM simulates a positive vegetation–precipitation feedback in the North African region, this feedback is not strong enough to produce multiple equilibrium climate-ecosystem states on a regional scale.

scholarly journals Interannual Influences of the Surface Potential Vorticity Forcing over the Tibetan Plateau on East Asian Summer Rainfall

2022 ◽  
Author(s):  
Chen Sheng ◽  
Bian He ◽  
Guoxiong Wu ◽  
Yimin Liu ◽  
Shaoyu Zhang
Keyword(s):  
Surface Potential ◽  
Potential Vorticity ◽  
East Asian ◽  
Summer Rainfall ◽  
Circulation Pattern ◽  
The Tibetan Plateau ◽  
East Asian Summer Rainfall ◽  
Asian Summer ◽  
Heating Mode ◽  
Anomalous Heating

AbstractThe influences of interannual surface potential vorticity forcing over the Tibetan Plateau (TP) on East Asian summer rainfall (EASR) and upper-level circulation are explored in this study. The results show that the interannual EASR and associated circulations are closely related to the surface potential vorticity negative uniform leading mode (PVNUM) over the TP. When the PVNUM is in the positive phase, more rainfall occurs in the Yangtze River valley, South Korea, Japan, and part of northern China, less rainfall occurs in southern China, and vice versa. A possible mechanism by which PVNUM affects EASR is proposed. Unstable air induced by the positive phase of PVNUM could stimulate significant upward motion and a lower-level anomalous cyclone over the TP. As a result, a dipole heating mode with anomalous cooling over the southwestern TP and anomalous heating over the southeastern TP is generated. Sensitivity experiment results regarding this dipole heating mode indicate that anomalous cooling over the southwestern TP leads to local and northeastern Asian negative height anomalies, while anomalous heating over the southeastern TP leads to local positive height anomalies. These results greatly resemble the realistic circulation pattern associated with EASR. Further analysis indicates that the anomalous water vapor transport associated with this anomalous circulation pattern is responsible for the anomalous EASR. Consequently, changes in surface potential vorticity forcing over the TP can induce changes in EASR.

Skeletochronology and mark–recapture assessments of growth in the North African agamid lizard (Agama impalearis)

1999 ◽  
Vol 249 (4) ◽  
pp. 455-461
Author(s):  
El Hassan El Mouden ◽  
Mohammed Znari ◽  
Richard P. Brown
Keyword(s):  
North African ◽  
Mark Recapture ◽  
Agamid Lizard ◽  
The North
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