Supplementary Material
Article Title: Impact of Cold Air Surges on Rainfall Variability in the Sahel and Wet African
Tropics: A Multi-scale Analysis
Journal:
Climate Dynamics
Authors:
Edward K. Vizy* and Kerry H. Cook
Affiliation: Department of Geological Sciences
Jackson School of Geosciences, The University of Texas at Austin
1 University Station C1100
Austin, TX 78712
e-mail address: ned@jsg.utexas.edu
The supplemental material provided is meant to clarify how individual cold air surge
events are selected for compositing in section 4 of the paper. Recall that two criteria are used.
First, cases in which 850 hPa cold temperature anomalies averaged between 10°E - 30°E move
equatorward from 30°N are selected, with no constraints imposed for the cold anomaly
magnitude or the equatorward reach. Here, cold temperature anomalies are calculated as the
daily temperature departure from the 1979-2011 climatological daily mean.
Second, the equatorward surge of cool air must be accompanied by an amplification of
the 500 hPa geopotential height pattern over the Mediterranean Sea, with ridging in the west and
troughing to the east that enhances the mid-level negative zonal height gradient. This
amplification of the height pattern is diagnosed by creating western and eastern Mediterranean
500 hPa geopotential height anomaly indices by area-averaging daily geopotential height
departures between 30°N - 40°N; 5°W - 10°E and 30°N - 40°N; 15°E - 35°E, respectively.
Again, the geopotential height anomalies are calculated as the daily height departure from the
1979-2011 climatological daily mean. The western and eastern Mediterranean geopotential
height anomaly indices are then examined to identify times when the western Mediterranean
height anomaly is positive and the eastern Mediterranean height anomaly is negative
concurrently.
Supplemental Fig. 1 below provides an example to demonstrate the selection process.
Supplemental Fig. 1a shows the 850 hPa July – September cold temperature anomalies (K)
averaged between 10°E – 30°E for 1998. Note warm temperature anomalies are omitted to
improve clarity in interpretation of the figure. Supplemental Fig. 1b shows the area-averaged
500 hPa western Mediterranean (blue) and eastern Mediterranean (red) geopotential height
anomalies.
In the approach employed, the first step is to use the 850 hPa temperature anomaly
Hovmöller to identify times when the temperatures are (1) cold first at 30°N, and (2) the
Hovmöller indicates equatorward movement of the cold air over the subsequent next few days.
These potential cases are then evaluated further by examining the 500 hPa geopotential height
anomalies over the eastern and western Mediterranean. Only the cold surge events that exhibit
positive 500 hPa geopotential height anomalies over the western Mediterranean and negative 500
hPa geopotential height anomalies over the eastern Mediterranean are selected for use. These are
denoted in Supplemental Fig. 1 by the bold arrows.
Supplemental figure 1. 1998 July – September daily (a) 850 hPa cold temperature anomalies (K)
averaged between 10°E – 30°E, and (b) area-averaged 500 hPa geopotential height anomalies
(m) over the western (blue; 5°W - 10°E; 30°N - 40°N ) and eastern (red; 15°E - 35°E; 30°N 40°N) Mediterranean. Bold arrows denote cold air surge events selected for compositing.