Land Drought Index Trend

The Standard Precipitation-Evapotranspiration Index (SPEI) data gives us anomaly drought conditions over land spanning from 1901 to 2018 (inclusive) in monthly 0.5 degree latitude/longitude format. Today I combined all this grid data into a global land-only drought anomaly index and show its trend over time. Result:

SPE Index

Looks like it’s getting dryer over land, but it also looks cyclical. Time will tell.

Enjoy 🙂 -Zoe

Chart data archived here.


Northern Hemisphere
Southern Hemisphere

Drying in NH. Tiny drying in SH. Drying in the tropics. Wetter at the poles.

Note: Tropics = abs(latitude) < 23.5, Poles = abs(latitude) > 66.5


# Zoe Phin, 2021/05/19
# File:
# Run: source; require; download; alltime; plot
# Output: spei.csv, spei.yoy, spei.png

require() { sudo apt-get install -y nco gmt; }

download() {
    wget -O --no-check-certificate

onetime() {
    ncks -HC --trd -v spei -d time,$1,$1 | sed \$d | awk -F '[= ]' '
    $8 != "_" { 
        a=6378.137; e=1-6356.752^2/a^2; r=atan2(0,-1)/180;
        SA+=A; S+=$8*A
    } END { print S/SA }'

alltime() {
    for t in {0..1415}; do
        awk -vt=$t 'BEGIN{printf "%6.2f ", 1901+t/12+1/24}'
        onetime $t
    done | tee spei.csv

annual() {
    cat spei.csv | sed \$d | awk '{
        Y[substr($1,1,4)] += $2/12
    } END {
        for (y in Y) printf "%4d %.4f\n", y, Y[y]

yoy() {
    cat spei.csv | cut -c9- | tr '\n' ' ' | awk -vp=$1 '{
    for (i=0;i<p/2;i++) print ""
    for (i=p/2;i<=NF-p/2;i++) { s=0
        for (j=i-p/2; j<=i+p/2; j++)
        printf "%.4f\n", s
    }}' > spei.yoy

plot() { 
    yoy 120; paste -d ' ' spei.csv spei.yoy > plot.csv
    echo "set term png size 740,620
        set key outside top center horizontal
        set ytics format '%4.2f'
        set mxtics 2; set mytics 5
        set xrange [1900:2020]
        set grid xtics mxtics ytics
        plot 'plot.csv' u 1:2 t 'Wetter' w filledcurve above y1=0 lw 2 lc rgb '#0000DD',\
                     '' u 1:2 t 'Dryer'  w filledcurve below y1=0 lw 2 lc rgb '#DD0000',\
                     '' u 1:3 t '10yr CMA' w lines lw 3 lc rgb 'black'		
    " | gnuplot > spei.png 

Published by Zoe Phin

4 thoughts on “Land Drought Index Trend

  1. Well in that case its just as well that CO2 is more available to plants. It would be interesting to see the geographic and hemispheric distribution. Moisture comes from equatorial latitudes predominantly from areas with a high leaf area index where there is much evapotranspiration as seen here:,1.68,410/loc=-141.844,-32.810
    Pressure changes unequally between the hemispheres primarily driven from the Antarctic trough which acts as a global sink and the source of the most intense frontal activity. Could be simply a waxing and waning of precipitation in high southern latitudes affecting the amount of water vapour available over land masses.


  2. Zoe, thanks for the question, My interpretation is below and it gives rise to some hypothesis that could be tested against the data that you so cleverly analyze.

    There is greater pull from both poles as surface pressure has declined in the respective sinks, that in the northern hemisphere being the Aleutian and the Icelandic lows. Increase in precipitation at the Poles would be expected with the influx of warmer wetter air to the relative exclusion of cold dry air that descends from above, especially so in winter. Looks to me like land in the Southern hemisphere as a whole suffers a smaller decline in precipitation than the Northern Hemisphere as would be expected given the relative strength and all year round cyclone activity in the Antarctic trough which lowers surface pressure from 90 South through to about 40 South latitude, more in winter than summer. So the enhanced precipitation at the poles should be occurring mainly in the winter season, and more so in the Northern than the Southern Hemisphere.

    If the land is warming more than the sea, which happens in summer, the relative humidity of the air above the land should fall and with it cloud cover and precipitation in summer. Given that the rate of evapotranspiration from expanded leaf area is increasing due to enhanced CO2, precipitation should be rising over the sea as it falls over the land. Especially in summer when plants are more actively photosynthesizing.

    This is part of the rich texture of natural climate change associated with the annular modes phenomenon whereby atmospheric mass is shifted according to the vigor of polar cyclone activity that is strongest outside the Arctic and Antarctic circles over the Oceans. This changes the distribution of the atmosphere, surface pressure and the planetary winds. The atmosphere has electromagnetic properties, it exists in a magnetic field that is influenced by processes that are internal to the core of the Earth and those processes that are internal to the sun that influences the stuff that it throws off into space, all heavily charged, magnetized material. So, in winter when the polar atmosphere is most heavily ionized, it rotates in the same direction as the Earth but faster, more so aloft than at the surface. This conditions polar cyclone activity. That’s the way I see it based on twenty years study of reanalysis data that captures the character of the atmosphere at seventeen pressure levels from pole to pole.

    Liked by 1 person

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