1. Defining and predicting the West African Monsoon Onset
Rory Fitzpatrick1, Doug Parker1, John Marsham1, Peter Knippertz2, Caroline Bain3
University of Leeds, United Kingdom
Karlsruhe Institute of Technology, Germany
UK Met Office, United Kingdom

2. Motivation

3. Definitions of onset length scale
Local onset – occurring at the grid-cell to country wide scale
Regional onset – occurring over a large longitudinal range (typically 10W-10E). Data is zonally averaged across the region
Semi-local (hybrid) onset

4. The West African Monsoon
Figure 1 – Lafore et al. 2010
Figure 2 c and d – Sultan and Janicot 2003b

5. Definitions Local Regional Le Barbé et al. 2002
Walter (1967)
Kowel and Knabe (1972)
Benoit (1977)
Olaniran (1983)
Sivakumar (1988)
Omotosho et al. (2000)
Le Barbé et al. (2002)
Zeng and Lu (2002)
Marteau et al. (2009)
Vellinga et al. (2012)
Yamada et al. (2012)
Le Barbé et al. 2002
Sultan and Janicot 2003
Fontaine and Louvet 2006
Fontaine et al. 2008
Gazeaux et al. 2011
Vellinga et al. (2012)
Nguyen et al. (2014)
This list does not include locally determined or traditional methods. There may be many more!

6. Datasets compared ERA-Interim 1998-2012 , 1.5°x1.5° resolution
GPCP , 1°x1° resolution
TRMM 3B42 v , 0.25°x0.25° resolution
TRMM 3B42 v , 0.25°x0.25° resolution

7. Sultan and Janicot (2003)
Regional Definition

8. Sultan and Janicot 2003 (SJ)
TRMM v
Sultan and Janicot 2003 (SJ)
Onset occurs when zonally-averaged, time-smoothed precipitation (10W-10E) decreases at 5N (red line) simultaneous with increases at 10N (blue line) and 15N (green line)

9. Such a date is not always evident
Good Example TRMM7 2002
Bad Example TRMM7 2010
Selection of an onset date is not always unambiguous if definition is taken verbatim. It is therefore useful to create an objective version of regional onset date SJ.

10. Objective onset SJ_10 Objective onset of SJ: Can differ greatly
the first date for which precipitation at 5N is less than zonally averaged precipitation at 10N for 7 consecutive days (SJ_10)
(blue vertical line)
Can differ greatly
from the subjective version
(black line)
and the point where
precipitation at 5N falls
below precipitation values at
15N (green line)
TRMM v7 2010

11. SJ_10 onset dates: Dataset comparison
Definition and Dataset
Mean onset date
Standard deviation
Sultan and Janicot 2003
24 June 8
ERA Interim
26 June
13.9
GPCP
29 June
9.3
TRMM v6
28 June
9.9
TRMM v7
5 July
9.6
GloSea5
7.0
Should I add the coarse grained TRMM or maybe show the GloSea5 results here?

12. Datasets can greatly disagree on onset dates

13. Marteau et al. 2009
Local Definitions

14. Marteau et al. 2009 (Mart) TRMM v6 1998-2010
Local onset is triggered on the first rainy day (>1mm) of two consecutive rainy days (total rain > 20mm) with no 7 day dry spell of total rain < 5mm occurring in the subsequent 20 days

15. Inconsistency in onset across datasets
TRMM v
GPCP
Fix colorbars
There are few locations across West Africa where TRMM v7 and GPCP local onset dates are correlated.

16. Spatially sporadic correlation between SJ_10 and Mart
The regional onset date is only sporadically correlated with local onsets.
Regional onset may only have limited use in predicting local onset.

17. GLOSEA5 ONSET PREDICTABILITY
Brief results
GLOSEA5 ONSET PREDICTABILITY

18. Mean onset of GloSea5 captures general local onset pattern
TRMM v7
GloSea5

19. But inter-annual variability is not well captured
TRMM 2002
GloSea5 2002

20. TRMM 2003
GloSea5 2003
Although GloSea5 has a good representation of mean onset date, it does not capture inter-annual variability.

21. Can GloSea5 correctly predict early or late local onsets?
Definition of prediction
Observed onset
Predicted onset
Complete hit
> 1 s.d. prior to mean
Partial hit
Between 0.5 and 1 s.d. prior to mean
Partial miss
> 1 s.d. prior to mean
Between 0.5 s.d. prior and 0.5 s.d. after mean
Complete miss
> 0.5 s.d. after mean
False hit
< 1 s.d. prior to mean
s.d. – standard deviation

22. Not exceptionally.
A similar pattern is found for late onsets.

23. Conclusions
There is a big difference between local and regional onsets of the West African Monsoon. Are different dynamics involved?
Datasets agree on mean regional onset date, but do not show inter-annual correlation.
Skilful prediction of regional onsets may have little practical use for local onset forecasts.
GloSea5 can recreate the mean local onset date but does not capture inter-annual variability.

24. Because everything can be described by The Simpsons
“Lisa, your word to spell is weather.”
“Can you use that in a sentence please?”
“Certainly. I don’t know whether the weather will be good today.”