Modulation of Daily Precipitation Over Southwest Asia by the Madden-Julian Oscillation
M. Barlow, M. Wheeler, B. Lyon, and H. Cullen
2005: Mon Wea Rev., 133, 3579-3594.
Abstract
Analysis of daily observations shows that wintertime (Nov-Apr) precipitation over S
outhwest
Asia is modulated by Madden-Julian Oscillation (MJO) activity in the eastern Indian
Ocean, with
strength comparable to the interannual variability. Daily Outgoing Longwave Radiati
on (OLR)
for 1979-2001 is used to provide a long and consistent, but indirect, estimate of p
recipitation, and
daily records from 13 stations in Afghanistan reporting at least 50% of the time fo
r 1979-1985
are used to provide direct, but shorter and irregularly-reported, precipitation dat
a. In the station
data, for the average of all available stations, there is a 55% increase in daily p
recipitation when
the phase of the MJO is negative (suppressed tropical convection in the eastern Ind
ian Ocean).
The distribution of extremes is also affected such that the 10 wettest days all occ
ur during the
negative MJO phase. The longer record of OLR data indicates that the effect of the
MJO is quite
consistent from year to year, with the anomalies averaged over Southwest Asia more
negative
(indicating more rain) for the negative phase of the MJO for each of the 22 years i
n the record.
Additionally, in 9 of the 22 years the average influence of the MJO is larger than
the interannual
variability (e.g., the relationship results in anomalously wet periods even in dry
years and vice
versa). Examination of NCEP/NCAR reanalysis data shows that the MJO modifies both t
he local
jet structure and, through changes to the thermodynamic balance, the vertical motio
n field over
Southwest Asia, consistent with the observed modulation of the associated synoptic
precipitation. A simple persistence scheme for forecasting the sign of the MJO sugg
ests that the
modulation of Southwest Asia precipitation may be forecastable for three-week perio
ds. Finally,
analysis of changes in storm evolution in Southwest Asia due to the influence of th
e MJO shows
a large difference in strength as the storms move over Afghanistan, with apparent r
elevance for
the flooding event of April 12-13, 2002.