I did a brief study about the factors affecting monsoon precipitation in Nepal few months ago for a class project. I am sharing the slides as it is relevant to the recent flooding in north India and Nepal.

1. Factors affecting Monsoon
Precipitation in Nepal
- Sagar P. Parajuli
05/01/2013

2. Motivation
• Agriculture is the backbone of economy in the
Indian subcontinent including
Nepal, India, Bangladesh
• Agriculture of this region mainly depends on
monsoon precipitation
• Natural disasters such as flooding, landslides and
lightening are frequent
• Drought affect local water resources and worsen
water shortage

4. Fig. 1. ISM, Indian Summer Monsoon; EASM, East Asian Summer
Monsoon. Arrows show 60-year mean summer (June, July, August) wind
fields based on NCEP/NCAR reanalysis. Adapted from (Wang et al. 2010)
Indian Summer Monsoon

5. Figure 2. 13 years average JA precipitation: TRMM composite climatology(0.5 *0.5 degree)
http://gcmd.gsfc.nasa.gov/KeywordSearch/Metadata.do?Portal=GCMD&KeywordP
ath=[Project%3A+Short_Name%3D%27TRMM%27]&EntryId=GES_DISC_TRMM_T
CC_V6&MetadataView=Full&MetadataType=0&lbnode=mdlb5

6. Factors considered
• ENSO/Pacific QDO
• Topography
• Tibetan plateau heating
• Aerosols
ENSO Topography Tibetan Plateau Aerosols

7. Precipitation trend in Nepal
• Decadal oscillation (11 years cycle) seen in Nepal
precipitation trend (Shrestha et al. 2007)
• Pacific QDO’s relation with Nepal precipitation is not
instantaneous but the maximum (minimum) precipitation
anomalies occur 1–2 years prior to the cold (warm) extremes
of the Pacific QDO (Wang et al. 2013)
ENSO/QDO Topography Tibetan Plateau Aerosols

8. Station-based
mean annual
precipitation in
Nepal (Baidya S.
2007)
TRMM Mean
monthly
precipitation
(0.25*0.25 degree) in
Nepal
ENSO/QDO Topography Tibetan Plateau Aerosols
http://apollo.lsc.vsc.edu/classes/met130/notes/chapter10/elnino.html

9. Relation between ENSO and Indian Monsoon
ENSO/QDO Topography Tibetan Plateau Aerosols
(Kumar, K. et al. 2007)

10. Figure 2. Composite SST anomaly for drought (shaded) and drought-free
(contours) El-Nino years
ENSO/QDO Topography Tibetan Plateau Aerosols
(Kumar, K. et al. 2007)

11. Fig 3. Model simulation results (CAM 3.5.18, 1992-1997) of
average JJA Surface wind (arrows) and precipitation (shaded) for
(a) Standard topography, (b) All topography removed
(a) (b)
ENSO/QDO Topography Tibetan Plateau Aerosols
(Boos et al. 2010)

12. Figure 4. (c) Model simulation results of average JJA surface wind (arrows) and
precipitation (shaded) with Tibetan topography removed (d) Precipitation and
850 hpa wind anomaly for JJA relative with Tibetan plateau albedo = 1.
(c)
(d)
ENSO/QDO Topography Tibetan Plateau Aerosol
(Boos et al. 2010)

13. ENSO Topography Tibetan Plateau Aerosols
Figure 5. (a) Time-latitude cross section of TOMS AI anomalies during high aerosol
loading years over Himalayan region (b) same as (a) but for precipitation (mm/day)(c)
Area mean daily aerosol index over Himalayan region (Lau et al. 2006)

14. ENSO Topography Tibetan Plateau Aerosols

15. Figure 6. 200 hpa (left) and 925 hpa (right) geopotential height (contours)
and wind (arrows) for July/August 2012 from ERA-Interim reanalysis. Closer
phi contours means colder and wider means warmer region.
ENSO Topography Tibetan Plateau Aerosols

16. Conclusions
• Himalayan topography, moisture flux from bay of
Bengal, tropospheric absorbing aerosols, strong summer heating
over the Tibetan plateau and Pacific QDO all affect precipitation
in Nepal in varying degrees
• The concentrated and highly convective monsoon precipitation
along the Nepal Himalayas is difficult to simulate with climate
models (Shrestha et al. 2006)
• Precipitation trend in Nepal is unique and uncorrelated with that
of India (Wang et al. 2010)
• Accurate forecasting of monsoon precipitation requires
consideration of all of the above-mentioned factors

17. Thank you!
Comments?