spells of the ISM through the northward-propagating
30–60-day mode and the westward-propagating
10–20-day mode (e.g. Krishnamurti and Bhalme
1976; Keshavamurty and Sankar Rao 1992). Monsoon
intra-seasonal oscillation has a seminal role in
influencing the seasonal mean, and its interannual
variability (e.g. Goswami and Chakravorty 2017).
Synoptic systems
In the south Asian monsoon region, the synoptic
systems are mainly of tropical nature. They play an
important role for the onset and advance of the monsoon during June, distribution of rainfall during the
peak phase of the monsoon in July and August and
withdrawal of the monsoon from mid-September to
mid-October. Much of the monsoon rainfall over the
central plains of India is associated with the
low-pressure systems which develop over the north
Bay of Bengal and move onto the subcontinent along a
west-northwesterly track (see Chap. 7). The lowfrequency Madden–Julian Oscillations (MJO) (Madden and Julian 1972) moving eastward on the 30–
40-day scale along near-equatorial belts, on several
occasions trigger northward-moving organized convective episodes. These systems sometimes interact
with extratropical systems of the northern hemisphere
and produce extremely heavy rainfall in some parts of
northern India (Pisharoty and Desai 1956; Ramaswamy 1962).
Orographic precipitation
Capacious rainfall rates are generally noticed over the
Western Ghats (WG) and north and northeast region of
India during the summer monsoon season. These
regions have unique characteristics of mountainous
terrain that acts as a barrier to southwesterly winds
(e.g. Patwardhan and Asnani 2000a, b; Tawde and
Singh 2015). The windward side of WG receives
highest intense rainfall and leeward side of the WG is
a strong rain shadow region. Rain shadow areas differ
from one region to another along Karnataka, Maharashtra and Kerala due to the complexity of mountain
terrains. Intense orographic rainfall is confined up to
800 m height in the WG (e.g. Rahman et al. 1990).
High rainfall spells over the west coast of India are
associated with warm Sea Surface Temperature (SST),
low-level convergence, high CAPE and low convective inhibition (CIN) (e.g. Maheskumar et al. 2014).
The Himalaya mountain range acts as a barrier by
blocking the warm moist monsoon air masses primarily on south-facing slopes and preventing their
migration on the other side creating a prominent rain
shadow contrast (see Chap. 11). Nearly 80% (20%) of
the annual rainfall occur in the Himalayas due to
southwest (winter) monsoon. The nature of the convective systems varies dramatically from the western
to eastern foothills of Himalayas.
Box 3.2 Precipitation teleconnections with modes
of climate variability
ENSO and IOD
The year-to-year variability of Indian monsoon rainfall
(Fig. 3.1b) is governed by the slowly varying surface
features. El Nino conditions in the Pacific play a major
role in modulating the interannual variability of ISM
rainfall (Sikka 1980, 1977; Pant and Parthasarathy
1981; Rasmusson and Carpenter 1983; Webster et al.
1998). Almost 50% of the droughts are associated with
ENSO (see Chap. 6), however in the last few decades
the ENSO-Monsoon relationship has been weakened
(e.g. Kripalani and Kulkarni 1997; Krishna Kumar et al.
1999), frequency and intensity of droughts have been
increased and some of them are not associated with
ENSO.
The coupled mode in the Indian Ocean (Indian
Ocean Dipole; IOD; Saji et al. 1999) is also known to
modulate interannual variability of ISM rainfall.
A positive relationship between IOD and ISM rainfall
is well known (Ashok et al. 2004; Saji et al. 1999; Saji
and Yamagata 2003). The positive (negative) IOD
significantly dilutes the influence of El Nino (La Nina)
on the Indian monsoon (Ashok et al. 2004; Chowdary
et al. 2015). There are more frequent positive IOD
events in recent decades due to the rapid warming of
the Indian Ocean (e.g. Cai et al. 2018). In addition to
IOD and ENSO, there is a strong link between ISM
rainfall and the equatorial Indian Ocean oscillation
(EQUINOO; Gadgil et al. 2004). In general, positive
phase of the EQUINOO is favourable for a good
monsoon. Association between EQUINOO and ENSO
also determines the variations in ISM rainfall on the
interannual time scale.
Eurasian snow cover
Eurasian snow cover also plays a major role in the
year-to-year variability of ISM rainfall (Blanford
1884). Generally, positive Eurasian snow cover
anomalies during winter and spring tend to be followed by an anomalous deficit rainfall over the Indian
subcontinent in the subsequent summer monsoon
season, while negative snow cover anomalies tend to
be followed by abundant rainfall (Bhanu Kumar 1987;
Bamzai and Kinter 1997). It has been observed that all
non-ENSO related droughts over India have been
50
A. Kulkarni et al.
30–60-day mode and the westward-propagating
10–20-day mode (e.g. Krishnamurti and Bhalme
1976; Keshavamurty and Sankar Rao 1992). Monsoon
intra-seasonal oscillation has a seminal role in
influencing the seasonal mean, and its interannual
variability (e.g. Goswami and Chakravorty 2017).
Synoptic systems
In the south Asian monsoon region, the synoptic
systems are mainly of tropical nature. They play an
important role for the onset and advance of the monsoon during June, distribution of rainfall during the
peak phase of the monsoon in July and August and
withdrawal of the monsoon from mid-September to
mid-October. Much of the monsoon rainfall over the
central plains of India is associated with the
low-pressure systems which develop over the north
Bay of Bengal and move onto the subcontinent along a
west-northwesterly track (see Chap. 7). The lowfrequency Madden–Julian Oscillations (MJO) (Madden and Julian 1972) moving eastward on the 30–
40-day scale along near-equatorial belts, on several
occasions trigger northward-moving organized convective episodes. These systems sometimes interact
with extratropical systems of the northern hemisphere
and produce extremely heavy rainfall in some parts of
northern India (Pisharoty and Desai 1956; Ramaswamy 1962).
Orographic precipitation
Capacious rainfall rates are generally noticed over the
Western Ghats (WG) and north and northeast region of
India during the summer monsoon season. These
regions have unique characteristics of mountainous
terrain that acts as a barrier to southwesterly winds
(e.g. Patwardhan and Asnani 2000a, b; Tawde and
Singh 2015). The windward side of WG receives
highest intense rainfall and leeward side of the WG is
a strong rain shadow region. Rain shadow areas differ
from one region to another along Karnataka, Maharashtra and Kerala due to the complexity of mountain
terrains. Intense orographic rainfall is confined up to
800 m height in the WG (e.g. Rahman et al. 1990).
High rainfall spells over the west coast of India are
associated with warm Sea Surface Temperature (SST),
low-level convergence, high CAPE and low convective inhibition (CIN) (e.g. Maheskumar et al. 2014).
The Himalaya mountain range acts as a barrier by
blocking the warm moist monsoon air masses primarily on south-facing slopes and preventing their
migration on the other side creating a prominent rain
shadow contrast (see Chap. 11). Nearly 80% (20%) of
the annual rainfall occur in the Himalayas due to
southwest (winter) monsoon. The nature of the convective systems varies dramatically from the western
to eastern foothills of Himalayas.
Box 3.2 Precipitation teleconnections with modes
of climate variability
ENSO and IOD
The year-to-year variability of Indian monsoon rainfall
(Fig. 3.1b) is governed by the slowly varying surface
features. El Nino conditions in the Pacific play a major
role in modulating the interannual variability of ISM
rainfall (Sikka 1980, 1977; Pant and Parthasarathy
1981; Rasmusson and Carpenter 1983; Webster et al.
1998). Almost 50% of the droughts are associated with
ENSO (see Chap. 6), however in the last few decades
the ENSO-Monsoon relationship has been weakened
(e.g. Kripalani and Kulkarni 1997; Krishna Kumar et al.
1999), frequency and intensity of droughts have been
increased and some of them are not associated with
ENSO.
The coupled mode in the Indian Ocean (Indian
Ocean Dipole; IOD; Saji et al. 1999) is also known to
modulate interannual variability of ISM rainfall.
A positive relationship between IOD and ISM rainfall
is well known (Ashok et al. 2004; Saji et al. 1999; Saji
and Yamagata 2003). The positive (negative) IOD
significantly dilutes the influence of El Nino (La Nina)
on the Indian monsoon (Ashok et al. 2004; Chowdary
et al. 2015). There are more frequent positive IOD
events in recent decades due to the rapid warming of
the Indian Ocean (e.g. Cai et al. 2018). In addition to
IOD and ENSO, there is a strong link between ISM
rainfall and the equatorial Indian Ocean oscillation
(EQUINOO; Gadgil et al. 2004). In general, positive
phase of the EQUINOO is favourable for a good
monsoon. Association between EQUINOO and ENSO
also determines the variations in ISM rainfall on the
interannual time scale.
Eurasian snow cover
Eurasian snow cover also plays a major role in the
year-to-year variability of ISM rainfall (Blanford
1884). Generally, positive Eurasian snow cover
anomalies during winter and spring tend to be followed by an anomalous deficit rainfall over the Indian
subcontinent in the subsequent summer monsoon
season, while negative snow cover anomalies tend to
be followed by abundant rainfall (Bhanu Kumar 1987;
Bamzai and Kinter 1997). It has been observed that all
non-ENSO related droughts over India have been
50
A. Kulkarni et al.