associated with excessive snow depth over Eurasia
(Kripalani and Kulkarni 1999).
Other than ENSO, the Atlantic, western North
Pacific circulation changes (e.g. Chowdary et al. 2019;
Srinivas et al. 2018) also play a role in monsoon
interannual and decadal variability (Sankar et al. 2016;
Yadav 2017). The variabilities in the rainfall, as well
as the teleconnections of monsoon, could be natural,
but there is an intriguing possibility of global warming
to modulate these variations. It is suggested that the
weakening linkage between ENSO and ISM, despite
the increase in ENSO activity, could be due to global
warming (Krishna Kumar et al. 1999). Also the
anomalous warming over the Eurasian land mass and
enhanced moisture conditions over the Indian region
in a global warming scenario could have contributed to
the weakening of the influence of warm ENSO events
on ISM rainfall (e.g. Ashrit et al. 2001). Moreover, the
warming of the Indian Ocean at a faster rate than the
global oceans (Roxy et al. 2014) has implications on
the variability of rainfall over India, by playing a
major role in the declining trend of ISM rainfall
(Preethi et al. 2017).
Decadal Variations
Variations in ISM rainfall are characterized by distinct
epochs typically of about three decades, of above and
below normal monsoon activity (e.g. Parthasarathy et al.
1991a, b; Kripalani and Kulkarni 1997). The observational, paleo-climatic and simulated datasets show
increased (decreased) ISM rainfall during the positive
(negative) phase of the Atlantic Multidecadal Oscillation (AMO) (e.g. Goswami et al. 2006; Joshi and Rai
2015; Krishnamurthy and Krishnamurthy 2015). The
leading mode of SSTs in the North Pacific Ocean,
Pacific Decadal Oscillation (PDO) with periodicities of
15–25 years and
50–70 years (e.g. Mantua and Hare 2002), could negatively impact ISM rainfall (e.g. Krishnan and sugi
2003; Krishnamurthy and Krishnamurthy 2013). The
high correlation between the inter-decadal component
of variability of ISM with that of Nino-3 SST highlights
the importance of El Nino-Monsoon relationship (Parthasarathy et al. 1994; Kripalani et al. 1997; Kripalani
and Kulkarni 1997; Mehta and Lau 1997; Krishnamurthy and Goswami 2000). This indicates that
low-frequency modulation of summer monsoon could
largely influence rainfall over the Indian subcontinent.
Along with this, a strong multi-decadal variability with
alternate wet (above normal) and dry (below normal)
epochs of monsoon rainfall has been observed in the
instrumental records extending back to 150 years
(Kripalani and Kulkarni 2001; Joseph et al. 2016; Preethi et al. 2017).
Northeast and Winter Monsoon precipitation
While most parts of India receive the major share of the
annual rainfall during southwest/summer monsoon
season (from June to September), southeast peninsular
India falls under the rain shadow region during this
season. During the northeast monsoon season from
October to December (OND), the zone of maximum
rainfall migrates to southern India and the prevailing
winds become northeasterly (Fig. 3.1a; middle panel)
(e.g. Ramaswamy 1972; Dhar and Rakhecha 1983;
Singh and Sontakke 1999; Balachandran et al. 2006;
Rajeevan et al. 2012). The northeast monsoon rainfall
shows strong interannual variability (28%), which is
more than twice the variability of southwest monsoon
rainfall (11%) (e.g. Nageswara Rao 1999; Sreekala
et al. 2011). The normal date of the northeast monsoon
onset is 20th October with a standard deviation of 7–
8 days (Raj 1992).
The relationship between ENSO and northeast
monsoon has been strengthened during 1979–2005
(Kumar et al. 2007) while it is weakened in the decade
of 2001–2010 (Rajeevan et al. 2012). Local air-sea
interaction within the Indian Ocean also modulates the
northeast monsoon rainfall (Yadav 2013). IOD-related
circulation is found to be an important local forcing
mechanism for northeast monsoon (Kripalani and
Kumar 2004).
During winter (December to February), cold air
masses originating from the Siberian High move
southward (Fig. 3.1; bottom panel), and lead to interaction between high northern latitudes and the tropics
(Wang et al. 2003). Western disturbances from the
Mediterranean to Central Asia transport moisture to
the Indian winter monsoon contributing significantly
to annual precipitation in the Himalaya region (Dimri
2013; Dimri et al. 2015).
3 Precipitation Changes in India
51
(Kripalani and Kulkarni 1999).
Other than ENSO, the Atlantic, western North
Pacific circulation changes (e.g. Chowdary et al. 2019;
Srinivas et al. 2018) also play a role in monsoon
interannual and decadal variability (Sankar et al. 2016;
Yadav 2017). The variabilities in the rainfall, as well
as the teleconnections of monsoon, could be natural,
but there is an intriguing possibility of global warming
to modulate these variations. It is suggested that the
weakening linkage between ENSO and ISM, despite
the increase in ENSO activity, could be due to global
warming (Krishna Kumar et al. 1999). Also the
anomalous warming over the Eurasian land mass and
enhanced moisture conditions over the Indian region
in a global warming scenario could have contributed to
the weakening of the influence of warm ENSO events
on ISM rainfall (e.g. Ashrit et al. 2001). Moreover, the
warming of the Indian Ocean at a faster rate than the
global oceans (Roxy et al. 2014) has implications on
the variability of rainfall over India, by playing a
major role in the declining trend of ISM rainfall
(Preethi et al. 2017).
Decadal Variations
Variations in ISM rainfall are characterized by distinct
epochs typically of about three decades, of above and
below normal monsoon activity (e.g. Parthasarathy et al.
1991a, b; Kripalani and Kulkarni 1997). The observational, paleo-climatic and simulated datasets show
increased (decreased) ISM rainfall during the positive
(negative) phase of the Atlantic Multidecadal Oscillation (AMO) (e.g. Goswami et al. 2006; Joshi and Rai
2015; Krishnamurthy and Krishnamurthy 2015). The
leading mode of SSTs in the North Pacific Ocean,
Pacific Decadal Oscillation (PDO) with periodicities of
15–25 years and
50–70 years (e.g. Mantua and Hare 2002), could negatively impact ISM rainfall (e.g. Krishnan and sugi
2003; Krishnamurthy and Krishnamurthy 2013). The
high correlation between the inter-decadal component
of variability of ISM with that of Nino-3 SST highlights
the importance of El Nino-Monsoon relationship (Parthasarathy et al. 1994; Kripalani et al. 1997; Kripalani
and Kulkarni 1997; Mehta and Lau 1997; Krishnamurthy and Goswami 2000). This indicates that
low-frequency modulation of summer monsoon could
largely influence rainfall over the Indian subcontinent.
Along with this, a strong multi-decadal variability with
alternate wet (above normal) and dry (below normal)
epochs of monsoon rainfall has been observed in the
instrumental records extending back to 150 years
(Kripalani and Kulkarni 2001; Joseph et al. 2016; Preethi et al. 2017).
Northeast and Winter Monsoon precipitation
While most parts of India receive the major share of the
annual rainfall during southwest/summer monsoon
season (from June to September), southeast peninsular
India falls under the rain shadow region during this
season. During the northeast monsoon season from
October to December (OND), the zone of maximum
rainfall migrates to southern India and the prevailing
winds become northeasterly (Fig. 3.1a; middle panel)
(e.g. Ramaswamy 1972; Dhar and Rakhecha 1983;
Singh and Sontakke 1999; Balachandran et al. 2006;
Rajeevan et al. 2012). The northeast monsoon rainfall
shows strong interannual variability (28%), which is
more than twice the variability of southwest monsoon
rainfall (11%) (e.g. Nageswara Rao 1999; Sreekala
et al. 2011). The normal date of the northeast monsoon
onset is 20th October with a standard deviation of 7–
8 days (Raj 1992).
The relationship between ENSO and northeast
monsoon has been strengthened during 1979–2005
(Kumar et al. 2007) while it is weakened in the decade
of 2001–2010 (Rajeevan et al. 2012). Local air-sea
interaction within the Indian Ocean also modulates the
northeast monsoon rainfall (Yadav 2013). IOD-related
circulation is found to be an important local forcing
mechanism for northeast monsoon (Kripalani and
Kumar 2004).
During winter (December to February), cold air
masses originating from the Siberian High move
southward (Fig. 3.1; bottom panel), and lead to interaction between high northern latitudes and the tropics
(Wang et al. 2003). Western disturbances from the
Mediterranean to Central Asia transport moisture to
the Indian winter monsoon contributing significantly
to annual precipitation in the Himalaya region (Dimri
2013; Dimri et al. 2015).
3 Precipitation Changes in India
51