The Indian Ocean dipole is a phenomenon that occurs in the Indian Ocean region and impacts weather patterns in surrounding areas like Australia and India. It involves changes in sea surface temperatures between the western and eastern parts of the tropical Indian Ocean. During a positive dipole phase, sea surface temperatures are cooler in the west and warmer in the east, while a negative dipole phase has the opposite temperature pattern. This impacts wind and rainfall patterns in Australia and monsoon patterns in India. The dipole can occur in neutral, positive, or negative phases and typically develops between May and October.

1. Indian ocean dipole is a situation
that occurs in Indian oceanic region. This
phenomenon is leads to change weather
patterns of countries which are closer to
Indian ocean. Mainly this can influence
weather in Australia & India. It is one of
the key drivers of Australia's climate and
can have a significant impact on
agriculture and other activities in Australia.
Indian monsoon also affected by IOD.
Changes of different sea surface
temperature (SST) of the tropical western
and eastern Indian Oceanic zones is the
main process that drive this IOD situation (Figure 1). This situation is similar to the El Nino and La
Nina conditions that occurs in Pacific Ocean. The IOD has three phases as Neutral, Positive and
Negative. These events are usually starting around May or June, higher between August and October
and then again rapidly decrease when the monsoon arrives in the southern hemisphere at the end of
spring.
This was first identified with SST anomaly and south Africa rainfall anomaly and both observations and
model simulations. when Sea surface temperature (SST) of One horizontal end of the Indian ocean is
cooling more than other end and this leads to increase evaporation of water in higher SST region, leading
to make low pressure zone and air is sinking in the other end where the cooler SST and also high-
pressure zone. This will lead air transport through sea surface from high pressure to low pressure.
Ascended warmer air will cool down by raining in higher SST regions and will travel to lower SST
region by higher atmosphere. The other end has very sunny, cooler environment with or without less
clouds and rain. This is how this phenomenon is occurring. There are three phases of IOD,
Slightly higher SST in eastern
region because water flows from Pacific
Oceanic region and flows from islands of
Indonesia. Air is ascended above this area
and falls to western half of the ocean and
descended (Figure 2). This SST are closer
to normal and this phase of IOD has little
impact on weather conditions.
1994 1996 1997 1998 2006 2011 2010 2012 2014 2015 2016
Figure 1 West & East regions of Indian ocean
Figure 2 Diagram of positive IOD phase.
IOD – INDIAN OCEAN DIPOLE
Neutral phase
Negative IOD Positive IODHistory of Indian Ocean Dipole (Ref: http://www.bom.gov.au/climate/iod/ )

2. Negative phase
Western wind will drive cooler water in
western region and hot water in eastern
region and this will perform higher
evaporation rates and more clouds,
more precipitations. Air is ascended
above this higher SST area and falls to
western end of the ocean and descended
(Figure 3). This will impact on Australia
climate and increase rain. winds
become more westerly, bringing
increased cloudiness to Australia's
northwest and more rainfall in the Top
End and southern Australia. also impact
on Indian monsoon negatively.
positive phase
Western wind was weakened and
Eastern wind became strong and it drives
cooler water in western region and hot
water in estern region and this will perform
higher evaporation rates and more clouds,
more precipitations in western region. Air
is ascended above this higher SST area and
falls to eastern end of the ocean and
descended (Figure 4). This will impact on
Australia weather system coming from
Australia west, also resulting in less rainfall
and higher than normal temperatures over
parts of Australia during winter and spring
(draught like situation & less rainfall over southern Australia and the Top End.). Positive IOD condition
is stimulating Indian oceanic monsoon and it will cause for precipitation to India. It is beneficial for
Agriculture also impact on Indian monsoon.
This positive or Negative IOD
effects will occur approximately
every 3-5 years. If Positive IOD
correlated with the El Nino effect.
This will lead a very arid condition in
the Eastern part of the Indian ocean
(Australia). Also, if Negative IOD
correlated with La Nina effect. most
parts of the Australia will be with
heavy rains and storms will result
flooding and failure of Agriculture
works.
Figure 3 Diagram of negative IOD phase.
Figure 4 Diagram of positive IOD phase.
Figure 5 SST anomalies with map of Indian oceanic zone
Positive phase
Negative phase
Ref: Saji_etal1999, N. H. Saji, B. N. Goswami, P. N. Vinayachandran & T. Yamagata, A dipole mode in the tropical Indian Ocean (1999).