Extremes of the Indian Summer Monsoon Rainfall,

1
Extremes of the Indian Summer Monsoon
Rainfall, ENSO and Equatorial Indian Ocean
Oscillation Francis P. A.
Center for Atmospheric and Oceanic
Sciences Indian Institute of Science Bangalore,
INDIA
29 September 2004
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Plan of talk

• Plan of talk
• Evolution of the problem
• Indian Ocean Dipole mode (IOD)
• Interannual variation of Indian Summer Monsoon
  Rainfall (ISMR)
• El Nino and Southern Oscillation (ENSO)
• ENSO and ISMR
• ISMR, ENSO and IOD
• Equatorial Indian Ocean Oscillation (EQUINOO)
• ISMR, ENSO and EQUINOO
• Conclusions

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Evolution of the problem
Northward propagations in the Tropical
Convergence zone
Tropical Convergence zone
Offshore convective system
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Evolution of the problem
Correlation between west coast rainfall and ISMR
Northward propagations in the Tropical
Convergence zone
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Evolution of the problem

• On many days in June and July 2003, there were
  deep clouds and intense rainfall all along the
  west coast of India.
• Initially it was attributed to the offshore
  troughs, which is quite often at this time of
  the year.
• However, detailed analysis of the satellite
  images showed that these could systems are not
  isolated, but linked to very intense convection
  over the western parts of equatorial Indian
  Ocean.
• Further analysis suggested that while the western
  parts of equatorial Indian Ocean was convectively
  very active, convection over the eastern parts of
  equatorial Indian Ocean was highly
  suppressed.
• Similar cases were noticed in 1994, 1997 and
  1982.
• Interestingly, all the above years were positive
  phases of much talked INDIAN OCEAN DIPOLE (IOD)
  MODE (Saji et al, Nature, 1999 Webster et al,
  Nature, 1999).
• This lead to the conclusion that a positive IOD
  was developing.

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Evolution of the problem
Daily GPI rainfall (20030615)mm/day
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Evolution of the problem
METEOSAT-5 IR image at 1200UTC on 15th June, 2003
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Evolution of the problem
OLR anomaly patterns for July 2003 and 1994
A positive IOD in 2003?
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Indian Ocean Dipole Mode

• Positive IOD events are characterized by
• Warm (cool) SST in the western (eastern) parts of
  Indian Ocean
• Enhanced (reduced) convection over the western
  (eastern) parts of Indian Ocean and
• Anomalous surface easterlies along the central
  equatorial Indian Ocean

• Negative IOD events are characterized by
• Warm (cool) SST in the eastern (western) parts of
  Indian Ocean
• Enhanced (reduced) convection over the eastern
  (western) parts of Indian Ocean and
• Anomalous surface westerlies along the central
  equatorial Indian Ocean

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Indian Ocean Dipole Mode
July-August mean OLR and surface wind anomalies
for 1994
July-August mean SST anomaly during 1994
WEIO (50-70E, 10S-10N)
EEIO (90-110E, 10S-EQU)
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Indian Ocean Dipole Mode
Time series of dipole mode index (DMI) along with
Nino 3 index and equatorial wind index.
Dipole mode index (DMI) is defined as the
normalized difference in the SST anomaly between
WEIO and EEIO Based on the DMI, major positive
IOD events during 1958-1999 are 1961, 1967, 1972,
1982, 1994 and 1997
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Indian Ocean Dipole Mode
Composite anomalies of SST, wind and
precipitation during different seasons in the
evolution of positive IOD events.
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Indian Ocean Dipole Mode
Correlation between DMI and CMAP precipitation

• Good correlation in the equatorial Indian Ocean
  but very poor correlation over Indian region!
• However, Ashok et al (GRL, 2001) argued that
  excess rainfall in 1994 and normal rainfall in
  1997, despite of the occurrence of the strongest
  El Nino of the century, are due to the positive
  dipole events in the Indian Ocean.

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Interannual variation of ISMR
Interannual Variation of the anomaly of
All-India summer monsoon rainfall (as of the
mean)
std dev about 10 of mean
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Interannual variation of ISMR
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Interannual variation of ISMR
Northward propagation of the convective systems
which form over the warm equatorial Indian Ocean
onto the Indian subcontinent and culminate around
20-25oN in the interval of 2-6 weeks is one of
the prominent features of the intraseasonal
variation of Indian summer monsoon
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Interannual variation of ISMR
Lows/ depressions which form over the warm
waters of Bay of Bengal and move towards the
subcontinent also modulate the monsoon rainfall
variability
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Equatorial Indian Ocean Oscillation
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Monsoon and ENSO
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El Nino and Southern Oscillation Introduction

• For many years, coastal residents of Peru had
  noticed a strange feature of the eastern Pacific
  Ocean waters that border their home.
• This region of tropical yet relatively cool water
  is host to one of the world's most productive
  fisheries and a large bird population.
• In the first months of each year, a warm
  southward current usually modified the cool
  waters.
• But every few years, this warming started early
  (in December), was far stronger, and lasted as
  long as a year or two.
• Torrential rains fell on the arid land as one
  early observer put it, "the desert becomes a
  garden."
• Warm waters flowing south brought water snakes,
  bananas, and coconuts from equatorial rain
  forests.
• However, the same current shut off the deeper,
  cooler waters that are crucial to sustaining the
  region's marine life and drastic reduction in
  fish catch.

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El Nino and Southern Oscillation Introduction

• This is El Niño, "the Christ child," so named
  because of its frequent late December appearance.
  
• Once thought to affect only a narrow strip of
  water off Peru, it is now recognized as a
  large-scale oceanic warming that affects most of
  the tropical Pacific.
• The meteorological effects related to El Niño and
  its counterpart, La Niña (a cooling of the
  eastern tropical Pacific), extend throughout the
  Pacific Rim to eastern Africa and beyond.
• El Niño is normally accompanied by a change in
  atmospheric circulation called the Southern
  Oscillation.
• Together, the ENSO (El Niño - Southern
  Oscillation) phenomenon is one of the main
  sources of interannual variability in weather and
  climate around the world. For example, it
  produces flooding in north western America and
  forest fires in Indonesia.

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El Nino and Southern Oscillation Introduction
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El Nino and Southern Oscillation Observation
NORMAL CONDITION
El- NINO CONDITION
LA-NINA CONDITION
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El Nino and Southern Oscillation Observation-TAO
Project
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El Nino and Southern Oscillation Introduction
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Monsoon and ENSO
NINO 3 Index and Southern Oscillation index
Major El Nino events since 1960 are 1972, 1982,
1987, 1991-1993, and 1997 Major La Nina Events
since 1960 are 1975, 1988 and 1998
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Monsoon and ENSO
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Monsoon and ENSO
Interannual Variation of the anomaly of
All-India summer monsoon rainfall (as of the
mean)
Excess rainfall years such as 1961, 1970, 1983
and 1994 were not La Nina Droughts of 1951,
1965, 1968, 1979, 1985 and 1986 were not El Nino
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Monsoon and ENSO
21 year moving correlation between ISMR and Nino3
index
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Monsoon, ENSO and IOD
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Monsoon 2002 and 2003

• MONSOON 2002
• Hence, the failure of the monsoon in 2002 was
  not anticipated, even though it was known that a
  weak El Nino was developing. This drought was
  not predicted either by empirical models or GCMs.
• From the experience of 1997 and 2002 it is clear
  that we are yet to understand completely the
  impact of El Nino on the monsoon.
• COMPARISON WITH 2003
• Fortunately, the monsoon of 2003 has turned out
  to be far better (all India monsoon rainfall 2
  above average). In particular, whereas there was
  an unprecedented deficit of 49 in all-India
  rainfall, in July 2003 there was excess of 7.
  Comparison of the OLR anomaly patterns for July
  2002,2003 is revealing.

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Equatorial Indian Ocean Oscillation
Jul 2002
Jul 1997
OLR and surface wind anomaly patterns
Aug 1986
Jul 1994
Correlation patterns rel. to equatorial Indian
Ocean
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Equatorial Indian Ocean Oscillation
OLR anomaly patterns
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Equatorial Indian Ocean Oscillation
Time-Longitude diagrams of surface zonal wind
averaged over 2.5oS-2.5oN
EQWIN Negative of the surface zonal wind anomaly
averaged over 60oE-90oE, 2.5oS-2.5oN, normalized
by standard deviation is used an index for EQUINOO
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Equatorial Indian Ocean Oscillation

• LINK TO EQUATORIAL INDIAN OCEAN
• Convection over the eastern Arabian Sea and
  western parts of Indian subcontinent is
• Positively correlated to western parts of
  equatorial Indian Ocean (WEIO 50-70E,10S-10N)
  and
• Negatively correlated to the eastern parts of the
  equatorial Indian Ocean (EEIO90-110E, 0-10S)

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ENSO and Indian Monsoon
El Nino
La Nina
OLR anomaly patterns
ENSO index We use negative of Nino 3.4 SST
(170oW-120oW 5oS-5oN) anomaly, normalized by
standard deviation as an index of ENSO
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ISMR, ENSO and EQUINOO
ISMR anomaly (normalized by std.), EQWIN and ENSO
index for extreme ISMR years (shown the ascending
order of ISMR)
All the extreme ISMR anomalies can be understood
in terms of favourable/unfavourable phases of
either ENSO or EQUINOO or both.
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ISMR, ENSO and EQUINOO
ISMR anomalies in the phase plane of EQWIN and
ENSO index
-0.25 lt ISMRA lt 0.25 -1.0 lt ISMRA lt -0.25 0.25 lt
ISMRA lt 1.0 -1.5 lt ISMRA lt -1.0 1.0 lt ISMRA lt
1.5 ISMRA lt -1.5 ISMRA gt 1.5
Composite index is a linear combination of EQWIN
and ENSO index
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ISMR, ENSO and EQUINOO

• ISMR, ENSO and EQUINOO
• The most striking feature of the distribution of
  extreme years is the clear separation between the
  years with excess and deficit with the surplus
  years located above a certain line L in the
  phase plane of EQWIN and ENSO index and deficit
  years below the line.
• This suggests that the appropriate index would be
  a composite index, which is a linear combination
  of the ENSO index and EQWIN.
• To study the relation between the composite index
  and ISMR, we use order statistics, i.e.we order
  years according to the composite index. We
  consider all the possible pairs of years, each
  pair comprising one year with a large excess and
  one with a large deficit. Then we find out the
  fraction of such pairs in which the drought years
  has a lower composite index than the years with
  surplus.
• We see that, each of the 11 drought years have
  lower composite index than each of the 7 surplus
  years.The probability of this happening by pure
  chance (i.e., when ISMR is unrelated to composite
  index) is 7!11!/18! ie., 0.00134, which is
  exteremly small!

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ISMR, ENSO and EQUINOO

• ISMR, ENSO and EQUINOO
• If we consider the relationship of the extreme
  years to ENSO index (or EQWIN) alone, we find
  that of the possible 77 pairs, 11 (14) pairs have
  deficit years with larger values of the index
  than that for the excess years.
• Hence, the relationship of extreme ISMR to the
  composite index is far stronger than that to the
  index of either the ENSO or the EQUINOO.
• We note that the coefficients of both the indices
  in the composite index is almost equal so that
  the influence of EQUINOO on ISMR is as strong as
  that of ENSO
• Of the 63 pairs of years having small deficits
  and small excess (magnitude between 0.25 and 1
  std.) ISMR, with one surplus and deficit, in 30
  cases, surplus year has smaller composite index
  that the corresponding deficit year.
• The same holds for the relationship to separate
  ENSO index or EQWIN.
• Thus it appears that there is no relationship
  between ISMR anomalies and either the ENSO index
  ore EQWIN or the composite index, when the
  variation of ISMR is within one standard
  deviation.

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ISMR, ENSO and EQUINOO

• CONCLUSIONS
• The east west asymmetry in the convection
  anomaly in the eastern and western equatorial
  Indian Ocean and associated anomalies in central
  equatorial Indian Ocean, which is the atmospheric
  component of Indian Ocean dipole mode is termed
  as equatorial Indian Ocean oscillation (EQUINOO).
• There is a very strong relation between large
  excess/deficit in ISMR and a composite index
  based on indices of ENSO and EQUINOO.
• Events of exceptionally large deficits/excess
  occur when the amplitude of composite index is
  large and when the two modes act in phase so that
  they reinforce one another.
• The relationship between composite index and ISMR
  anomaly with in (/- 1) std. but above (/-) 0.25
  std, if any is very weak.

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References

• SOME RELEVENT REFERENCES
• Gadgil, S., P. N. Vinayachandran, P. A.Francis,
  and S. Gadgil (2004), Extremes of the Indian
  summer monsoon rainfall, ENSO and equatorial
  Indian Ocean oscillation, Geophys. Res. Lett.,
  31, L12213, doi10.1029/2004GL019733.
• Gadgil, S., P. N. Vinayachandran, and P. A.
  Francis (2003), Droughts of Indian summer
  monsoon Role of clouds over the Indian Ocean,
  Curr. Sci., 85, 1713 1719.
• Saji, N. H., B. N. Goswami, P. N. Vinayachandran,
  and T. Yamagata (1999), A dipole mode in the
  tropical Indian Ocean, Nature, 401, 360 363.
• Vinayachandran, P. N., S. Iizuka, and T. Yamagata
  (2002), Indian Ocean dipole mode events in an
  ocean general circulation model, Deep Sea Res.,
  Part II, 49, 1573 1596.
• Kumar, K. K., B. Rajagopalan, and M. A. Cane
  (1999), On the weakening relationship between the
  Indian monsoon and ENSO, Science, 284, 2156
  2159.
• Webster, P. J., A. M. Moore, J. P. Loschnigg, and
  R. R. Leben (1999), Coupled oceanic-atmospheric
  dynamics in the Indian Ocean during 1997 98,
  Nature, 401, 356 360.

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