iogoos: november 4-9, 2002 peter j. webster: georgia institute of technology variability of the...
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IOGOOS: November 4-9, 2002 IOGOOS: November 4-9, 2002
peter j. webster: georgia institute of technology
Variability of the
Indian Ocean monsoonsystem on timescales
of weeks to years
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Outline:
Brief description of coupled phenomena
•Weather•Intraseasonal varaibility•Interannual variability
•Regulation•Predictability
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resource
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and hazard………
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description
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mean august and january precipitation
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Annual cycle of convection (OLR) in the Indian Ocean-South Asia region
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annual cycle of surface winds over Indian Ocean and South Asia
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Monsoon regions are the largest region where precipitation exceeds evaporation.
This excess water must come from somewhere …
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…….which defines the scale of the monsoon.The monsoon is intrinsically inter-hemispheric with the
winter hemisphere being the source of moisture
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Monsoon tied to both zonal & meridional land-sea contrastsMonsoon tied to both zonal & meridional land-sea contrasts
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To a large degree the system seems“well behaved” and appears to actas a large scale sea-breeze system.
But there are surprises and issuesthat are difficult to understand
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Time Scales of Monsoon Variability
• Interannual variability: Variations on the annual cycle of the monsoon generated by SST/land surface variations in PO and IO.
• Monsoon Weather: Monsoon lows and depressions, tropical cyclones (hurricanes) and etc. Produce short-lived local flooding (or drought), erosion, high winds and etc.
• Intraseasonal Variability: “Envelopes”: or clusters of weather events producing 20-40 day droughts or flood periods. Arguably most important but also most difficult to forecast
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Persistence of SST over 6-month periodPersistence of SST over 6-month period
North Indian Ocean shows strong persistence (I.e., if you know theSST now, you know it 6 months from now). Much more persistent
than in Pacific Ocean which contains springtime “persistence barrier:Inference: predictability exists in Indian Ocean.
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El Nino influences Indian Ocean Changes SST, sea surface slope and regions of maximum precipitation
El Nino influences Indian Ocean Changes SST, sea surface slope and regions of maximum precipitation
But we are uncertain how this influence is manifestedand large amounts of variance are unexplained
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Relationship Between Indian Monsoon Rainfall and ENSORelationship Between Indian Monsoon Rainfall and ENSO
El Nino
La Nina
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Decadal Changes in Monsoon-ENSO RelationshipsDecadal Changes in Monsoon-ENSO Relationships
Correlations over 125 years average about -0.6 but there are long periods where the correlation drops tovery small numbers.
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also………..also………..
Four near normal years show very different precipitation patterns. ENSO offers only “broad-brush” prediction of
rainfall over India. Does rainfall over India define a strong or weak monsoon?
Are fine “fine brush” forecasts possible?
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Other modes of variability?Other modes of variability?
• are there other modes of variability besidesEl Nino?
• Is there predictability in the these modes?• Are these phenmena independent of El Nino?• Is there a basic building block of variability in
the Indian Ocean/monsoon regime?• Can understanding these phenomena help design n observing array in the Indian Ocean?
• are there other modes of variability besidesEl Nino?
• Is there predictability in the these modes?• Are these phenmena independent of El Nino?• Is there a basic building block of variability in
the Indian Ocean/monsoon regime?• Can understanding these phenomena help design n observing array in the Indian Ocean?
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Indian Ocean SST and All-India Rainfall
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Indian Ocean Zonal Mode or Dipole
Indian Ocean Zonal Mode or Dipole
• “Recently” discovered mode of variability following 1997-1998 period (earlier evidence exists, though)
• Exhibits strong coupled characteristics between ocean and atmosphere
• Appears linked to ENSO and to vigor of monsoon• Strongly tied to annual cycle as is El Nino in Pacific
Ocean• Strongly tied to climate variability especially equinoctial
“short rains” in East Africa• Vigorous for decades and occasionally takes a statistical
vacation
• “Recently” discovered mode of variability following 1997-1998 period (earlier evidence exists, though)
• Exhibits strong coupled characteristics between ocean and atmosphere
• Appears linked to ENSO and to vigor of monsoon• Strongly tied to annual cycle as is El Nino in Pacific
Ocean• Strongly tied to climate variability especially equinoctial
“short rains” in East Africa• Vigorous for decades and occasionally takes a statistical
vacation
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most unusual events in 1997
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Coupled mechanism for the Indian Ocean Zonal Mode
Webster et al. (1999)
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Composite growth of the positive phase of IOZM (i)
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Composite growth of the positive phase of IOZM (ii)
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Composite growth of the negative phase of IOZM (i)
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Composite growth of the negative phase of IOZM (ii)
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Indian Ocean Zonal Mode: Inherent mode of Indian Ocean invoked by outside
perturbations including ENSO
Indian Ocean Zonal Mode: Inherent mode of Indian Ocean invoked by outside
perturbations including ENSO
Time series of E-W SST gradient
frequencyspectra
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Sea-level variations associated with IOZM:Positive phase: warm SST WIO, low SSH EIO
Negative phase: warm SST EIO, high SSH EIO
Sea-level variations associated with IOZM:Positive phase: warm SST WIO, low SSH EIO
Negative phase: warm SST EIO, high SSH EIO
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Intraseasonal variability in Indian Ocean
Intraseasonal variability in Indian Ocean
• Predominant feature of IO system• Arguably the most important time scale for
prediction (variability on 20-40 day scales far larger than interannual
• Possess features very similar to interannual variability
• Shows great robustness in behavior, effect and duration
• A chaotic instability? Predictability? • Is intraseasonal variability a coupled ocean-
atmosphere phenomenon?
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Intraseasonal Modes: Impact on RainfallIntraseasonal Modes: Impact on Rainfall
• Intraseasonal variability imposes a distinct form to precipitation.
• Histograms of precipitation, shown for 9 years show distinct wet periods with lulls in between.
• Lower diagram shows the distinct spatial character
Precipitation histograms in central India
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Pattern of Intraseasonal ModesPattern of Intraseasonal Modes
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Temporal evolution of MISO along 90E: 1995Temporal evolution of MISO along 90E: 1995
Active phases of the monsoon commence near the equator andpropagate northward (and southward) across South Asia.
commenceactive phase
northward propagation
Active phase
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Latitude-time section of OLR along 90E: 1988Latitude-time section of OLR along 90E: 1988
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Sequence of Monsoon Intraseasonal Variability
Sequence shows the evolution of the “average” MISOover a 30-day period
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Composite wind stress during intraseasonal oscillation
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0°
10ºN
20ºN
20ºS
10ºS
Composite zonally & vertically heat flux during intraseasonal oscillation
0-5-10-15 +5 +10 +15
day
lati
tud
e
-0.7PW+0.5PW
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Can we use composite structures to forecast ISO?
Can we use composite structures to forecast ISO?
• Intraseasonal oscillations almost absent from numerical models, coupled or otherwise.
• Empirical forecasts possible if ISO is resilient and reproducible
• Following scheme uses 6 features of ISO and makes 25 day forecasts
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(pentads)
Example of 25 day empirical forecast for 1999Ganges Valley precipitation
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Ganges Valley 25-day forecasts 1991-1994
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Ganges Valley 25-day forecasts 1995-1998
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The coupled ocean-atmosphere system in the
Indian Ocean
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Heat budget of the North Indian Ocean
• Heat budget comprises of change in heat storage, flux into system surface and transport across equator• Annual mean small southward transport (-0.25 PW)• Annual cycle between -2PW and -2 PW, opposite to atmospheric heat flux and wind• Heat transferred by ocean from summer to winter hemisphere indicating regulation o by negative feedbacks
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Heat Storage Components
• Change in heat storage can be broken down into two parts due to: changes in temp (H.dT/dt) changes in thickness (T dH/dt)
•Through these two processes we can understand the actual physics that determine the annual cycle of upper ocean temperature
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Interannual variability of ocean heat transport
• There is a large interannual variability in IO ocean transport• Also, large intraseasonal variability associated with atmospheric component of ISO• Note that heat transport is seamless at equator. Can Ekman transport account for that? Probably not!
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Differences between IO meridional heat flux as function of ENSO
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Differences between IO meridional heat flux as function of monsoon strength
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Details of heat transport at equator and 10S
Wave and boundary heat transport? Ekman and boundary transport?
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Cross-equatorial heat transport for 1987 and 1988
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Ocean transport and monsoon strength Ocean transport and monsoon strength
weak monsoon strong monsoon
weak southward ocean heat transport
strong southwardheat transport
anom warms NIO anom cools NIO
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Strong and weak monsoons invoke different upwelling effects creating SST gradients. Then …….
Impact of anomalous monsoon seasons
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Some important points
• The monsoon is a coupled system• The system possesses great predictability• Probabilistic forecasts on all time scales
are possible• But, there is insufficient data for initialization• All ideas presented here are fanciful and will
remain so until substantiated with solid data