The influence of Indian Ocean SST on tropical East

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					    The influence of Indian Ocean SST on
    tropical East African rainfall: Insights
        from observations and GCMs
                  Emily Black
   Centre for Global Atmospheric Modelling,
             University of Reading
with help from Julia Slingo, Hilary Spencer and Ken Sperber



                                        emily@met.rdg.ac.uk
                                     www.met.rdg.ac.uk/~emily
                      Questions


1. How well can the relationship between East
   African rainfall and the Indian Ocean Dipole be
   simulated by an atmosphere only GCM?
                      Questions


1. How well can the relationship between East
   African rainfall and the Indian Ocean Dipole be
   simulated by an atmosphere only GCM?
2. To what extent is the simulation affected by the
   lack of ocean-atmosphere coupling in an
   AGCM?
                 Talk outline
1. Background
2. The observed relationship between the Indian Ocean
   SST and East African rainfall
3. The simulation of the relationship between the Indian
   Ocean SST and East African rainfall by atmosphere
   only models
      - Integrations forced with GISST data
      - Integrations forced with idealised anomalies
4. The impact of the lack of coupling on the simulation
      - Comparison between CGCM and AGCM forced
        with CGCM SST
                 Talk outline
1. Background
2. The observed relationship between the Indian Ocean
   SST and East African rainfall
3. The simulation of the relationship between the Indian
   Ocean SST and East African rainfall by atmosphere
   only models
      - Integrations forced with GISST data
      - Integrations forced with idealised anomalies
4. The impact of the lack of coupling on the simulation
      - Comparison between CGCM and AGCM forced
        with CGCM SST
                    Background I – the Seasonal Cycle
East African rainfall                                     Indian Ocean SST




                                         Rainfall
                                Month




                                                    difference
                                  Short rains




                                                    SST
                                                                    Month
    Rainfall




                                                            SST difference = WIO - EIO

               Month
                 Short rains
    Background II – the Indian Ocean dipole and
              East African rainfall




                                           Wind field




Zonal wind anomalies   Vector wind field
                Observed relationship between rainfall and
                           Indian Ocean SST
                                  1. Non-dipole years with
                 Nino 3              warm SST in the region of
                                     interest
                                  2. Non-dipole years with
Rainfall (mm)




                 WIO                 cold SST in the region of
                                     interest


                 EIO




                 SST anomaly
                Observed relationship between rainfall and
                           Indian Ocean SST
                                  1. Non-dipole years with
                 Nino 3              warm SST in the region of
                                     interest
                                  2. Non-dipole years with
Rainfall (mm)




                 WIO                 cold SST in the region of
                                     interest
                                  3. Dipole years with warm
                                     SST in the region of
                 EIO                 interest
                                  4. Dipole years with cold
                                     SST in the region of
                                     interest
                 SST anomaly
                 Talk outline
1. Background
2. The observed relationship between the Indian Ocean
   SST and East African rainfall
3. The simulation of the relationship between the Indian
   Ocean SST and East African rainfall by atmosphere
   only models
      - Integrations forced with GISST data
      - Integrations forced with idealised anomalies
4. The impact of the lack of coupling on the simulation
      - Comparison between CGCM and AGCM forced
        with CGCM SST
HadAM3 forced with observed SST


                 Observed rainfall



                 Modelled rainfall



            •the model captures some
            aspects of the rainfall variability
            •BUT the extremely high rainfall
            seen in strong dipole years is not
            replicated
Dipole composite winds – observed and modelled
Observed              Modelled
        Long term mean
                                 Compare modelled and
                                 observed winds:
                                 •observed reduction in
                                 the flow of wind away
                                 from the African coast is
    Dipole composite anomaly     not simulated by the
                                 model
                                 •observed along shore
                                 Sumatran coast
                                 anomalies are not
                                 simulated
Dipole composite winds – observed and modelled
Observed             Modelled
        Long term mean
                                Compare modelled and
                                observed winds:
                                •observed reduction in
                                the flow of wind away
                                from the African coast is
     Dipole composite mean      not simulated by the
                                model
                                •observed along shore
                                Sumatran coast
                                anomalies are not
                                simulated
                 Talk outline
1. Background
2. The observed relationship between the Indian Ocean
   SST and East African rainfall
3. The simulation of the relationship between the Indian
   Ocean SST and East African rainfall by atmosphere
   only models
      - Integrations forced with GISST data
      - Integrations forced with idealised anomalies
4. The impact of the lack of coupling on the simulation
      - Comparison between CGCM and AGCM forced
        with CGCM SST
Forcing HadAM3 with idealised SST anomalies
                                       Model:
                                       •HadAM3 (3.75 x 2.5 lateral
                                       resolution and 19 vertical levels)
                                       •Run in perpetual October mode for
                                       150 days


 Experiments
 1. Western Indian Ocean forcing (anomalies -2, -1.5, -1, -0.5, -0.25, -
    0.1, +0.1, +0.25, +0.5, +1, +1.5, +2)
 2. Eastern Indian Ocean forcing (anomalies –2, -1.5, -0.5, +0.5, +1,
    +1.5, +2)
 3. Symmetrical dipole forcing (DMI –4, -2, -1, +1, +2, +4)
  The effects of Western anomalies, Eastern anomalies
         and the SST gradient on rainfall on land

DMI vs rainfall for all runs
                                                       •Dominant effect is from the
                                                       Western Indian ocean (r=0.91!)
                                                       •Eastern Indian Ocean has no
                                                       discernable effect
WIO SSTA vs rainfall for all runs
                                                       •The only impact of the SST
                                                       gradient (dipole forcing) is
                                                       weakening of the impact WIO
                                                       negative SST anomalies
                               EIO SSTA vs
                               rainfall for all runs
                Observed relationship between rainfall and
                           Indian Ocean SST
                                  1. Non-dipole years with
                 Nino 3              warm SST in the region of
                                     interest
                                  2. Non-dipole years with
Rainfall (mm)




                 WIO                 cold SST in the region of
                                     interest
                                  3. Dipole years with warm
                                     SST in the region of
                 EIO                 interest
                                  4. Dipole years with cold
                                     SST in the region of
                                     interest
                 SST anomaly
                Atmosphere only runs
•Atmosphere only model can simulate some aspects of the
variability of wind and rain within the Indian Ocean basin
BUT
•Atmosphere only model cannot simulate the observed
relationship between the Indian Ocean dipole, low-level wind
within the Indian Ocean basin and extreme East African rainfall
                Atmosphere only runs
•Atmosphere only model can simulate some aspects of the
variability of wind and rain within the Indian Ocean basin
BUT
•Atmosphere only model cannot simulate the observed
relationship between the Indian Ocean dipole, low-level wind
within the Indian Ocean basin and extreme East African rainfall
Question:

Is the lack of coupling between ocean and
atmosphere a significant factor?
       - investigated by comparing coupled model data with an
atmosphere only run forced with coupled model SST….
      HadAM3 forced with HadCM3 SST

 5
                         HadAM3
1
5
                         HadCM3
1
4
                         Difference
-5
  0        10       20
       Model Year
                    Comparison:
                    •in most years the difference is small (and
                    climatology is very similar)
                    •BUT in some years there is a big
                    difference eg year 4…
          HadAM3 forced with HadCM3 SST
             Year 4 (strongest “dipole”)
Atmosphere only              Coupled
          Total wind field
                                       Differences in the wind
                                       fields:
                                       •CGCM has stronger
                                       westerly anomalies over the
            Wind anomaly               whole basin
                                       •AGCM has a more
                                       localised response to the
                                       strongest gradients
         Precipitation anomaly
             Conclusions
1. The observed relationship between the Indian
   Ocean dipole and East African rainfall cannot
   be represented by an atmosphere only GCM
2. A comparison between AGCM and CGCM
   simulations suggests that this may in part be
   due to the AGCM’s lack of ocean-atmosphere
   coupling