Click to edit Master title styleTimeseries of dynamical and precipitation indices

• Black: observations; RGB: ensemble members for each start date case running left to right.

• Dynamical indices other than DMI show poor predictability.

AIR Webster-Yang DMI

Goswami MHI

Wang-Fan circulation

Click to edit Master title styleMean state representation #1 (precip,SST)

Obs Model

Model

Obs• Indian precipitation pattern ‘reasonable’.

• Clear westward bias in mean state SST (JJA): confined warm pool and extended cold tongue.

Click to edit Master title styleMean state representation #2 (circulation)

Obs

Model

Obs

Model

• Trade winds too strong in the western Pacific in GloSea4.• Mean Walker circulation is not too bad although slight

westward bias (particularly in descending regions).

Click to edit Master title style

• Initially, hindcast precipitation response over India does not look unreasonable.

• However, largest signals are west of the coast and over the Himalaya and may not be reflected in observed AIR.

• As we shall see, response in some individual cases is not ideal.

Regressions onto Niño-3 #1

Click to edit Master title styleRegressions onto Niño-3 #2

Obs Obs

Model Model

• Evidence of westward bias of SST and perturbations to the large scale circulation.

Click to edit Master title style

• 1997 and 2002 chosen owing to their diversity and unusual effects on the monsoon (hence a tough test of the seasonal forecast model).

• 1997: large, east Pacific El Niño, normal monsoon (102% AIR).

• 2002: more moderate, central Pacific El Niño, monsoon drought (81% AIR).

Case studies for 1997 and 2002

Click to edit Master title styleCase studies: 1997 (SST & vp200 anomalies)

Model

Obs

• Warm El Niño anomalies extend ~30° too far west.• Anomalous ascent over warm SST pushes into the Maritime

Continent, shifting anomalous subsidence west over India.

Click to edit Master title styleCase studies: 2002 (SST & vp200 anomalies)

Model

Obs

• Warm El Niño anomalies again extend too far west.• Anomalous subsidence over Indian is shifted too far west, as

far as East Africa.

Click to edit Master title styleCase studies: 2004. More unusual case of El Niño followed rapidly by small La Niña.

Anomalous ascent in East Pacific: responding to La Niña instead of El Niño!

Obs

Model

Click to edit Master title style

Clear but not unexpected improvement in mean state using June start.

1997 case study using different start dates (1 April, 1 May, 1 June)

Click to edit Master title styleSummary

• While dynamical teleconnections are slightly better represented than AIR, this might reflect larger domain used; rainfall teleconnection is still poorly represented.

• Coupled biases are shown in mean state 850hPa wind (easterlies penetrate too far west) & SST (equatorial cold tongue extends too far west), known to be of detriment to teleconnection simulation in coupled models (Turner et al., 2005; Annamalai et al., 2007).

• SST biases develop rapidly in the equatorial Pacific following initialisation.

• Anomalous descent situated too far west in both 1997 and 2002 case studies.

• Future experiments: test sensitivity to mean state in these cases using wind stress corrections.