Results (continue)

Ensemble Mean Surface Pressure

Figure 4 (animation) shows the change of the time-longitude cross section of surface pressure at the equator by increasing the size of ensemble set.

Figure 4: Time-longitude cross section of surface pressure at the equator (hPa). The change is shown as an animation with increasing the size of ensemble set as 1, 2, 4, 8, 16, 32, 64, 128, and 200. The unit of time axis is day with day 0 as the onset day of the warm SST area.

As the increase of the size of ensemble set, there appear the following characteristics. These characteristics become clearer with the increase of the ensemble size.

1 run

One can recognize a low pressure anomaly over the warm SST area (located at 180E). There are eastward propagating disturbances which do not have any relationship with the low pressure anomaly. No feature of east-west asymmetry can be observed.

2 run mean

4 run mean

One can recognize that, high pressure anomaly appears to the west, while low pressure anomaly appears to the east of the warm SST area.

8 run mean

16 run mean

It is clearly recognized that Eastward propagation of low pressure anomaly to the east of the warm SST area.

32 run mean

As the size of ensemble set is increased, the characteristics mentioned above become clearer.

64 run mean

128 run mean

200 run mean

The ensemble mean operation with the size of larger than 32 reproduces the tendency that the low (high) pressure anomaly appears to the east (west) of the warm SST area. This is the characteristics of surface pressure anomaly shown by Hosaka et al. (1998) with the long term mean data. The ensemble mean operation revealed that this characteristics establishes about 30 days after the switch-on of the warm SST area.

The eastward propagation of low pressure anomaly appears just after the switch-on of the warm SST area. On the other hand, the high pressure anomaly to the west of the warm SST area does not seem to establish until 30 days after the switch-on, and it seems to propagate westward after that date. A more detailed analyses will reveal the development process of the high pressure anomaly to the west of the warm SST anomaly.