thought i'd toss this one up first since i'm watching it against our current weather in the u.s. and the status of the atlantic ridge. i got this info from steve in metairie, la... and he from watching joe bastardi.
SOI is a southern oscillation index.. it's related to pressure anomalies in the south pacific. i'm not really certain of the details, but what i could glean from steve's info is how changes in the index correspond to changes in the global circulation. the timing and amplitude of troughs seems closely linked to changes in SOI index. here's the 30-day index from a site called 'the long paddock' from australia.
Date Tahiti Darwin Daily** 30 day 90 day
Av.SOI Av.SOI
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11-Aug-2003 1015.34 1011.95 10.70 -0.37 -6.25
12-Aug-2003 1015.19 1012.80 4.60 -0.18 -6.24
13-Aug-2003 1015.44 1013.30 3.10 0.13 -6.26
14-Aug-2003 1014.41 1012.45 2.00 0.76 -6.20
15-Aug-2003 1013.34 1011.15 3.40 1.33 -6.22
16-Aug-2003 1013.19 1011.80 -1.40 1.27 -6.33
17-Aug-2003 1013.78 1012.95 -4.80 0.85 -6.32
18-Aug-2003 1013.34 1012.95 -7.40 0.24 -6.19
19-Aug-2003 1013.41 1012.00 -1.20 -0.33 -6.05
20-Aug-2003 1014.91 1010.90 14.50 -0.37 -5.69
21-Aug-2003 1014.98 1011.60 10.60 -0.27 -5.43
22-Aug-2003 1016.10 1012.15 14.10 0.41 -5.12
23-Aug-2003 1015.69 1011.95 12.80 0.82 -4.80
24-Aug-2003 1014.24 1011.45 7.00 0.83 -4.33
25-Aug-2003 1012.45 1012.40 -9.50 0.28 -3.93
26-Aug-2003 1014.19 1013.55 -5.90 0.05 -3.64
27-Aug-2003 1014.33 1013.95 -7.50 0.01 -3.54
28-Aug-2003 1014.91 1015.25 -11.80 -0.23 -3.54
29-Aug-2003 1013.76 1014.10 -11.80 -0.61 -3.50
30-Aug-2003 1013.83 1013.95 -10.50 -0.98 -3.40
31-Aug-2003 1014.00 1013.40 -6.10 -0.69 -3.31
1-Sep-2003 1014.25 1013.35 -8.40 -0.27 -3.41
2-Sep-2003 1014.30 1014.15 -12.90 -0.22 -3.56
3-Sep-2003 1013.59 1014.85 -21.20 -0.58 -3.61
4-Sep-2003 1014.36 1015.50 -20.50 -1.24 -3.65
5-Sep-2003 1015.15 1014.45 -9.60 -1.71 -3.59
6-Sep-2003 1015.41 1013.60 -3.00 -1.80 -3.50
7-Sep-2003 1014.54 1013.40 -7.00 -1.91 -3.39
8-Sep-2003 1015.39 1013.50 -2.50 -2.25 -3.23
9-Sep-2003 1016.20 1012.75 6.70 -2.47 -2.90
the middle number, 6.70, is the daily SOI value. note that it is the first positive value since august 24th.
now, the limited ability i have to interpret it is from what steve told me.. the value correlates to the amount of amplitude of troughing in the eastern united states roughly 15 days later. negative values tend to correspond to a flat, zonal pattern (like in NAO positive), while positive ones correspond to troughing in the east, common to NAO negative.
now, here's what we've had. the troughing that took fabian out corresponded to the positive SOI values from august 20th to 24th (fabian recurved around september 3-6, roughly two weeks later). today there is generally flat ridging across the atlantic.. and we correspond to the negative SOI value of august 26th. note that SOI was negative from august 25th to september 8th.. with a strong signal for much of the period, with a slightly weaker signal in the middle and a slow taper at the end. that weak middle signal was around aug31-sep1.. corresponding to sep 14-15.. the time the shortwave is forecast to weaken the ridge temporarily. since SOI was strongly negative in the following days, i'd suspect the ridge will be flat and established next week.. and quite possibly take isabel all the way westward.
this is a crude method, but these signals do correspond to real weather events.. we can watch how isabel progresses over the next few days and see how much merit it has in this case.
HF 2247z09september