Lab 8 – The Geostrophic Wind and ENSO
All questions subsets are worth 5 points.
1.) Recall that the Coriolis force can be computed as where V is the wind speed (in ms-1), is the earth’s angular rotation rate about its axis, and is the latitude.
On an isobaric map, the pressure gradient force is computed as where g is the gravitational constant of 9.8 ms-2, Z is altitude, is the difference between two altitude lines, and d is the distance between the altitude lines.

The geostrophic wind is balance between CF and PGF. To avoid sign confusion, and since wind is always a positive value, we’ll use absolute values. Solving for V gives where represents the geostrophic wind.
Consider this 300-mb map from Nov 1, 0000 UTC. The fastest winds are surrounded by shaded isotachs. Label the polar jet with PJ and the subtropical jet with SJ. (5 pts)
If one zooms into Washington
Compute the geostrophic wind. Assume the latitude is 48.2486ºN, m, and d = 175000 m.
Convert this value to knots, where 1 m s-1 = 1.94 kts. How well does the observation match the calculation?
Draw two arrows depicting the balance between the Coriolis force and pressure gradient force on the map in 2b.
If this situation were in a trough, the observed wind will be than the geostrophic wind.
2.) The Troup SOI is used by the Australian Bureau of Meteorology to assess ENSO conditions. The equation is:
= (average Tahiti MSLP for the month) –
= long-term average of for the month in question
= long-term standard deviation of for the month in question
Tahiti’s average pressure is usually larger than Darwin’s average pressure, and the differences also vary monthly.
Hence, it’s the monthly pressure anomaly differences, normalized by their monthly PDIFF standard deviation, which best measures the Southern Oscillation. The multiplication by 10 simply provides a more relatable value range.
Using this convention, the Troup SOI ranges from about –35 to about +35. It is often rounded to a whole number. Sustained negative values of the Troup SOI < −8 indicate an El Niño episode. Sustained positive values of the Troup SOI > +7 indicate a La Niña episode. Sustained values between -8 and +7 indicate “neutral” ENSO conditions.
Column B in the assignment spreadsheet contains for January from 1866 to 2022. Years are listed in Column A.
What is for January? Show the answer here and the uploaded spreadsheet computation in Cell E3.
What is for January? Show the answer here and the uploaded spreadsheet computation in Cell E4.
in January 2021 was 7.9 mb. Show the Troup SOI calculation below.
In January 2021, was there an El Niño episode, La Niña episode, or a neutral ENSO condition? Assume this value’s magnitude has been sustained for a few months.
Read https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf . What is February 2023’s ONI? Is there currently an El Niño episode, La Niña episode, or a neutral ENSO condition?