### Convert this value to knots, where 1 m s-1 = 1.94 kts. How well does the observation match the calculation?

Environmental Science

**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 ( more less ) 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) – (average Darwin 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?