The Effects of the North Atlantic Oscillation on Northern Hemisphere Precipitation

Gidon Eshel
Dept. of the Geophysical Sciences
The Univ. of Chicago
5734 S. Ellis Ave.
Chicago, IL 60637

Tel: (773) 702-0440
Email: geshel@uchicago.edu
Home Page: http://geosci.uchicago.edu/~gidon

Hi,

I am looking forward very much to meeting you all, and am very excited about the opportunity to visit your class in particular and Williams in general.

Very important: if you have any problem carrying out any part(s) of this assignment, please, please, please, do not hesitate to contact me by phone or email; I will be delighted to help!

The basic idea of this assignment is to familiarize you with one of the myriad effects the North Atlantic Oscillation (NAO) has on Northern Hemisphere (NH) climate, modulation of NH monthly-mean precipitation by the ebb and flow of the NAO.

There are two datasets you will need to download:

• monthly-mean NAO index, given in the file NAO.index. It is a simple text (ASCII) file, containing a single row comprising 249 values, the monthly-mean NAO index beteween January 1979 and September 1999. This index is the difference of normalized sea-level atmospheric pressure between Ponta Delgada, Azores Islands, and Reykjavik, Iceland. You can read more about this index online, e.g., here or here.
• monthly-mean NH precipitation, averaged spatially on a 3 degrees latitude by 3 degrees longitude grid, given in the file NHprecip.tsv. To learn more about this dataset, check out this link. The 120 longitude grid points are located at 1.5, 4.5 ... 355.5, 358.5 degrees (where longitude is positive to the east of the Greenwich meridian, i.e., longitudes between 0 and 180 are east longitudes, while between 180 and 360 they are west ones). The grid's 15 latitudes are all northern latitudes, spanning 21.5, 24.4 ... 60.5, 63.5 degrees north. The file structure is as follows. Each row corresponds to a specific latitude/month combination. Within a given row, data values corresponding to different longitudes are separated by tabs. Data values corresponding to different latitudes are in different rows. Each row has 120 values, for the 120 longitudes starting from 1.5E and going east all the way around the globe until 358.5E, better known as 1.5W... Every group of 15 rows (lines) constitute a single time (a single month), of which there are 249 (so the file has 249x15=3735 rows overall). This file sould be readable by any program that manipulates data, spreadsheets in particular. Just to be absolutely clear,
  • longitude: 1.5E, 4.5E ... 4.5W, 1.5W (120 points)
  • latitude: 21.5N, 24.5N ... 60.5N, 63.5N (15 points)
  • time: January 1979, February 1979 ... August 1999, September 1999 (249 months)

To do:

• compute the simultaneous correlation over all months between the NAO index and the precipitation at each of the grid's nodes. By `simultaneous' here I mean that the NAO index value of each month is aligned with the precipitation index at each one of the grid points at the same month
• plot (contour) the above (simultaneous) correlations at all grid points superimposed on a map of the NH. Make sure the title of the plot defines unambiguously what the plot displays
• repeat the same calculation, only allow for a 2- and 4-month lag between the NAO index and the precipitation value. This should yiled 4 plots: 2 in which the NAO index leads (comes chronologically before) the gridded precipitation values, and 2 in which the NAO index lags behind (comes chronologically after) the gridded precipitation values
• plot (contour) the above 4 correlation fields at all grid points superimposed on a map of the NH