Analyzing The Interaction Between Mean Monthly SSTs and Winds off the Coast of West Africa

1. Analyzing The Interaction Between Mean Monthly SSTs and Winds off the Coast of West Africa Holly A. Anderson Linden Wolf Physics of Air-Sea Interaction Dr. Mark Bourassa December 6, 2007

2. Objectives • Investigate the impact the curl of the wind stress has on SST in terms of upwelling and downwelling through Ekman pumping. • Conduct a qualitative, climatological look into Ekman pumping and the related changes in SST in the sub-equatorial region off west Africa.

3. Relevance • Upwelling of cooler, nutrient-rich oceanic water leads to increased biomass in the euphotic zone. • Since increased biomass means increased living organisms, upwelling is a vital process for the success of the fishing industry. • Upwelling and downwelling also has significant effects on the oceanic energy budget and oceanic circulation through its transportive effects.

4. Wind-Induced Vertical Motion • The Coriolis force deflects wind to the left in the Southern Hemisphere. • Therefore, cyclonic (anticyclonic) motion will result in mass divergence (convergence), and upwelling (downwelling) will occur. We would expect cooling (warming) of SSTs to occur in this situation. Upwelling and therefore cooling of SSTs would occur.

5. Ekman Pumping Vertical velocity in the Ekman layer is proportional to the curl of the wind stress. If Ekman transport causes divergence, there must be upwelling to replace the negative outflow. In the Southern Hemisphere, negative (positive) curl will correspond to positive (negative) vertical velocities, or upwelling (downwelling) of cooler (warmer) SSTs.

6. Data • QuikScat/SeaWinds Scatterometer Psuedostress Fields • Global • 0.5 x 0.5 degree grid • Monthly • Converted into equivalent neutral wind speed • 10m reference height • QuikSCAT/NCEP Blended Curl of the Wind Stress • Global • 0.5 x 0.5 degree grid • 6 hour time steps • 10m reference height • NCDC Extended Reconstructed Sea Surface Temperatures • Global • 2 x 2 degree grid • Monthly

7. Methodology • QuikScat/SeaWinds Psuedostress Fields • QSCAT/NCEP Blended Curl of the Wind Stress • NCDC Extended Reconstructed SST • These three fields were averaged over August 1999-June 2006 to provide a “pseudo-climatology” of winds, stresses, and SST over an area west of the continent of Africa for the 7 years that all three sources of data were available. • The mean monthly fields were then plotted on a domain from 10 N and 40 S and from 25 W and 35 E. This area is off the west coast of central and south Africa. • For this project, it is assumed that there is no surface current. That is, the change in SST is related to the change in the curl of the wind stress alone. • Density is also assumed to be constant through seasons, as it relates to the Ekman equation.

9. Equatorial Cold Tongue • The equatorial cold tongue (ECT) arises in the Southern Hemisphere winter when a zonal westward wind over the equator leads to mass vertical transport of water through upwelling. • Winds north of the equator are deflected northward and winds south of the equator are deflected southward. Upwelling of cooler water is needed to replace the resulting outflow. • The ECT obtains maximum amplitude during the Southern Hemisphere winter and surrounding months – June, July, August, and September.

18. Area 1 10W, 2S to 8W, 2N Area 2 4W, 27S to 0, 23S

19. Area 1 10W, 2S to 8W, 2N

20. SSTs and the curl of the wind stress both have a cyclical trend. • The SSTs closely follow that of the curl of the wind stress. • This is exactly as expected. In Area 1, we see that negative values of the curl of the wind stress are related to cooling SSTs (upwelling). Likewise, positive values of the curl of the wind stress are related to warming SSTs (downwelling).

23. SST Change Prediction in Area 1 • Changes in the magnitude of the curl of the wind stress do appear to be related to the magnitude change in SSTs, although not perfectly. • The low correlation may be due to sampling errors, as well as density differences during different parts of the year.

24. Area 2 4W, 27S to 0, 23S

25. In Area 2, we again see cyclical trends in SSTs. Unfortunately, we see little correlation between positive and negative values of the curl of the wind stress related to warming (downwelling) and cooling (upwelling) of SSTs, respectively.

28. SST Change Prediction in Area 2 • Whereas Area 1 was relatively well correlated, Area 2 is disappointing. The magnitude change in the curl of the wind stress does not appear to be well correlated to the magnitude change in SST whatsoever. • What could affect this? • This could be due to poor sampling of mid-latitude cyclones in the area, or due to eddy circulations in the area that spin off of South Africa. • Also, it could be due to oceanic currents.

29. Ocean Currents off South Africa Area 2 is right above the Benguela Current, known for upwelling cold water. http://www-das.uwyo.edu/~geerts/cwx/notes/chap11/safrica.html

30. Conclusions • Calculating a monthly mean “psuedo-climatology” for the oceanic region west of Africa shows the presence of an equatorial cold tongue during winter months, as well as the monthly influence of Ekman pumping due to changes in the curl of the wind stress on SSTs. • The prediction of SST change for more northern latitudes in this domain is decent. • The relation between the magnitude change of both curl of the wind stress and SST is less obvious for latitudes farther south. This may be due to underlying oceanic currents in the South Atlantic.