WIND FORCING OF THE ATLANTIC THERMOCLINE ALONG 32-DEGREES-N AT LOW-FREQUENCIES

The Bermuda tide gauge record extends back to the early 1930s. That se a level fluctuations there are highly coherent with dynamic height fro m hydrographic data has two interesting implications. First, it should contain information about the low-frequency circulation of the Atlant ic. Furthermore, because dynamic height contains information on heat s torage, it might, on the limited timescales accessible in the record, also contain clues about climate. A simple model of wind forcing of th e Atlantic from the African coast to Bermuda uses the Levitus mean den sity data to estimate the long Rossby wave speed as a function of long itude. Sea Level and thermocline variability estimated this way are in remarkably good agreement with observations at periods of more than a few years duration. The peak-to-peak sea level signal is similar to 1 8 cm, which is nearly 25% of the slope across the Gulf Stream at this latitude. The model results suggest that the variability is largest so mewhat to the east of Bermuda; fluctuations of similar to 10 cm extend as far east as similar to 35 degrees W. One surprising result is that at the longest periods in the GOADS data, the wind curl has a double- peak structure in longitude. That is, there is a significant amount of power on the eastern side of the ocean as well as near Bermuda. There fore, it is essential to use the full horizontal resolution of the win d data; using the mean curl across the Atlantic turns out not to be a good way to estimate thermocline variability. One might wonder if the wind data are reliable at these long periods were it not for the good agreement between the results and observed sea level. The power in win d variability increases out to similar to 500 months, although with li ttle statistical reliability. Sea level variability however, appears t o peak at somewhat shorter periods. Although it is pushing the resolut ion of the data, this result is a limitation imposed by the basin widt h scale. The power in the model ocean's response to wind forcing is ne arly an order of magnitude larger during the first half of the record (1952-69) than during the second (1970-86). It is likely that signific ant changes in buoyancy forcing by the atmosphere are coherent with ch anges in wind. Nevertheless,these results suggest that the variability in sea level-and so in deep temperature-can perhaps be accounted for without invoking changes in stored heat of the deep ocean.