Physical Oceanography Division

BENCHMARKS

FOR ATLANTIC OCEAN CIRCULATION

 

 

 

 

 

 

Gulf Stream position

NOAA Research presently manages the U.S. contribution to the global Voluntary Observing Ship (VOS) network.  This portion of the VOS network provides temperature profiles using expendable bathythermographs (XBT). Several VOS transects provide data from the late-1960's that cross the subtropical gyre of the North Atlantic (e.g., the line from New York to Bermuda, Figure 1. By adding earlier data obtained using mechanical bathythermographs (MBT) it is possible to obtain a 50+-year record of temperature changes above 200m. This time-series can provide a measure of Gulf Stream position. To obtain this position, the depth-averaged temperature of the upper 150m (T150) was estimated for each MBT and XBT profile. The individual T150 values were averaged on to a 0.2 degree of latitude from (35N to 40N) by two-month from (January - February, 1940 to May-June, 2001) grid. For each bi-month, the position of the Gulf Stream was taken as the latitude of maximum horizontal gradient in T150.

 

• Mean position: Over the 50+time period of this record, the mean position of the axis of the Gulf Stream is 37.5N at an approximate longitude of 72W.

 

• Decadal signals: The bimonthly time-series of Gulf Stream position was smoothed using a 3-year running box car filter and normalized by the record standard deviation. The resulting time series is given in Figure 2. (solid line) and shows a multi-decadal signal with a southernmost position in the 1960's followed by a northern shift of some 100 km during the 1970's and 1980's . The Stream shifts southward and then northward in the 1990's. Hurrell's NOA index is also shown in Figure 2. (dots). As described in Joyce et al (2000) and Curry and McCartney (2001), the two time-series are visually correlated at the longer time-scales. However, because of the brevity of the oceanographic record causative relations can't be defined. Large SST signals, order 2-4°C, associated with these migrations suggests the potential for air-sea coupling. Through NOAA/OGP support. A10 continues to be occupied and the position time-series will be updated every 6-months. 

 

• Annual cycle: An annual cycle in Gulf Stream position at this longitude is not discernible from these data. Mean bi-montly positions are either at 35.5N (4) or 35.7N (2) the resolution of the grid. Standard deviations range from 0.4 to 0.5 degrees.

• Decadal signal: A smoothed and normalized (by the standard deviation) time-series of transport is also given in Figure 2. In addition, the Hurrell's NOA - index is plotted in this figure. The three time-series have a very similar long-term signal in the records. The position and NAO values are relatively high in the late 1950's decreasing to low values in the 1960's. The transport time-series begins in the late 1960's and the three series rise to high values in the late 1980's, early 1990's. All three series decrease in the late 1990's. Additional analyses is underway to ascertain if there had been a regime shift in the structure of both the ocean and atmosphere in the late 1990's. Superimposed on the long-period unresolved signal are higher frequency (order decades) signals in all three records.

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Last Updated: 05/2402