Monthly charts of the north wall of
the Gulf Stream, derived from aircraft, satellite and surface observations, have been
published by the U.S. Naval Oceanographic Office in The Gulf Stream Monthly Summary from
1966 until 1974, and by the U.S. National Oceanic and Atmospheric Administration in Gulf
Stream from 1975 until 1980 and in Oceanographic Monthly Summary from 1981 to 1994. Since
1994 charts have been obtained from the US Navy. The latitude of the north wall was read
from each chart at six longitudes : 79ºW, 75ºW, 72ºW, 70ºW, 67ºW and 65ºW (the
charts were incomplete east of 65ºW). Principal components analysis was used to find the
pattern of variation common to the six longitudes.
This is preferable to simple averaging of the six latitude measurements which would
emphasise the eastern longitudes where the north-south movements of the Stream are
largest, possibly obscuring smaller but coherent north-south shifts further west.
Correlation coefficients were calculated between the time-series at the six longitudes and
principal components calculated from the resulting correlation matrix. Each principal
component is a weighted average of the six latitude series whose weightings can be
expressed as correlation coefficients between the component and the original series. The
first principal component of the position of the north wall has positive correlation
coefficients of 0.15, 0.54, 0.49, 0.62, 0.57 and 0.48 with the latitude of the north wall
at the six longitudes from 79ºW to 65ºW respectively and can therefore be regarded as
representing coherent variations from month to month in this section of the Gulf Stream.
(This principal component is virtually indistinguishable from that obtained when the
annual cycle is removed beforehand.) This principal component is the Gulf Stream north wall index (the GSNW index).
The eigenvalue corresponding to the first principal component has a value of 1.5 so
that the component accounts for 25% of the total variance of the six variables. This
percentage has undoubtedly been reduced by the short period meandering of the Gulf Stream,
for there will be little correlation between meanders that are widely separated in
longitude. The correlation coefficients quoted show that the north-south shifts described
by the component are evident at all the longitudes. The variation of latitude associated
with the first principal component has a standard deviation at the six longitudes from
79ºW to 65ºW of 0.04, 0.12, 0.25, 0.26, 0.34 and 0.37 degrees, respectively.