Variations of the Gulf Stream's north wall east of Cape Hatteras

REMOTE SENS. ENVIRON. 47:303-310 (1994)

Variations of the Gulf Stream's North Wall East of Cape Hatteras Fred M. Vukovich* Z e l v e years (1977-1988) of NOAA infrared imagery data were used to study the variations of the north wall displacements of the Gulf Stream east of Cape Hatteras (i.e., between 74 Wand 65 W). The variations of the north wall displacements were compared with the variations in the Straits of Florida transport, which was estimated using a multifrequency model based on the sea-level differences between Siboney, Cuba and Key West, Florida for the period 1977-1988, and with transport measurements based on the submarine cable between Jupiter, Florida, and Settlement Point in the Bahamas for the period 1982-1988. The north wall reached a maximum northward position in 1985 when the cable data indicated that the Straits of Florida transport was a maximum. The north wall reached a maximum southward position in 1981. Cable data were not available before 1982, but the available data suggested that there may have been a minimum transport in 1981. A weak annual cycle of the north wall was found with a maximum northward position in the fall and a maximum southward position in the spring. No relationship was found between the annual cycle of the Gulf Stream's north wall and the annual cycle of the Straits of Florida transport.

INTRODUCTION The Gulf Stream current flows from the south/southwest to the north / northeast along the East Coast of the United States south of Cape Hatteras. In that region, the Gulf Stream is generally found at the Shelf Break. At Cape Hatteras, however, the Gulf Stream usually separates from the shelf slope and turns eastward, mov-

* Science Applications International Corporation, Raleigh, North Carolina. Address correspondence to Fred M. Vukovich, Science Applications International Corporation, 615 Oberlin Rd., Suite 300, Raleigh, NC 27605. Received 18 December 1992; revised 17 April 1993. 0034-4257 / 94 / $7.00 ©Elsevier Science Inc., 1994 655 Avenue of the Americas, New York, NY 10010

ing away from the coast. Large wavelike meanders develop on the Gulf Stream downstream from Cape Hatteras (Fulgister, 1963; Hansen, 1970; Richardson, 1981; Cornillion, 1986). Figure 1 is an NOAA/AVHRR infrared image which provides an example of the Gulf Stream behavior described above. Generally, the wavelike meanders have periods on the order of 7-21 days. Many of these waves become unstable, separate from the Gulf Stream, and produce large warm core rings (WCR) in the Slope Sea or cold core rings in the Sargasso Sea (Auer, 1987; Brown et al., 1986). Fulgister (1963), Hansen (1970), Richardson (1981), Cornillion (1986), Auer (1987), and others have noted that long period oscillations exist in the Gulf Stream system east and north of Cape Hatteras, but Casagrande (1987) also noted that there was a pattern of northward and southward displacements of the Gulf Stream in this region that had periods on the order of at least a year. These longer period north / south displacements of the Gulf Stream are the focus of this article. Twelve years (1977-1988) of NOAA infrared imagery were used to study the long period oscillations (i.e., periods greater than 1 month) in the Gulf Stream east of Cape Hatteras between 65 W and 75 W. The satellite data were combined with a 12-year (1977-1988) estimate of the transport in the Florida Current obtained using sea-level differences between Key West, Florida and Havana, Cuba (Maul and Vukovich, 1993; Maul et al., 1990). DATA PROCESSING

The variations of the Gulf Stream's northern boundary between 65 W and 75 W were obtained using the "wave staff" technique. Surface frontal analyses were developed in that region on relatively clear-sky days using NOAA/AVHRR (10 /~m band) infrared images. Only those images that had sufficiently clear skies in the region of interest, so that there was little doubt of the position of the Gulf Stream front, were used for this study (i.e., some of the images were contaminated by

303

304

Vukovich

80

~" iss"

3

f

60

70

40

\

1

30

Figure 2. The measurement scheme for determining the Gulf Stream's (crosshatched region) north wall variations. The Slope Sea region is also indicated.

Figure 1. NOAA/AVHRR infrared image of the region off the east coast of the United States. The Gulf Stream and its north wall are indicated.

clouds, but these were small clouds which provided no difficulty in interpolation of the frontal position in the region of the cloud). The north / south distance (i.e., the distance along a longitude line) was measured between a fixed point and the Gulf Stream's northern boundary in that region. The fixed points were: 1) 40 N, 74 W; 2) 40 N, 70 W; and 3) 40 N, 65 W (see Fig. 2). All distances along a given longitude line obtained in a given month were averaged to produce monthly averaged values of the distance. The wave staffs were designed to detect as accurately as possible the northsouth displacements of the Gulf Stream in this region which were significant in the period. Since they are not exactly perpendicular to the isobaths, especially at 74 W, the measure of the amplitude of the waves along the front was not precise. This article, however, places more emphasis on the characteristics of the time variations of the displacements rather than the magnitude of those displacements. Anywhere from four to 10 frontal analyses were available each month over the 12-year period. The average number of images per month was approximately 6.0, and the standard deviation was a little greater than ± 1.0. A time series of monthly average distances was

developed for each of the three fixed points (i.e., 75 W, 70 W, and 65 W). The 12-year average distance was calculated and subtracted from each of the monthly average values to produce displacements about the mean. The subsequent displacements were multipled by ( - 1 ) in order to create a sign convention so that positive displacements were displacements to the north and negative displacements were displacements to the south. Annual averages of the displacements were calculated for each year and subtracted from the monthly values of the displacements for that given year to remove interannual trends. Figure 3 shows the time series of demeaned and detrended Gulf Stream north wall displacements at 65 W, 70 W, and 74 W. The data shown on the figure have been filtered using a 5-month lowpass filter. The Straits of Florida volume transport was estimated using a multifrequency model (Maul et al., 1990) from sea-level data at Siboney, Cuba, which is near Havana, and Key West, Florida. The relationship between sea level and transport was previously noted by Blaha (1984). These data were used instead of the transport measurements made by submarine cable from Jupiter, Florida to Settlement Point in the Bahamas (Larsen and Sanford, 1985) because the application of the sea-level data provided transport estimates over the entire 12-year period. No transport measurements from the cable were available before 1982. Monthly mean sea-level data were used. Gaps in the data were filled using a least square process. For example, the gaps in the Key West data were filled by fitting these data to the Miami sea-level data and replacing the missing data through the best fit equation. The high linear correlation between Key West and Miami sea-level data (r--0.91) assured meaningful data replacements (see Maul and Vukovich, 1993). The sea-level difference (SLD), Siboney minus Key West, was then bandpass filtered, and the volume transport was estimated. The model parame-

Variations of the Gulf Stream's North Wall 305

60,

it i

I

o

0

4-

[ v; I'

I

2-

I

lItlll.l '

A

I.~

-4O

i

, . . . . .

;

',3'

i

1

t

:,,!

L

i

',,:

-80

0 r~ t/I Z er

I-

MONTH 60

.ff

~

4o

ul

20

~

o

77

a

79

81

83

-4

o

~

~

,~

=

o

~

~

~

=

o

~

,,

MONTH

~'

4o

- 77

79

81

83

83

87

-60

Figure 4. The time series of the demeaned and detrended monthly transport (Sv) in the Straits of Florida based on the sea-level difference (SLD) between sea-level measurements at Key West, Florida and Siboney, Cuba. Tp is the major spectral period for each time series.

MONTH 80i

i

f.ltll

,,.=

u. ~

-40 i 7

9

ftl

83

85

i 87

-60 -

MONTH

Figure 3. The time series of the demeaned and detrended monthly Gulf Stream north wall displacements (km) for 74 W, 70 W, and 65 W.

ters are given in the article by Maul and Vukovich (1993). The time series of mean monthly transport in the Straits of Florida based on the SLD are shown in Figure 4. The SLD based transport data are departures from the mean, which are the results provided by the model. As previously noted, these data are used in this study, instead of the cable transport data, because the SLD data provided transport estimates versus time over the entire 12-year period. However, in part of this study, the study of interannual trends, annual averaged transport data were required. In those cases, the cable transport data were used, even though they provided

information for just a portion of the period (i.e., 19821988). Figure 5 shows the spectral variance for the Straits of Florida volume transport based on 12 years of sealevel data and that based on 7 years of cable data. The predominant period (Tp) in both sets of data is the annual period. Furthermore, the variance for the transport based on the SLD data is about a factor of 2 greater than that from the cable data (i.e., the variance of the transport from the SLD data is 18 Sv2 and that for the cable data is 9 Sv2). These data together with those shown by Maul and Vukovich (1993) and by Maul et al. (1990) would suggest that the transport derived from the SLD provide a reasonable, though by no means perfect, representation of the time variability of the transport in the Straits of Florida. TRENDS

The interannual variations of the Gulf Stream north wall displacements at 74 W, 70 W, and 65 W were examined using the annual mean data (Fig. 6). Though the year-toyear variations of the northern boundary are different at each of the longitudes, the maximum northward displacement occurs in 1985 and the maximum southward displacement in 1979 at each of the three longitudes. The variation of the annual means nearly represents a sine wave at each of the longitudes. The amplitude of the waves was almost identical at 74 W and 70 W (i.e.,- 20 km and - 2 1 km, respectively), and somewhat

306

Vukovich

]!

5-

! i

....~ . .

4-

../'

8 .__ 3 >

-2o ]

8.2-.

03

' :"¢,~.r ' ' "

/

40

:

-60 t 1976

1-

:

1978

1980

1982

i

?

:

i i

1984

1986

--

1988

1990

YEAR

0-1L0.0

Figure 6. The interannual variations of the Gulf Stream north wall displacements (km) for 74 W, 70 W, and 65 W.

.__ 0 2

0.4

0.6

0.8

1.0

Hz

30

O-

~

0-

t

/

B-

I

.....

x

:.

a.

6~

-

- 34 .

,,

i"~

0"--

-10 - - -

/

N

"" **°Y

• 39

/"

I

o

' 32

-20 4-30 1976

2

2-

1980

1982

1984

1986

1988

-31 1990

YEAR

0; 0.0

1978

0.2

0.4

0.6

0.8

1.0

HZ

Figure 5. The variance-preserved spectra for the Straits of Florida transport based on 12 years of SLD data and that based on 7 years of cable data. The principal periods for each spectrum are indicated.

larger at 65 W (i.e., - 3 4 km). Figure 7 presents the interannual variation of the Gulf Stream north wall displacements based on the average of the annual mean data for the three longitudes. The figure also contains the interannual variations of the Straits of Florida transport (dashed line), which was derived from the cable data, and which only represents the period 1982-1988. In the mean data, the maximum northward displacement of the Gulf Stream occurred in 1985, precisely the same year when there was a maximum in the Straits

Figure 7. The interannual variations of the Gulf Stream north wall displacements (km) based on the averages for the mean data at the three longitudes and interannual variations of the Straits of Florida transport (Sv) based on the cable data.

of Florida transport. The maximum southward displacements of the Gulf Stream occurred in 1979. It is not known if there was a minimum Straits of Florida transport at that time. The available transport data would suggest that the maximum southward displacement of the Gulf Stream occurred in a period when the Straits of Florida transport had a relatively low or minimum value. The correlation analysis using these two sets of data provided a correlation coefficient of 0.72 with no phase lag. Figure 8 shows the average annual cycle of the Gulf Stream's northern boundary at 65 W, 70 W, and 74 W based on the 12-year data set. The time of maximum northward displacement and of southward displacement (i.e., October and March respectively) are the same at 65 W and 74 W. The annual cycle at 70 W, however,

Variations of the Gulf Stream's North Wall 307

40

I

I

I

2.0

I

74:77-88 1.5 70:77-88 65:77-88

s S --l~

.

1.0

I

-40

0.0-

t ii

-20

• - i - 2

6

o

R-SLD-65

0.5

iii-lii!:!i

>

l

-0.5 i

-1.0 10

8

11

12

( I I iI.-.l'JYIYI'I

MONTH

Figure 8. The average annual cycle of the Gulf Stream north wall displacements (km) at 74 W, 70 W, and 65 W based on the 12-year data set. 3

15

YEAR

Figure 10. The correlation between the annual cycle of the Gulf Stream north wall displacements (km) at 74 W, 70 W, and 65 W and the annual cycle of the Straits of Florida transport (Sv) for each year.

Ave Ann Cycle

10

I I i /

2

Transport

8

5

~ .

~ ,

E

;=,

J"

0

~"-

•

0

E -5

-2

-10

-3

-15 2

3

4

5

6

7

8

9

10

11

12

MONTHS

Figure 9. The average annual cycle of the Gulf Stream north wall displacements (km) based on the averages for the mean data at the three longitudes and the average annual cycle of the Straits of Florida transport (Sv) based on the SLD data.

has a maximum northward displacement in December and southward displacement in August. The amplitude of the annual cycle at either 74 W, 70 W, or 65 W was not remarkable, and they were about 10 km, 10 km, and 15 km, respectively. The relationship between the annual cycle for the Gulf Stream north wall displacements and that for the Straits of Florida transport is shown in Figure 9. The figure shows the average annual cycle for the north wall based on the data at the three longitudes. The correlation coefficient is small and negative (i.e., r = - 0 . 1 9 ) for these data as it is at each individual longitude (i.e., at 74 W, r = -0.19; at 70 W, r = -0.29; and at 65 W, r-- -0.14) when no phase lag is considered. Positive correlation coefficients are found

when a 4-month phase lag is considered, but these positive values are all less than r = 0.40. There was no consistent relationship from year-to-year between the annual cycle of the Gulf Stream north wall displacements at the three longitudes and the Straits of Florida transport (Fig. 10). Large positive correlation coefficients occasionally were found (e.g., r = 0.88 at 74 W in 1987) as were large negative values (e.g., r = - 0 . 8 7 at 70 W in 1980), but these large values seldom persisted for more than 1-2 years. The data indicated that there was a major difference in the behavior of the annual cycle of the Gulf Stream north wall displacements between the first half (i.e., 1977-1982) of the 12-year period and the second half (i.e., 1983-1988) (Fig. 11). In the first half, the annual cycle is weakly defined at 70 W and 65 W. As a matter of fact, there was no annual cycle, on the average, at 70 W. The annual cycles at 74 W and at 65 W had their maximum northward displacements in August, which is considerably different from that for the overall average (see Fig. 8), and their maximum southward displacements in March. The amplitude at 74 W was about 9 km and at 65 W, about 12 km, which are slightly less than their overall amplitudes in both cases. In the second half, there were well-defined annual cycles at all three longitudes. The character of the annual cycle at 74 W was only different from the overall average in the time of maximum northward displacement (i.e., December instead of October). At 70 W, the amplitude was 15 km, or 5 km larger than the overall average. The maximum northward displacement was in January, and the maximum southward displacement was in June, as opposed to December and August in the overall

308

Vukovich

40

2

'"

-20 ' " ~

L

74:77-82 74:83-88

",,

J

St

A r"

..... "

/ I d!72

o.. -20

"-----

- ....

40 6

1

10

* ....

sld 8 3 - 8 8

11

-2 1

70:77-82

2

3

4

5

6

7

8

9

10

11

12

MONTH

20

Figure 12. Six-year averages (i.e., 1977-1982 and 1983-1988) of the annual cycle of the Straits of Florida transport (Sv).

0

7

4.

-20 -

7

'° ,

1 ~,

0

9

10

11

12

III

~E 20

8

~

65:77-82

,

65:83-88

",

. . . . .

@

-20

-40 1

2

3

4

5

6

7

8

9

10

II

12

Figure 11. Six-year averages (i.e., 19771982 and 1983-1988) of the annual cycle of the Gulf Stream north wall displacements (km) at 74 W, 70 W, and 65 W.

was in August (December), and the secondary maximum (minimum) was in March (May). The relationship, if any, between the differences in the annual cycle of the Gulf Stream north wall displacements and those of the Straits of Florida transport could not be determined from the available data. Figure 13 shows the variance spectra for the north wall at the three longitudes. The variability of the Gulf Stream north wall displacements both in terms of periodicity and magnitude increased eastward away from the coast. The integrated variance increased eastward (i.e., 4612 km 2 at 74 W, 7276 km 2 at 70 W, and 23,952 km 2 at 65 W), and the number of significant periods doubled from 74 W to 65 W. The period with the largest variance is the 3-month period at 74 W, the ll.5-month period at 70 W, and the 5-month period at 65 W. The annual peak is represented by the 11.5month period, which was present at all longitudes, since the discrete nature of the monthly sampling would not allow discrimination of an 11.5 month period. SUMMARY AND DISCUSSION

average. At 65 W, the amplitude was 19 km, also 5 km larger than the overall average, and the maximum southward displacement was in April, as opposed to March in the overall average. The time for the maximum northward displacement occurred in October as it did in the overall average. There were also major differences in the annual cycle of the Straits of Florida transport between the first and second half of the period (Fig. 12). In the first half, the maximum transport occurred in May and the minimum in November. In the second half, the annual cycle was bimodal. The primary maximum (minimum)

Twelve years of NOAA satellite infrared images (19771988) were used to examine the variations of the Gulf Stream north wall displacements between 65 W and 75 W (i.e., 65 W, 70 W, and 74 W). The variations of the Gulf Stream north wall displacements were compared with the variations of the Straits of Florida transport estimated from the sea-level differences between Siboney, Cuba, which is near Havana, and Key West, Florida using a multffreuqency model (Maul et al., 1990). These data covered the entire 12-year period in the form of departures from the mean and were used to study intraannual variations. Transport measurements made

Variations of the Gulf Stream's North Wall 309

500"~ ;I

74W [ Tl=ll.5 T2=5.2 [ T3--4.0 T4=2.8I

450 ~ 8

500-

"

1

70w I T1=11.5 T2=9.5 I T3=5.2 T4=4.5] T5=3.0 T~=2.7[

450 0

,~ 400

=

> aso

-

¢'

(~ 300 -. E 250-

~

:

~2oo-~

~- 2 0 0 a

E150

E150~oo50-

0.0

0.2

1600"3

0.41_1:,0.6

0.8

1.0

0- i 0.0

0.2

0.4

0.6

0.8

1,0

[T~=16.0 T2=I1.5

I T3=9.5

1400 ~

3"4=5.2

I T5--4.7 T6=2.8

oool i i °°l

I

4oo ll L ° °iL 0.0

0.2

0.41120.6

0.8

1.0

by the submarine cable that stretches across the Florida Current between Jupiter, Florida and Settlement Point in the Bahamas provided annual averaged data, which were used to study the interannual variations. The data showed that the Gulf Stream's northern boundary reached a maximum northward position in 1985 and a maximum southward position in 1981, on the average. The amplitudes of the frontal displacements on an interannual basis were from 20 km at 74 W and 70 W to about 35 km at 65 W. The transport in the Straits of Florida also reached a maximum in 1985. Data

Figure 13. The variance-preserved spectra for the monthly Gulf Stream north wall displacements (km) for 74 W, 70 W, and 65 W. The principal periods for each spectrum are indicated. The major periods (subscripted T's) are indicated for each spectrum.

from the submarine cable were not available before 1982 so that it was not possible to determine if there was a minimum transport in 1981, but the increase in the transport between 1982 and 1985 suggest that there may have been a minimum in the transport around 1981. Comparison between interannual variation of the Gulf Stream north wall displacements averaged over the 3 observation points (65 W, 70 W, and 74 W) with the interannual variation of the transport in the Straits of Florida provided a correlation coefficient of 0.72 with no phase lag. These data suggested that transport may

310

Vukovich

have influenced the north-south displacements of the Gulf Stream's northern boundary east of Cape Hatteras on an interannual basis. An annual cycle was found to exist for the Gulf Stream north wall displacements at all three longitudes. The amplitude of displacements associated with the annual cycles and the total variance in the time series of the displacements increased eastward from the coast. An annual signal was the predominant period in the variance spectra of the displacement time series for the northern boundary at 70 W only. At 74 W and 65 W, the predominant periods were the 3-month period and the 5-month period, respectively. In the annual cycle, the maximum northward and southward displacement times at 65 W and 74 W were in the fall and in the spring, respectively, but those at 70 W were in the winter and in late spring or early summer, respectively. Amplitude of the annual signal was small, on the average, ranging from about 10 km at 74 W and 70 W to about 15 km at 65 W. The maximum northward displacement occurs in the fall, early winter time frame, a period when the mean wind has a strong component from the north at the surface over the east coast of the United States; and the maximum southward displacement occurs in the spring and early summer, which is generally characterized by a wind component from the south. The annual cycle of the wind off the east coast is more likely to impede the annual cycle of the northern boundary. The comparison between the average annual cycle for the Gulf Stream north wall displacements with that for the Straits of Florida transport provided a correlation coefficient of r-- -0.19 with no phase lag. The maximum correlation between the annual cycles for the transport and the Gulf Stream north wall displacements was r = 0.40 with a 4-month phase lag. The 4-month phase lag results from the time differences, on the average, between the maximum and minimum transport (July and November, respectively) and the peak northward and southward displacements (October and March, respectively), which is 4 months. The poor correlation between the annual cycles suggests little or no relationship between the annual cycle for the transport and that for the Gulf Stream north wall displacements. It was noted, however, that there was a significant difference in the characteristics of the annual cycle of the Gulf Stream north wall displacements between the first half of the 12-year period and the second half. In the first half, the annual cycle was nonexistent at 70 W; and at 74 W and 65 W, the maximum northward displacement occurred in the summer (August) instead of the fall (October). In the second half, the annual

cycles were better defined at all three longitudes. The months at which the maximum northward and southward displacements were found, were different from the period average for each longitude. Similarly, there were major differences in the annual cycle for the transport between the first half and the second half of the period. In the first half, there was a single peak (May) and a single minimum (November). In the second half, there were two peaks. The primary maximum (minimum) was in August (December) and the secondary, in March (May). Though the data were not sufficient to determine whether these two events were related, the fact that these events occurred simultaneously is not easily dismissed. The author wishes to express his appreciation to Dr. George A. Maul (NOAA/ AOML) for providing the sea-level difference transport data. REFERENCES

Auer, S. J. (1987), Five-year climatological survey of the Gulf Stream system and its associated rings, J. Geophys. Res. 92(C 11):11,709-11,726. Blaha, J. P. (1984), Fluctuations of the monthly sea level as related to the intensity of the Gulf Stream from Key West to Norfolk,J. Geophys. Res. 84(C5):8033-8042. Brown, O. B., Cornillion, P. C., Emmerson, S. R., and Carle, H. M. (1986), Gulf Stream warm rings: a statistical study of their behavior, Deep Sea Res. 33:1459-1473. Casagrande, C. E. (1987), Study of Physical Processes on the U.S. Mid-Atlantic Continental Slope and Rise Final Report. Volume II: Technical Presentation, Minerals Management Service / Atlantic OCS Office. Cornillion, P. (1986), The effect of the New England seamounts on Gulf Stream meandering from satellite IR imagery, J. Phys. Oceanogr. 16:386-389. Fulgister, F. C. (1963), Gulf Stream 60~, Prog. Oceanog. 1:265373. Hansen, D. V. (1970), Gulf Stream meanders between Cape Hatteras and the Grand Banks, Deep Sea Res. 17:495-511. Larsen, J. C., and Sanford,T. B. (1985), Florida current volume transports from cable voltage measurements, Science 227: 302-304. Maul, G. A., and Vukovich, F. M. (1993), Aspects of the relationship between the subseasonal cycle of the loop current and the Straits of Florida volume transport, J. Phys. Oceanogr. (May), forthcoming. Maul, G. A., Mayer, D. A., and Bushnell, M. (1990), Statistical relationships between local sea level and weather with Florida-Bahamas cable and Pegasus measurements of Florida current volume transport, J. Geophys. Res. 95(C3): 3287-3296. Richardson, P. L. (1981), Gulf Stream trajectories measured with free drifting buoys, J. Phys. Oceanogr. 11:999-1010.