Satellite Investigations on the Fluctuations of the Subtropical and Antarctic Convergence Zones

Karl-Heinz S z e k i e 1 d a UDC 551.465.45:551.465.53;Indian Ocean

Summary

Investigations have been made using GOSSTCOMP* data in the area of the subtropical and antarctic convergence zone. The study showed that the position of both boundaries undergoes oscillations in E-W direction and it is assumed that the circumpolar current may act on both boundaries of the convergence zones. Since it seems that the antarctic convergence zone is not as stable as has been postulated before, it is suggested that unstable waves, gyres and interaction of different water bodies are the major cause of the observed changes in the location of the two zones.

Satellitenuntersuchungen fiber die Fluktuation der subtropischen und antarktischen Konvergenzzonen (Zusammenfassung)

Mit Hilfe von GOSSTCOMP*-Daten wird untersucht, wie verfinderlich die Lage der subtropischen und antarktischen Konvergenzzonen ist und vermutet, dag der Zirkumpolarstrom und das Zusammenwirken verschiedener Wassermassen die hauptsfichliche Ursache f/Jr die beobachteten Wellen und Wirbel sind.

Recherches effectu~es par satellite sur ies fluctuations des zones de convergence subtropicales et antarctiques (R~sum~)

On a procEd6 ~ des recherches en utilisant des donn6es GOSSTCOMP* dans le domaine des zones de convergence subtropicale et antarctique. L'6tude a montrd que la position des deux fronti6res se trouve soumise ~ des oscillations dans la direction E-O et on suppose que le courant Circumpolaire peut agir sur les deux limites des zones de convergence. Comme il semble que la zone de convergence antarctique n'est pas aussi stable qu'il avait 6t6 postul6 par le pass6, on sugg6re que les ondes instables, les tourbillons et l 'interaction de diffdrentes masses d'eau sont la cause principale des changements observ6s dans la situation des deux zones.

1 Introduction

The Antarctic Ocean can be divided into two separate regions with waters near the Antarctic continent and the subantarctic waters. Both water masses can easily be recognized by their sea surface temperatures alone and have boundaries associated with the antarctic and subtropical convergence zones. The subantarctic water extends at the surface south of the subtropical convergence zone to around 52 to 53 ~ S where the antarctic convergence is located. Although the subantarctic waters exist throughout the whole year, significant temperature changes can be recognized.

* Global Ocean Sea Surface Temperature Computer Program

26 Dt. hydrogr. Z. 36, 1983. H. 1. S z e k i e 1 d a, Satellite Investigations

The subtropical convergence with its position of 10 ~ north of the antarctic convergence may show a temperature range of 10 to 14 ~ in winter and 14 to 18 ~ in summer, but its location may vary. For instance, the convergence is found further south on the western sides of the oceans where warm water is carried to the south. This transport of warm water by the western boundary currents has been demonstrated early with satellite data for the Agulhas current by W a r n e c k e , A l l i s o n , M c M i l l i n and S z e k i e l d a [1971] and the temperature gradients connected by S z e k i e 1 d a , S h e n k and S a 1 o m o n s o n [1974]. The latter study showed that these temperature grfidient meanders were stronger than one may expect from historical data or satellite composite data over a period of 14 days. This was confirmed by H a r r i s , L e g e c k i s and F o r e s t [1978], who also showed that data with high resolution imagery material reveals details about the eddies connected with the Agulhas current. However, the sharp temperature gradient fluctuations were also observed outside the Agulhas current with earlier data obtained by the Nimbus satellites (S z e- k i e l d a , S h e n k and S a l o m o n s o n [1974], which shows that besides the Agulhas current, another mechanism must be responsible for the sea surface temperature (SST) gradients and variations observed earlier.

Separated eddies from the Agulhas current have been reported in the vicinity of the subtropical convergence (D u n c a n [1968]; D i e t r i c h [1957]) and have been detected with satellite data by S z e k i e l d a [1976] and by H a r r i s , L e g e c k i s and F o r e s t [1978]. Duncan (cited above) observed an eddy in the subtropical convergence southwest of Africa and concluded that the subtropical convergence is not necessarily continuous, exact conditions probably depending on the strength of the seasonal variations of the Agulhas current.

It has been pointed out by several authors that the sinking process of the heavier surface water at the convergence zone is not a continuous process; rather, it seems to appear in individual mass intrusions at different places. The infrared data reported by S z e - k i e I d a , S h e n k and S a 1 o m o n s o n [1974] indicate strong oscillations of the tempera- ture gradients which could certainly not be resolved with conventional ship observations. Therefore, the synoptic coverage of infrared data would indicate the fluctuation and location of boundaries which are associated with the converging zones within the subantarc- tic zone. The following study therefore attempted to give more insight into the temperature gradients and the possible mechanisms involved in their distribution.

2 Validity of satellite sea surface temperature and data interpretation

Satellite infrared measurements have b e e n m o r e widely used during the last years for monitoring temperature gradients connected with current systems and upwelling events. For summaries on ocean fronts, see for instance S z e k i e 1 d a [1976] and L e g e c k i s [1978]. Sea surface temperature does not necessarily reflect at all seasons the location or boundaries of eddies and currents. This is due to isothermal surface layers during winter months caused by vertical mixing or due to the conditions prevailing in tropical regions where isothermal conditions exist throughout the year.

If cloudfree conditions are present over an area to be observed, satellite infrared images can readily be used to detect thermal gradients. For large scale mapping, however, statistical methods have to be applied to eliminate the atmospheric contribution to the measurements. So far, the most advanced large scale mapping system is the GOSSTCOMP which is providing data on a large grid, but compared to ship measurements in the southern oceans, has a very dense monitoring grid. Small scale features such as coastal upwelling or narrow currents can hardly be identified, although the temperature field on a large scale can easily be resolved.

Mathematically, the component sources of thermal radiation from the ocean obtained at satellite altitudes through a clear atmosphere can be written as:

Dt.hydrogr.Z.36, 1983. H. 1. S z e k i e I d a , Satellite Investigations 27

/ e~ (~,)/3 (2~, ts) dZ , Ns

where N~ is the total energy per cm 2 emitted by a radiating surface in a bandwidth (~.1, ~,2) at a temperature t~, fi is the Planck radiance, and es is the surface emissivity, and

;~2 0

~ dr ( '~ 'P) dP d'~ N~ = [3 (~., tp) dp '

,t.1 po

where N~ is the positive energy contribution of the atmosphere in the bandpass ( Z 1 , Z 2 ) , tp is the temperature of the atmosphere at pressure level p, and r ()~, p) is the atmospheric transmittance from pressure level p to the top of the atmosphere, p0 ist the atmospheric pressure at the surface. The total energy, Nt, over the bandpass measured by the sensor viewing an ocean surface is then given by

N~ =, N~r~ + N~, - N~ - (N~a~ - N~) ,

where a~ and Ts are the absorptivity and transmittance, respectively, of the total atmosphere. Expressed in terms of equivalent temperatures one obtains

[bb - - Is - - At,

where tbb is the sensor observed brightness temperature, ts is the surface temperature , and At is the atmospheric correction. The value of At is obtained from soundings with a vertical temperature profile radio meter (VTPR) aboard the same satellite.

T a b l e 1

10.5- to 12.5%tm atmospheric attenuation corrections

Absorber At Range

H20 0 to 9.0 ~

CO2 0.1 to 0.2 ~

03 0.1 ~

Aero.sols 0.1 to 0.95 ~

As seen in Table 1 at tenuation by wa te r vapor far exceeds that by any of the other absorbers. Other residual absorption, such as that produced by salt and dust particles, has little effect unless there is high particle concentration. Ice crystals near the t ropopause are assumed to be opaque.

Brightness temperatures associated with sea surface temperature measurements are corrected for atmospheric attenuation with coefficients derived from VTPR processing of a coincident temperature and moisture profile sounding. The coefficients are derived by integrating the effect of absorption and emission of radiance by each layer of the atmo- sphere. This integration is based on both the VTPR sounding and theoretical transmittance functions. The computed coefficients are used to correct both for the straight-down single atmosphere absorption and the absorption through a slanted atmospheric path.

28 Dr. hydrogr. Z.36, 1983. H. 1. S z e k i e I d a, Satellite Investigations

The ability to eliminate cloud contaminated scenes is the most significant factor in determining the eventual accuracy of surface temperature values. Clouds are themselves radiating surfaces. C1ouds absorb the radiation emitted from the ocean surface and emit radiation as specified by the Planck function, in a single-channel model, no effective means are available to infer a surface radiance value in cloud contaminated scenes. For this reason the viewed scene must be limited to cloud free areas. Considerable effort has gone into the design of the sea surface temperature model, both in the retrieval process and in post analysis, to eliminate any cloud contaminated brightness temperatures.

Original research on sea surface temperature retrieval techniques using scanning radiometer infrared (SRIR) data was done by S m i t h , R a p , K o f f l e r et al. [1970]. A modification of this original technique, first suggested by S m i t h and K o f f 1 e r [1970], was later developed into the present technique by L e e s e , P i c h e 1, G o d d a r d et al. [1971]. The current method of retrieving sea surface temperatures from satellite SRIR data employs a statistical technique to extract sea surface temperatures from blocks of raw data organized into histograms.

For a uniform-scene temperature field, with only random noise affecting the sensor output, a frequency distribution of the data elements from this field will be Gaussian. Simply calculating the arithmetic mean or locating the modal class of this histogram will give one the actual scene temperature. Since most scenes have some clouds present, one must be able to find the true surface scene temperature when cloud contaminated samples are present in the histogram. The warm side of the histogram (higher IR counts) is unaffected by the clouds. If one could find an uncontaminated histogram whose cold side is a mirror image of the warm side, the mean would be the surface scene temperature.

Because there is a certain degree of non-random noise in the data, two operations are performed to insure the accuracy of the calcula~ed mea~:

1. The histogram is smoothed before attempting to retrieve a mean; and 2. since a single calculation of the mean from a slightly non-Gaussian histogram could

be in error, repeated calculations of the mean are made from all possible combinations, taken three at a time, of the classes (each class being one IR response count wide) constituting the warm side of the histogram. These estimates of the mean are distributed into a second histogram called the mean estimate histogram (MEH). The uncontaminated mean of the data histogram is then calculated from the MEH by finding the mean value of all mean estimates which fall within two classes of the mean value of all mean estimates which fall within two classes of the mode of the MEH.

The accuracy of the data obtained can be estimated with the daily mean difference and standard deviation around the mean for verification against ships for the NOAA-5 satellite as given in Table 2 and Fig. 1.

In addition to the comparison between ship observations and satellite SST an attempt was made to evaluate the quality of the data for the area of investigation without having access to additional ship data. Within the test site at 55 ~ E and 25 ~ S data were collected over the period from March 1979 to December 1980 in order to estimate the possible error in the SST monitored f~om satellites. The area chosen has rather low seasonal changes; therefore, variations around the monthly mean were also expected to be low. The principal idea was to estimate the variations around the monthly fluctuations which can be expected to be about one degree per month. Therefore, any variation greater than the expected natural changes would indicate an error introduced by the data received from satellites and based on their processing.

The results from these studies are given in Table 3 and Fig. 2 which indicate that even small yearly fluctuations can be monitored with a high accuracy. If one considers that the daily cycle as well as the seasonal cycle superimposes the analyzed variation, it is obvious that the temperature measurements used at present have a high accuracy to follow tempera- ture gradient variations even over a range of a few ~ Therefore, changes as observed in the strong temperature gradients connected with the Agulhas current and the subtropical and antarctic convergence exceeding more than one ~ per degree latitude can be easily monitored with GOSSTCOMP.

Dt.hydrogr .Z .36 , 1983. H. 1. S z e k i e I d a , Satellite Investigations 29

T a b l e 2

NESS-SST verification for April through December 1978

The tab le displays the m i n i m u m and m a x i m u m daily m e a n d i f fe rence and s t a n d a r d dev ia t ion a r o u n d the m e a n for ver i f ica t ion aga ins t ships for the N O A A - 5 satel l i te . All quan t i t i e s are

in ~ and a nega t ive d i f fe rence m e a n s tha t N E S S - S S T is colder .

R a n g e of m e a n R a n g e of s t a n d a r d M o n t h d i f fe rence dev ia t ion

Apri l - 0 . 8 8 to - 0 . 2 5 ~ 1.34 to 2.09 ~

May - 0 . 4 2 to 0.11 1.81 to 2.36

J u n e - 0 . 4 4 to 0.54 2.13 to 3.11

July - 0 . 3 5 to 0.44 2 . 2 2 t o 2.92

Augus t - 0 . 2 7 to 0.56 2.38 to 2.85

S e p t e m b e r - 0 . 1 6 to 0.82 2.03 to 2.51

O c t o b e r - 0 . 3 2 to 0.0 1.59 to 2.24

N o v e m b e r - 0.90 to - 0.09 1.86 to 2.40

D e c e m b e r - 1.06 to - 0 . 5 7 1.91 to 2.42

t At:

t At:

i .

+ i.o~ "C- 1978 / '~ /! f'.. O . . . . . . . . . . . . . . . . . . r~ -~'-=-"-- 'J~-~,- --,"r - ' .w-~-~,=,' , - -

" "" " " . . . . i3 ~J 'v"

1.0

+1.o- �9 C -

O -

1 .0

+1.o-

�9 c -

0 -

1 . 0 -

I I APFI M A Y d U N

. , t,. ..'l , . . , .

t~_ " - " " ......... i "" ....... " " ' " J " ' ' ~ " ~ ' '" .......... " " " - - -~-=-~=~'-2"F''~'v" . - - - ' - . . . . . . . . . . . . . . . . . . . . . . - ' - " 2 2 - :

l I J U L A U 6 S E P

�9 - ~ z : . z . , - - - ~ - - z . = , , a = ~ ( 2 2 - - - - ' . . . . . . . . . . . . . . .

; / " " . ' ' . , ,."'~' "., ~ .,--,,A

I I O C T N O V D E C

~ime >

Fig. 1. Daily mean difference in ~ between satellite and synoptic reportings of ship temperatures for 1978. A negative value means that the satellite derived temperature is colder.

The measurements have been taken at 12ZT. Measurements at 00ZT showed a ~imilar trend. " Data from National Environmenta l Satellite Service (NOAA)

30 Dt. hydrogr. Z.36, 1983. H.I. S z e k i e 1 d a, Satellite Investigations

T a b l e 3

Standard deviation around monthly average temperature fin and difference between monthly average at 55 ~ E and 25 ~ S Afm

Standard [m A fr. deviation

March 1979 25.8 ~ 0.6 0.5 ~

April 25.3 0.9 0.9

May 24.4 1.0 0.6 June 23.4 0.6

1.0 July 22.4 0.1 0,5

August 22.3 0.1 0.6

September 22.4 0.9 0.7

October 23.3 0.7 0.3

November 23.6 0.8 1.3.

December 24.9 1.5 1.3

January 1980 26.2 0.3 0.7

February 26.5 0.7 0.5 March 25.8 0.8

0.6 April 25.2 0.8 0.8

May 24.4 0.7 0.6 June 23.7 0.1

1.2 July 22.5 0.5 0.6

August 22.0 0.5 0.7

September 21.5 0.1 1.2

October 21.6 1.0 2.4

November 24.0 2.4 1.4

December 25.4 1.4

L T 27 I - *cl , ~ . . . ,

T 25 1980

2 1 M A M d O A S 0 N 0 Id 7 M A M J a A 0 N D J.

t~me

Fig. 2. Monthly average SST obtained from GOSSTCOMP at 55 ~ E and 25 ~ S for 1979 and 1981). Included is the standard deviation of data around the monthly average

Dt. hydrogr. Z. 36, 1983. H. 1. S z e k i e I d a, Satellite Investigations 31

Based on the average monthly differences compared to the standard deviation it can be assumed that in most cases the standard deviation of the methodological errors may be less than one ~

3 Sea surface temperature distribution

In the use of SST maps one has to keep in mind the standard deviations as outlined before and also that SST maps are commonly combined from data obtained during different clays. Changes in sensor parameters as well as differences in other parameters used for ~dibration may change the quality of the data from one day to another. The true surface temperature may also undergo changes from one day to another. For the whole period of investigation, only a few synoptic SST maps which are based on a one-day observation have been obtained of which some selected samples will be presented here.

For the period March 1979 to August 1979, the antarctic (ACZ) and subtropical convergence zones (STCZ) are well separated and can be identified by the sea surface temperature field. However, based on the location of the gradients, one recognizes that the convergence zones are not static; rather, they undergo drastic changes in their position. In September, the STCZ is very well developed but as a consequence of the heating cycle, it loses its surface temperature characteristics in the G o s s T C O M P data for the following month, and the ACZ only can be recognized by its strong temperature gradient.

Figure 3 shows the strong temperature gradient covering a temperature range of about 20 ~ with a southward deflection at around 20 ~ E through action of the Agulhas current. ~:!milar southward extension of warm water has been observed for all seasons (see the examples shown in Figures 3 through 8). At around September the strong temperature gradient starts to shift further south and in January 1980 a sharp front covering a tempera- ture range of 9 to 15 ~ has been observed. This strong temperature gradient which has been found in January 1980 has been confirmed in 1982 for the same season as shown in Fig. 9. Temperature gradients at that time are as high as 9 ~ per degree latitude. From the temperature distribution it is clear that the temperature gradient located at around 47 ~ S partly covers the range for the STCZ and the ACZ and it seems that portions of both convergences are located close to each other.

The distribution of temperature especially in the vicinity of the Agulhas current is influenced by the interaction between the southward transport of warm water and the modes of retroflexion of the Agulhas current. H a r r i s, L e g e c k i s and F o r e s t [1978] ~bserved that the Agulhas current appears to have two modes of retroflexion, one at about 4 ~ E and one, more commonly, further to the east, at about 20 ~ E off Cape Agulhas.

Apart from the more westward retroflexion mode there is evidence that at certain times of the year, a branch of the Agulhas current penetrates into the South Atlantic close to the continent. Planetary waves, probably induced by the presence of the Agulhas plateau, also occur in the Agulhas return current.

South of Africa isolated warm patches have been observed several times which could be identified in the temperature distribution. Such isolated temperatures have been reported before in connection with eddies separated from the Agulhas current.

In 1978 the presence of warm eddies could be detected with the GOSSTCOMP data. The data south of Africa over a period of four weeks, during which several one-day coverages were obtained, are shown in Fig. 10.

At the beginning of this sequence, on October 7, the southward transport of warm ~-'ter by the Agulhas current can be identified with the sea surface temperature isoline of i=; ~ A similar observation was made almost two weeks later and again on October 24. Three days later, on October 27, a different distribution pattern existed which indicated the non-static conditions of the location of the boundary between the STCZ and the Agulhas current. The sequence of data from October 27 to November 7 indicates that most probably wave propagation may be the cause for the separation of warm eddies from the Agulhas

32 Dt. hydrogr. Z. 36, 1983. H. 1. S z e k i e I d a , Satellite Invest igat ions

4 0 ~

5 o ~

1 0 ~ 2 0 ~ 3 0 ~ 4 0 ~ 5 0 ~ s

Fig. 3. Satellite sea surface t empera tu re in ~ for 6 Apri l 1979 obta ined within 24 hours

3 0 ~ S

4 0 o

5 0 ~

1 0 ~ 2 0 ~ 3 0 " 4 0 ~ 5 0 ~

Fig.4. Satellite sea surface t empera tu re in ~ for 1 June 1979 obta ined within 24 hours

Dt.hydrogr .Z.36, 1983. H. 1. S z e k i e I d a , Satellite Investigations 33

6 d ,5

4 o ~

5 o ~

70 ~ 2 0 ~ J 0 ~ 4 0 ~ 5 0 ~ E

Fig. 5. Satellite sea surface temperature in ~ for 17 August 1979 obtained within 24 hours

10 ~ 20 ~ 50 ~ 40 ~ 50 ~ E

Fig. 6. Satellite sea surface temperature in ~ for 11 September 1979 obtained within 24 hours

34 Dt. hydrogr. Z. 36, 1983. H. 1. S z e k i e I d a , Satellite Investigations

30 ~

40 ~

50 ~

f O ~ 20 ~ 30 ~ 40 ~ 50~

Fig. 7. Satellite sea surface temperature in ~ for 12 November 1979 obtained within 24 hours

30 ~

S

400

50 ~

10 ~ 20 ~ 30 ~ 40 ~ 50 ~ E

Fig. 8. Satellite sea surface temperature in ~ for 18 January 1980 obtained within 24 hours

Dt. hydrogr. Z.36, 1983. H. 1. S z e k i e 1 d a, Satellite Investigations 35

40 ~

5oOl ~

4 0 ~ - -

2 0 ~ 5 0 ~ 4 0 ~ 5 0 ~ E

Fig. 9. Temperature distribution and temperature gradients (expressed in degrees celsius per degree of latitude)

36 Dt . hydrogr . Z. 36, 1983. H. 1. S z e k i e I d a , Satelli te Inves t iga t ions

, - - - - - - W 2 W

No oar, ,

Fig. 10. Sepa ra t ion of wa rm eddies f rom the A g u l h a s cu r ren t and wavel ike p rogress ion into the east . T h e a reas covered in each d i ag ram ranges f rom 34 ~ S to 44 ~ S and 10 ~ E to 40 ~ E.

T e m p e r a t u r e in ~

Dt. hydrogr. Z. 36, 1983. H. 1. S z e k i e 1 d a, Satellite Investigations 37

current and the transport of warm water in east direction. If this is the case, a set of observations has to be timed in a sequence which is shorter than the frequency of the observed wave" propagation. Unfortunately, the GOSSTCOMP data does not give a com- plete daily coverage over a large area. However, the fluctuations, as observed, and the location and shape of the temperature field suggest rather fast changes. From data recorded in 1979 two locations were selected in order to follow the temperature fluctuations. The first location is in the near vicinity of the Agulhas current at 40 ~ S and 24 ~ E, while the second position is located at 45 ~ S and 40 ~ E. The observations showed that both locations depict periodic variations in temperature which exceed the error of the measurements as discussed ~arlier. The frequency of the occurring temperature maxima shows its periodicity at about ou r t een days based on a sampling frequency of about every three days. As can be seen in Fig. 11 at the position 40 ~ S and 24 ~ E, the high temperature pulses are clearly superimposed over the yearly cycle in the temperature fluctuations.

For identification of frictional stresses and the nature of disturbances which have been observed, temperature gradients have been analyzed for a period of over two years. The data along 40 ~ E have been presented in Fig. 12 which shows the areas having temperature gradients greater than 1 ~ per degree latitude. This presentation of SST's further eliminates uncertainties in the data and indicates the position of the STCZ and the ACZ. The analysis reveals that the region between the two convergence zones as located by their northern and southern limits, respectively, are frequently separated by pulses with temperature gradients less than one degree per latitude, and that disturbances with wavelike structures are seen along the boundaries.

Because of the high cloud coverage in the investigated area the STCZ starts to be "Asible through the GOSSTCOMP data early in February/March but later in the year, it can hardly be recognized in the presentation of temperature gradients. Therefore, only for a rather short period, from March to September, SST data can be used for studying the dynamics of the convergence zones with satellite derived SST's. During the southern summer months, the cloud conditions do not allow any investigations for several months. This indeed limits the investigations for only a short period of the year.

The displacement of the convergence zones has been studied during a period when cloud conditions allowed complete SST coverage within two days, whereby the greatest part of the data was acquired within one day. A sequence of data analyzed during the period between June 8, 1979 and July 20, 1979 showed that the position of the ACZ strongly oscillated to the south on June 28, 1979 and back about one week later (Fig. 13). In the vicinity of the Agulhas current, both convergence zones are closed to each other as indicated ~n the gradient maps. However, it does not necessarily mean that they are not separated from each other; moreover, the used field of temperature gradients indicates only the range in which the convergence zones may be located. One limitation of the N - S gradient is that in the case of strong oscillations the isolated maxima in temperature gradients do not necessarily mean a discontinuity in the convergence zones. By using the position of the maximum gradient rather than the isogradients, the position of the convergence zones can be more easily recognized, and it is easier to follow the oscillations of the gradients as well as the changes in relative positions of the convergence lines. Figs. 12 to 16 demonstrate the pulsing nature of the position of the convergence zones, which is probably caused by a periodic wind transport and a resulting perturbation of the temperature gradients.

More important seem to be the dynamics of the circumpolar current which may act on the subtropical and antarctic convergence zone. The stress which the wind exerts on the sea ~rface cannot be balanded by a piling-up of water in the direction of flow because the current is continuous around the earth, and therefore it must be balanced by other frictional stresses at the lateral boundaries of the current. The torque exerted by these stresses along the quasi-vertical boundary surfaces of the current must balance the torque exerted by the wind on the surface.

38 D t . h y d r o g r . Z . 36, 1983. H. 1. S z e k i e I d a , Sa te l l i te I n v e s t i g a t i o n s

20 ~ T ,8 t ~6

- ~ 1979 / ~

APR MAY dUN JUL ALl6 SEP OCT NOV DEC

'0 I" 1 T ,8 .~ 46

t 4

, 2 t ~ m e )

Fig. 11. Sea s u r f a c e t e m p e r a t u r e at 40 ~ S, 24 ~ E, a n d 40 ~ S, 40 ~ E

3 0 o

S

'~0" 500

B

f979

o , o

I I I ii'i

i : iiiiiiiiiiiiiiii iiiiiiiii iiir : : : : :~.: : : : :~:~ "'~::, ~::. o;:::'

I

i I ;~I

400 S

500

......: ~ 9 ~ . . . . . . , , ...7. ,:::::., ::. :/-:..:.:. :'.7"-.'-.'. .... ::-'..-';,

--:::, .:.. ...--,....:.-:.-:::':.":'::-.":::::.-~.:.::..":."'.~.-'-:":::::- . . . . . . ~,',"-~ ggo',', ',",', ', '•o~ 'g~ ; ; ' .V , ' .% ' . " , " , ' ,V , ' , ; ' , " , ~ ,~",~176176176

~ - 1 ~ ' - " . "-::-"." . . . . . .~:.:.A':'.--::.'-'."-:': ":": ' . . . .:::: ' . ' ::-..-:::-: ' : ' .":"': ' : '":"~ - . . . . .x . : - : .>- . . - .,--..-------.-,--.:---:...~.~.:..:.:--..'.'.'.-:---:'.:'-':.:- f ~ I .... r - ' -~ ' " "7-.', . . . . . . . . . . . . . . . . . . . . . .

I.::.::-: ..~.:~.".'" "1::. " '" "..".7." "::..:::..... - I i ~ " ' " " ":" ~-:.':':.":" ""i::".':'; I

I I I . : : : -" - . : ' " I I

I~ I ) I I I . . . . . 1 i ~', MAR APR MAY dUN dUL AU6 SER

• �9

Fig. 12. T e m p e r a t u r e g r a d i e n t s a l o n g 40 ~ E . S h a d e d a r e a s r e p r e s e n t v a l u e s g r e a t e r t h a n o n e ~ p e r d e g r e e l a t i t ude

Dt. hydrogr. Z.36, 1983. H. 1. S z e k i e 1 d a , Satellite Investigations 39

50 �84

12JUNE i979

5d

#5JUNE t979

4@ S

50'

40' 5

50"

i 40"

50"

22JUNE 1979

28JUNE t979

~OJULY 1979

I I I I 10 ~ 20 ~ 50 ~ 4 0 ~ 50 ~ E

Fig. 13. Location of the maximum temperature gradient in N-S direction indicating the gradients for the subtropical and antarctic convergence zone

40 Dt. hydrogr. Z. 36, 1983. H, 1. S z e k i e I d a , Satellite Investigations

4 0 ~ S

45 ~

50"

JO ~

3 )2 JUNE 19 79

i I I I 20 ~ 30 ~ 40 ~ 50 ~ E

Fig. 14. Location of temperature gradients in N-S direction in ~ per degree latitude

Dt. hydrogr. Z. 36, 1983. H. 1. S z e k i e I d a , Satellite Investigations 41

4 0 ~ S

4 5 ~

5 0 ~

_ o / ~2 JUNE ,9,9

~ _ ~ . ~ ] 28 JUNE /979

t0 ~ 20 ~ 30 ~ 4 0 ~ 5(::1' E

Fig. 15. Location of the temperature gradients in N-S direction in ~ per degree latitude

42 Dr. hydrogr. Z. 36, 1983. H. 1. S z e k i e 1 d a , Satellite Investigations

4 0 ~ 3

4 5 o

5 0 ~

~ ' 1 17JULY I979

I I I t 20 o 50 ~ 40" 50~ E

Fig. 16. Location of the temperature gradients in N-S direction in ~ per degree latitude

Dt. hydrogr. Z. 36, 1983. H. 1. S z e k i e 1 d a , Satellite Investigations 43

A c k n o w l e d g e m e n t s

The digital data used in this study have been furnished by the U. S. D e p a r t m e n t of C o m m e r c e N O A A / N E S S . Imagery mater ia l has been purchased f rom E D I S / S D S D . The description of the ret r ieval technique has been extracted f rom B r o w e r , G o h r b a n d , P i c h e 1, et. al. [1976].

Especial ly I would like to thank Dr. Pichel for his support and reading this manuscript .

References

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S z e k i e l d a , K.-H., W. E. S h e n k and V . V . S a l o m o n s o n , 1974: Variability of sea-surface temperatures of the southern In- dian Ocean. J. Cons. int. Explor. Mer. 35, 143 148.

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W a r n e c k e , G., L. L. J. A l l i s o n , M. M c M i l l i n , andK. H. S z e k i e l d a , 1 9 7 1 : Remote sensing of ocean currents and sea surface temperature changes derived from the Nimbus 2 satellite. J. Phys. Oceanogr. 1, 45-60.

Eingegangen am 7. Juli 1982

Angenommen am 25. M/irz 1983

Anschrift des Verfassers: Dr. Karl-Heinz Szekielda, 20 Waterside Plaza, Apt. 20 F, New York, New York 100 10, U.S .A.