Some Aspects of Evaporation and Sensible Heat Flux of the Tropical Atlantic
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Some Aspects of Evaporation and Sensible Heat Flux of the Tropical Atlantic*)
D. Henning and H. Flohn
Meteorologisches Institut, Bonn
(Manuscript received 2.11.1979, in revised form 29.4.19801
Abstract:
Fluxes of water vapour and sensible heat are estimated from routine measurements (1953-1965) along the sea
routes from Europe to Africa (E) and South America (W). Seasonal and latitudinal patterns are highly influ
enced by the upwelling regions just south of the equator and within the area of the Canaries Current (south
of latitude 20 ON). Mean evaporation is less than 25 mm/month at 13 oN, E-rt., in March and April and like
wise at lOS, E-rt., in July. The 20 °N-to-10 aS E-r1. averages of 900 mm/year, evaporation, and of 2.85 W/m2,
flux of sensible heat, amount to 78 percent and 67 percent of the respective Wort. figures.
Both fluxes exhibit the highest interannual variability within the two upwelling regions at the E-route during
the respective seasons. During the year of 1963 with a large-scale circulation anomaly in the northern hemi
sphere in winter particularlY strong anomalies are observed in both upwelling regions with evaporation ex
ceeding its respective averages by more than 80 percent between 0° and 4 Os from June to October and by
more than 40 percent between 16 ON and lOoN from February to April.
According to HASTENRATH and LAMB (1 978b) even the mean annual flux of sensitrle heat is negative over
laIge areas south of the equator. This cannot be confirmed here. Probably, there exists a data problem.
Finally, the possibility is considered whether the upwelling-induced reductions of evaporation and their varia
tions might give some insight into the mechanisms of climatic fluctuations.
Zusammenfassung: Verteilung und Veranderlichkeit der Verdunstung und des Stroms fUhlbarer Warme Uber
dem tropischen Atlantik
Aus Schiffsbeobachtungen (1953-1965) langs der Seefahrtswege von Europa nach Afrika (E) und SUdameri
ka (W) werden die Strome von fiihlbarer Warme und Wasserdampf abgeleitet. Die Kaltwasser-Auftriebsgebiete
siidlich des Aquators und im Kanaren-Strom (siidlich 20 ON) bestimmen den Jahresgang und die Abhangigkeit
von der Breite. Dabei sinkt langs der E-Rt. im Marz-April urn 13 ON, im Juli urn 1 oS - die mittiere monatli
che Verdunstung unter· 25 mm ab, die Jahressumme unter 700 bzw. 650 mm. 1m Mittel iiber die ganze Zone
20 ON -10 aS erreichen die Jahreswerte langs der E-Rt. 900 mm bzw. 2.85 W/m 2 ; das sind 78 % bzw. 67 % der
Werte langs der WoRt.
Beide Fliisse weisen ihre hochste interannuelle Veranderlichkeit im Bereich der beiden Kaltwassergebiete, be
sonders entlang der E-Rt. in den betreffenden Jahreszeiten auf. 1m Jahre 1963 mit einer gro~raumigen win
terlichen Zirkulationsanomalie auf der Nordhalbkugel treten in den beiden Aufquellgebieten besonders star
ke Anomalien auf: Zwischen 0° und 4 oS lag die Verdunstung von Juni bis Oktober urn mehr als 80%, zwi
schen 16° und 10 ON von Februar bis April urn tiber 40 % tiber dem Durchschnitt.
Die teilwelse noch im Jahresmittel abwarts gerichteten Strome fiih1barer Warme, die HASTENRATH und
LAMB (1978 b) stidlich des Aquators gefunden haben, werden durch unser Material nicht bestlitigt; hier scheint
ein Daten-Problem vorzuliegen.
Zum Absch1~ wird die M(iglichkeit diskutiert, mittels der auftriebs-bedingten Anderungen der Verdunstung
ein vertieftes Verstandnis fUr den Mechanismus der Klimaschwankungen zu gewinnen.
*) Dedicated to Herbert Riehl on the occasion of his 65th birthday.
430~"lff Contributions to Atmospheric Physics Vol. 53 No.3, August 1980
0005-8173/80/030430·12 $ 02.00/00 © 1980 Friedrich Vieweg & Sohn Verlagsgsellschaft mbH
i
\ ..PALTRIDGE, G. W., and PLATT, C.M.R., 1976: Radiative Processes in Meteorology and Climatology, Chap. m. Develo
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PERRIN DE BRICHAMBAUT, 1963: Rayonnement solaire et echanges radiatifs naturels. Gauthier·Villars.
PROSPERO, J. M. and CARLSON, T. N., 1972: Vertical and aeral distribution of Saharan dust over the Western Equato
rial North Atlantic ocean. 1. Geophys. Res. 77,5355-5365.
PROSPERO, 1. M., NEES, R. T., 1976: Dust concentration in the atmosphere of the Equatorial North Atlantic: pos
sible relationship to the Sahelian Drought. Personal Report.
PROSPERO, J. M., NEES, R. T. and SAVOIE, D.: Atmospheric aerosol measurements during GATE. Univ. Miami Tech.
Rept. TR 76-5.
PROSPERO, J. M., CARLSON, T. N., SAVOIE, D. and NEES, R. T., 1977: Atmospheric turbidity over the Northern
Equatorial Atlantic during the summer of 1974: the impact of North African Aerosols. Personal Report.
RANGARAIAN, S., 1972: Wavelength exponent for haze scattering in the tropics as determined by photoelectric photo- .' . .
meters. Tellus XXIV, 56-64. ...
SAVOlE, D. and PROSPERO, 1. M., 1976: Sahara aerosol transport accross the Atlantic ocean: characteristics of the
input and the output. Bull Amer. MeteoroL Soc. 57,145.
SCHOEPP, W., 1949: Die Bestimmung der Komponenten der atmosphiirischen TrUbung aus Aktinometermessungen.
Springer Verlag, Wien.
VAN DE HULST, H., 1952: Scattering in the atmospheres of the earth and the planets, In Kuiper: The Atmospheres
of the Earth and planets. The University of Chicago Press. 2nd edition.
VOLl, F. E., 1959: Photometer mit Selen-Photoelement zur spektralen Messung der Sonnenstrahlung und zur Bestim·
mung der WellenUingenabhangigkeit der Dunsttriibung. Arch. Meteor. Geophys. Bioklim. B 10, 100-131.
VOLl, F. E. and GOODY, R. M., 1962: The intensity of twilight and upper atmospheric dust. 1. Atmos. ScL 19,
385-406.
VOLl, F. E., 1966: Volcanic dust and a global tWilight network. 1. Rech. Atmos. 2, 327-330.
VOLl, F. E., 1969: Twilight and stratospheric dust before and after the Agung eruption. Appl. Opt. 8, 2505-2517.
VOLl, F. E., 1969: Stratospheric dust striations. Bull Amer. MeteoroL Soc. 50, 16.
VOLl, F. E., 1970: On dust in the tropical and midlatitude stratosphere from recent twilight measurements, 1.G.R.
75, 1641-1646.
VOLl, F. E., 1970: Spectral skylight and solar radiance measurements in the Caribbean: maritime aerosols and Sahara
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429Resume: Repartition et variabilite de I'evaporation et du flux de chaleur sensible sur l'Atlantique tropical
Les flux de vapeur d'eau et de chaleur sensible sont estimes a partir d'observations de bateaux (1953-1965)
sur les trajets d'Europe vers I' Afrique (E) et vers l'Amerique du Sud (W). Les regions de remontee d'eau froide
au sud de l'equateur et dans Ie courant des Canaries (au sud de 20 ON) influencent fortement les phenomenes
a
au cours de I'annee et en fonction de 1a latitude. Sur Ie trajet E, I'evaporation moyenne est inferieure 25 mm
a a a
par mois 13 0 N en mars et avril et 1 ° 5 en juillet; la somme annuelle est respectivement 700 et 650 mm.
En moyenne sur toute la zone de 20 ° N a 10 0 5, les valeurs annuelles atteignent, Ie long tu trajet E, 900 mm
pour l'evaporation et 2,85W/m 2 pour Ie flux de chaleur sensible, ce qui repCllsente respectivement 78% et 67%
des valeulS relatives ala route W.
Les deux flux ont 1a plus grande variabilite interannuelle dans les deux regions de remontee d'eau fro ide, sur la
route E durant les saisons concernees. Au cours de l'annee 1963, ou apparail: une anomalie de circulation a grande
echelle dans l'hemisphere nord en hiver, des anomalies partieulierement fortes sont observees dans les deux
regions de remontee, l'evaporation depassant les valeurs moyennes respectives de plus de 80 % entre 0 0 et 4 oS
a
de juin octobre et de plus de 40 % entre 16 "N et 10 ON de fevrier it avril.
Suivant HASTENRATH et LAMB (197 Bb), me me Ie flux moyen annuei de chaleur sensibles est negatif sur de
vastes regions au sud de l'equateur; ceei n'est pas confllme par cette etude, iI s'agit probablement d'un probleme
de donnees.
En terminant, on examine la possibilite d'arriver a une meilleure comprehension du mecanisme des fluctuations
a
climatiques I'aide des variations de l'evaporation induites par les remontees.
1 Introduction
Routine meteorological data from the thirty 2° X 5° fields shown in Figure 1 are used in order to
estimate individual monthly averages of evaporation and sensible heat flux during the period 1953-1965.
The data were made available to Prof. H. RIEHL in 1967 by the Seewetteramt (Deutscher Wetterdienst)
in Hamburg and then evaluated by one of us (H); they served for a study of interrelations between struc
ture and intensity of the Intertropical Convergence Zone (ITCZ) and hurricane frequency. A report written
on this subject has never been published; only a few results were presented by FLOHN (1972, Figures 3
and 4).
During the last two decades climatologically relevant estimates of surface energy fluxes for the Atlantic
iI..
?cean were carried out by ALBRECHT (I 960, 1961), BUDYKO (1963), BAUMGARTNER and REICHEL
(1975), BRITTON et al. (I976), BUNKER (1976), BUNKER and WORTHINGTON (I976), and most thor
oughly by HASTENRATH and LAMB (I 978b). The results presented here in part also have climatological
character, featuring some conspicuous permanent quasi-meridional as well as zonal gradients of the sur
face energy fluxes in some detaiL Beyond this, the paper deals with the year-to-year changes of these
fluxes.
2 Data
The data available were area-averaged individual monthly mean values of sea-surface tempera
ture, air and dewpoint temperatures, wind speed, and surface pressure from the two sea routes shown in
Figure I which shall be referred to as Wort. and E-rt. (rt. = route). The mean values of saturation and of
actual specific humidity were derived from the averages of air and dewpoint temperatures, wind speed
was converted from Beaufort units (B) into metric units (v, in m/s) by application of
v = .836 a B3/2. (1)
With a = 1, (1) is the formula given by List (I 966). Here a = 1.1 was used in order to avoid underestima
tes owing to the non-linearity of the Beaufort scale.
431• Figure 1
Area of investigation
• BUd 1
Untersuchungsgebiet
The total amount of data from which the monthly mean values had been derived exceeded 130000 in the
case of sea-surface and dew-point temperature respectively, and was about 150000 for each of the remain
ing quantities; 64 (36) percent of these data sets were observed along the E- (W-rt.). The minimum num
ber of original data per rectangle and individual month typically varies between 15 and 65. However, be
cause most results are presented for groups of adjacent months and rectangles, these figures mUltiply.
3 Procedure
Evaporation (E) as well as the flux the sensible heat (H) were estimated by application of bulk
aerodynamic formulae as
(2)
and
H== CHPCp(tw-t)v. (3).1
P stands for the density of moist air, qs and q are the specific humidities of "air in contact with salt water"
(BUNKER, 1976) and in a standard height assumed to be 10 m, v stands for the wind speed in this height,
c p is the specific heat of the air at constant pressure, tw and t represent sea-surface and air temperatures
(t in 10m), and CE and CH are the transfer coefficients for water vapour and sensible heat, respectively,
which are assumed to be equal, but variable. They were taken, with a reduction of 10 percent, from
BUNKER's Table 4 given 1976 which contains coefficients for 63 combinations of density stratification
and wind speed. The 10 percent reduction seems to be appropriate because of different assumptions
ff concerning the assessment of wind speed. So, for near neutral stratifications and winds around 8 mls
~ in 10m a transfer coefficient of 1. 296 X 10-3 is applied. This agrees quite well with the results of experi
ments carried out in the Tropical Atlantic by KROGERMEYER (1975) and by HASSE et aI. (I978), respec
tively, who, for conditions as above, give averages of CE = (1.20 ± .16) X 10- 3 and CE = (1.15 ± .22) X 10- 3 ,
respectively. Under the same conditions a CE == 1.13 X 10-3 was recan tly found by MERLIVA T (1978)
applying an isotopic method based on distributions of deuterium and oxygen 18.
The mean values of Figure 2 were calculated by averaging the individual monthly fluxes. The 15 E-rt. rec
tangles occupy nearly 1.64 X 106 km 2 compared with 1.80 X 10 6 km 2 occupied by those of the W-rt. Of
course, the ship's observations are not evenly distributed over the areas.
4324 Error estimates
The figures discussed below may be erroneous to a degree generally adherent to evaluations of
ordinary ship data by bulk formulae. However, the original data had been checked scrupulously as a
matter of routine, by staff members of the Seewetteramt, Hamburg, before their delivery to H. RlEHL.
Also,judging from the travelling speeds, it seems quite unlikely that more than three measurement series
were made on board of the same vessel within a special rectangle (s. Figure 1) during a special month.
At about 25°C to 26 °c a possible error of sea-surface temperature of .2 °c causes the evaporation calcu
lation to be uncertain by about 7 percent. An error of 6 percent is introduced by a dewpoint temperature
around 22°C which fails by .2 °c. So, because the temperature measurements were made by psychrome
ter and because we deal with large numbers of observations, it seems to be realistic to assume that the
lr.,ncertainty of our estimates, because of data shortcomings, remains below 10 percent.
There are still two other sources of systematical errors. First, the factor a 1.1 in expression (1) might be
up to 4 percent too large in ~ with high frequencies of low wind speeds. Secondly, the neglect of cor
relation products like (qs - q) 'v' (where the primes designate the deviations of the daily means from their
monthly averages) causes, due to KRAUS and MORRISON (1966), errors of either sign up to 15 percent,
however, these products seem to be negative more often than positive. Thus, there is a tendency that our
estimates - though they are rather conservative - still might be too large by about 5 percent. At any rate,
the internal homogeneity of the data set seems to be sufficient to allow reliable comparisons in the space
and time domain.
5 Results - Climatology
With respect to the climate of the tropical Atlantic and the identification of the ITCZ based on
different parameters (including the horizontal wind components), reference can be made to investigations
of KUHLBRODT (1942), HOFLICH (1974), and to those of HASTENRATH (1976, 1977), LAMB (1977) and
HASTENRATH and LAMB (1977a, 1977b, 1978a); many of the analyses given could be reproduced by our
data. The seasonal oscillation of the ITCZ and related phenomena between 19 ~ (mean surface pressure
minimum in July, E-rt.) and 7 Os (mean sea-surface and air temperature maxima in April, W-rt.; the maxi
'. 'llum convergence shifts between about 18 ~ and lOS) are dominating the tropical Atlantic's climate.
r..t is the importance of this zone for the dynamics (FLOHN, 1957), and above all for the energetics (RIEHL
and MALKUS, 1958) - which might impart special interest to estimates of energy fluxes from the surface
of this latitude belt.
The time-latitude cross-sections of Figure 2 show the average patterns of evaporation and sensible heat
flux along the two sea routes. The presentation of evaporation in mm/month*) instead of latent heat flux
has been preferred in order to stress its nature as a component of the water balance. - The features along
the E-rt. (Figures 2b and 2d) have a somewhat more organized appearance than those along the W-rt.,
which is due to two cold-water phenomena: the coastal upwelling within the Canaries Current north of
8 ~ between December and June and the upwelling water just south of the equator, mainly from June to
October. In recent years investigations of the Canaries Current and of the wind field above were carried
out by MITTELSTAEDT (1976), WOOSTER et a1. (1976), HUYER (I 976), and SPETH et al. (1978), with the
most thorough description of the phenomenon given by WOOSTER et aI. (1976). The seasonal cycle of the
surface temperature is developed most between latitudes 10 ~ and 20 ~ with the minimum reached in
March (with less than 18°C at and north of 20 ~) and the strongest meridional temperature gradient be
tween latitudes 8 ~ and 13 ~. Both features are nearly verified by our data: the minimum sea-surface
temperature occurs at 19 ~ in April with less than 19°C.
*) At 2S °c, 100 mm/month "" 92.9 W/m 2 •
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'" 0 GO 0 GO 0 CDThe second phenomenon, the equatorial upwelling has not found much attention so far in the Atlantic. It
can be realized e.g. from the respective sea-surface-temperature charts of MAZEIKA (1968) or HASTEN
RATH and LAMB (1977a) that a cold-water tongue stretches westwards just south of the equator - a famil
iar phenomenon also in the eastern Pacific Ocean, off the Ecuador-Peru coast - from the coast of Gabon,
Zaire and Angola during the northern hemisphere summer with surface temperatures below 23°C in July
as far west as 12 CW. Our data show less than 22.5 °c as mean surface temperature of the rectangle imme
diately south of the equator, E-rt., in July. With temperatures above 28°C the surface is warmest here in
March and April, cf. also the "Climatic Atlas ..." of HASTENRATH and LAMB (1977a).
The restriction of this upwelling phenomenon to a relatively narrow belt south of the equator and espe
cially its quasi-seasonal amplitude changes suggest that it is mainly controlled by the surface wind field
.....ccording to
w =(pf)-l rot z i (4)
where w is the vertical velocity at the bottom of the oceanic Ekman layer, f the CorioIis parameter, and
rot z i the'wind stress curl (compare YOSHIDA and MAO, 1957, and HANTEL, 1972). There is strong evi
dence from our data which is confirmed by the respective maps of HASTENRATH and LAMB (l977a)
compare also FLOHN (1957, Figures 9 and 10) for the existence just at the southern border of the ITCZ,
of a belt of zero relative vorticity with positive values to the north and negative ones to the south. While
from January to May relative vorticity is negative only south of about 1 "N, the area with negative relative
vorticity extends considerably farther north simultaneously with the northward shift of the ITCZ
during the northern hemisphere summer months so that, while fin (4) changes its sign across the equator,
rot z i does not within a range of about 8 degrees latitude on each side of the equator from June to Sep
tember/October. An expansion of negative relative vorticity during May to September up to latitude 7 ON
was demonstrated already by FLOHN (1957, Figure 9) and had been interpreted as the result of a trans
equatorial transport of absolute vorticity along the streamlines. The remarkably strong gradients across
the equator during this period in Figures 2b and 2d (E-rt.) document the abrupt disappearance of upwell
ing north of the equator. Minimum values of sensible and latent heat flux just south of the equator occur
also along the Wort. (Figures 2a and 2c) their smallness, however, demonstrates that upwelling here is
much less effective.
,-"nnual time-integrals of Figure 2 are compiled in Table 1. Along the E-rt., the equatorial minimum at
1 Os is lower than that of the coastal upwelling at 13 "N. These 646 mm/year at lOS, E·rt., are about
equal to the mean annual evaporation of the central Baltic Sea, between 55° and 58 "N (HENNING, un
published results), or, according to BAUMGARTNER and REICHEL (1975), to the mean oceanic evapora
tion of the latitude belt 50° -55 "N. However, according to TREMPEL (1978), the evaporation also
derived from ship data - of the Peru Current at latitude 5 °S-O° does not even exceed 330 mm/year.
• Figure 2 a) Mean flux of sensible heat [W/m2) along the W-rt., 1953-1965
b) Mean flux of sensible heat [W/m2) along the E-rt., 1953-1965.
c) Mean evaporation [mm/month] along the W-rt., 1953-1965
d) Mean evaporation [mm/month] along the E-rt., 1953-1965
Isolated - (+) sign means minimum (maximum)
Bild 2 a) Mittlerer Flul),fiihlbarer War me [W/m2] entlang der W-rt., 1953-1965
b) Mittlerer Flu~ fiihlbarer Warme [W/m2] entlang der E-rt., 1953-·1965
c) Mittlere Verdunstung [mm/Monat) entlang der W-rt., 1953-1965
d} Mittlere Verdunstung Imm/Monat] entlang der E-rt., 1953-1965
Isolierte - (+) Zeichen bedeuten Tiefst- (Hochst-)werte
435• Table 1 Mean annual amounts of evaporation [mm) (E), mean annual averages of sensible heat flux IW1m2) (H), and the
Bowen ratio values, [% I (Bow) of the mean annual fluxes
• Tabelle 1 Mittlere jahrliche Verdunstungshohen [mm) (E), Jahresmittel der FlUsse fiihlbarer Warme (W/m2) (H), und die
Werte des Bowen-Verhiiltnisses der Jahresmittelwene [%1 (Bow).
E H Bow
Lat. W-rt. E-rt. W-rt. E-n. Wort. E-n.
19 ON 1187 953 2.59 .99 2.8 1.3
17 1202 889 4.38 -.11 4.7 -.2
15 1196 756 4.41 -.29 4.8 -.5
13 1119 683 4.07 .69 4.7 1.3
11 1080 703 4.17 2.31 5.0 4.2
9 1155 697 4.98 3.50 5.6 6.5
7 1146 798 4.87 4.71 5.5 7.6
5 1114 962 4.80 5.99 5.6 8.1
3 1091 1083 5.20 6.22 6.2 7.4
1 ON 944 847 3.89 4.25 5.3 6.5
1 Os 971 646 3.63 1.03 4.8 2.1
3 1160 784 3.56 .56 4.0 ~
5 1279 1043 3.86 2.23 3.9 2.8
7 1355 1203 4.69 3.15 4.5 3.4
9 Os 1356 1279 4.36 4.34 4.2 4.4
20° -10 ON 1156 792 3.92 .86 4.4 1.4
lOON_0° 1090 882 4.75 4.96 5.6 7.3
00-10 0S 1219 990 4.00 2.26 4.2 3.0
I 200N-I0 Os 1154 899 4.23 2.85 4.7 4.1
From our estimates, the W-E-rt. averaged mean evaporation between 10 Os and 5 Os - a latitude belt
fully within the regime of the SE-trades - amounts to 1270 mm/year. The estimate of HASTENRATH
and LA MB (1978b) based on data from the total area of this latitude belt at the Atlantic is 1396 mm/year,
that of BAUMGARTNER and REICHEL (1975) 1480 mm/year. The difference between 1396 and 1270
year could be fully accounted for by the application of different transfer coefficients, the choice of ...
ml
HASTENRATH and LAMB (1 978b) was CE = 1.4 X 10-3 =constant.
As can be seen from Table 1 the minima of sensible heat flux south of the equator as well as at the Canar
ies Current, both E-rt., are met "upwind" of the respective evaporation minima, and this is true also in
most of the relevant months: the air experiences its maximum of heat loss where the strongest "reverse"
sea-surface-temperature gradients occur, while evaporation reaches its minimum farther north, where the
air has somewhat cooled down (or failed to warm up). Likewise, the sensible heat flux minimum south
of the equator, E-rt., in June precedes the respective evaporation minimum, compare Figures 2b and 2d.
During the period of intense upwelling south of the equator, E-rt., the latitude belt of the sea-surface
temperature minima is indicated by the evaporation minima and not by those of sensible heat flux.
In spite of negative sensible heat flux in the equatorial upwelling region between June and October, on
the annual average the ocean here also supplies sensible heat to the air, see Table 1. This, however, is not
in accordance with the data given by HASTENRATH and LAMB (1978b). Chart 44 of this atlas shows be
tween longitudes 28 OW and 3 OW a wide latitude belt from the equator southward where the mean annual
flux of sensible heat is downwards. Similar areas are documented each month which cannot be identified
with our data. These differences might be a data problem. HbFLICH (1974) suspects that till the 1930's
air temperature, at least on German vessels, might have been measured up to .7 °c too high due to measur
ing techniques which have gradually been improved thereafter.
4366 Results - Variability
Some of the results concerning the individual years are summarized in Figure 3 as well as in
Table 2. Figure 3 shows the estimates for the area and the season which are most affected by the equa
torial upwelling. Mainly these figures, their W/E-rt. differences and their year-to-year variability - with
special respect to 1963 - suggest some palaeo-climatological considerations which can only be mentioned
very short in the last chapter. The J3-years averages applying to Figure 3 are evaporation heights of
504 mm, W-rt., and 219 mm (:;:,: 44 mm/month), E-r1., respectively, and sensible heat fluxes of 3.02 W/m 2
and -1.50 W/m 2 for the respective Wort. and E-rt. rectangles. The accompanying variation coefficients
(standard deviations divided by the arithmetic means) amount to 9.5 and 26.4 percent as for W- and E-rt.
...::aporation, and to 43.4 and 68.1 percent as for the respective fluxes of sensible heat. The largest ano
aly occurs in 1963; only in this year sensible heat was transferred from the ocean to the atmosphere,
and the E-rt. evaporation then reached W-rt.levels (data base: more than 360 ship observations). At the
W-rt., evaporation for June to October varied between 609 mm (:;:': 122 mm/month) in 1953 and 439 mm
in 1963 (!). The smallest 5-months total of evaporation at the E-rt. was estimated to 163 mm (= 32.6 mm/
month) for 1964. The smallest evaporation in a single month is 12 mm, obtained for July 1962 (based on
62 observations). -- -.
Columns I and II of Table 2 are supplements to the lower part of Figure 3. Like column II, column III
shows 3-months evaporation heights for the period of most intense upwelling, however, at the Canaries
Current; the reduction of evaporation here is stronger, the year-to-year variability less than south of the
equator. Very conspicuous are the high amounts of evaporation in 1963 at both areas: this coincidence
draws the a~tention to the 1963 anomalous circulation patterns which were observed over large parts of
the northern hemisphere (compare DIE GROSSWETTERLAGEN MITTELEUROPAS, 1963) until June,
resulting in a very low index situation, especially from January to March over the North Atlantic and
Europe.
2
"'W/m
6
4
2
a
-2
• Figure 3
1953 54 55 56 57 58 59 60 61 62 63 64 65 Flux of sensible heat - upper graph - and
evaporation - lower graph - averaged over
W/m2 mm/month latitudes 4 ·S-O· and the months June..Qctober
120 - Blank columns: Wort.
120 Hatched columns: E-rt.
100 •.. -
100
--~ ~
80 1---- • BUd 3
80
Flue, ftihlbarer Warme oberes BUd - und
60 60 Verdunstung unteres Bild -,jeweilsge
40 40 mitteIt tiber die Breiten 4 ·S-O· sowie die
20
0 II II 20
o
Monate J uni-Oktober
We~e Siiwen; W-Route
Schraffierte Siiwen: E-Route.
437• Table 2 Evaporation E in mm and flux of sensible heat H in W/m2 for individual seasons and latitude belts, as indicated
below
• TabeUe 2 Verdunstung E in mm und Strom flihlbarer Wiirme H in W/m2 in den einzelnen Jahren fiir die unten angege
benen Jahreszeiten und Breitenbereiche
I II III IV V VI VII VIII IX I
Yr./rt. W E E E W E W E W E W E E W E
1953 113 501 423 189 208 290 296 276 160 - 5.4 .2
1954 - - 83 - 494 424 208 253 337 322 254 147 - 4.6 .6
1955 - 76 - - 418 185 250 305 323 232 156 - b! .6
1956 83 484 402 211 244 284 308 272 133 - 7.6 1.4
1957 - - 116 98 407 398 168 199 235 285 228 146 -4.6 3.4 1.0
1958 1073 570 89 80 476 391 217 232 336 318 246 121 - 3.0 3.9 =.~
1959 1088 713 122 58 470 408" 227 223 285 314 240 168 -.8 6.3 .7
1960 1183 742 106 83 497 399 229 214 318 299 290 137 -4.5 11.5 .8
1961 1055 728 107 93 460 400 166 211 303 293 279 164 - 4.3 5.7 1.4
1962 960 705 74 85 388 237 - 305 224 103 5.8 5.7 - 3.7
1963 956 966 281 123 446 439 215 197 231 304 233 2164.8 4.4 3.2
1964 1010 646 91 78 478 390 218 220 283 311 247 147
-6.4 4.5 1.3
1965 1087 738 99 76 490 407 214 237 321 301 261 149
- 5.2 3.6 1.1
Mean 1052 726 111 86 473 407 204 225 294 306 • 253 150 -4.4 .6
5.3\
I (]* .071 .156 .483 .207 .058 1.038 .105 .083 .116 .038 J.085 .177 -.38 .44 2.67
I: § at Eq-4 oS, year; II: § at Eq-4 oS, June-Aug.; Ill: § at 16 ° -10 "N, Feb.-Apr.;
IV: § at 20 °N_lO ·S, June-Oct.; V: § at 20° -lOoN, July-Sept.; VI: § at 10 "N-Eq, June-Aug.
VII: § at Eq-10 oS, July T Aug.; VIII:!:! at 18 °-12 ON, Feb.-Apr.; IX:!! at Eq-10 ·S, July T Aug.
Eq: equator, (]*: standard deviation divided by the arithmetic mean.
In columns IV to VI of Table 2 the year-to-year variability of the E-rt. evaporation heights is always
smaller than that obtained for the respective Wort. figures. When averaging over the entire latitude belt
(20 "N-lO oS, column IV, E-rt.) 1963 still turns out as the year of maximum evaporation; the 1963 It/JI
evaporation estimate, however, represents the minimum in the case of column V (20° -10 "N, E-rt.).
Column VI covers the area and season of strongest E-rt. evaporation. In columns VII and IX the Eq-l 0 Os
averages of both evaporation and sensible heat flux are compiled for the two-months period of July and
August; the figures of sensible heat flux of column VIII are the parallels to the evaporation estimates of
column III, both series representing fluxes in the core region of the Canaries Current.
7 Conclusions and Outlook
Our data show that upwelling phenomena near the equator, while occurring fairly regularly,
show rather different intensities during northern hemisphere summer seasons, when the southern hemi
sphere circulation reaches its greatest intensity and pushes the Intertropical Convergence Zone into its
most northerly position. In contrast to this, during northern winter - when the temperature gradients
equator-pole are nearly equal at both hemispheres (FLOHN 1967) and the ITCZ is situated near the
equator - nearly no upwelling is observed. Obviously, this cannot be explained in terms of divergence of
the Ekman flow at both sides of the equator.
The spatial coherency of equatorial anomalies is apparently relatively weak at the western parts of both
Atlantic and Pacific, as indicated by the seemingly low correlation between our data along both routes.
438This feature may perhaps be due to the fact that both oceans are half-closed basins near the equator, and
that zonal gradients - as described by WYRTKI (1975) - prohibit correlations like this; further investiga
tions are needed.
Available data (ALLISON et al. 1971, BARNETT 1977, WYRTKI 1977) from the Pacific seem to indicate
that the interannual variability there is greater than at the Atlantic (cf. however Table 2, Column II);
the enormous fluctuations of rainfall e.g. at Nauru (0.5 oS, 169 "W), varying between 95 mm/year and
more than 5000 mm/year, are nowhere surpassed. DOBERITZ (1969) found some evidence for a negative
correlation between the sea-surface-temperature anomalies in the El-Nmo region of the Pacific and those
at the Atlantic. Seasonal oscillations of upwelling in the eastern equatorial Pacific, however, seem to be
less pronounced than those at the eastern equatorial Atlantic .
....le interannual variations of evaporation are sufficiently great to suggest weak statistical variations of the
tropical, perhaps even of the global water budget. This should be investigated in the equatorial Pacific,
where these variations are probably stronger and the area of simultaneous occurrence of upwelling (down
welling) extends over 100°-120° longitude. At the Atlantic E-rt. the annual evaporation in the belt
2 ~-8 Os varies in only eight years, between 765 mm (1958) and 1134 mm (I963). Recent palaeo
climatic data suggest a remarkable negative correlation between the extension of tropical rainforests in
both Africa and South America and the intensity (and/or frequency) of upwelling along the equator
(SHACKLETON 1977, FLOHN 1979). This, together with the dependence of the atmospheric CO 2 increase
on the interannual variability of upwelling/downwelling in tropical oceans (NEWELL et al. 1978) suggests
that equatorial upwelling may have a greater importance for the understanding of the mechanisms of cli
matic fluctuations (FLOHN 1979) than hitherto has been realized.
Acknowledgement
A financial support for this investigation by the Deutsche Forschungsgemeinschaft is gratefully
acknowledged .
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441Short Contribution
Calibration of the METEOSAT IR·Channel by Ground Measurements
Peter Koepke
Meteorological Institute, University of Hamburg!)
(Manuscript received 8.4,1980, in revised form 25.4,19801
Abstract:
IR-radianccs at METEOSA T have been determined from radiative transfer calculations, basing on measured
values of the relevant optically acting parameters. The calibration of the IR-channel of the METEOSAT radio
meter was made by comparing the calculatcd radiances with the coincident radiometer counts. The calibration
yields temperatures more than 2 K less than those from the latest IR-calibration table of ESOC (MORGAN,
1979/6 and 1980/7). Different sources of this discrepancy ate discussed.
Zusammcnfassung: Eichung des METEOSAT IR-Kanals mittels Bodenmessungen
Basicrend auf gemesscncn Werten der relevant en optisch wirksamen Parameter wurden IR-Strahldichten am
METEOSAT berechnet. Durch Vergleich dieser Strahldichten mit zugehOrigen METEOSAT-Grauwerten wur
de der IR-Kanal geeicht. Diese Eichung ergibt urn mehr als 2 K geringere Temperaturen als nach der letzten
IR-EichtabclIe des ESOC (MORGAN, 1979/6 und 1980/7). Verschiedene Grtinde fUr den Unterschied werden
diskutiert.
Method
Spectral radiances emerging from the atmosphere into the direction of METEOSAT can be ...".,
calculated, jf the relevant optically acting parameters are known. Weighting with the spectral response
of the radiometer and integration over the wavelength yields radiances LSAT , which can be compared
with the respective radiometer counts. As usual in the IR, the radiances are expressed in terms of the
equivalent blackbody temperatures T SAT. Thus the radiometer counts are calibrated in temperatures.
The measurements have been made on c10udfree days at the research platform "Nordsee" at 54.7 "N and
7.2 °E, 70 km off-shore. Thus the atmosphere and the underlying surface can be treated as horizontally
homogeneous, a presupposition of the radiation model. The results in this article are based on measure
ments at 4 days with most suitable conditions during summer 1979.
The sea surface temperature has been determined with a Barnes PRT 5 radiometer, with an infrared res
ponse similar to that of the METEOSAT IR-channel (MORGAN, 1979/5). The radiation coming from
the sea surface as well as the radiation from a well stirred temperature controlled sea water reservoir have
been measured (HI NZPETER , 1967; GRASSL, 1976), So it was possible to eliminate the reflected down
ward infrared radiation using a reflection coefficient calculated after DOWNING and WILLIAMS (I 975).
The measurements have been performed under the same direction (elevation angle 27.2°), under which
METEOSAT saw the platform. In addition, the water temperature at about 10 cm below the sea surface
1) Now with Meteorological Institute, University of Munich.
442 Contributions to Atmospheric Physics Vol. 53 No.3, August 1980
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