In the absence of horizontal flow, the surface salinity of any part of the ocean is mainly determined by three processes: Decrease of salinity by precipitation P, increase of salinity by evaporation E, and change of salinity by vertical mixin

Jacobs, Woodrow C.

English

Yale University

      P.O. BOX 208118 | NEW HAVEN CT 06520-8118 USA | PEABODY.YALE. EDU   JOURNAL OF MARINE RESEARCH The Journal of Marine Research, one of the oldest journals in American marine science, published important peer-reviewed original research on a broad array of topics in physical, biological, and chemical oceanography vital to the academic oceanographic community in the long and rich tradition of the Sears Foundation for Marine Research at Yale University.  An archive of all issues from 1937 to 2021 (Volume 1–79) are available through EliScholar,  a digital platform for scholarly publishing provided by Yale University Library at  https://elischolar.library.yale.edu/.  Requests for permission to clear rights for use of this content should be directed to the authors, their estates, or other representatives. The Journal of Marine Research has no contact information beyond the affiliations listed in the published articles. We ask that you provide attribution to the Journal of Marine Research.  Yale University provides access to these materials for educational and research purposes only. Copyright or other proprietary rights to content contained in this document may be held by individuals or entities other than, or in addition to, Yale University. You are solely responsible for determining the ownership of the copyright, and for obtaining permission for your intended use. Yale University makes no warranty that your distribution, reproduction, or other use of these materials will not infringe the rights of third parties.  This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. https://creativecommons.org/licenses/by-nc-sa/4.0/   SOME EMPIRICAL RELATIONS BETWEEN SEASONAL1 VALUES OF (E-P) AND SURFACE SALINITIES OVER THE NORTH ATLANTIC 1 BY WOODROW C. JACOBS U.S. Weather Bureau, Washington, D. C. In the absence of horizontal flow, the surface salinity of any part of the ocean is mainly determined by three processes: Decrease of salinity by precipitation P, increase of salinity by evaporation E, and change of salinity by vertical mixing. Where (E-P) is a positive quantity, the effect on the vertical distribution of salinity is as if salt were being diffused into the volume through the sea surface and, when (E - P) is a negative quantity, the effect is as if salt were being diffused outward through the sea surface. In general, it is to be expected that regions of large (E - P) will be regions of high surface salinities and regions of small or negative (E - P) will be regions of low salinities. Therefore, where the horizontal transport term is small, a quantitative relationship between (E - P) and surface salinity might be expected. Wtist (1936) has found that the average annual surface salinities can be related to the average annual latitudinal values of (E - P) quite accurately by a simple empirical expression of the form: s. = K(E-P) + sk, (1) where s. is surface salinity, K is a constant, Sk is a constant salinity and (E-P) is given in cm/year. He finds it necessary, however, to assign different values for Kand Sk for different oceans. His formulae do not apply near the coasts where the horizontal transport term tends to be large and where the effects of runoff from land surfaces may be important. They also do not apply near the Equator. On the basis of previously derived seasonal evaporation and precipi-tation data for the oceans (Jacobs, 1948), it has been possible to obtain seasonal (E - P) values for the North Atlantic and North Pacific. It is considered of interest to determine whether or not a simple empirical relation can be found between the seasonal salinities and the seasonal qua~tities of (E-P), similar to the relations found by Wtist, when we consider mean annual values. Unfortunately, seasonal salinity data are not available for the Pacific, while (E- P) values are not available 1 Contributions from the Scripps Institution of Oceanography, New Series, No. 400. (330) 1948] Jacobs: Relation Between (E-P) and Salinities 331 for the South Atlantic because computations of evaporation have not been possible for the latter region. Since the region must be one within which the transport of surface waters is at a minimum, the choice of areas left for analysis is limited. An examination of current charts for the oceans (U.S. Hydrographic Office, 1945) indicates that the only completely suitable area is the one in the North Atlantic bounded by the 20th and 30th parallels and the 60° W and 20° W meridians. This section excludes the coastal areas with their pronounced N -S surface currents and is yet of sufficiently large lateral extent to minimize the effects of E-W flow. 2 37140 37 130 1 37.1 20 37 110 z :; 4 37 100 "' ::: 37 090 .. ... a: 37 080 37070 37.060 NORTH ATLANTIC I LATITUDE RANGE 200N - 30°N L0 NGITU0E ~ANGE 60°W -2o•w _ • I w / / V -I w - WINTER I S p - SPRING Su - SUMM ER V A - AUTUMN I _S p. I 19. 195 20 . 20. 5 21. 215 22. 22.5 2 3 23.5 2 4 . 245 25. 255 E-P ICmsl Figure 1. Surface salinity as a function of seasonal (E-P) in an area of the North Atlantic where the transport of surface waters Is at a minimum (Lat. 20°-30°N; Long. 60°-20° W) . The results of plotting the seasonal salinity values against the seasonal (E- P) values for this area are given in Fig 1. An alm_o~t perfect straight line relationship is shown such that the surface sahm-ties can be given very accurately by the formula: S0 = 0.0171 (E - P) + 36.730 ± 0.003 (0 /oo)• (2) The range of salinities computed through its use differs from the ob-served range of salinities by less than 10%. 2 The seasonal salinity values have been sealed from charts given by Bohnecke (1936). 332 Journal of Marine Research [VII, 3 Having obtained good results in the one area of the North Atlantic where the transport of surface waters is at a minimum such that the effect of advection on the surface salinity would be small, it was de-cided to test at least one other area even though a considerable advec-tive effect might be expected. The area chosen for this examinaiion is the one immediately to the north of the previous area, i. e., the area bounded by the 30th and 40th parallels and by the 20th and 60th meridians. 36 590 36 565 36 565 a: 36 560 36 550 Su • ~· Sp7 9 NORTH ·ATLANTIC L/\TITUOE RANGE 30°N-40°N LONGI1 UDE RANGE GO•W-20°W I ~. --I ...-I ) ...- ...- " >~ 9 0 0007 IE Pl ..-__ _,;;;;:--~ s - . I: w - Wl~T~R I Sp - SPRING Su - SUMMER I A - AUTUMN! 10 .. I 2 13 14 15 16 17 ,a 19 E-P (CmsJ Figure 2. Surface salinity as a function of seasonal (E -P) in an area of the North Atlantic where the horizontal transport of surface waters cannot be neglected (Lat. 30°-40° N; Long. 60°-20° W). The results of plotting the seasonal values of (E - P) against the seasonal salinities for this area are given in Fig. 2. Unfortunately, the seasonal (E - P) values and seasonal salinities for spring and summer are so nearly identical that, for practical purposes, they both consti-tute a single point. The winter and autumn points, on the other hand, are so widely dispersed that no reasonable curve can be drawn that would serve to connect all points. However, as first steps in analysis, it is possible to assume that the summer, spring and autumn values are correct and that high salinity water is being advected into the area during winter (curve A in Fig. 2) or that the summer, spring and winter values are correct and that low salinity water is being advected into the area during autumn (curve Bin Fig. 2). 1948] Jacobs: Relation Between (E-P) and Salinities 333 An examination of the salinity distribution over this portion of the North Atlantic, as given by Bi:ihnecke (1936), reveals that the hori-zontal salinity gradients are slight except along the 40th parallel be-tween the meridians 40° W and 60° W where very low salinity water exists. In order to determine the nature of the surface flow in this area of marked salinity gradients, monthly charts of the surface cur-rents prepared for one-degree squares by the U. S. Hydrographic Office (1945) have been examined and the surface transport across the 40th parallel determined. The midmonths of each of the seasons were accepted as representing the average seasonal flow, and the aver-age northerly component of fl.ow for the area between the 39th and 41st parallels and the 40° W and 60° W meridians was computed for each of these months. The results are as follows: Season Winter Spring Summer Autumn vii +0.73 cm sec -0.43 cm/sec -0.26 cm/sec - 2.89 cm/sec, where V II is the component of fl.ow across the 40th parallel, negative values indicating southerly fl.ow (importation of low salinity water) and positive values indicating northerly fl.ow (transport outward of waters across the 40th parallel) in accordance with usual notational practice. Given in Fig. 3A are the results of plotting the seasonal values of V 11 against the difference between the observed surface salinities, S, and the salinities computed on the assumption that the summer, spring and autumn values are correct (curve A in Fig. 2). An almost complete scattering of points occurs, indicating that the difference between the winter and autumn salinities cannot be accounted for by the advection of high salinity water during the winter season. In Fig. 3B are plotted the differences between the observed surface salinities and the salinities computed on the assumption that the sum-mer, spring and winter values are correct (curve B in Fig. 2) against the seasonal values of V 11• The three points representing the summer, spring and autumn months fall nicely along a straight line, but the winter value proves to be about 0.003 ° / 00 too high to be accounted for merely by an absence of advection of low salinity water. However, since the winter season is the only one which shows positive values for V 11 (transport toward the north across the 40th parallel), it is reason-able to construct the curve as shown in Fig. 3B by simply connecting the points by two straight lines which intersect at a common point 334 ~004 -00'2 000 <;>+.004 .,, +.006 .. .008 + 0 10 A Journal of Marine Research s,• 1MPQRTAT1()N C, LOW · SALINI TY WATER I . . 1MP OR TATION or HIGH · SALINITY WATE R So S0, 36.5598 + 00007 IE - Pl . VI 1- 0 12 +2.5 + 2.0 +1 5 T l0 +0.5 0 .0 - Q.5 -1.Q -1.S - 20 -25 - 3.0 - 3.5 - 4l0 - 0 12 - 0 10 ·008 v,0 -.004 I lt/) - 002 .000 +.002 +.004 B V, (Cms /Sec. ~ NJ S o , 36 5526 + 0 .00153 {E - P) 1/" / /i/ V So •1/ IMPORTATION Of LQw·SA1..1N1TY WATER . '!f ,,. I ;---y.so tMPORTATJON Of HIGH·SAUNITY >AM EF +2.5 ,._zo T LS ""LO + 0.5 0 .0 - 0 .5 - LO - 1.S - 2.0 - 2 5 -3.0 -3.5 - 4.0 V, (Cms/ Sec - N) ..... 0 12 •010 -.ooe -:;-.006 :-:? ~-004 J: I - .CO2 "" 000 +00 2 +.004 • .006 C +LO -S E ASONAL VAL U ES Of 8 - .02 .on + .0 2 + _n4 + .06 +.oe +1.0 I I W • WINTER So· SPRING Su· SUMMER S0 •36.5543+ 0 .00l34 (E -PI A · AUTUMN , _ I I ,___;-I ; l ~I 1/ s,~ IMPQRTATJQN OF LOW·SAUNITY WATER 'N _;.,---- . IMPORTATION OF HIGH-SALINITY So. WATER I I I I +o.s 0 .0 - 0 .5 - 1.0 - LS - 2 .0 - 2.s - 3.0 V,(Cms./Sec.-N) (Figure 3) [VII, 3 1948] Jacobs: Relation Between (E-P) and Salinities 335 along the line where V 11 = 0 and to make no assumptions regarding the nature of the surface transport of salinity during the winter season. However, since there is northerly transport of surface waters across the 40th parallel only during the winter season, and considering the fact that, except along the 40th parallel, the salinities of surrounding areas tend to be slightly higher than within the region under consider-ation, it is more reasonable to assume that water of higher salinity is being advected into the region during winter, the amount advected being proportional also to V 11• On the basis of this assumption the curve C in Fig. 2 has been constructed. As shown by Fig. 3C, V11 plotted against the differences between the observed seasonal salinities and the salinities computed on the basis of curve C indicate an almost perfect straight-line relationship. Curve C in Fig. 2 can be represented by the following formula: So = 0.00134 (E - P) + 36.5543 (0 / 00), (3) while the curve in Fig. 3C can be represented by the expressions: S - So = 0.00246VII + 0.00071 (% 0 ) (4) or V 11 = 406.25(S - So) - 0.2875 (cm/sec). (5) Combining expressions (3) and (4), a simple empirical formula is obtained which allows the surface salinity to be computed with excep-tional accuracy. In this case, So = 0.00I34(E - P) + 0.00246V~ + 36.555 ± 0.001 (%0 ), (6) where V11 is in cm/sec. Because of the paucity of subsurface current and salinity data in these regions and because of the complicated nature of the vertical and lateral mixing processes which are involved, it does not appear profit-able at this time to attempt to determine, on the basis of theoretical considerations, the significance of the salinity and (E - P) relation-ships which have been illustrated. Figure 3. Difference between the computed (So) and observed (S) salinities for the area in the North Atlantic Lat. 30° to 40° N and Long. 60° to 20° Was a function of the transport of surface waters across the 40th parallel between the meridians 60° Wand 40° W . A. Tbe relationship with the northerly component of surface flow, v •. assuming the rela-tion between S and (E-P) as ip.ven by curve (A) in Fig 2. B. The relationship with v •. assuming the relation between Sand (E - P) as given by curve (B) in Fig. 2. C. The relationship with v •. assuming the relation between Sand (E - P) as given by curve (G) In Fig. 2. The values for the ratio, b, between volume of water advected and total volume of water within the mixing layer (assuming v. constant with depth throughout the mixing layer) are entered along the top margin of graph. 336 BoHNECKE, GUNTHER Journal of Marine Research REFERENCES [VII, 3 1936. Atlas: Temperature, Salzgehalt and Dichte an der Oberflii.che des Atlant-ischen Ozeans. Wiss. Ergebn. dtsch. Atlant. Exped. Meteor, 1925-27, Bd. 5, Atlas: VII + 76. JACOBS, w. C. 1948. The energy exchange between sea and atmosphere and some of its conse-quences. 180 pp. ma. to be published. U. S. Hydrographic Office 1945. Monthly current charts-North Atlantic. H. 0 . Misc. No. 10,688-1-. WusT, GEORG 1936. Oberflii.chensalzgehalt, Verdunstung und Niederschlag auf dem Weltmeere; In, Lii.nderkundliche Forschung, Festschrift Norbert Krebs, pp. 347-359. J. Engelhorns Nachf., Stuttgart. 368 pp. 

Some empirical relations between seasonal values of (E–P) and surface salinities over the North Atlantic

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Some empirical relations between seasonal values of (E–P) and surface salinities over the North Atlantic

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