energy exchange charts ol the North Atlantic ocean

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energy exchnge chrts ol the North Atlntic ocen Abstrct Chrts of clculted energy exchnge cross the surfce of the North Atlntic Ocen hve been constructed.

1 energy exchnge chrts ol the North Atlntic ocen Abstrct Chrts of clculted energy exchnge cross the surfce of the North Atlntic Ocen hve been constructed. Wind nd temperture observtions obtined from 8 million ship wether reports were entered individully into the bulk erodynmic equtions with exchnge coefficients tht vried with wind speed nd stbility. The individul fluxes were verged to obtin monthly nd nnul mens of ltent nd sensible het momentum. Net rdition fluxes were clculted using Budyko's (1963) formuls. Monthly nd nnul verges for 32 yers hve been formed for 500 subdivisions of the ocen. Averges for ech month from 1941 through 1972 were computed for squres to study the vritions nd nomlies of the fluxes, meteorologicl vribles, nd se temperture. Chrts giving nnul verges of the net het gin by the ocen, evportion, sensible nd rditionl het exchnge, wind stress components, nd meteorologicl vribles re presented. A grph of the monthly vritions for Mrsden Squre 116 nd n nomly chrt for Jnury 1958 show the vribility of the fluxes nd the lrgescle nomly pttern. 1. Introduction Fluxes of energy nd mss through the ocen's surfce re the primry contcts tht the ocen hs with the outer world, which cn produce wter trnsformtions, fronts, nnul cycles, nd climtic vritions of wter chrcteristics. T o understnd observed chnges in wter msses, it is necessry to know the sptil nd temporl distribution of the exchnges of energy nd mss with the tmosphere nd the rditive exchnge of energy between the ocen, the tmosphere, the sun, nd spce. T h e present work ws designed to fulfill this need by forming flux, meteorologicl vrible, nd se temperture verges for ech month from 1941 to 1972 for 66 entire Mrsden Squres (10 squres) nd monthly nd nnul vlues verged over the 32 yers for 500 smller res. One my well sk why this work ws undertken when chrts of the exchnge, principlly by Budyko (1963), lredy exist. T h e nswer is threefold. First, Budyko used such corse grid of dt points tht his chrts cn not give the required sptil detil nd his extrpoltions of isopleths re, in some crucil res, erroneous. Second, reserch by mny workers on the turbulent trnsfer of energy nd mss hs shown tht the coefficients used in the exchnge formuls re not constnt but depend upon the stbility of the ir nd the wind speed. Lst, huge body of surfce mrine obi Contribution No from the Woods Hole Ocenogrphic Institution. 670 Andrew F. Bunker nd L. V. Worthington Woods Hole Ocenogrphic Institution Woods Hole, Mss servtions hs been collected, edited, nd mde vilble on mgnetic tpe by the Ntionl Climtic Center (NCC) of NOAA. These three considertions together with the present need for detiled informtion on the exchnges indicted tht the tsk should be crried out now. 2. The energy equtions nd exchnge coefficients The energy exchnge t the ocen surfce hs been computed by Budyko (1963) from the eqution: A = Q(1 ) I L S (1) where A is the monthly het gin by the ocen, Q(1 ) is the solr rdition bsorbed by the ocen, is the surfce lbedo, I is the net infrred rditionl exchnge t the surfce, L is the ltent het flux due to evportion, nd S is the sensible het exchnge with the tmosphere. T h e terms of the eqution were evluted for 280 points over the world ocen, nd monthly nd nnul chrts were constructed nd published. T h e bulk erodynmic exchnge equtions developed principlly by Sverdrup (1937), Jcobs (1942), nd Budyko (1963) cn be expressed in the following forms: T = pc U > D (2) = pc {q_ q )U s 5 = pc C (T p H (3) - T )U s (4) Here, r,, nd S re the monthly verges of the trnsfer of momentum, wter vpor, nd sensible het; p is the density of the ir; c is specific het of ir t constnt pressure; C, C, nd C re the exchnge coefficients of momentum, wter vpor, nd sensible het, respectively; U is the monthly verge wind speed t 10 m or ship nemometer level; q nd q re the monthly verge mixing rtios of the ir in contct with the wter nd t the 10 m or deck level; nd T nd T re the verge tempertures of the se surfce nd the ir t 10 m or deck level. All three uthors ssumed tht the coefficients, Co, C, nd C were identicl nd constnt. They used vlues rnging from 1.4 to 2.3 X 10~. After their works were completed, mny field experiments nd theoreticl studies hve been crried out tht show tht the three coefficients re not identicl nd tht they vry with wind speed nd stbility. These more recent determintions re bsed on observtions of flux nd grdient p h u s s H 3 Vol. 57, No. 6., June 1976

Bulletin Americn Meteorologicl Society 671 verged over short time intervls, e.g., minutes or hours, nd hence re pplicble to individul wether observtions.

2 Bulletin Americn Meteorologicl Society 671 verged over short time intervls, e.g., minutes or hours, nd hence re pplicble to individul wether observtions. The verges of vrible coefficients used with typicl monthly series of instntneous observtions will be less thn the corresponding constnt coefficient pplicble to monthly or sesonl mens. A study hs been mde (Bunker, 1975, 1976) of recent exchnge coefficient determintions, nd their vribility with wind speed nd stbility hs been estblished. It ws shown tht C X 10 during neutrl stbility conditions vries from 1.2 for light winds to 3.4 for hurricne-force winds, while C X 10 vries from 1.32 to For moderte winds nd ir temperture 5 K greter thn se tempertures, vlues of coefficients re reduced bout 50%. For ir tempertures 5 K less thn se tempertures, coefficients re incresed bout 22%. For very high winds the coefficients do not vry significntly with stbility. Tbles of C nd C s functions of speed nd stbility hve been constructed for computer use. The exchnge coefficient for het, CH, hs not been determined with the sme confidence s the other coefficients; hence, C is ssumed to be equl to C for computtionl purposes. In the present work, fluxes hve been computed by entering individul ship observtions of wind, temperture, nd mixing rtio insted of monthly verges into q. (2)-(4) nd choosing vlues of the coefficients from the speed-stbility tbles. Monthly flux verges re found by verging the individul fluxes. Solr nd infrred rditionl exchnge ws clculted following the formuls nd tbles of Budyko (1963). The method employs tble of solr rdition received t the surfce on cloudless dys nd clcultes empiriclly the diminution cused by the verge totl cloud cover. A minor chnge in the lbedo t high ltitudes ws mde by using the vlues of Pyne (1972). Reflection by se ice ws hnd clculted nd subtrcted from the mchine-computed bsorbed rdition mounts. Hydrogrphic Office (1946) chrts were used to obtin the monthly percentges of the re covered by ice. New, winter ice ws ssigned n lbedo of 0.8, wheres old, melting spring ice ws ssigned vlue of An indiction of the success of the method nd of the correctness of C is given by the greement of the determintions of the net nnul het loss of the North Atlntic by this method nd n ocenogrphic method. The loss of het ws clculted by summing the dvection of het cross the equtor nd cross line from Scotlnd to Icelnd, Greenlnd, nd Lbrdor by ocenic currents of known tempertures (Worthington, 1970, 1976). The bulk erodynmic method gives net het loss of 5.9 W m~, wheres the ocenic dvection method gives loss of 4.8 W m. This greement cn not be considered s proof of the method, since the ocen currents re not known exctly. Trnsports of 12 in- nd out-flowing currents hve been estimted to the nerest 10 m s~\ giving zero net flow. rrors for worst cse cn be estimted for flows cross the equtor. An error of 1 X 10 m s in the trnsport esti3 D 3 D H mtes of the 27 C surfce wter flowing northwrd nd of the 2.5 C bottom wter flowing southwrd would led to n error of ±2.4 W m~ in the net het loss. This error estimte shows tht the bulk erodynmic method gives het loss within the uncertinty rnge of the ocenic method. For comprison, verging Budyko's 1963 chrt of net nnul het losses over the sme re gives loss of 1.5 W m~. In ddition to the het fluxes, the wind stress nd verges of meteorologicl vribles were computed. To id in dignostic studies of the ir-se interctions, the long-term monthly verges of the fluxes nd sttistics of the vribles were computed for ech qudrnt from which the wind blew. An estimted 12 million ship wether observtions were red from the tpes in the course of the work. Only observtions with ll the vribles necessry to compute ll the fluxes nd tht could pss specified qulity checks were ccepted. Roughly 8 million observtions pssed these checks. As rule, ech Mrsden Squre ws subdivided into 10 res, but in regions of sprse dt, fewer thn 10 subdivisions were mde so tht significnt verges could be formed. xcept in rctic regions, the number of observtions vilble per subdivision rnged from more thn 1200 to nerly nergy exchnge chrts The principl gol of the work ws to determine the mgnitudes nd distribution of the monthly nd nnul net het gin by the ocen to id in the study of ocenic currents nd wter mss formtion. The contoured distribution of the nnul het gin is shown in the chrt on the cover with colors indicting rnges of het gin nd loss by the ocen. The chrt defines the principl res of het gin nd loss tht in generl coincide with the cold nd wrm ocenic currents. The mximum cooling occurs off the est cost of the United Sttes, where the wrm wter of the Gulf Strem is exposed to cold dry continentl ir in the wintertime. Worthington (1972) hs noted tht the exposure of the "wrm core" of the Gulf Strem to continentl polr ir in winter is not necessrily the sole cuse of the vst energy exchnges in this re. He hs suggested tht wter mss generlly known s 18 wter (Schroeder et l., 1959; Worthington, 1959; Istoshin, 1961) is formed t the se surfce south of the Gulf Strem nd penetrtes southwrd, t the 300 m level, s fr s the Cribben Se. In regions of net negtive nnul het exchnge the sesonl ocenic thermocline is ersed before the end of the winter. In such regions, wter msses cn often be formed by the convergence nd sinking of surfce wter, which flows, t depth, wy from the re of its formtion. The negtive het exchnge is mintined by wrm surfce wter tht flows in from the south to replce the sinking wter. A second region of net negtive nnul het exchnge occurs est of the Grnd Bnks, where the thermocline wter msses (15-4 C) of the western North Atlntic re formed ccording to Worthington

3 672 Vol. 57, No. 6, June 1976 r * i 1 U O U H < s s s W.5 S 2 * o 3 OH S 8 o

Bulletin Americn Meteorologicl Society (1972). However, his evidence for this is fr from conclusive. Between Norwy nd Svlbrd (77 N) the lredy cool wter is cooled more during the winter.

4 Bulletin Americn Meteorologicl Society (1972). However, his evidence for this is fr from conclusive. Between Norwy nd Svlbrd (77 N) the lredy cool wter is cooled more during the winter. There seems to be little doubt tht the formtion of North Atlntic Deep Wter tkes plce in this third mjor cooling re. As the cold Lbrdor Current flows southestwrd long the cost, its wter continues to lose het until it crosses the 51st prllel west of 50 W nd then gins het rpidly. Prt of the current turns southestwrd round Newfoundlnd nd continues to gin het s it flows towrd Cpe Htters. Much of this wter is entrined into the estwrd-moving slope wter nd never reches Cpe Htters. The finding tht there is wrming of the current from Nov Scoti to Htters is mjor difference between the present work nd tht produced by Budyko (1963). Figure 1 shows Budyko's chrt compred with the present verges expressed in kiloclories per squre centimeter per yer. His chrt shows only slight wrming between Newfoundlnd nd Nov Scoti with the mximum cooling for the North Atlntic in the wters of the continentl shelf between Cpe Htters nd New Jersey. To mintin this negtive het exchnge would require flow of wrm wter onto the continentl shelf comprble to tht of the Gulf Strem. This mistke occurred presumbly becuse of the few dt points used nd n unrelistic extrpoltion of isopleths. Another mjor difference between the chrts lies in the region of the intertropicl convergence zone (ITCZ) est of South Americ. Budyko's chrt shows net het gin by the ocen, wheres our chrt shows smll het loss. The net nnul het loss in this re is produced by strong evportion into the fresh N trdes during the wintertime nd diminution of solr rdition in the summer by the clouds ssocited with the ITCZ. The strong heting of the wter off the west cost of Afric is the result of the upwelling of cold wter. The cold wter inhibits evportion nd reverses the usul direction of the sensible het flux. The stbility of the ir prevents the formtion of mny cumulus clouds, nd insoltion reches North Atlntic mximum. Another but weker re of upwelling is noted long the cost of Venezuel. The net nnul het loss cn be better understood by noting the contributions of evportion, sensible het flux, nd rdition shown in Fig. 2. The evportion is given in centimeters of wter evported per yer. The vlues my be converted into wtts per squre meter pproximtely by multiplying by 0.8. The most conspicuous feture of this chrt is the gret evportion of the wrm Gulf Strem wter. By contrst, the evportion from the cold Lbrdor Current est of Newfoundlnd is very smll. Other regions of the chrt show vritions depending on locl conditions such s upwelling, strength of the wind, nd humidity of the ir. The evportion verged over the re of the North Atlntic enclosed by the equtor, the costlines, 673 nd the verge ice line in the north is 147 cm yr. This compres with n verge evportion by Budyko (1963) of bout 131 cm yr for the sme re. The sensible het flux pttern is very similr but removes less het from the ocen. In the trde wind region the sensible het flux produces only 5% of the totl het loss including net infrred rditionl loss, wheres over the Norwegin Se, it ccounts for 37%. vportive het loss is the lrgest het-budget term in both res, being 72% in the tropics nd 49% over the Norwegin Se. Only over the Africn upwelling zone is the nnul sensible het flux directed into the wter. Over the Lbrdor Current est of Newfoundlnd the nnul verge ir temperture minus se temperture is positive, but the nnul sensible het flux is directed out of the wter becuse during the winter, the winds re stronger when the ir temperture minus se temperture is negtive. The rdition chrt presents the nnul verge net solr nd infrred rdition bsorbed by the se surfce. The contours of equl rdition bsorption re strongly dependent upon the ltitude but re modified by the locl cloudiness nd, from Newfoundlnd to Icelnd nd Svlbrd, by the se ice. The 50 W m isopleth cuts cross the Norwegin Se nd menders SW to Newfoundlnd. The effect of the cloudiness shown in Fig. 3 ssocited with the ITCZ is pprent t 5 N. The mximum rdition bsorption occurs over the Africn upwelling region, where the cloud cover drops to 2.2 okts. The nnul verges of the sclr wind speed nd the x nd y components of the wind stress re presented in Figs. 3b-3d. In ddition to the dependence of the stresses upon the generl circultion of the tmosphere, severl regions of mximums nd minimums re noted. A mximum x-component stress of 0.21 P ner 50 N, 42 W, is produced by strong winds nd lrge ir-se temperture difference. A minimum of the x-component stress occurs in the Cribben Se, where the trde winds blow strongly with gret persistence from the est. An irregulrity in the y component occurs off the North Americn est cost nd over the Grnd Bnks. Here tongue of negtive stress sticks into the bnd of positive vlues. The negtive verges result from the dominnce of the wintertime strong NW winds over the weker summertime SW winds. The lrge negtive vlues responsible for the upwelling ner the cost of Afric re produced by the strong NN winds during the summer months. Comprisons of these stress verges with those found by Hellermn (1967) show the present vlues to be generlly higher. This is prticulrly true of locl mximums nd minimums nd is to be expected since Hellermn used verges over 25 squres, wheres the present vlues re bsed on verges over 10. The 0.1 P isopleths between 40 nd 60 N re nerly the sme but the 0.15 nd 0.20 P lines do not pper on Hellermn's chrt. The difference in this region of strong winds probbly rises from smller smpling res combined with the present method of computing stresses from individul wind reports, resolving the stresses into components, nd then forming the verges. _1-1 -2

5 674 Vol. 51, No. 6, June 1976 c * <L> '9 S. «. * '3 ^ s bp ' c (J 3C3 3 D C S3 fl < ^ *-» < rt X 0 4 S fn i>.3 fl

6 Bulletin Americn Meteorologicl Society 675 'S TO i; 13 A d rt ^ c ^ cpu < - 5 O.5 d ^ * ^.s «O <4-1 o.g o U u w 'S > fl OH TO P o 13 C fl U 2 < bo TO CO c c fl fl X) cu <u < ^

7 676 Vol. 51, No. 6, June 1976 FIG. 4. Monthly verges of fluxes nd meteorologicl vribles for MS 116 (30-40 N, W) from 1941 to 1972.

8 Bulletin Americn Meteorologicl Society 677 FIG. 5. Anomlies of net het gin by the ocen for Jnury Units re W M" 2. Wether ship vlues re plotted ner the sttions. Ares of positive nomlies re indicted by the lined shding. These few chrts revel the mny wys in which lrge- nd smll-scle processes cn combine to produce the locl interction between the ocen nd the tmosphere. Since mny thousnds of surfce mrine observtions hve been received from res never visited by ocenogrphic vessels, this technique offers vluble mens of finding regions of mximum interction nd determining the locl predominting processes operting in the tmosphere nd the ocen. Plns hve been mde to study the South Atlntic Ocen with this technique. 4. Monthly vritions of energy fluxes Climtic vritions over the North Atlntic cn be nlyzed using the verges formed for ech month of the period for ech entire Mrsden Squre. The computer plot of selected vribles for MS 116 (30-40 N, W) is presented in Fig. 4 to show the mgnitudes of the vritions nd the reltionships between the meteorologicl vribles nd the fluxes. The nnul cycle is the predominting feture of these grphs. In spite of the confusion to the eye cused by the nnul cycle, the grphs show lrge yer-to-yer summertime nd wintertime vritions. Only the net rdition nd summertime sensible het fluxes hve smll vritions. The lrge vritions of the net het exchnge cn be trced bck to lrge vritions of the ltent het flux, which, in turn, re cused by vritions in the mixing rtios nd the sclr wind speed. Although winters with gret or smll het loss my occur in groups of two, three, or four, no obvious regulrity is pprent. Further, the chrcter of winter my chnge during the seson, s it did in the winters of nd The chnges in both cses re relted to lrge chnges in the wind direction; this suggests tht there is dependence upon the men positions nd migrtions of the est cost pressure trough. Inspection of monthly men pressure chrts for these periods confirms strong dependence of the fluxes upon the strength nd position of the trough nd the Bermud-Azores high-pressure cell. The grph shows tht during Jnury 1958 the ocen in this squre lost n unusully lrge mount of het. The question nturlly rises s to whether ll prts of the Atlntic hd severe winter. To nswer this question, the verge Jnury net het exchnge for ech Mrsden Squre ws subtrcted from the Jnury 1958 men to determine the nomlous het exchnge. The results were plotted nd contoured in Fig. 5. It is seen tht the excessive het loss extended from Msschusetts SW into the Cribben Se nd estwrd pst OSV '.' st of this region of gret het loss there ws bnd

9 678 Vol. 57, No. 6, June 1976 of nomlously low het loss extending from the Mine nd Greenlnd region to the equtor. Frther est, nother bnd of high het loss extended from the Icelndic wters to the Gulf of Guine. The North Se nd Gibrltr nomlies give hint of nother mild-winter bnd. From this chrt it is obvious tht lrge-scle influences re controlling the ir-se interctions. Lrgescle nomly ptterns hve been observed in the lst few decdes in other res nd with other vribles tht show similr chrcteristics. Nmis (1951), for exmple, presented temperture nomly chrts over the United Sttes tht show similr rel continuity nd lterntion nd discussed their reltion to the Pcific nticyclone. The uthors refrin from giving ny explntion of the pttern t the present time. Much work will be required to study the nomlous behvior of other vribles in order to pinpoint the cuses of the pttern. Ptterns of other months must be drwn to determine their evolution nd the subsequent effect upon the se surfce temperture nomly. Acknowledgments. The uthors wish to thnk Mrgret Chffee, for the excellent job done in drfting the chrts, nd Roger Goldsmith, for his skillful writing of the computer progrm. Reserch ws supported by the Office of Nvl Reserch under Contrct No. N C ; NR References Budyko, M. I., 1963: Atls of Het Blnce of the rth. (In Russin) Acdemy of Sciences, Moscow, 69 pp. (Also, Guide to the Atls of the Het Blnce of the rth. Trnslted from Russin by I. A. Donehoo, U.S. Wether Bureu, WB/T-106, Wshington, D.C., 25 pp. Bunker, A. F., 1975: nergy exchnge t the surfce of the western North Atlntic Ocen. Unpublished mnuscript, WHOI-75-3, 107 pp., 1976: Computtions of surfce energy flux nd nnul ir-se interction cycles of the North Atlntic Ocen. Mon. We. Rev., 104(8), in press. Hellermn, S., 1967: An updted estimte of the wind stress on the world ocen. Mon. We. Rev., 95, (See correction, Mon. We. Rev., 96, ) Hydrogrphic Office, 1946: Ice Atls of the Northern Hemisphere. H.O. No. 550, U.S. Nvy, Wshington, D.C., 106 pp. Istoshin, Yu. V., 1961: Formtive re of "eighteen-degree" wter in the Srgsso Se. Okenologiy, 1, (Deep Se Res., ngl. Trnsl., 9, ) Jcobs, W. C., 1942: On the energy exchnge between se nd tmosphere. J. Mrine Res., 5, Nmis, J., 1951: The gret Pcific nticyclone of winter : A cse study in the evlution of climtic nomlies. J. Meteor., 8, Pyne, R.., 1972: Albedo of the se surfce. J. Atmos. Sci., 29, Schroeder,., H. Stommel, D. Menzel, nd W. Sutcliffe, Jr., 1959: Climtic stbility of eighteen degree wter t Bermud. J. Geophys. Res., 64, Sverdrup, H. U., 1937: On the evportion from ocens. J. Mrine Res., 1, Worthington, L. V., 1959: The 18 wter in the Srgsso Se. Deep Se Res., 5, , 1970: The Norwegin Se s Mediterrnen bsin. Deep Se Res., 17, , 1972: Negtive ocenic het flux s cuse of wtermss formtion. J. Phys. Ocenogr., 2, , 1976: On the North Atlntic Circultion. Johns Hopkins Ocenogr. Stud., Bltimore, Md., Johns Hopkins Press, in press. nnouncements U.S.A. workshop on GAT centrl progrm A three-week U.S.A. Workshop on the GAT Centrl Progrm will be held t the Ntionl Center for Atmospheric Reserch in Boulder, Colo., during the period 1-19 August The objectives of the workshop re: 1) to provide forum for reporting nd evluting the U.S. GAT reserch effort; 2) to foster communictions between reserchers nd to strengthen nd to develop joint reserch efforts; 3) to provide synthesis of the structure of the tropicl tmosphere nd upper ocen bsed on GAT observtions; 4) to ssemble t the time of the workshop preprint volume of the contributions of individuls nd groups prticipting in GAT reserch; nd 5) to help mp out course of reserch nd synthesis ctivities tht will led to n effective ttck on the GAT Centrl Progrm objectives, prticulrly in the relm of interctions nd prmeteriztion in numericl models. The progrm will be divided into three min topic res focused on the scle of phenomen nd fourth topic to integrte workshop ctivities nd provide guidnce for the continuing U.S. GAT reserch progrm. The session titles nd principl orgnizers re: 1) Generl circultion, tropicl wves nd ITCZ phenomen of both the tmosphere nd ocen (R. Burpee); 2) Cloud-cluster scle phenomen including undisturbed conditions (W. Gry nd D. Mrtin); 3) Mesoscle nd smller-scle phenomen, emphsizing moist convection (M. Grstng nd W. Seguin); nd 4) Where do we go from here? (R. Reed). (Continued on pge 685)

The heat budget of the atmosphere and the greenhouse effect

The heat budget of the atmosphere and the greenhouse effect The het budget of the tmosphere nd the greenhouse effect 1. Solr rdition 1.1 Solr constnt The rdition coming from the sun is clled solr rdition (shortwve rdition). Most of the solr rdition is visible light