ROBERT R. DICKSON and JEROME NAMIAS

Atmospheric Climatology and ItsEffect on Sea Surface Temperature­Winter 1977 to Winter 1978

The previous report In this series(Dickson and Namias, In press) hasdescribed the progressive amplificationof the Northern Hemisphere circula­tion in 1976 compared with theabnormally strong westerly now of thepreceding 5 years. In 1977 this ampli­fied circulation type was maintained,characterized in addition by episodesof extreme circulation anomaly and byelements of great persistence.

All three of these characteristics­high amplitude now, extreme abnor­mality, and tenacious persistence­were displayed during winter 1976-77.The mean distribution of 700 mbheight anomaly for winter (Fig. IA)leaves little doubt about the strongamplification of planetary waves in theupper westerlies in that season; theprincipal centers of height anomaly­an intense Pacific trough south of theAleutians (-520 feet), the meridionalridge overlying the west coast of NorthAmerica (+250 feet), the deep responsetrough downstream over eastern NorthAmerica, and a major ridge (notshown) over northern Siberia-allrepresented departures of three stan­dard deviations or more from the longterm seasonal "normal."

Robert R. Dickson is with the FisheriesLaboratory. Ministry of Agriculture. Fisher­ies. and Food. Lowestoft. Suffolk. England

R33 OHT. Jerome amias is with theScripps Institution of Oceanography. La Jolla.CA 92093.

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As Wagner (1978) pointed out, suchintense abnormalities are unlikely tooccur in a given location more thanonce per century. The dislocatingeffect on climate in this sector (includ­ing record breaking cold over easternNorth America) was compounded bythe extreme persistence of these cells.Since their establishment in fall 1976,the major troughs and ridges intensi­fied but showed little change inposition through most of the succeed­ing winter. Of the changes which didoccur between fall and winter theprincipal feature was the establishmentof an immense blocking high over thepolar region in early January, encour­aging a southward displacement of thezonal westerlies over much of thehemisphere which amounted to arecord expansion of the circumpolarvortex (Angell and Korshover, 1978).

Figure 2 illustrates the resultingsympathetic shift in the principal stormtrack over the Atlantic sector byshowing the total (3-month) change inwinter storm activity from the previouswinter (1975-76) to the winter underdiscussion. As shown, storm activitydecreased markedly at high latitudes,principally along a zone connecting theeast-central United States, southGreenland, and Iceland where thedeficit exceeded 10 storms per season.The main zone of increased stormactivity lay further south along theAmerican Atlantic seaboard and east­ward to western Europe.

The possible causes of such extremeand protracted climatic behavior havebeen discussed by Namias (1978). First,

it is evident that the anomaloustendencies of the atmospheric circula­tion during fall and winter were almostcompletely in phase with the normaltrends of seasonal forcing so thatunderlying seasonal changes did notoppose (and may have amplified) thefactors responsible for generating theabnormal windfield. Thus a deepsummer trough off the western sea­board and a downstream ridge overcentral North America, such as wereobserved in summer 1976, will normal­ly retrogress to the east central Pacificand west coast respectively in fall, andfrom fall to winter there is normally atendency for the Pacific trough and thewest coast ridge to intensify further inthese locations, accompanied by asouthward shift of the main westerlywind axis and storm belt. The mainpoint at issue here of course is thereason why these apparently normaltendencies of the circulation wereexaggerated into features of extremeanomaly.

The factors at work appear to haveincluded the following. First, retro­gression of the high pressure anomalycell to western orth America in fallbrought a strong southerly air flow tothe eastern Pacific, inhibiting coastalupwelling, reducing the loss of sensibleand latent heat to the atmosphere andboosting northward advection ofwarm water along the western sea­board. As a result, there occurred aprogressive and dramatic anomalouswarming (or more correctly a lack ofnormal seasonal cooling) of eastPacific waters from summer 1976 towinter 1976-77, by which time coreanomalies off the west coast exceeded+3.5°F (Fig. IB).

In sharp contrast, a vast zonaltongue of abnormally cold surfacewater (in places three standard devia­tions from normal) persisted in thewest and central North Pacific, aremnant of the cold water belt ofrecord extent which had prevailed inthe summer of 1976 ( amias, 1978).The maintenance of these cold condi­tions into fall and winter under anintense and persistent Aleutian troughis readily explained by the enhancedcloud cover, increased sensible and

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700mb HTOM

SSTOMFigure I.-Mean distribution of (A) 700 mb height anomaly (feet -;- 10) and (8) sea surface temperature anomaly (OF) for winter

1976-77.

May-June 1979 2/

latent heat 10SSI, cold frontal activity,and surface water divergence that sucha cell implies.

Between these two extremes ofPacific surface temperature anomaly, asharpening zone of east-west tempera­ture contrast developed from fall towinter along long. 1400 -150 0 W (Fig.I B) and the baroclinic effect of thisgradient (itself extreme) is suggested byNamias (1978) to have played asignificant role in stabilizing theanomalous circulation pattern. sharp­ening frontal and cyclonic activity,augmenting the upper level southerlyflow over the east Pacific and thusreinforcing the west coast ridge down­stream.

The above discussion has concentra­ted on only two (If the mechanismswhich might have lent a degree ofstability and persistence to the anomal­ous circulation of the fall and winter inquestion (i.e., compatibility withseasonal forcing and the feed backeffect of extreme surface temperaturegradients). Other possible contributorymechanisms have also been suggested;among these are the possible remoteinfluence of EI Nino conditions in theeastern tropical Pacific (present fromsummer 1976 to spring 1977) inmaintaining the intensity of the deepAleutian low via the mechanismdescribed by Bjerknes (1966) and latersimulated by Rowntree (1972, 1975).

Elsewhere, as Namias (1978) indicat­ed, the recurrent outbreaks of Arcticair over eastern North America in thelee of the west coast ridge and therefrigera ti ng effect of the grea ttyextended snow cover (the most exten­sive for at least 10 years; Matson. 1977)enhanced the baroclinic contrast at theeastern seaboard, leading to an intensi­fication of east coast storm activity andthus to a reinforcement of the meantrough in this sector also. (In January,during its period of strongest develop­ment. upper westerly wind speeds werr

Figure 7..- -Change in the total number of storms from winter 1975-76 to winter1976-77. Contours represent an overall change of 5 or 10 storms per season withareas of decrease shaded. Based on monthly storm track charts published in theMariners Weather LOR.

'In January. Wagner (1977) reported that thewesterlies at 700 mb level were as much as IIm/sec stronger th~n normal in the vicinity ofthe Pacific trouj!h.

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as much as 8 m/sec above normal offthe southern Atlantic coast (Wagner,1977).) The question as to which of

these feedback loops exerted a signific­ant control on the circulation is atpresent unknown; nevertheless, it is

Marine Fisheries Review

hard to avoid the suggestion that acting(as they did) in unison, their combinedeffect was at least partly responsible forthe tenacious persistence of the anom­alous circulation through two consecu­tive seasons.

The effects of this intense and pro­tracted atmospheric forcing on sea sur­face temperature (SST) over the NorthPacific has already been described.In the Atlantic sector (Fig. IB) themajor centers of SST anomaly layoffthe Atlantic seaboard where a tongueof abnormally warm water (+4.0° F)followed the axis of the Gulf Stream,and off south Greenland where a broadzone of cold surface water (-3.2°F)extended southward and eastwardtowards Europe. The former centermay not simply be due to the strongsoutherly circulation around theAmerican coastal trough; Worthing­ton (1977) has presented evidence thatintense cold air outbreaks from theAmerican continent in winter maybring about a radical deepening of thethermocline south of the Gulf Streamand hence an intensification of warmwater transport by the Gulf Streamsystem. The cooling in the northernNorth Atlantic is the expected result ofstrong anticyclonic circulation overGreenland which tends to occur (as inthis case) when a well-developed baro­clinic zone at the American Atlanticseaboard encourages a south-eastwardshift in the mean position of the IcelandLow (Dickson and Namias, 1976).

The last week of winter finally sawthe breakdown of the hemisphericcirculation pattern which had domina­ted the fall and winter seasons but thespring circulation which replaced itdisplayed features of almost equivalentabnormality. As Figure 3A shows, thecirculation anomaly pattern whichemerged was almost exactly out ofphase with the pattern of the precedingfall and winter over much of theNorthern Hemisphere. Centers ofpositive height anomaly abruptlyreplaced the preexisting intensetroughs over the forth Pacific, eastern

orth America, and western Europe,while a narrow salient of negativeheight anomaly extended over westernNorth America from a deep trough

May-June /979

centered on the Bering Sea. Onceestablished, however, this new planet­ary wave pattern persisted with almostas much regularity as had the winterpattern. As Wagner (1978) pointed out,such persistence is even more unusualin spring than winter since the "transi­tion season" of spring is characterizedby rapid changes in solar angle, in thelength of day, and in thermal contrastbetween continent and ocean.

Although the principal anomalycells of the spring circulation wereconsiderably less extreme than those ofwinter (only the Great Lakes ridgeapproached three standard deviationsfrom normal), the reversed sense of thecirculation brought a rapid modifica­tion of surface temperature anomaliesover both oceans. The elimination ofthe mean trough from the centralNorth Pacific (coincident with thecessation of El Nino conditions in thetropics), brought a relative changetoward less cloudy skies, settling air,surface water convergence, and aslackening of mid latitude westerlyflow; as a result the long establishedcold water tongue in the west CentralPacific was considerably eroded, withcore anomalies warming by 1°Fcompared with winter (Fig. 3B). Offthe American west coast where theanomalous temperature field was lesswell entrenched, the change fromanomalous southerly to northerly airflow (Fig. 3A) led to a reversal of thewarming processes there (including aresumption of coastal upwelling) andhence to the almost total destruction ofthe preexisting warmth in the easternPacific. Together, these new tendenciestowards warming in the west andcooling in the eastern Pacific alsobrought the virtual elimination of thestrong east-west surface temperaturegradient which had been an importantfeature of the winter season. In theAtlantic sector the only center of SSTanomaly to persist unaltered intospring was the tongue of anomalouswarmth overlying the Gulf Stream axisoff the Atlantic seaboard. If real, thiscenter most probably reflects thecontinued influence of strong GulfStream transports initiated by theextreme cold air outbreaks of the

winter season. In April the volumetransport of 95 sverdrups was thelargest value ever computed for theGulf Stream (Worthington, 1977).

In summer 1977 (Fig. 4A) thehemispheric circulation became pro­gressively less amplified than hithertowith weak tongufls and ridges progress­ing rather rapidly from month tomonth. In the seasonal mean, however,a zonal belt of below-normal heightsextended from south of the Aleutiansto eastern Canada and this, coupledwit h modera te subtropical ridgesfarther south, gave rise to a flat andfairly fast flow across the North Pacificand along the U.S.-Canadian border(where an active frontal zone withenhanced storm activity developedbetween the cool Canadian air and thestagnant heat of the South).

In the Pacific sector this enhancedwesterly flow maintained (or ratheramplified) the cool surface tempera­tures in the west central region andcompleted the demolition of the pre­existing warmth at the Americanseaboard so that an unbroken belt ofcool surface water now followed themain west-wind axis from coast tocoast (Fig. 4B). In the Atlantic sectorthe eastward spread of warm surfacetemperatures from the eastern sea­board continued, and a less dramaticbut more extensive surface warmingtook place farther south beneath theexpanded subtropical ridge. Tropicalstorm activity was exceptionally weakin both Pacific and Atlantic sectors.

The fast zonal westerly circulationgenerally continued into fall, disruptedsomewhat by a recurrent blockingtendency over eastern Canada and thewestern Atlantic (Fig. SA). As a result,the patterns of air temperature ano­maly over the American continent andsea temperature anomaly over bothoceans changed relatively little fromsummer to fall (Fig. 4B, 5B). Thesummer warming of the Bering Seaunder the polar anticyclone was,however, extended southward in fall tothe northernmost North Pacific as anisolated remnant of this ridge settledover the Aleutians. This, in turn,brought a narrowing of the cold waterbelt at midlatitudes and together with

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A'0,'

".

Figure .\. Mean disl nbul ions of (A) 700 mh height anomaly (feet -, 10) and (R)sea surface temperature anomaly (0 F) for spring1977.

24 Marine Fisheries Rel'iew

SUMMER 1977

A

B

700mb HTOM

SSTOMFigure 4.-Mean distributions of (A) 700 mb height anomaly (feet -;- 10) and (B) sea surface temperature anomaly (OF) for

summer 1977.

May-June /979 25

A

B

700mb HTDM

SSTDM

26

Figure 5.-Mean distribution of (A) 700 mb height anomaly (feet -:- 10) and (8) sea surface temperature anomaly (OF) for fall1977 •

Marine Fisheries Review

WINTER 1978

A

B

700mb HTOM

SSTOMFigure. 6-Mean distributions of (A) 700 mb height anomaly (feet -:--10) and (B) sea surface temperature anomaly (0 F) for winter

1977-78.

May-June 1979 27

Figure 7.-Mean 700 mb zonalwind speed profiles for the westernhalf of the Northern Hemispherefor (A) January 1977 and (B) Feb­ruary 1978. Solid lines show theobserved profiles, dashed lines showthe normal. (From Wagner, 1977,and Dickson, 1978, respectively.)

annual distribution of Pacific SSTanomaly (Fig. 9A). Colder water thannormal occupies all but the Bering Seaand the southeastern Pacific, with theprincipal cold water belt (core anoma­lies -2.4° F) extending zonally beneaththe displayed axis of westerly windsand storms.

The general cooling at the Americanseaboard in spring, summer, and fallhas left little trace of the preexistingwinter warmth beyond a slight north­ward inflexion of the 0° isopleth, butfurther south off Baja California anisopleth center remains which is theproduct of local warming in all fourseasons (see Fig. IB, 3B, 4B, 5B). In theAtlantic sector (Fig. 9B) the onlymarked feature is the tongue ofabnormally warm surface water(+3.2° F) which persisted with littlechange off the Atlantic seaboardthroughout 1977. In this location,where SST gradients are at their mostextreme, there is always the possibilityof error through uneven distribution ofsampling, and certainly there areoccasions throughout thl: Yl:ar wherethis persistence of warmth seems atvariance with the prevailing sense ofthe circulation. Nevertheless the very

JANUARY 1977

90 .---......~+_--+--_+_-...,w 70

§ 60f­f= 50«...JIf) 40WWa: 30<.0Wo

o 4 a '2 '6ZONAL WIND SPEED (MS- 1)

FEBRUARY 1978

20_':-4--0!"'------;4>-----1a>----:"2:----:"6·

ZONAL WIND SPEED (Ms-I)

90 ...-r-t-o<:-->--------<>--------<>------,w 70o::> 60f-

~ 50...J

If) 40ww~ 30wo

B

A

displaced Pacific westerlies werestronger this winter and were able tobreak through the southern half of thewest coast ridge. The eastward exten­sion of the Pacific trough over Cali­fornia (as in the seasonal mean, Fig.6A) brought frequent storm activityand twice-normal precipitation tobreak the 2-year drought in thesouthwest, aided by a moisture-ladensoutherly flow around the southernremnant of the west coast ridge(centered over Baja California). Local­ly, this southerly flow and the anti­cyclonic conditions over Baja Cali­fornia were also able to bring aboutsome renewal of ocean warming offsouthern California and Mexico (Fig.6B).

Continuing on their low-latitudetrack, the disturbances in the sub­tropical westerlies picked up additionalmoisture crossing the Gulf of Mexicoand, invigorated by the sharp baro­clinic contrast at the Atlantic coast,continued eastwards along a southerlypath towards southwest Europe. Thisexcess of kinetic energy above normalat low latitudes, the deficit at higherlatitudes between lat. 45° and 60° N,and the similarity (in this regard)between the two winters under discus­sion is well illustrated by Figures 7Aand 7B (from Wagner, 1977, andDickson, 1978, respectively) whichcompare the extreme conditions ofJanuary 1977 with those of February1978. There is little evidence of thisvigorous southerly storm track in theSST anomaly distribution for theNorth Atlantic (Fig. 6B); it is perhapsreflected in the new cooling center westof Spain, but in the western Atlanticthe warmth along the Atlantic sea­board showed little change from thefall.

Finally, Figures 8 and 9 summarizethe anomalous tendencies of atmos­phere and ocean for the year 1977 as awhole. As expected (Fig. 8) the meanannual distribution of 700 mb heightanomaly is mainly a reflection of theextreme developments of the winterseason, with its polar ridge, Pacifictrough, pan-Atlantic trough, andsubtropical ridge anomalies all clearlydisplayed. The influence of the winterseason is also evident in the mean

enhanced storminess off British Co­lumbia, drew the center of coolingcloser to the American coast.

These changes, however, were largelya feature of the early fall. As fallprogressed the normal retrogression oftroughs and ridges initiated the esta­blishment of a different but ratherfamiliar circulation anomaly patternwith the west coast trough movingoffshore to intensify south of theAleutians, with a distinct ridgingtendency over the western seaboardand with a lee trough developingdownstream in a variable locationbetween the Appalachians and theMississippi Valley. As yet thesefeatures were relatively weak orintermittent and, occurring late in theseason, were not even evident in theseasonal mean, but they were signifi­cant precursors of the forthcomingwinter circulation.

As Figure 6A demonstrates, thewinter circulation of 1977-78 was onceagain characterized by a well amplifiedwave train at mid- and high latitudes,and its component pressure anomalycells-while less extreme-did displaycertain marked similarities to those ofthe previous winter. Once again, a deepfull-ocean trough (-330 feet) centeredsouth of the Aleutians brought wide­spread cooling to the surface waters ofthe northern North Pacific (Fig. 6B).Once again (Fig. IA, 6A), a zonal beltof negative height anomaly extendedbetween deep troughs over easternNorth America and the eastern Atlan­tic; and, once again, an intense polarridge over west and central Canadahelped displace these mid latitudetroughs and the main west wind axisfar to the south of normal, aided by theSST anomaly pattern in the centraland eastern Pacific which was compa­tible with major storminess. However,the two winter circulation patterns diddisplay important differences alsowhich were particularly significantover west and southwest America.Although a full-latitude west coastridge was able to form on limitedoccasions throughout the winter, thenear record cold in the midwest andeastern states was not due to this causebut to severe Arctic outbreaks from thehigh pressure center over Canada. The

28 Marine Fisheries Review

1977 MEAN ANNUAL 700mb HEIGHT a ITS ANOMALY(feet+IO)Figure 8.-Mean annual distribution of 700 mb height and its anomaly (feet -;- 10) for 1977.

extent and persistence of the warming and winter of 1976-77, and attendant vortex Rev. 106:746-751.does suggest that the data are meaning- displacement. Mon. Weather Rev. 106: 137- , and J. Namias. 1976. North Amer-

142. ican influences on the circulation and climateful, and if so a possible explanation Bjerknes, J. 1966. A possible response of the of the North Atlantic sector. Mon. Weathermight concern the record Gulf Stream atmospheric Hadley circulation to equatorial Rev. 104:1255-1265.transport following the frigid Arctic anomalies of ocean temperature. Tellus , and In press. Atmo-

18:820-829. spheric climatology and its effect on seaoutbreaks of the winter season. Dickson, R. R. 1978. Weather and circulation surface temperature - 1976. In J. R. Goulet

Literature Cited of February 1978-record or near-record cold and E. D. Haynes (editors). Ocean variabilityAngell, J. K., and J. Korshover. 1978. The ex- east of the Continental Divide with a major in the U.S. Fishery Conservation Zone. 1976.

panded north circumpolar vortex of 1976 blizzard in the north east. Mon. Weather p. 19-33. U.S. Dep. Com mer" NOAA Tech.

May-June 1979 29

A

,I

.--t

B

1977

Figure 9.-Mean annual distribution of sea surface temperature anomaly (OF) in 1977 over the Pacific and Atlantic sectors.

Rep. NMFS Circular.Matson, M. 1977. The satellite snow cover

record: status, trends and relationships. Proc.NOAA Climate Diagnostics Workshop,Wash., D.C.

Namias, J. 1978. Multiple causes of the NorthAmerican abnormal winter 1976-77. Mon.Weather Rev. 106:279-295.

30

Rowntree, P. R. 1972. The influence of tropicaleast Pacific Ocean temperatures on the atmos­phere. Q. J. R. Meteorol. Soc. 98:29()"321.

____ . 1975. Thermal and orographic forc­ing of the Northern Hemisphere winter circu­lation in a numerical model. In Proc.WMO/IAMAP Symp. Long-Term ClimaticFluctuations, p. 355-364. World Meteorologi­cal Organization, Geneva, Switzerland.

Wagner, A. J. 1977. Weather and circulation ofJanuary 1977, the coldest month on record inthe Ohio Valley. Mon. Weather Rev. 105:553­560.

____ . 1978. The circulation and weatherof 1977. Weatherwise 31:25-31.

Worthington, L. V. 1977. Intensification of theGulf Stream after the winter of 1976-77.Nature 270:415-417.

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