101 the Effect of Protracted Spring

VOL. 33, No. 3, MARCH, 1952 101

The Effect of Protracted Spring Thaws on Ice Conditions in Hudson Bay *

G. A. MACKAY Canadian Meteorological Service, Gander, Nfld.

ABSTRACT

The extent of ice cover in Hudson Bay was investigated during the Spring of 1948 by the RCAF and interested governmental departments. A series of reconnaissance flights over the Bay in March and April disclosed that it was virtually ice-bound from shore to shore. Large areas of open water were observed during the period May 3rd to May 6th. An inspection of the synoptic weather charts disclosed that weather conditions over the Bay both prior to and during this period were abnormal. This immediately suggests that the ice conditions observed might not be representative. For an appraisal of the observations it is necessary to determine the effects of the abnormal weather on the ice. The observed ice conditions are mentioned and the causes of open water areas briefly discussed. The severity of the winter at Churchill was investigated to determine the normalcy of the ice development. A protracted thaw over the eastern parts of Hudson Bay is disclosed by reference to the weather charts. The correlation between the mean monthly temperatures for the spring months and the date of ice clearance at Churchill Harbour is then investigated to determine the effects of a protracted thaw on the ice.

FAIRLY persistent shore lead that en- OPEN WATER IN WINTER circles Hudson Bay during the winter Open water may result from any of the fol- gave rise to the belief that the center of A lowing processes: (1) strong wind action; (2) Hudson Bay was clear of ice during the winter. Churchill was made into an airbase on the "Crim- son Route" in World War II. Flights made over the Bay at this time provided the first evidence of the solid ice cover in the center during the winter. Corroborating this, climatic records of weather stations fringing the Bay do not reveal the presence of an adjacent large area of open water in mid-winter. Sorties out of Churchill to about 150 miles over the Bay in January, 1948 and flights across the Bay on March 11 and April 14th to 17th, 1948 disclosed virtually complete ice cover. The shore lead was in evidence in all cases and its width ranged from zero to ten miles. The large areas of open water observed in the first week of May are shown in FIGURE 1. East- erly winds prevailed over the Bay for the week prior to the taking of these observations and con- sequently the large open water areas lie off the east coast and to the west of the Belcher Islands. Smaller areas of open water were noted at the mouths pf the larger rivers and inlets along the west coast. The shift of the ice to the west had rendered the shore lead along the west coast non- existent and a belt of small pans of close pack ice was forced tightly against the land-locked ice. FIG. 1. Map showing open water areas observed on * Published by permission of the Controller, Meteoro- Hudson Bay by RCAF aircraft flights during the pe- logical Division, Department of Transport, Canada. riod May 3-6, 1948.

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FIG. 2. Synoptic situations in Hudson Bay region April 28-May 4, 1948, and the normal pressure distributions for April and May.

Unauthenticated | Downloaded 10/05/21 06:37 AM UTC VOL. 33, No. 3, MARCH, 1952 103 strong currents and up welling; (3) tidal action; Hudson Bay where they are responsible for and (4) decay due to radiational and conductive patches of open water that persist throughout the gains in heat. year. Tidal currents produce open water areas Wind produces a compression of the ice field throughout the winter in many parts of the Arctic to the leeward and is largely responsible for the archipelago as do the river currents in such north- shore lead that encircles the Bay. The shore lead ern inland locations as Kettle Rapids, the Kazan would tend to reach a maximum width along the River, Great Bear River and Baker Lake. Cur- windward coast with a persistent and strong rents were probably responsible for the patches wind. The momentum imparted the larger pans of open water observed at the river mouths along and fields of ice by the winds undoubtedly causes the west coast. a decrease of the actual ice area by subsequent Spring tides frequently produce flooding over rafting and hummocking. the land-locked ice and the tidal flats. This water Currents are strongest in the northern part of breaks through the hinge where the land-locked

FIG. 3. Curves of maximum and minimum daily temperatures at stations around Hudson Bay, April-May 1948, and the normal trends for the same period. (The daily maximum and minimum temperatures for Chesterfield Inlet were not available and 1830 GMT and 0630 GMT data were used to obtain the curves; this accounts for the minima being shown higher than the maxima in some cases.)

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FIG. 4. Cumulative temperature-departure curves for maximum and minimum temperatures at Fort Churchill, computed from December 1, 1947 to May 5, 1948 (parts of curves prior to January 1 not shown). ice joins the floating ice or through thin ice which front became stationary over the central portions may exist at the river mouths. On February of the Bay as a low developed to the south of the 15th, 1946, the ice in the mouth of the Churchill region. From April 28th until May 3rd the River was flooded to a depth of about twenty southern half of the Bay was in an easterly cir- inches. Large sheets of shell ice were left at ebb culation of modified polar Atlantic air. Tem- tide. Judging from the frequency of fog occur- peratures again were maintained above normal. rence in similar locations that winter, the process Much colder air covered the northern and north- occurs frequently. Snow on the tidal flats along western sections of the Bay at this time. The the west coast that month was well glazed over weather charts for this period are shown in —a further suggestion of flooding. FIGURE 2. The extent of the open water in May suggests The maximum and minimum temperature that the ice had been subjected to large scale de- curves for this period for weather stations around cay, although currents along the eastern half of the Bay are shown in FIGURE 3. The lengthy the Bay may have aided by removing ice to Hud- mild spell with above freezing daytime tempera- son Straits. tures may be clearly seen from these diagrams. Although temperatures in the western sections WEATHER of the Bay were abnormally warm throughout In late April a slow moving front with waves this period they were generally below 30°F. was in an NNE to SSW orientation in the vicin- ity of Hudson Bay. A moderate southerly cir- ICE DEVELOPMENT DURING THE WINTER culation was produced in advance of this front To determine the nature of the winter experi- and this affected the eastern half of the Bay for enced in 1947-48 cumulative anomaly curves for the period April 25th to 27th. Temperatures the maximum and minimum temperature were were well above normal in this circulation. The constructed. These curves are shown in FIGURE

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4. The portion shown in the diagram covers the The following TABLE gives the break-up dates period after January 1st. The origin, however, and the mean monthly and mean annual tempera- was December 1st. tures for Churchill Harbour. Warmer than normal conditions were experi- Mean enced in the early winter months. For the period Mean Mean Mean Mean Mean Mean Date June ice Jan. Feb. March April Annual January 11th until the late April warm spell, Year May temp. cleared temp. temp. temp. temp. temp. (1-10) temp. continuously below normal minimums were ex- out °F °F °F °F °F °F °F

perienced. Over this period the minimum tem- 1928 June 17 -16.7 -11.5 - 2.3 9.3 29.0 20.2 peratures averaged about 7F° below normal. 1929 June 20 -19.3 -18.6 - 9.7 11.2 22.0 16.9 1930 June 9 -19.1 -15.3 - 5.0 10.3 34.2 21.0 Colder than normal maximum temperatures were 1931 June 11 -15.8 - 3.2 - 6.0 14.5 27.4 22.7 1932 June 7 -12.7 -20.2 -12.7 11.5 30.7 44.0 19.5 also experienced over the period February 13th 1933 June 17 -19.6 -19.9 - 5.4 7.7 28.5 33.5 15.2 1934 June 21 -16.8 -17.0 - 9.2 13.4 23.3 38.1 16.9 to April 22nd. 1935 June 8 -31.8 - 6.7 -10.0 13.1 36.5 16.7 1936 June 18 -26.5 -25.0 - 4.6 1.3 24.0 13.9 The abnormally low minimums are suggestive 1937 May 27 -23.0 -11.5 - 2.2 21.1 37.0 43.9 20.3 1938 June 5 -11.7 -14.1 - 0.0 8.4 34.0 36.4 23.2 of persistent anticyclonic conditions, high radi- 1939 June 11 -17.4 -24.3 -12.2 13.6 30.2 35.2 17.8 1940 June 4 - 6.2 - 9.4 - 1.6 15.1 31.5 39.5 21.5 ative losses and light winds. Ice formation would 1941 June 6 -14.6 - 6.9 - 0.6 15.0 29.3 41.7 19.8 certainly not have been retarded under these con- 1942 June 8 -11.9 -17.1 + 2.5 12.8 31.4 34.9 18.9 1943 June 17 -20.1 -13.8 -11.7 11.9 26.0 18.0 ditions. 1944 June 4 - 4.0 -17.9 - 7.8 18.0 31.8 20.7 1945 June 21 -16.7 -11.0 + 1.4 4.6 23.5 15.8 The severity of the winter beyond normal 1946 June 14 -21.5 -25.0 + 2.5 16.2 24.2 16.6 1947 June 20 -19.1 -14.1 -f- 2.3 5.9 25.5 would not affect the ultimate ice thickness ap- 1948 June 10 16.0 36.2

preciably. Sea ice tends to reach a maximum Normal 11 -19 -17 - 6 14 30 18 thickness of 1.5 meters due to the insulating effects of the ice itself and slight warming caused by the sea water. Greater ice thicknesses re- ported in the Bay are probably a result of rafting. Close to fifty percent of the surface winds at both Churchill and Port Harrison are from the northwesterly quadrant. The type of winter experienced at Churchill would therefore be generally representative of the winter experienced by the Bay as a whole. It would seem reason- able, therefore, to accept the ice conditions observed during March and April as being fairly close to normal.

THE CORRELATION OF THE MEAN MONTHLY TEMPERATURE AND THE BREAK-UP DATE AT CHURCHILL On April 14th to 16th the Bay was considered to be virtually ice covered. Open channels were observed along the coast in James Bay and off Cape Jones where the winds were southerly. Similarly a channel one-quarter to two miles in width was observed along the west coast and to the east of Mansel Island where there was a west- erly circulation. One-half month later the large areas of open water shown in FIGURE 1 were observed. Throughout most of the elapsed time, above normal temperatures and easterly winds were experienced in the localities where the open water was observed. FIG. 5. Scatter diagrams for relation of ice clearance date at Churchill and deviation of mean monthly tem- The correlation of the mean monthly tempera- peratures from normal: at the top for May temperature, ture for the spring months and the date of ice at the bottom for April temperature. The regression clearance at Churchill Harbour was then investi- lines were drawn to best fit by visual inspection; com- gated to determine the effects of such a protracted puted regression lines were unreasonable in slope ow- mild spell on the ice. ing to the small number of cases.

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A glance at the TABLE shows a marked rela- the break-up date and the mean temperature of tionship between the mean May temperature and that period of the year, when thawing is actually the date of ice clearance. This relationship is occurring, is of course to be expected. Further shown also in the scatter diagram (FIG. 5) where than this, however, the correlations also show that the deviation of the mean monthly temperature the type of winter probably does not materially from the normal is plotted against the actual affect the break-up date. break-up date for that year. The scatter dia- The half month of thawing temperatures ex- gram showing the relationship between the break- perienced in the southeastern sections of the Bay up date and the deviation from normal of the produced the large areas of water shown in FIG- mean April temperature is also shown in FIG- URE 1. Since the temperatures were abnormally URE 5. high in that area the extent of the open water This relationship was assumed to be a straight- would be well in advance for that time of year. line function and the correlation coefficients were then calculated for various combinations of the CORRELATIONS AT MONTREAL HARBOUR mean monthly temperature anomaly and the As a check on the correlations obtained for break-up date. The following correlation coeffi- Churchill, the break-up dates for Montreal Har- cients and confidence intervals were obtained: bour were investigated. These records are more

CORRELATION OF DATE OF ICE CLEARANCE AND DEPAR- complete, going back to 1908. Due to missing TURE OF MEAN MONTHLY TEMPERATURE FROM NORMAL, data only the periods 1916-1933 and 1937 to CHURCHILL HARBOUR 1942 were used. In 1931 ice-breaker operations into Montreal Harbour commenced. These do 95% Probability .01 No. not appear to have influenced the harbor-clear- Month r of true value of Signif. cases r being within: level ance date appreciably. For Montreal Harbour the following correla- Jan. -0.29 20 .17 & —.65 -.56 Feb. -0.27 20 .19 & -.64 -.56 tion coefficients between ice break-up and monthly March -0.04 20 .39 & - .47 -.56 mean temperature anomalies were obtained: Jan- April -0.67 21 -.50 & - .85 -.55 uary .45, March .85. Using half-monthly periods May -0.82 21 -.61 & -.92 -.55 in the ratio 1:2:5 for the period March 1st to April +May -0.87 21 -.71 & -.94 -.55 April +2 May -0.86 21 -.69 & - .94 -.55 April 17th a correlation of .91 was obtained. For May -bjune 1-10 -0.92 9 -.67 & - .98 -.80 the period after ice-breaker operation had commenced the correlation for the month of March The above are product-moment correlation coefficients. and the break-up date was calculated to be .90. Using the rank-difference method values of r of — .05, These results corroborate those for Churchill. — .83, — .86, — .85 and — .88, respectively, were obtained. The 95% probability ranges were determined by use of The improved correlation obtained by weighting 1 + f the half-monthly mean temperatures emphasizes Fisher's transformation, taking z — 1.151 log10 ^ _ with the effects of the protracted late spring thaws in = Vtf^T ice clearance.

It will be noted that the correlations with Janu- ACKNOWLEDGMENT ary, February, March and April temperatures are The author wishes to acknowledge the kind not only low but of small confidence; April plus assistance of Mr. K. Buckthought. double-weighted May temperatures gives the best correlation. REFERENCES The mean temperature for May at Churchill is [1] Burbidge, F. E.: Operation "Ice Cake" Meteoro- 30°F and that for April 14°F. A protracted thaw logical Service of Canada, Toronto. 1948. at these times would raise the mean monthly tem- [2] Hare, F. K., and Montgomery, M. R.: Ice, Open peratures substantially, the amount depending on Water, and Winter Climate in the Eastern Arctic of North America, Arctic, Vol. 2, No. 3, pp. 149- both its intensity and length. The rather good 164 (1948). correlations obtained for the mean temperatures [3] Lamont, A. H.: Ice Conditions Over Hudson Bay of the late spring months and the date of ice clear- and Related Weather Phenomena. Meteorologi- ance show that the presence or absence of pro- cal Service of Canada, Toronto, 1948. [4] Climatic Summaries for Selected Meteorological tracted spring thaws is all important in deciding Stations in Canada, Newfoundland and Labrador. the break-up date. Such a correlation between (Canadian Met. Serv.)

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