Canadian Water Resources Journal ISSN: 0701-1784 (Print) 1918-1817 (Online) Journal homepage: http://www.tandfonline.com/loi/tcwr20 SPATIAL ANALYSIS OF TEMPERATURE AND PRECIPITATION ANOMALIES ON THE CANADIAN PRAIRIES DURING TWO STRONG EL NINO EVENTS James M. Byrne & Aaron A. Berg To cite this article: James M. Byrne & Aaron A. Berg (1998) SPATIAL ANALYSIS OF TEMPERATURE AND PRECIPITATION ANOMALIES ON THE CANADIAN PRAIRIES DURING TWO STRONG EL NINO EVENTS, Canadian Water Resources Journal, 23:3, 231-243, DOI: 10.4296/cwrj2303231 To link to this article: https://doi.org/10.4296/cwrj2303231 Published online: 23 Jan 2013. Submit your article to this journal Article views: 461 View related articles Citing articles: 3 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalinformation?journalcode=tcwr20
SPATIAL ANALYSIS OF TEMPERATURE AND PRECIPITATION ANOMALIES ON THE CANADIAN PRAIRIES DURING TWO STRONG EL NINO EVENTS Submitted October 1997; accepted June 1998 Written comments on this paper will be accepted until March 1999 James M. Byrnel and Aaron A, Bergl Abstract Many authors have argued that El Nino events are responsible for major climate disruptions on a global basis. This study examines the effects of strong El Nino events on the Canadian prairies. There is a substantive warming over all of the prairies in winter during strong El Nino events and an increase in the latitudinal precipitation gradient in which the far southern regions experience moderate decreases in seasonal precipitation. This condition is reversed in the central prairies and parkland where seasonal precipitation is elevated slightly above normal conditions. R6sum6 De nombreux auteurs attribuent au ph6nomdne El Nino lesperturbations climatiques majeures d l'6chelle mondiale.letude se penche sur les retomb6es des puissants ph6nomenesel Nino dans les Prairies canadiennes. Un r6chauffement considerablese produit dans l'ensemble des Prairies en hiver au cours oes puissantsph6nomenes El Nino et on constate une augmentation dans le gradientde pr6cipitation en latitude, par lequel les r6gions les plus au Sudconnaissent des baisses moder6es des pr6cipitationssaisonnidres. Ce ph6nomene est invers6 dans les Prairiescentrales et le foret-parc ou les pr6cipitations saisonnidressont l6gerement plus 6levees que les conditions normates. lntroduction El Nino and Southern Oscillation (ENSO) events are considered to have a substantive impact on regional climates around the globe (Ropelewski and Halpert, i987a, 1987b; Moran and Morgan, 1997; and many others). El Nino events occur on a fairly regular basis. Forty seven strong or very strong El Nino events occurred between 1525 to 1987; seventeen strong or very strong events and thirty two moderate events between 1806 and 'l 987 (Quinn et al., 1987). On average, the El Nino recurrence interval is about four years; and a strong or very strong event occurs once in ten years. There has been much discussion of the regional impacts of El Nino events on specific areas of the world. lt has been stated that the 1982-83 event and associated extreme weather cycles caused massive flooding in Equador and California, and six tropical storms in French Polynesia in just three months. Other persistent climate patterns associated with El Nino are extensive droughts in eastern Australia, Indonesia and South Africa, and intensified drought in the African Sahel (Moran and Morgan, 1997). In Nofth America, El Nino is also associated with snowfall anomalies in the Rocky Mountains. During the 1982-83 1. Water Resources Institute, University of Lethbridge, Lethbridge, AB Canadian Water Resources Journal Vol. 23, No. 3. 1998 231
event snowfall was very low in western Canada and the U.S., and skiing was very poor as many resorts had little or no snow. The variations in regional climate attributed to El Nino events, while compelling, are usually generalized to large regions; and typically the repofis are for areas that are dramatically impacted. An exception to the latter is the report of Ropelewski and Halpert (1986) in which it is stated that ENSO typically warms all of North America; and has a positive effect on total precipitation over the continent. Their analysis considered a very large geographic region with highly varied spatial data resolution; and hence could easily have missed regional signals not supportive of their conclusions. Preliminary analysis carried out in the present paper indicate that the Canadian prairies do show consistent temperature and precipitation anomalies in response to ENSO events. In the case of precipitation, L2 Ero our analysis demonstrates substantive reduction, a result very different than that of Ropelewski and HalPert. Methodology Discrete Location AnalYses The analysis was carried out in two phases. Initially, climate data for the period 1960-1989 for three locations on the prairies were integrated into monthly data. These data were used to develop simple descriptive statistics of the deviation of monthly averages of mean daily maximum temperature, and monthly precipitation for two El Nino years, 1983 and 1987. Figure 1 shows the change in daily maximum temperature, averaged over the month, for locations near Red Deer, AB; Saskatoon, SK; and Brandon, MN. The data show the average variation from the -r-i--l ;jli-r @ e6 I A o.+ Rn I AB w SA ra Eo b0 (). U -4 +- 1 2 a 4 6 7 8 9 101112 month = MAN Figurel: Deviation of Average Daily Maximum Air Temperature from 1960-89 Normals Based on Average conditions During Two El Nino Events (1983 and 1987) for Discrete Locations Across the Prairies 232 Revue canadienne des ressources hydriques Vol.23. No.3, 1998
1960-89 normal daily maximum temperature for daily maximum temperatures in 1983 and 1987 for these locations. The data show strong warming trends in all winter months in all three locations, except for December in Alberta. The warming relative to normal conditions intensifies eastward. January near Red Deer in Alberta in 1983 and 1987 had maximum temperatures about 6"C warmer than normal, while in Saskatchewan and Manitoba, January maxrmum temperatures were 10.C and 11"C warmer than normall A substantial cooling (greater than -2"C) occurred in July and August in Saskatchewan and Manitoba. Alberta does not show any significant variation from normal conditions in July or August. Figure 2 presents the variation in average monthly precipitation for 1983 and 1987 compared to the 1960-89 monthly precipitation normals. Trends in the precipitation data are not as clear as those for temoerature (Figure 1). In general, events of 1983 and 1987 show a substantive reduction in annual precipitation in Alberta (86 mm below normal) and Manitoba (72 mm below normal) but annual precipitation in Saskatchewan was essentially normal (a difference of only -1.7 mm). The spatial variation in precipitation decline is somewhat anomalous. Based on the temoerature variations discussed above and intuitive analysis, one would expect the Saskatchewan precipitation anomaly (or more correctly, the lack thereof) to align more closely with either neighboring province. The weakness of this analysis is the issue of discrete locations where individual events can distorl the overall trend(s). The data in Figures 1 and 2 led us to believe that detailed analysis of the spatial impacts of ENSO events on the Canadian prairies would be appropriate; would allow us to identify the temporal-spatial variations in temperature and precipitation in years with strong or very strong El Nino events; and provide an illustrated forecast of the impacts that a strong ENSO event may have on the region. F 930 Fzo 810 :t ito r -10 *r fr -zo F E -30 2 3 4 5 6 7 8 9 10rlr2 month I AB N SA il MAN Figure 2: Deviation From Monthly Precipitation Conditions During Two El Nino Events Locations Across the Prairies Normals Based on Average (1983 and 1987) for Discrete Canadian Water Resources Journal Vol.23, No. 3. 1998 z.t5
Spatial Analysis of El Nino lmpacts The current El Nino event in the Pacific is classif ied as one of the strongest on record. Hence, we decided to carry out the analysis based on the impacts of past strong El Nino events in 1972-73 and 1982-83. In the analysis we used the Nat Christie Climate and Agriculture Research Program historical database (NCRPD). The database was developed under contract for the University of Lethbridge Water Resources lnstitute by the Agriculture and Agri-Food Canada Lethbridge Research Station (Barr and McGinn, 1993). The database variables used were daily maximum and minimum temperatures and daily preclpitation. The NCRPD is an interpolated gridded database covering the agricultural areas of the Canadian prairies at a grid spacing of approximately 50 km. For this analysis, temperature and precipitation data were integrated to monthly averages and monthly totals respectively, using Fortran and C coding. Where data were missing (most common along the northern perimeter of the prairies) the grid point was eliminated from the spatlal analysis. The monthly normals for average temperature and total precipitation were calculated for each grid poini for the 1960-89 period. Monthly average temperature and total precipitation for January through Seotember were calculated for the second year of El Nino events based on reports in Quinn ef al. (1987). For example, for the 1982-83 El Nino, the analysis evaluated 1983 iemperature and precipitation departures f rom the normals. During the 1960-89 time period, there were two strong El Nino events (1972-73) and (1 982-83) and three events classified as medium intensity 1965, 1976 and 1997 (Quinn et al..1987). Spatial and Temporal Analysis of Temperature Our discussion of spatial and temporal trends in temperature and precipitation 234 focused on the agricultural regions only, since the database used limited data in northern Manitoba and Saskatchewan, and we had less confidence in the spatial trends in those areas. Figures 3, 4 and 5 show the spatial deviation in average monthly temperature from the 1960-89 normal temperature based on the average of the departures from the mean for the two strong El Nino years (1972-73) and (1982-83). The spatial distribution of El Nino-year monthly temperatures appears to agree generally with the trends identified in the analysis of discrete locations in each province. The greatest relative warming (almost 6"C) occurred in JanuarY over southern Manitoba and Saskatchewan (Figure 3). Southern Alberta, and much of central Saskatchewan and Manitoba warmed by about 4'C in strong El Nino years. A similar trend continued in February, with relative deoartures from mean conditions of 2-3"C in southern Alberta and 3-4'C in southern Saskatchewan and Manitoba. The same trend continued in March. Central Manitoba and Saskatchewan showed departures of 3-4'C above normal conditions, and warming in Alberta ranged from 1"C in the north to almost 3'C in the south. The increase in relative warming from west to east notable in January and March was reversed in April and May (Figure 4). The temperature gradient from west to east in the spring months had a lower magnitude, but opposite direction, to that of the winter months. Strong El Nino events did not appear to have a substantive impact on late spring andior early summer temperatures, as evldenced by the June and July maps of temperature departures from monthly normals (Figures 4 and 5). ln both months, the temperature patterns approximate normal conditions. However, August conditions showed substantive warming, and the spatial pattern of relative warming was similar to that in January and February (greater relative warming from northwest to southeast) (Figure 5). The September map Revue canadienne des ressources hydriques Vol. 23. No. 3, 1998
ri.: r j1,::ii:.lirirat jr jrf,r,rf-:tr.:.if r.a4 ' [n3:r&1!stt:-lw]ffi January Temperature Anomalies February Temperature Anomalies March Temperature Anomalies 300 km --"-- Figure 3: Deviation in Average Monthly Temperature ("c) from the 1960-1989 Normal Temperatures Based on the Average Departures from the Mean for the 1973 and 1983 El Nino Events Canadian Water Resources Journal 235 Vol.23, No. 3, 1998
April Temperature Anomalies 300 km -:- Figure 4: Deviation in Average Monthly Temperature ("G) from the 1960-1989 Normal Temperatures Based on the Average Departures from the Mean for the 1973 and 1983 El Nino Events Revue canadienne des ressources hydriques Vol.23. No.3, 1998
July Temperature Anomalies August Temperature Anomali es September Temperature Anomalies 300 km ] Figure 5: Deviation in Average Monthly Temperature ("c) from the 1960-19g9 Normal Temperatures Based on the Average Departures from the Mean for the 1973 and 1983 El Nino Events Canadian Water Resources Journal 297 Vol.23, No.3. 1998
showed a slight cooling over most of the prairies; and it appears that the greatest cooling occurred in the northern regions. Spatial and Temporal Analysis of Precipitation Chang ef ai. (1990) demonstrated that winter precipitation is correlated strongly with spring soil moisture conditions in southern Alberta, and probably in the rest of the prairies. Spring and summer precipitation is also critical to crop development and yield. Therefore, an assessment of possible El Nino impacts on precipitation throughout the year is informative for the agriculture community. Figures 6, 7 and 8 show the relative impacts of two strong El Nino events on January to September monthly precipitation. Analysis of the spatial trends in precipitation shows the following patterns. There appears to be a wide impact band across the southern prairies in which January precipitation was 5-1 0 mm lower than the 1960-89 normal. There are several large pockets where precipitation was more than 10 mm below normal. (This translates to >10 cm of snow, given 1 cm of water is assumed to equal 10 cm snow.) In February, a similar but not as intense spatial trend is evident. There is a large band approximating the parkland region in which February snowfall was 4 cm below normal; other bands to the north and south with snowfall 4-8 cm below normal; and one oocket in south-central Saskatchewan where the deviation is greater than B cm below normal. March precipitation again showed a negative trend. Large bands of below normal precipitation dominated most of Alberta (excepting an anomalous area of slightly above normal precipitation extendlng from Banff to the Lloydminister region), all of Saskatchewan and most of Manitoba (except a 4 mm positive anomaly over the Lake Winnipeg area). In April (Figure 7), the general negative trend is reversed, as most of the prairies received 0-5 mm above normal precipita- 238 tion. There were some below normal precioitation bands in northern Saskatchewan, southeastern Manitoba, and the extreme south of Alberta and Saskatchewan. In May, much of central Saskatchewan and Manitoba experienced above normal precipitation (10 mm in a few locations) but the southern prairies, including central Alberta, again received lower than normal precipitation. A large cell in the Calgary west region received 10-1 5 mm below normal. In June the dry trend reversed in the central regions of all three provinces, and large areas received 10-20 mm above normal precipitation. The exception was southwestern Saskatchewan and southwest Alberta where precipitation remained 10 mm below normal. In July (Figure 8), large areas of the prairles experienced positive precipitation deviations of up to 20 mm. This additional precipitation would probably be welcomed, but timing would be critical; if received late in the month on crops already suffering moisture stress, then minimal benefit to growth and yield would be realized. Another notable feature of the July map is a large dry cell in west-central Alberta where precipitation deviated by as much as 10 mm below normal. BY August, many grain crops are ripening, and in late August and September, precipitation is disruptive of harvest and can degrade crop quality. In August, precipitation fluctuated slightly around the normal condition; but the southern orairies received 5-1 0 mm below normal. There was a large zone of above normal precipitation over much of eastern Alberta. During September, there were large belts of positive precipitation over southern Manitoba and southeastern and central Saskatchewan while most of Alberta and southwestern Saskatchewan had lower than normal precipitation. Overall, several ironies exist in the El Nino precipitation patterns seen in Figures 6, 7 and B. Regions typically facing the greatest crop water deficits (the Palliser Triangle) received lower than normal precipitation during strong El Nino events; Revue canadienne des ressources hydriques Vol. 23. No. 3, 1998
- -.10.s. tc% i t,,' March Precipitaiion Anomalies Figure 6: Deviation in Average Monthly Precipitation (mm) from the 1960-1989 Normal Precipitation Based on the Average Departures from the Mean for the 1973 and 1983 El Nino Events Canadian Water Resources Journal 239 Vol. 23, No. 3, 1998
April Preci pi tation Anomalies Mav Precinitation Anomalies Figure 7: Deviation in Average Monthly Precipitation (mm) from the 1960-1989 Normal Precipitation Based on the Average Departures from the Mean for the 1973 and 1983 El Nino Events Revue canadienne des ressources hydriques Vol.23. No.3. 1998
July Precipitatior: Anomaiies 300 km -- Figure 8: Deviation in Average Monthly Precipitation (mm) from the 196G-1989 Normal Precipitation Based on the Average Departures from the Mean for the 1973 and 1983 El Nino Events Canadian Water Resources Journal 241 Vol.23, No.3, 1998
Figure 9: Sum of Precipitation Anomalies (mm) Across the Prairies, January to June whereas the parkland regions that typically do not suffer substantial crop water deficits received normal or above normal precipitation in most months. In August and September, there were large bands of positive precipitation anomalies that could interfere with the harvest and degrade crop quality. These positive anomaly bands are most notable over central regions that probably would have reasonable yields, given these regions would have received normal or slightly above normal January to July precipitation. Figure 9 illustrates the precipitation pattern that strong El Nino events bring to the Canadian prairies. The trend is very clear but not extreme in terms of absolute values. Substantial negative precipitation anomalies dominate the southern plains, including much of southern Manitoba and southwestern Alberta, where anomalies range 30-45 mm below normal. The deviation below normal generally increases southward. A oositive trend exists across the central and northern orairies where January-June precipitation is 15-30 mm above normal. z+z Summary We have shown the spatial impacts that may be expected on the Canadian prairies from strong El Nino events. The analysis of these impacts indicate a strong latitudinal gradient in precipitation and temperature. Overall, El Nino impacts on the Canadian prairies are not extreme; but some of the winter temperature anomalies are very large. Our analysis also shows some very large effects on mean maximum temperatures. The most notable regional impacts are an overall elevation of winter temperatures and an enhancement of latitudinal precipitation gradients. The extreme southern prairies typically have the greatest crop water deficit, and precipitation in this region is somewhat depressed in El Nino years. The central prairies and parkland areas typically do not have significant soil water deficits, and received a moderate increase in precipitation during El Nino years. Acknowledgements Phil Braun contributed to the data management required for the study. Agriculture and Revue canadienne des ressources hydriques Vol.23. No. 3, 1998
Agri-Food Canada prepared the historical database under contract with the Universitv of Lethbridge. References Barr, A.G. and S.M. McGinn. 1993. lnformation Manual for the Nat Christie Weather Database. Lethbridge Research Station Publication, Agriculture and Agri- Food Canada. Chang, C., T.G. Sommerfeldt, T. Entz and D.R. Stalker. 1990. "Long Term Soil Moisture Status in Southern Alberta." Canadian Journal of Soil Science. 70: 1 25-1 36. Moran, Joseph M. and Michael D. Morgan. 1997. Meteorology: The Atmosphere and the Science of the Weather. th ed. Toronto: Prentice-Hall. Quinn, W.H., V.T. Neal and S.E. Antunez De Mayola. 1987. "El Nino Occurrences over the Past Four and a Half Centuries." Journal of Geophysical Research, 92: 14449-14461. Ropelewski, C.F. and M.S. Halpert. 1987a. "North American Precioitation and Temoerature Patterns Associated with the El Nino/Southern Oscillation (ENSO)." Monthly Weather Review, 11 4: 2352-2362. Ropelewski, C.F. and M.S. Halpert. 1987b. "Global and Regional Scale Precipitation Patterns Associated with the El Nino/Southern Oscillation (ENSO)." Monthly Weather Review, 115: 1606-1 626. Canadian Water Resources Journal Vol. 23, No. 3. 1998 243