TELECONNECTIONS BETWEEN EL NIN O AND LA NIN A EVENTS AND SUMMER EXTENDED DRY SPELLS ON THE CANADIAN PRAIRIES

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 19: (1999) TELECONNECTIONS BETWEEN EL NIN O AND LA NIN A EVENTS AND SUMMER EXTENDED DRY SPELLS ON THE CANADIAN PRAIRIES B.R. BONSAL a, * and R.G. LAWFORD b a Climate Research Branch, Atmospheric En ironment Ser ice, Downs iew, Ontario, Canada M3H 5T4 b NOAA/OGP, Sil er Spring, MD 20910, USA Recei ed 28 April 1998 Re ised 26 February 1999 Accepted 1 March 1999 ABSTRACT Teleconnections among El Niño and La Niña events, North Pacific sea surface temperature anomalies, and summer (June, July, August) extended dry spells on the Canadian Prairies are documented and analysed. For the period , results show the majority of El Niño events associated with a persistent North Pacific SST anomaly pattern consisting of anomalously cold water in the east-central North Pacific and anomalously warm water along the west coast of North America. The average number of summer extended dry spells on the Prairies associated with El Niño events is significantly higher than for non-el Niño periods. This relationship occurs during the second summer following the mature stage of El Niño events. Further analysis reveals that La Niña events are associated with opposite North Pacific SST anomaly patterns that also tend to persist for several seasons. The average number of summer extended dry spells associated with these events is significantly lower than for non-la Niña periods. Even though the relationships among El Niño, LaNiña, North Pacific SST anomalies, and extended dry spells show high variability, the results of this study may be considered as an initial step towards contributing to a long-range forecasting technique of summer extended dry spells on the Canadian Prairies. Copyright 1999 Royal Meteorological Society. KEY WORDS: teleconnections; Canadian Prairies; extended dry spells; El Niño; La Niña; PNA patterns; 500 hpa anomalies; North Pacific SST anomalies 1. INTRODUCTION Environmental processes and economic activities on the Canadian Prairies are highly dependent on precipitation, especially during the summer season (June, July, August) when the majority of annual precipitation is received (Longley, 1972; Dey, 1982). Summer precipitation is highly variable in space and time often resulting in extreme precipitation variations including wet spells and extended dry spells leading to droughts (Dey and Chakravarti, 1976; Dey, 1982). The general synoptic characteristic of summer dry (wet) periods is a persistent mid-tropospheric ridge (trough or zonal flow) and associated positive (negative) anomalies centred over the Prairie region (Dey, 1977; Knox and Lawford, 1990). The causes of this persistence are not fully understood. Anomalous surface boundary conditions such as sea surface temperature (SST), soil moisture, and snow cover anomalies have the potential to induce persistent anomalies of mid-tropospheric circulation patterns, mainly through changes in latent and sensible heat fluxes from the surface (Walsh et al., 1985; Ripley, 1988). Bonsal et al. (1993) found a significant relationship between persistent SST anomalies in the North Pacific Ocean and extended dry spells over the Canadian Prairies during the growing season of May August. The severity of the dry spells was defined by their duration, spatial extent, and strength of the 500 hpa anomalies over the Prairies. The associated SST patterns were characterized by anomalously * Correspondence to: Climate Research Branch, Atmospheric Environment Service, Downsview, Ontario, Canada M3H 5T4. CCC /99/ $17.50 Copyright 1999 Royal Meteorological Society

2 1446 B.R. BONSAL AND R.G. LAWFORD cold water in the east-central North Pacific and anomalously warm water along the central west coast of North America (Figure 1). It was suggested that the persistent SST anomaly patterns aided in the development of the positive 500 hpa anomalies and, thus, extended dry spells over the Prairies. The possible causes of these SST anomaly patterns were not determined. Several studies have shown relationships between El Niño (La Niña) events and North Pacific SST anomaly patterns similar (opposite) to that shown in Figure 1 (e.g. Emery and Hamilton, 1985; Weber, 1990). Individual El Niño and La Niña events can differ significantly in their timing and duration. Typical events tend to develop during summer to early autumn, mature during winter, and terminate during the following spring (Deser and Wallace, 1990). The North Pacific SST anomaly patterns are normally observed near the mature stages (i.e. autumn and winter) of these events. The development of the SST patterns appears to be related to the different phases of the Pacific North American (PNA) pattern which have also been associated with El Niño and La Niña (e.g. Hamilton, 1988). In particular, positive (negative) PNA patterns occur during the winter following the onset of El Niño (La Niña) events. It has been suggested that the North Pacific SST anomalies are generated by the surface circulation associated with the Aleutian low which is stronger (weaker) during positive (negative) PNA patterns (Emery and Hamilton, 1985; Yongping and McBean, 1991). The preceding suggests that if North Pacific SST anomaly patterns (similar to Figure 1) are related to El Niño, and extended dry spells on the Prairies are related to these SST anomalies (Bonsal et al., 1993), then extended dry spells may be related to El Niño events. The main objective of this study is to determine if there are any significant relationships between El Niño events and summer extended dry spells on the Canadian Prairies during the period Relationships among atmosphere ocean teleconnections including PNA patterns, North Pacific SST anomalies, and mid-tropospheric circulation patterns over the Canadian Prairies are also documented. Since La Niña events are often associated with opposite responses in terms of PNA patterns (e.g. Yarnal and Diaz, 1986), North Pacific SST anomalies (e.g. Emery and Hamilton, 1985), and in general, temperature and precipitation patterns around the globe (e.g. Ropelewski and Halpert, 1986), this study also analyses extended dry spells on the Prairies associated with these events. The investigation is essentially an extension of Bonsal et al. (1993) in that it examines possible effects of El Niño and La Niña events on North Pacific SST anomalies and extended dry spells on the Prairies. The data and methodology are detailed in the second section and results presented in the third section. A discussion of the variability in the results and concluding remarks are provided in the fourth and fifth sections, respectively. Figure 1. Anomalies of North Pacific SSTs and 500 hpa heights associated with growing season extended dry spells on the Canadian Prairies (from Bonsal et al., 1993)

3 EL NIN O, LA NIN A AND CANADIAN DRY SPELLS 1447 Figure 2. Canadian Prairie study area and precipitation stations used in this study 2. STUDY AREA, VARIABLE DESCRIPTIONS, AND METHODOLOGY 2.1. Extended dry spells The study area for the extended dry spell analysis is the agricultural region of the Canadian Prairies which includes the southern portions of Alberta, Saskatchewan, and Manitoba (Figure 2). June, July, and August (JJA) daily precipitation data for the 63 stations shown in Figure 2 are obtained from the Atmospheric Environment Service (AES) archives in Downsview, Ontario. Missing data are handled by substituting from the nearest AES station that had precipitation values for the missing period. This ensured a complete precipitation time series for each station. Given that the majority of the replacement stations are within 50 km of the actual stations, and the highest number of missing stations for any given day is only three (3/63=4.8%), the identification of large scale dry spells should not be significantly affected. Dry spells (consecutive days without precipitation) have frequently been used to analyse deficits in growing season precipitation on the Canadian Prairies (Dey and Chakravarti, 1976; Dey, 1982). This analysis defines an extended dry spell as a period of 10 consecutive days during which no measurable precipitation is recorded. The definition is similar to that utilized by Bonsal et al. (1993) who outlined individual growing season extended dry spells on the Prairies. For this study, a monthly dry spell value is calculated for every June, July, and August during the study period. May is excluded since summer responses over the Prairies are the focus of this investigation. Monthly extended dry spell values are calculated by first determining the number of dry spells for each station. If a station has only one consecutive 10-day period without precipitation, the value is 1. If it has two separate periods with this condition, the value is 2, and so on. If the consecutive days without precipitation exceed 10, the number of extended dry spells increases. For example, if a station has 11 consecutive dry days, the value is 2 ; 12 consecutive days, the value is 3 ; and so on. As a result, the longer the consecutive dry period (i.e. greater the likelihood it will have severe impacts on the Prairies), the higher the designated number of extended dry spells. The number for the entire Prairies is determined by summing the values for each station. A JJA total is also computed. Higher values indicate large scale extended dry periods over the Prairies while smaller values indicate few extended dry periods (and possibly large scale wet periods). The dry spell variable is representative of precipitation frequency (i.e.

4 1448 B.R. BONSAL AND R.G. LAWFORD periods with or without precipitation) on the Prairies. Several investigations have indicated that the longer these spells persist, the greater the impact on activities such as agriculture which can lead to drought conditions over the region (e.g. Dey, 1982) Mid-tropospheric circulation patterns Northern Hemisphere mid-tropospheric circulation data consist of monthly 500 hpa geopotential heights for the Northern Hemisphere (15 N 90 N) on a 455 point, 5 latitude by 10 longitude grid as outlined by Shabbar et al. (1990). Monthly 500 hpa anomalies (relative to the base period) are used to generate anomaly maps for the entire Northern Hemisphere. From these maps, the areally-averaged 500 hpa anomaly over the Canadian Prairie study area is calculated using the Geographic Information System SPANS. Larger positive values indicate mid-tropospheric ridging over the Prairies while larger negative values indicate a more zonal flow. The 500 hpa data are also used to identify PNA patterns. As alluded to previously, the main focus is on the development of these patterns during the winter following the onset of El Niño and La Niña events (although other seasons are also examined). The following winter PNA index (Wiche et al., 1991) is, therefore, used: PNA=1/3[ Z(45 N, 160 W)+Z(55 N, 110 W) Z(35 N, 80 W)] where Z is the normalized monthly 500 hpa anomaly. The three main centres correspond closely to those used in several other investigations (e.g. Leathers et al., 1991). Seasonal PNA indices are determined by averaging the three monthly values within each season. The seasons include winter (December, January, February); spring (March, April, May); summer (June, July, August); and autumn (September, October, November). Larger positive PNA values indicate a strong meridional flow over the North Pacific and North America with negative anomalies over the east-central North Pacific, positive anomalies over western Canada, and negative anomalies over the south-eastern US. Larger negative values indicate more west east directed flow over this region with the anomalies being opposite to the positive cases North Pacific SST anomaly patterns North Pacific SST data are obtained from the Scripps Institution of Oceanography in La Jolla, California. They consist of monthly values on a 5 latitude by 5 longitude grid and are based on ship observations compiled by the Bureau of Commercial Fisheries, information from the US Weather Bureau, and the National Marine Fisheries Service (Namias, 1970; Davis, 1976). Monthly SST anomalies are calculated by subtracting the 44-year average value from the actual value at each grid point. North Pacific SST anomaly patterns are quantified using the procedure designed by Bonsal et al. (1993). They determined that SST anomaly gradients between the east-central North Pacific (30 N 40 N, 165 W 135 W) and the central west coast of North America (45 N 55 N, 130 W 125 W) were associated with extended dry spells on the Canadian Prairies (see Figure 1). The gradients were quantified by subtracting the largest absolute anomaly in the central North Pacific region from that along the west coast of North America. Positive gradients occur when the west coast anomalies are higher than those in the central North Pacific (e.g. Figure 1) and ice ersa. Seasonal SST anomaly gradients are determined (in C) by averaging the three monthly values within each season El Niño and La Niña e ents This study incorporates the El Niño and La Niña events defined by Kiladis and Diaz (1989). They used seasonal normalized SOI values and tropical Pacific SST anomaly indices for the region 4 N 4 S and 160 W to the South American coast. For an El Niño (La Niña) event, the SST anomaly in this region had to be positive (negative) for at least three consecutive seasons and be at least 0.5 C above (below) the mean for at least one of these seasons. The seasonal SOI also had to remain below 1.0 (above +1.0)

5 EL NIN O, LA NIN A AND CANADIAN DRY SPELLS 1449 Table I. El Niño and La Niña events used in this study (from Kiladis and Diaz, 1989) El Niño: La Niña: Note: Each year refers to onset of the event. for the same three seasons. Based on this definition, ten El Niño and seven La Niña events occurred from 1948 to 1991 (Table I). These years refer to the onset of the events (year 0) Methodology North Pacific SST anomaly gradients, PNA indices, and 500 hpa anomalies and extended dry spells over the Prairies are composited for the El Niño and La Niña events in Table I. The purpose is to determine if there are any significant differences between El Niño and non-el Niño (including La Niña) periods and La Niña and non-la Niña (including El Niño) periods for each variable. The standard two-tailed Student s t-test (e.g. Ebdon, 1985) and the non-parametric Wilcoxon rank-sum test (Bethea et al., 1995) are used. Significance is tested by a Monte Carlo procedure. For each variable, the time series is randomly shuffled times and a Student s t-value calculated, to produce a random distribution of t-values. The original t-values are compared to this empirically derived distribution to assess their statistical significance at the 5% level. 3. RESULTS 3.1. North Pacific SST anomaly patterns Composite North Pacific SST anomaly gradients associated with El Niño, LaNiña, and the difference between the events are given in Table II. The gradients are analysed from the winter of the year coinciding with the onset of the events to the second autumn following the onset (12 seasons). Table II shows many positive gradients associated with El Niño and negative with La Niña. For El Niño events, a positive gradient is first observed during autumn (0) and is strongest during winter ( +1). The gradient weakens slightly during spring (+1) and even further during summer (+1) (but remains positive). Autumn (+1) shows a strengthening of the gradient, and this stronger gradient persists through winter ( +2) and spring Table II. Composite North Pacific SST anomaly gradients ( C) associated with El Niño and La Niña events Season El Niño La Niña Difference (El Niño La Niña) Winter (0) Spring (0) Summer (0) Autumn (0) Winter (+1) * +2.2* Spring (+1) * +2.7* Summer (+1) * +1.8* Autumn (+1) Winter (+2) Spring (+2) Summer (+2) Autumn (+2) * Significant at 5% level using both the Student s t and Wilcoxon rank-sum tests. Numbers in parentheses refer to year with respect to the onset of El Niño or La Niña events.

6 1450 B.R. BONSAL AND R.G. LAWFORD Table III. Composite PNA indices associated with El Niño and La Niña events Season El Niño La Niña Difference (El Niño La Niña) Winter (0) Spring (0) Summer (0) Autumn (0) * Winter (+1) +2.8* 4.3* +7.1* Spring (+1) Summer (+1) Autumn (+1) Winter (+2) Spring (+2) Summer (+2) * Significant at 5% level using both the Student s t and Wilcoxon rank-sum tests. Numbers in parentheses refer to year with respect to the onset of El Niño or La Niña events. ( +2). The gradient then weakens during the following summer and autumn. Positive SST anomaly gradients, therefore, occur from autumn (0) until summer (+2); a total of eight seasons. There are no significant differences between any of the seasonal SST anomaly gradients associated with the ten El Niño events and all other SST anomaly gradients during the study period. A likely explanation is that these types of North Pacific SST anomaly patterns also frequently occur in the absence of El Niño events (i.e. they can be forced by other factors) (Namias et al., 1988; Zhang and Wallace, 1996). La Niña events are associated with negative SST anomaly gradients throughout the entire period. A stronger gradient appears to develop during autumn (0) and strengthens to become significantly negative in winter ( +1), spring ( +1), and summer ( +1). It weakens slightly during the following season, but continues to persist through autumn ( +2). The preceding indicates that for the most part, North Pacific SST anomalies associated with El Niño and La Niña tend to be opposite with the gradients persisting for several seasons in both cases PNA patterns Composite PNA indices associated with El Niño, LaNiña, and differences between the events are provided in Table III. During El Niño events, positive PNA patterns develop in autumn (0) and strengthen during winter ( +1). They disappear during the following season. Conversely, a very strong and significant negative PNA index occurs during the winter ( +1) associated with La Niña events. It also disappears during the following season. The results in Table III indicate that the occurrence of strong PNA patterns coincides with the strengthening of both the positive and negative SST anomaly gradients (Table II). It, therefore, appears that PNA patterns likely influence the development and/or strengthening of the North Pacific SST anomaly patterns Summer 500 hpa anomalies and extended dry spells o er the Canadian Prairies Table II shows that positive SST anomaly gradients (during El Niño) are associated with summer (+1), but also persist (although weaker) until summer (+2). The SST gradients associated with La Niña have similar characteristics (with an opposite sign). Monthly 500 hpa anomalies and extended dry spells over the Prairies are, therefore, examined during the first and second summers following the mature stages of El Niño and La Niña events (Table IV). The composite number of extended dry spells are expressed as a percentage of the 44-year average. If a value is larger than 100%, it experienced a greater than normal number of extended dry spells and ice ersa. Entire summer (JJA) extended dry spell and 500 hpa anomaly values are also given.

7 EL NIN O, LA NIN A AND CANADIAN DRY SPELLS 1451 Regarding summer ( +1), all periods associated with El Niño (La Niña) events have smaller negative (positive) 500 hpa anomalies. The number of extended dry spells during this summer correspond with the 500 hpa anomalies in that El Niño (La Niña) events are associated with fewer (more) dry spells. The only significant value is that associated with El Niño during June ( +1). Summer ( +2) shows several significant extended dry spell values. June, July, and JJA have significantly more dry spells associated with El Niño. August also experiences more extended dry spells, but this increase is not statistically significant. Conversely, La Niña events have significantly fewer dry spells for every summer month, as well as the entire summer. The differences between El Niño and La Niña events are also significant during the entire summer ( +2). In connection with 500 hpa anomalies, smaller positive (negative) composite values are associated with El Niño (La Niña) events during all time periods. However, none of these composite values is statistically significant. It, therefore, appears that El Niño and La Niña events (and associated persistent positive and negative SST anomaly gradients) are related more to extended dry spells during summer (+2) as opposed to summer (+1). The reasons for this are not clear. One possible factor is that the preceding results are based on composite or average responses for the period Responses to individual El Niño and La Niña events may show considerable variability. Another consideration is that El Niño and La Niña events sometimes occurred in consecutive years (e.g El Niño, 1964 La Niña, 1965 El Niño) (see Table I). As a result, the timing of the events was such that summer (+2) occasionally coincided with the start of another El Niño or La Niña. The following section examines the responses associated with individual El Niño and La Niña events including those of consecutive events. 4. INDIVIDUAL RESPONSES AND DISCUSSION Table V presents the magnitude and sign of PNA indices and North Pacific SST anomaly gradients associated with individual El Niño and La Niña events. Consecutive events (i.e. El Niño and La Niña in consecutive years) are denoted by an asterisk. Eight of ten El Niño events were associated with either Table IV. Composite areally-averaged 500 hpa anomalies and number of extended dry spells on the Canadian Prairies associated with El Niño and La Niña events Season El Niño La Niña Difference (El Niño La Niña) 500 hpa anomalies (dam) June (+1) July (+1) August (+1) JJA (+1) June (+2) July (+2) August (+2) JJA (+2) Extended dry spells (% of normal): June (+1) 54.7* July (+1) August (+1) JJA (+1) June (+2) 136.2* 67.2* +69.0* July (+2) 141.0* 63.0* +78.0* August (+2) * +69.2* JJA (+2) 135.8* 61.1* +74.7* * Significant at 5% level using both the Student s t and Wilcoxon rank-sum tests. Numbers in parentheses refer to year with respect to the onset of El Niño or La Niña events.

8 Table V. PNA indices and North Pacific SST anomaly gradients associated with individual El Niño and La Niña events PNA Index Autumn (0) to Winter (+1) North Pacific SST anomaly gradient Autumn (0) Winter (+1) Spring (+1) Summer (+1) Autumn (+1) Winter (+2) Spring (+2) Summer (+2) El Niño event * * * * La Niña event ** ** * Indicates El Niño event was followed by a La Niña event during the next year. ** Indicates La Niña event was followed by an El Niño event during the next year. Responses are categorized based on the normalized value of each variable as follows: 1.0 is very strong negative ( ); 1.0 to 0.5 is strong negative ( ); 0.5 to 0.0 is weak negative ( ); 0.0 to +0.5 is weak positive (+); +0.5 to +1.0 is strong positive (++); +1.0 is very strong positive (+++) B.R. BONSAL AND R.G. LAWFORD

9 EL NIN O, LA NIN A AND CANADIAN DRY SPELLS 1453 strong or very strong positive PNA indices during autumn (0) to winter (+1). All seven La Niña events experienced negative PNA indices, with six being strong or very strong. This indicates a consistent relationship between El Niño and La Niña events and the occurrence of strong PNA patterns. These results are similar to numerous other studies (both observational and modelling) that examined relationships between El Niño/La Niña events and mid-tropospheric circulation patterns over the Northern Hemisphere (e.g. Horel and Wallace, 1981; Weber, 1990). The physical basis for the relationships has also been well-documented (e.g. Hoskins and Karoly, 1981; Yarnal and Diaz, 1986). Table II suggests that positive (negative) SST anomaly gradients develop and/or strengthen during periods when strong positive (negative) PNA patterns are observed. All eight El Niño events having positive PNA patterns also experienced strong or very strong positive North Pacific SST anomaly gradients during autumn (0) to winter (+1). The two events without positive PNA (1951 and 1972) did not have positive SST anomaly gradients. In association with La Niña, all six events with strong/very strong negative PNA patterns also had strong negative SST anomaly gradients. The preceding strongly suggests that PNA patterns associated with El Niño and La Niña events influence the development of North Pacific SST anomalies. Several studies have also observed these SST anomaly patterns during the autumn and winter following the onset of El Niño and La Niña events in association with the different phases of the PNA. The SSTs were influenced by changes in the strength and location of the Aleutian low (which is directly related to the PNA) (Alexander, 1990, 1992; Yongping and McBean, 1991). Positive gradients persisted from winter ( +1) through summer ( +2) during six El Niño events (1953, 1957, 1965, 1976, 1982, 1986). In 1963 and 1969, strong positive gradients were observed during autumn (0) and winter (+1); however, they did not persist. One possible factor is that both were followed by a La Niña event during the next year. For La Niña, negative SST anomaly gradients persisted from winter (+1) through summer (+2) during only three events (1949, 1970, 1973). The events of 1964, 1975, and 1988 had stronger negative SST anomaly gradients during autumn (0) to winter (+1), but they did not persist through summer (+2). The 1964 and 1975 events were followed by an El Niño event during the next year which may have influenced the persistence of the SST anomaly patterns. Tables VIa and VIb provides the summer 500 hpa anomaly and extended dry spell responses during El Niño and La Niña events. The table is divided into events associated with persistent North Pacific SST anomaly gradients and those not associated with these gradients. Only strong and very strong responses are identified. In terms of El Niño, it is evident that strong positive 500 hpa and extended dry spell responses are associated more with summer ( +2) as opposed to summer ( +1). During the second Table VIa. 500 hpa anomaly and extended dry spell responses to El Niño and La Niña events El Niño event Positive 500 hpa More extended Positive 500 hpa More extended anomalies dry spells anomalies dry spells Summer (+1) Summer (+1) Summer (+2) Summer (+2) Events associated with persistent positive North Pacific SST anomaly gradients: 1953* July a, August a June a, July June, July a, August a August a June a, July a, August a 1976 June 1982 August a August, June a, July 1986 June a June a June a Events not associated with persistent negative North Pacific SST anomaly gradients: 1951 July July, August July, August 1963* July 1969* June a July a, August a August a July a, August 1972* June a * Indicates El Niño event was followed by a La Niña event during the next year. a Indicates a very strong response. Only strong and very strong responses are listed (see Table V for response categories).

10 1454 B.R. BONSAL AND R.G. LAWFORD Table VIb. Same as Table VIa except for La Niña events La Niña event Negative 500 hpa Fewer extended Negative 500 hpa Fewer extended anomalies dry spells anomalies dry spells Summer (+1) Summer (+1) Summer (+2) Summer (+2) Events associated with persistent negative North Pacific SST anomaly gradients: 1949 June a July, August a 1970 July June July June 1973 August a August June a, August a June, August Events not associated with persistent negative North Pacific SST anomaly gradients: * June June June, July, August 1975* June June, July, August a July a, August a June, July a, August 1988 July, August June June, July * Indicates La Niña event was followed by a El Niño event during the next year. a Indicates a very strong response. Only strong and very strong responses are listed (see Table V for response categories). summer, six events experienced strong positive 500 hpa anomalies for at least 1 month. For these six events, strong positive extended dry spell responses were also observed with the majority occurring for more than 1 month, and in some cases the entire summer. The only strong positive extended dry spell response not associated with strong positive 500 hpa anomalies occurred in association with the 1972 El Niño. Of the six events with strong positive 500 hpa anomalies and more extended dry spells, four were associated with persistent positive SST anomaly gradients (1953, 1965, 1982, 1986) while the other two (1951 and 1969) were not. The 1957 and 1976 events had persistent positive SST anomaly gradients, but no strong positive responses in terms of 500 hpa anomalies or extended dry spells during summer (+2). For summer ( +1), seven events were associated with strong positive 500 hpa anomalies during at least 1 month and four of these were associated with persistent positive SST anomaly gradients. However, with the exception of 1969, none experienced more extended dry spells. La Niña events are similar to El Niño in that the majority of the stronger responses occurred during summer ( +2). Six events experienced strong negative 500 hpa anomalies during at least 1 month of summer (+2) and all had below normal number of extended dry spells for most of if not the entire summer. Three were associated with persistent negative SST anomaly gradients in the North Pacific Ocean (1949, 1970, 1973). The events of 1964, 1975, and 1988 had strong negative 500 hpa anomaly and extended dry spell responses, but were not associated with persistent negative gradients. Unlike El Niño, most La Niña events were associated with strong negative extended dry spell responses during summer (+1). However, they were not as consistent as summer (+2). It is clear that the majority of El Niño and La Niña events are associated with strong extended dry spell responses during most of summer ( +2) (thus giving the significant values in Table IV). However, the physical connections between the occurrence of these events and the extended dry spells are not as clear. It is hypothesized that the relationship occurs with persistent North Pacific SST anomalies acting as a link. Many studies have observed relationships between North Pacific SST anomaly patterns similar to those associated with positive SST anomaly gradients, and long standing Rossby wave patterns consisting of ridging over western, and troughing over eastern North America (e.g. Namias, 1978; Dixon and Harnack, 1986). If the SST anomalies were reversed, the areas of ridging and troughing were also opposite (Namias, 1972). From this analysis, four of six El Niño events with persistent positive SST anomaly gradients were associated with strong positive 500 hpa anomalies over the Prairies during at least 1 month of summer ( +2). All three La Niña events with persistent negative SST anomaly gradients were associated with strong negative 500 hpa anomalies over the Prairies during at least 1 month of summer ( +2). Figure 3a and b show the composite Northern Hemisphere 500 hpa anomalies associated with these periods. The maps show El Niño (La Niña) events having very strong positive (negative) 500 hpa anomalies over the Canadian Prairies and strong negative (positive) anomalies over the eastern North

11 EL NIN O, LA NIN A AND CANADIAN DRY SPELLS 1455 Figure 3. Composite 500 hpa anomalies for summer (+2) months having strong 500 hpa anomalies and associated with persistent North Pacific SST anomaly gradients. a) El Niño and b) La Niña. Contour interval is 1.0 decametres. Negative contours are dashed Pacific (near the region of the SST anomaly gradients). A modelling study by Palmer and Sun (1985) showed that in the mid to upper troposphere (including the 500 hpa level), atmospheric pressure tended to increase (decrease) over and to the east of positive (negative) SST anomalies. This suggests that in these cases the persistent North Pacific SST anomaly gradients may have had an influence on the 500 hpa flow pattern over the eastern North Pacific, as well as downstream over the Canadian Prairies (similar to that shown in Figure 3a and b). There were also instances where the relationship did not occur. For example, it is unclear why there were relatively no strong 500 hpa anomaly or extended dry spell responses during summer ( +1) (which was also associated with strong positive/negative North Pacific SST anomaly gradients). Regarding summer ( +2), there were occurrences of persistent North Pacific SST anomalies and no strong 500 hpa anomaly responses over the Prairies (1957, 1976 El Niño). Strong 500 hpa anomalies and summer ( +2) extended dry spell responses also occurred without persistent SST anomaly gradients during the 1951 and 1969 El Niño events, and 1964, 1975, and 1988 La Niña events. One consideration is that El Niño and La Niña events sometimes occurred in consecutive years (as denoted by asterisks in Tables V, VIa and VIb). As alluded to previously, it appears that consecutive events do have an influence on the persistence of the North Pacific SST anomaly gradients. Strong extended dry spell responses were observed during summer ( +2) in association with three of the four El Niño events followed by a La Niña, and for both La Niña events followed by an El Niño. It, therefore, appears that the timing of the El Niño and La Niña events may have an influence on the summer extended dry spells. However, there were also several instances where strong extended dry spell responses occurred during non-consecutive events. The results in Tables VIa and VIb also indicate that 500 hpa anomalies and associated extended dry spells during summer ( +2) have an equal chance of occurring regardless of persistent North Pacific SST anomaly gradients. This would suggest that it is El Niño/La Niña affecting the extended dry spells and not North Pacific SSTs as concluded by Bonsal et al. (1993). However, the current study only focused on those summers associated with El Niño and La Niña to determine the effect of these events on North Pacific SSTs, 500 hpa heights, and extended dry spells over the Prairies. Other summers with severe extended dry spells (e.g. 1961, 1980) occurred in the absence of El Niño but were associated with persistent positive SST anomaly gradients in the North Pacific Ocean (see Bonsal et al., 1993). Therefore, El Niño/La Niña events are not a necessary condition to generate dry spell enhancing circulation over the Prairies.

12 1456 B.R. BONSAL AND R.G. LAWFORD It is likely that both tropical and North Pacific SSTs affect circulation patterns over North America but due to the complexity of ocean atmosphere interactions, it is difficult to separate these effects. Relationships between these two SST regions have been the subject of many observational and modelling studies with the majority focusing on the cold season (e.g. Zhang et al., 1997). Results have generally indicated that both the tropics and the North Pacific affect circulation over North America with those in the mid-latitudes acting on longer (i.e. decadal) time scales. A recent study by Ting and Wang (1997) showed both tropical and North Pacific SST influences on Northern Hemisphere mid-tropospheric circulation patterns and associated precipitation extremes over the US during the summer (JJA) season on interannual and interdecadal scales. Clearly, further research is required into the physical and dynamical causes of extended dry spell enhancing circulations over the Prairies. This should also include more localized phenomenon such as soil moisture conditions (e.g. Oglesby and Erickson, 1989) and even orographic influences generated by the western Cordillera (Bolin, 1950). Note that the extended dry spell variable is representative of precipitation frequency on the Canadian Prairies. Analyses are also carried out on monthly areally-averaged precipitation anomalies over the Prairies (not shown) which are representative of precipitation amounts. Even though the signs correspond to the dry spell results (i.e. negative precipitation anomalies with El Niño and ice ersa), they are not statistically significant. This indicates that El Niño and La Niña events are related more to precipitation frequency (i.e. consecutive dry days) as opposed to actual amounts on the Prairies. In summary, the examination of individual events shows a good correspondence between El Niño/La Niña and strong PNA patterns and North Pacific SST anomaly gradients during autumn (0) to winter (+1) (eight El Niño and six La Niña events). It also indicates that for many of these events (six El Niño and three La Niña), the SST anomaly gradients persisted for several seasons. Of the six El Niño events, four (1953, 1965, 1982, 1986) experienced both positive 500 hpa anomalies and more extended dry spells over the Prairies during summer (+2). These four had very strong and consistent responses in terms of more summer extended dry spells on the Canadian Prairies. Of the three La Niña events with persistent negative SST anomaly gradients, all experienced both negative 500 hpa anomalies and fewer extended dry spells over the Prairies during summer (+2). The analysis of individual El Niño and La Niña events also shows high variability with respect to summer 500 hpa anomaly and extended dry spell responses. 5. CONCLUSIONS This study shows some significant relationships between El Niño and La Niña events and summer extended dry spells on the Canadian Prairies. More (fewer) extended dry spells tend to occur during the second summer following the mature stage of the El Niño (La Niña) events. This finding is important for several reasons. First, more extended dry spells often have severe impacts on activities such as agriculture, while fewer extended dry spells are generally representative of more favourable conditions. Second, the long lags between the mature stage of El Niño (La Niña) and the occurrence of more (fewer) extended dry spells, may contribute to a long-range forecasting technique of these spells. Since severe extended dry spells are a potential major economic and environmental concern, the ability to anticipate or forecast their occurrence (or non-occurrence) could provide many benefits to the Canadian Prairie economy. For the most part, this analysis only provides associations among the variables with some larger scale physical explanations of the atmosphere ocean teleconnections being proposed. There is evidence that the relationship can occur through a series of atmosphere ocean teleconnections including PNA patterns, North Pacific SST anomalies, and upper-atmospheric anomalies over the North Pacific/North American region (in particular, over the Canadian Prairies). It is recognized that there are many other physical processes that require further investigation before a full explanation of the observed relationships can be provided. More analyses into the separation of tropical versus mid-latitude SST effects on anomalous weather conditions over North America, characteristics of individual and consecutive El Niño and La Niña events (e.g. intensity, duration, spatial extent), as well as more regional analyses of extended dry spells within the Prairies may provide further insight. This study can be considered as a preliminary step

13 EL NIN O, LA NIN A AND CANADIAN DRY SPELLS 1457 in identifying the potential causes, and contributing to long-range forecasting of summer extended dry spells on the Canadian Prairies. ACKNOWLEDGEMENTS The authors are grateful to Amir Shabbar and Walter Skinner of the Atmospheric Environment Service for their helpful comments. We would also like to thank the two anonymous reviewers for their useful suggestions toward an improved version of the manuscript. REFERENCES Alexander, M.A Simulation of the response of the North Pacific Ocean to the anomalous atmospheric circulation associated with El Niño, Climate Dyn., 5, Alexander, M.A Midlatitude atmosphere ocean interaction during El Niño. Part I: the North Pacific Ocean, J. Climate, 5, Bethea, R.M., Duran, B.S. and Boullion, T.L Statistical Methods for Engineers and Scientists, Marcel Dekker, Inc., New York, pp Bolin, B On the influence of the earth s orography on the general character of the westerlies, Bull. Am. Meteorol. Soc., 31, Bonsal, B.R., Chakravarti, A.K. and Lawford, R.G Teleconnections between North Pacific SST anomalies and growing season extended dry spells on the Canadian Prairies, Int. J. Climatol., 13, Davis, R Predictability of sea surface temperature and sea level pressure anomalies over the North Pacific Ocean, J. Phys. Oceanogr., 6, Deser, C. and Wallace, J.M Large scale atmospheric circulation features of warm and cold episodes in the tropical Pacific, J. Climate, 3, Dey, B Nature and possible causes of heavy summer precipitation in the Canadian Prairies, Geogr. Perspect., 40, Dey, B Nature and possible causes of droughts on the Canadian Prairies case studies, J. Climatol., 2, Dey, B. and Chakravarti, A.K A synoptic climatological analysis of summer dry spells in the Canadian Prairies, Great Plains Rocky Mountain Geogr. J., 5, Dixon, K.W. and Harnack, R.P The effect of intraseasonal circulation variability on winter temperature forecast skill, Mon. Wea. Re., 115, Ebdon, D Statistics in Geography, Basil Blackwell Ltd., Oxford, 233 pp. Emery, W.J. and Hamilton, K Atmospheric forcing of interannual variability in the north-east Pacific Ocean; connections with El Niño, J. Geophys. Res., 90, Hamilton, K A detailed examination of the extratropical response to tropical El Niño/Southern Oscillation events, J. Climatol., 8, Horel, J.D. and Wallace, J.M Planetary scale atmospheric phenomena associated with the Southern Oscillation, Mon. Wea. Re., 110, Hoskins, B.J. and Karoly, D.J The steady linear response of a spherical atmosphere to thermal and orographic forcing, J. Atmos. Sci., 38, Kiladis, G.N. and Diaz, H.F Global climatic anomalies associated with extremes of the Southern Oscillation, J. Climate, 2, Knox, J.L. and Lawford, R.G The relationship between Canadian Prairie dry and wet months and circulation anomalies in the mid-troposphere, Atmos. Ocean, 28, Leathers, D.J., Yarnal, B. and Palecki, M.A The Pacific/North American teleconnection pattern and United States climate. Part I: regional temperature and precipitation associations, J. Climate, 4, Longley, R.W The Climate of the Prairie Pro inces, Climatological Studies c13, Atmospheric Environment Service, Department of the Environment, Ottawa, 79 pp. Namias, J Macroscale variations in sea surface temperatures in the North Pacific, J. Geophys. Res., 75, Namias, J Experiments in objectively predicting some atmospheric and oceanic variables for the winter of , J. Appl. Meteorol., 71, Namias, J Multiple causes of the North American abnormal winter , Mon. Wea. Re., 106, Namias, J., Yuan, X. and Cayan, D Persistence of North Pacific sea surface temperatures and atmospheric flow patterns, J. Climate, 1, Oglesby, R.J. and Erickson, D.E Soil moisture and the persistence of North American drought, J. Climate, 2, Palmer, T.N. and Sun, Z A modelling and observational study of the relationship between sea surface temperatures in the north west Atlantic and the atmosphere general circulation, Q. J. R. Meteorol. Soc., 111, Ripley, E.A Drought Prediction on the Canadian Prairies, Report 88-4, Canadian Climate Centre, AES, National Hydrology Research Centre, Saskatoon, Sask, 137 pp. Ropelewski, C.F. and Halpert, M.S North American precipitation and temperature patterns associated with El Niño/Southern Oscillation (ENSO), Mon. Wea. Re., 114, Shabbar A., Higuchi, K. and Knox, J.L Regional analysis of Northern Hemisphere 50 kpa geopotential heights from 1946 to 1985, J. Climate, 3, Ting, M. and Wang, H Summertime U.S. precipitation variability and its relation to Pacific sea surface temperature, J. Climate, 10,

14 1458 B.R. BONSAL AND R.G. LAWFORD Walsh, J.E., Jasperson, W.H. and Ross, B Influences of snow cover and soil moisture on monthly air temperature, Mon. Wea. Re., 113, Weber, G.R North Pacific circulation anomalies, El Niño and anomalous warmth over the North American continent in : possible causes of the 1988 North American drought, Int. J. Climatol., 10, Wiche, G.J., Knox, J.L. and Welsh, L Relations between upper air flow patterns, climate, and hydrologic variability in the Red River north basin; North Dakota, South Dakota, Minnesota, Manitoba, and Saskatchewan, Draft version, United States Geological Survey. Yarnal, B. and Diaz, H.F Relationships between extremes of the Southern Oscillation and the winter climate of the Anglo-American Pacific coast, J. Climatol., 6, Yongping, Z. and McBean, G.A Seasonal North Pacific sea surface temperature patterns and their relations to the atmospheric circulation, Mar. Sci., 3, Zhang, Y. and Wallace, J.M Is climate variability over the North Pacific a linear response to ENSO?, J. Climate, 9, Zhang, Y., Wallace, J.M. and Battisti, D.S ENSO-like interdecadal variability: , J. Climate, 10,