4. Regional climate highlights

a. North America

1) Flooding of the Red River Valley and Tributaries during April 1997

Substantial flooding occurred in the Northern Plains of the United States and southern Manitoba, Canada during April 1997, with many rivers reaching record high levels during the month (Fig. 33). In the Red River Valley of eastern North Dakota and west-central Minnesota flooding persisted from early April through mid-May. At Fargo, North Dakota the Red River peaked at more than 6.6 m above flood stage during this period, a level reached only once previously in the past 100 years. Farther north, river levels at East Grand Forks also peaked at more than 6.6 m above flood stage, slightly exceeding the 500-yr statistical recurrence interval at that site. Record flooding also occurred in all major tributaries of the Red River during April, including the Wild Rice River and the Maple River (a record 1.8 m above flood stage), the Sheyenne River (a record 2.5 m above flood stage) and the Buffalo River (a record 1.1 m above flood stage).

The primary cause of this flooding was a highly abnormal thaw during March and April of substantial winter snow and river ice. Abnormal characteristics of the thaw included its timing, duration and areal extent, as well as the diurnal temperature changes during the periods of substantial snow and ice melt.

(i) Normal climate conditions

The atmospheric conditions that contributed to the record flooding of the Red River and its tributaries are best appreciated by first reviewing the geographic and normal climate conditions for the region. The Red River flows slowly northward within a broad, gently sloping flood plain from its source region in northeastern South Dakota/west-central Minnesota to Lake Winnipeg in southern Manitoba, Canada. In the southern part of the Red River Basin, daily mean temperatures normally remain below freezing from mid-November through late February/early March, and then increase to almost 10°C by the end of April. In the northern part of the basin, daily mean temperatures throughout the winter and spring typically average 3°-6°C cooler than those in the south, remaining below freezing from late October to early April, then increasing to 7°-8°C by the end of April.

This unique relationship between climate and geography makes the Red River Valley particularly susceptible to flooding during March and April (personal communication, Allen Voelker, National Weather Service meteorologist, Grand Forks, N. D.). In particular, the normal annual cycle in air temperature favors a south-to-north progression of the spring thaw, which is characterized by the melting of snow and river ice in the south during March while the downstream river channel remains frozen. These conditions favor flooding of the Red River and a backfill of the runoff into the river's tributaries.

The springtime thaw is particularly sensitive to modest departures from normal in the annual cycle of air temperature, with increased flooding typically observed during heavy snow years in which the thaw is delayed and confined to the southern part of the basin. In contrast, less flooding is typically observed during years in which the thaw is well established by March throughout the entire basin. Flooding is also influenced by the diurnal freeze/thaw cycle. A more gradual thaw, and therefore reduced flooding, occurs when daily high temperatures during March are above freezing and daily minimum temperatures continually drop below freezing.

(ii) Contributing factors to the April 1997 record floods

The important factors that set the stage for potential significant flooding of the Red River and its tributaries during April 1997 included greatly enhanced snowfall during the winter and a substantial buildup of river ice throughout the northern half of the Red River. These conditions resulted from a series of major cold-air outbreaks and winter storms from September 1996 to April 1997. During this period more than 200% of normal snowfall was observed over most of North Dakota, western Minnesota and northeastern South Dakota, with 125%-200% of normal snow covering the remainder of the upper Midwest, the northern Plains, Montana and most of Wyoming (Fig. 34 ). The floods were then directly initiated by a highly abnormal thaw of this snowpack and river ice.

This abnormal thaw occurred during March-April, and is highlighted using time series of estimated daily maximum and minimum temperatures in both the northern (Fig. 35a) and southern portions of the basin (Fig. 35b). The period 1-20 March featured a continuation of freezing conditions throughout the basin, with daily minimum temperatures often dropping below ­10°C in the south and below ­15°C in the north. These abnor
mally cold temperatures reflected a large influx of polar air into central North America in response to an amplified ridge and blocking anticyclone over the Bering Sea (Fig. 36a).

The first major melt of snow and ice then occurred in the southern part of the basin during 21 March-5 April, with daily maximum temperatures reaching 15°C on many days during the period and minimum temperatures remaining above freezing on one-half of the nights (Fig. 35b). Also during this period temperatures averaged 2°-4°C above average across the central United States and northern Plains States (Fig. 36b), which also contributed to significant snow melt and flooding in other river basins across North Dakota, South Dakota and Minnesota (Fig. 33). This large-scale warming resulted from a strong transport of mild Pacific air into central North America, in association with the breakdown of the high-latitude block and the establishment of broad westerly flow throughout the region (Fig. 36b). Farther north, freezing conditions and significant river ice persisted in the northern portion of the Red River Basin throughout the period, as daily maximum temperatures remained near freezing and daily minimum temperatures averaged ­5° to ­10°C. These conditions exacerbated flooding in the south by significantly impeding the normal northward flow of the river and allowing runoff to backfill into the river's tributaries across eastern North Dakota and western Minnesota.

This warm period was followed by a major storm and cold frontal passage on 5-6 April, which brought heavy rainfall to the Red River Basin. These conditions were followed during 6-17 April by a return to abnormally cold temperatures (Fig. 36c ) which froze standing water throughout the southern portion of the basin. This drop in temperatures occurred in association with a large influx of polar air throughout central North America, in response to the redevelopment of an amplified ridge and blocking anticyclone over Alaska (Fig. 36c).

A second major warming then occurred throughout the entire Red River basin during 17-30 April (Figs. 35a, b), which resulted in further snow and ice melt and to a second period of substantial rises in river levels. This warming was also associated with a large-scale pattern of above-normal temperatures throughout the western half of the United States and south-central Canada (Fig. 36d), and was similar to the previous thaw (20 March-5 April) in that it was linked to a strong transport of mild Pacific air into central North America. These conditions also followed the breakdown of a high-latitude block and the establishment of broad westerly/southwesterly flow throughout the eastern North Pacific and western North America.

This highly unfavorable March-April 1997 thaw in the Red River Basin can be put into perspective by comparing it with the very favorable or "ideal" thaw of 1994 (Fig. 37), a year in which there was only minor flooding despite record or near-record snow fall at many locations during October 1993-February 1994. The 1994 thaw featured four periods of substantial basin-wide warming during March, along with significant refreezing at night. These conditions produced a much more uniform melt of snow and river ice throughout the basin, and resulted in a substantial reduction of the winter snowpack prior to the onset of the major April warming.

(iii) Discussion

A combination of atmospheric conditions spanning many space and time scales generally contributes to most major climate events, and the same is true of the record Red River flooding of April 1997. The primary cause of the 1997 flooding was a highly abnormal thaw of winter snow and river ice, following a winter season that featured much above-normal snowfall across the Northern Plains. The buildup of snow and river ice resulted from repetitive winter storms and cold-air outbreaks that were heavily controlled by two prominent and sometimes related atmospheric phenomena: 1) recurring high-latitude blocking and block evolution in the vicinity of the Bering Sea and Alaska (see section 3b; see also Halpert and Bell, 1997), and 2) substantial variability in the eastward extent of the East Asian jet stream over the eastern North Pacific.

Similar atmospheric circulation features prevailed throughout the 1995/96 cold season, which resulted in abnormally cold temperatures and above-normal snowfall throughout the Red River Valley. This combination of recurring high-latitude blocking and abnormally cold air temperatures tends to be more prevalent during tropical Pacific cold episodes (La Niña), and much less prevalent during tropical Pacific warm episodes (El Niño) (Chen and van den Dool 1997). Also, it is likely that the blocking activity and increased intensity of the undercutting flow observed throughout the cool season was influenced by the strong MJO activity across the tropical Pacific. The strong undercutting coincided with periods of eastward propagating convection from Indonesia to the central equatorial Pacific (see section 3b). These observations are consistent with previous analyses of the relationship between strong MJO activity and changes in both the wintertime East Asian jet stream and the occurrence of blocking activity over the high latitudes of the North Pacific (Higgins and Mo 1997).

Despite the apparent external influence on the extratropical atmospheric circulation features by the La Niña and the MJO, it is evident that the timing of both the block evolution and the undercutting jet stream was a crucial contributing factor to the abnormal 1997 thaw and subsequent flooding. Thus, predicting the magnitude and areal extent of future Red River flooding is strongly dependent on forecasts of the timing of the March-April thaw. Unfortunately, long-lead (monthly) predictions of blocks and their life-cycles, as well as reliable predictions of the extratropical atmospheric response to the MJO, are presently not possible.

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