2. Factors Controlling the Climate of the West Coast of North America 33
positive or negative river stream-flow fluctuations. They concluded that
British Columbia stream-flow variability did not respond consistently to
broad-scale Pacific atmospheric circulation indices but is related to
regional atmospheric anomaly features over the eastern North Pacific.
Yarnal and Diaz (1986) found that precipitation anomalies were sensitive
to minor fluctuations in the general placement and orientation of the
PNA pattern. Yarnal and Diaz showed that during warm-event winters,
precipitation in California and southeast Alaska were negatively correlated , but northern British Columbia and Vancouver Island precipitation
were not significantly correlated with either.
To examine relationships between precipitation over British Columbia
and atmospheric circulation, circulation patterns when the precipitation is
anomalous were examined, splitting the data into winter and summer
half-years. The average 50 kPa heights for all cases when the standardized
anomaly index (anomaly-mean divided by standard deviation) was either
greater than 1 or less than -1 were computed. For the winter half-year
and both north and south coastal regions, high precipitation occurs when
the atmospheric flow is toward the coastal mountains, with 50kPa heights
lower in the Gulf of Alaska by up to 60m and increased over California
by 20m to 30m , as shown in Figure 2.3 (part A) . For low-precipitation
events in winter for the coastal regions, a ridge along the coast is
dominant, as shown in Figure 2.3 (part B). Low-precipitation anomaly
patterns are not opposite of high-precipitation pattern anomalies. For
both coastal regions, low precipitation occurs with more than 100m
increases in 50 kPa height along the British Columbia coast, and 40 m to
60m height falls nearer the dateline and over the Great Lakes. As the ttest shows, the anomaly patterns for low precipitation are a tripole of
changes (-, +, -) approximately along the 5400m contour of the 50kPa
height, with the height increases over British Columbia being about twice
as large as the decreases . In comparison , the high-precipitation cases can
be described as a dipole pattern with its orientation along the coast
(across the flow). Thus, high precipitation occurs with an increase in flow
strength as well as some redirection, while low precipitation corresponds
to a major flow curvature resulting in ridging over the region. For the
interior regions, flow anomalies occurring with high precipitation are
similar to their coastal counterparts.
Because the summer gradients are smaller, the height anomalies
corresponding to summer precipitation anomalies also tend to be smaller
in an absolute sense, but the statistical significance can be as high. Positive
summer rainfall anomalies in the coastal regions occur with reduced
heights (60m to 70m) along the British Columbia coast. Changes
elsewhere are less significant. The pattern of significant height changes
for summer low-precipitation months for the coastal regions is more
widespread. There are large height increases (80m-90m) along the
positive or negative river stream-flow fluctuations. They concluded that
British Columbia stream-flow variability did not respond consistently to
broad-scale Pacific atmospheric circulation indices but is related to
regional atmospheric anomaly features over the eastern North Pacific.
Yarnal and Diaz (1986) found that precipitation anomalies were sensitive
to minor fluctuations in the general placement and orientation of the
PNA pattern. Yarnal and Diaz showed that during warm-event winters,
precipitation in California and southeast Alaska were negatively correlated , but northern British Columbia and Vancouver Island precipitation
were not significantly correlated with either.
To examine relationships between precipitation over British Columbia
and atmospheric circulation, circulation patterns when the precipitation is
anomalous were examined, splitting the data into winter and summer
half-years. The average 50 kPa heights for all cases when the standardized
anomaly index (anomaly-mean divided by standard deviation) was either
greater than 1 or less than -1 were computed. For the winter half-year
and both north and south coastal regions, high precipitation occurs when
the atmospheric flow is toward the coastal mountains, with 50kPa heights
lower in the Gulf of Alaska by up to 60m and increased over California
by 20m to 30m , as shown in Figure 2.3 (part A) . For low-precipitation
events in winter for the coastal regions, a ridge along the coast is
dominant, as shown in Figure 2.3 (part B). Low-precipitation anomaly
patterns are not opposite of high-precipitation pattern anomalies. For
both coastal regions, low precipitation occurs with more than 100m
increases in 50 kPa height along the British Columbia coast, and 40 m to
60m height falls nearer the dateline and over the Great Lakes. As the ttest shows, the anomaly patterns for low precipitation are a tripole of
changes (-, +, -) approximately along the 5400m contour of the 50kPa
height, with the height increases over British Columbia being about twice
as large as the decreases . In comparison , the high-precipitation cases can
be described as a dipole pattern with its orientation along the coast
(across the flow). Thus, high precipitation occurs with an increase in flow
strength as well as some redirection, while low precipitation corresponds
to a major flow curvature resulting in ridging over the region. For the
interior regions, flow anomalies occurring with high precipitation are
similar to their coastal counterparts.
Because the summer gradients are smaller, the height anomalies
corresponding to summer precipitation anomalies also tend to be smaller
in an absolute sense, but the statistical significance can be as high. Positive
summer rainfall anomalies in the coastal regions occur with reduced
heights (60m to 70m) along the British Columbia coast. Changes
elsewhere are less significant. The pattern of significant height changes
for summer low-precipitation months for the coastal regions is more
widespread. There are large height increases (80m-90m) along the