32
G.A. McBean
during the year, about the same as the average value for the coastal
regions.
Orographic enhancement is a factor in the south interior region , due to
the influence of the Columbia and Rocky Mountains, which run approximately north-northwest/south-sourtheast along the eastern boundary of
the province. However, the eastward-flowing air has already precipitated
much of its moisture in passing over the Coast Mountains and , consequently, precipitation amounts are not as high as for the coastal regions.
Annual mean monthly precipitation ranges from 25 to 69mm/month.
As noted previously, the seasonal pattern shows two peaks, with the
winter one being slightly larger. The coefficient of variation is largest in
the late summer-early fall (reaching 0.56 in October) and smallest in the
spring (0.30 in March).
Time series of regional precipitation show different patterns of decadal
and shorter-period fluctuations. North coastal precipitation was consistently below the long-term average until about 1962, when there was a
change to consistently above-average conditions . The south coastal region
has a trend similar to that of the north coastal region. The south interior
region time series exhibits a long period of generally below-mean conditions, with a sudden recovery beginning in 1980.
Several authors have examined relationships between precipitation and
atmospheric circulations. Klein and Bloom (1987) related monthly mean
precipitation over the United States to the 70kPa height field and found
that the best statistical relationships occurred in winter (DecemberFebuary) and for the west coast. Station relationships were less successful
for regions just east of the continental divide and for summer. Redmond
and Koch (1991) correlated the Pacific North America (PNA) pattern
(Wallace & Gutzler, 1981) and the Southern Oscillation indices with
precipitation, temperature, and stream flow over the western United
States. The Pacific Northwest was identified as an area of higher correlation. Cayan and Roads (1984) also examined west coast precipitation in
relationship to localized circulation parameters, such as 70kPa height ,
winds, etc. Some studies found that precipitation frequency was better
correlated with circulation parameters than precipitation amount. Cayan
and Peterson (1989) looked at relationships between stream flow in
western North America and North Pacific atmospheric circulation. They
followed work by Namias (1975) and others who used sea-level pressure
variations in the central North Pacific Ocean as a variable for representing
atmospheric circulation. Their Central North Pacific index, sea-level
pressure anomaly averaged over 170 oE-150 oW, 35°N-55°N, was highly
correlated with the PNA pattern. Cayan and Peterson found that British
Columbia was close to the node of the atmospheric long-wave pattern
that emanates from the central North Pacific, so that small variations in
the position of the remote circulation anomaly center could yield either
G.A. McBean
during the year, about the same as the average value for the coastal
regions.
Orographic enhancement is a factor in the south interior region , due to
the influence of the Columbia and Rocky Mountains, which run approximately north-northwest/south-sourtheast along the eastern boundary of
the province. However, the eastward-flowing air has already precipitated
much of its moisture in passing over the Coast Mountains and , consequently, precipitation amounts are not as high as for the coastal regions.
Annual mean monthly precipitation ranges from 25 to 69mm/month.
As noted previously, the seasonal pattern shows two peaks, with the
winter one being slightly larger. The coefficient of variation is largest in
the late summer-early fall (reaching 0.56 in October) and smallest in the
spring (0.30 in March).
Time series of regional precipitation show different patterns of decadal
and shorter-period fluctuations. North coastal precipitation was consistently below the long-term average until about 1962, when there was a
change to consistently above-average conditions . The south coastal region
has a trend similar to that of the north coastal region. The south interior
region time series exhibits a long period of generally below-mean conditions, with a sudden recovery beginning in 1980.
Several authors have examined relationships between precipitation and
atmospheric circulations. Klein and Bloom (1987) related monthly mean
precipitation over the United States to the 70kPa height field and found
that the best statistical relationships occurred in winter (DecemberFebuary) and for the west coast. Station relationships were less successful
for regions just east of the continental divide and for summer. Redmond
and Koch (1991) correlated the Pacific North America (PNA) pattern
(Wallace & Gutzler, 1981) and the Southern Oscillation indices with
precipitation, temperature, and stream flow over the western United
States. The Pacific Northwest was identified as an area of higher correlation. Cayan and Roads (1984) also examined west coast precipitation in
relationship to localized circulation parameters, such as 70kPa height ,
winds, etc. Some studies found that precipitation frequency was better
correlated with circulation parameters than precipitation amount. Cayan
and Peterson (1989) looked at relationships between stream flow in
western North America and North Pacific atmospheric circulation. They
followed work by Namias (1975) and others who used sea-level pressure
variations in the central North Pacific Ocean as a variable for representing
atmospheric circulation. Their Central North Pacific index, sea-level
pressure anomaly averaged over 170 oE-150 oW, 35°N-55°N, was highly
correlated with the PNA pattern. Cayan and Peterson found that British
Columbia was close to the node of the atmospheric long-wave pattern
that emanates from the central North Pacific, so that small variations in
the position of the remote circulation anomaly center could yield either