30
G.A. McBean
coastal zone and generally show more complex patterns (but clearly
related to topography) than in other parts of North America.
Precipitation is one of the major features of climate and provides an
essential ingredient for terrestrial life forms. Precipitation also varies
dramatically in both time and space in mountainous regions such as
those that prevail along the mid-and higher-latitude west coasts of the
Americas. Studies under way to understand the nature and climatic
connections of precipitation in British Columbia will give some insight
into what is happening in similar regions of the Americas. Most of British
Columbia's precipitation is supplied by North Pacific air masses and is
liberated primarily during the cool season by a continual progression of
extratropical storms carried in the westerly flow. Mean annual precipitation exceeds 200mm/month for parts of the Coast Mountain region but
decreases to less than 100mm/month in some regions between
Vancouver Island and the mainland mountains. The gradients of precipitation along the slopes of the Coast Mountains are very pronounced. In
the interior plateau region of the province, precipitation decreases to 40
to 50 mm/month. Secondary maxima in precipitation, of 100mm/month - 1,
occur over the interior mountains and the Rock ies.
Despite these complexities, principal component analyses show that
large-scale variations in precipitation can be represented in terms of four
patterns: north coastal, south coastal, north interior, and south interior
regions (McBean & Hourston, 1994). Distinctions among these regions
are apparent in the box-plot summaries of the seasonal variation for each
regional precipitation series, as shown in Figure 2.2, as well as in other
statistics . Both the north coastal and the south coastal regions have
maximum monthly precipitation occurring -in October-January, when
cyclones are most frequent (e.g. , Murty, McBean, & McKee, 1983).
North coastal region values are higher than those for the south coastal
region, except for February. The onset of high winter values (above
about 230mm/month) starts in October for the north coastal region and
in November for the south coastal region, corresponding to the southward
shift of the westerly storm belt in the fall and winter . The west coast of
Washington state also has a pronounced annu al precipitation cycle,
with maximum in the winter (Finkelstein & Truppi, 1991). There is
considerable variability, especially during the winter months . During the
summer months, both coastal regions come under the influence of a
semipermanent ridge of high pressure, resulting in less cyclonic activity.
Precipitation in the months June, July, and August falls to below 80
mm/month. Although standard deviations are largest in the months
October to January, the relative variability (as given by the coefficient of
variation, defined as the standard deviation divided by the mean) is
higher in summer.
Both interior regions lie in the rain shadow of the Coast Mountains.
Mean annual precipitation is much lower, generally in the range of 30 to
G.A. McBean
coastal zone and generally show more complex patterns (but clearly
related to topography) than in other parts of North America.
Precipitation is one of the major features of climate and provides an
essential ingredient for terrestrial life forms. Precipitation also varies
dramatically in both time and space in mountainous regions such as
those that prevail along the mid-and higher-latitude west coasts of the
Americas. Studies under way to understand the nature and climatic
connections of precipitation in British Columbia will give some insight
into what is happening in similar regions of the Americas. Most of British
Columbia's precipitation is supplied by North Pacific air masses and is
liberated primarily during the cool season by a continual progression of
extratropical storms carried in the westerly flow. Mean annual precipitation exceeds 200mm/month for parts of the Coast Mountain region but
decreases to less than 100mm/month in some regions between
Vancouver Island and the mainland mountains. The gradients of precipitation along the slopes of the Coast Mountains are very pronounced. In
the interior plateau region of the province, precipitation decreases to 40
to 50 mm/month. Secondary maxima in precipitation, of 100mm/month - 1,
occur over the interior mountains and the Rock ies.
Despite these complexities, principal component analyses show that
large-scale variations in precipitation can be represented in terms of four
patterns: north coastal, south coastal, north interior, and south interior
regions (McBean & Hourston, 1994). Distinctions among these regions
are apparent in the box-plot summaries of the seasonal variation for each
regional precipitation series, as shown in Figure 2.2, as well as in other
statistics . Both the north coastal and the south coastal regions have
maximum monthly precipitation occurring -in October-January, when
cyclones are most frequent (e.g. , Murty, McBean, & McKee, 1983).
North coastal region values are higher than those for the south coastal
region, except for February. The onset of high winter values (above
about 230mm/month) starts in October for the north coastal region and
in November for the south coastal region, corresponding to the southward
shift of the westerly storm belt in the fall and winter . The west coast of
Washington state also has a pronounced annu al precipitation cycle,
with maximum in the winter (Finkelstein & Truppi, 1991). There is
considerable variability, especially during the winter months . During the
summer months, both coastal regions come under the influence of a
semipermanent ridge of high pressure, resulting in less cyclonic activity.
Precipitation in the months June, July, and August falls to below 80
mm/month. Although standard deviations are largest in the months
October to January, the relative variability (as given by the coefficient of
variation, defined as the standard deviation divided by the mean) is
higher in summer.
Both interior regions lie in the rain shadow of the Coast Mountains.
Mean annual precipitation is much lower, generally in the range of 30 to