5.1. Long-Term Contributions
Based on results like those presented above, it is increasingly understood that California’s most severe storms and much of its flood risks derive from land-falling ARs. What is less well recognized is the large extent to which California’s overall water resources derive from this same category of storms.
Figure 6 and
Figure 7 show the precipitation contributions, on a station-by-station basis, from the 1998–2008 SSM/I-derived AR events (
Figure 6) and from the 1951–2008 period of the Reanalysis-derived PE events (
Figure 7). During the recent SSM/I period (
Figure 6), AR storms have contributed anywhere from about 31 to 36% of overall precipitation at most cooperative weather stations in Central and Northern California, rising to as much as 46% (
Table 3). On average, precipitation during these events amounted to about two to three times the long-term average rates on the same days of year (disregarding AR occurrences). Many ARs making landfall north of California are included in this analysis, despite the fact that such storms are less likely to result in major precipitation contributions in California. Nonetheless the landfall of all ARs anywhere on the West Coast are found to have contributed between 30 and 45% of all precipitation in central and northern California. Fractions of total precipitation in Oregon and Washington deriving from landfalling ARs are similar at many, but not all, stations to these California fractions. When only AR episodes that made landfall in California are considered (not shown here), the fractions contributed remain roughly 30 to 45% of all precipitation in California because ARs making landfall farther north contribute little precipitation in California so that results are almost the same with or without them.
Figure 6.
Contributions of precipitation during wet-season (November–April) days on which atmospheric rivers made landfall on the West Coast (based on
Table 1) to overall precipitation from water year 1998 through 2008 at cooperative weather stations in the western US. Inset map shows the ratio of average precipitation on the AR days (including concurrent day and following day) to climatological means for the same combination of days. Concurrent and one following day are composited here to allow for a 1-day uncertainty between the GMT-based AR chronology and local-time cooperative precipitation observations.
Figure 6.
Contributions of precipitation during wet-season (November–April) days on which atmospheric rivers made landfall on the West Coast (based on
Table 1) to overall precipitation from water year 1998 through 2008 at cooperative weather stations in the western US. Inset map shows the ratio of average precipitation on the AR days (including concurrent day and following day) to climatological means for the same combination of days. Concurrent and one following day are composited here to allow for a 1-day uncertainty between the GMT-based AR chronology and local-time cooperative precipitation observations.
Table 3.
Ratios of precipitation and streamflow or runoff during AR and PE events, and subsequent days (as indicated), to overall precipitation, streamflow or runoff, among cooperative precipitation sites and VIC-gridded precipitation data, and streamflow or runoff, among HCDN streamflow gauging stations and gridded VIC simulations, respectively, in central and northern California (east of 120° W and south of 42° N). See
section 2 for descriptions of data sources.
Table 3.
Ratios of precipitation and streamflow or runoff during AR and PE events, and subsequent days (as indicated), to overall precipitation, streamflow or runoff, among cooperative precipitation sites and VIC-gridded precipitation data, and streamflow or runoff, among HCDN streamflow gauging stations and gridded VIC simulations, respectively, in central and northern California (east of 120° W and south of 42° N). See section 2 for descriptions of data sources.
| | Maximum ratio among central and northern California sites and grid cells shown in
Figure 6, Figure 7 and Figure 8 | 75th-percentile ratio among central and northern California sites and grid cells shown in
Figure 9 and Figure 10 | 25th-percentile ratio among central and northern California sites and grid cells shown in
Figure 9 and Figure 10 |
| AR events | PE events | AR events | PE events | AR events | PE events |
| Precipitation (days 0 and +1) |
| Cooperative precipitation observations | 46% | 22% | 36% | 17% | 31% | 13% |
| VIC-gridded precipitation | 45% | 21% | 40% | 16% | 33% | 12% |
| | Streamflow or Runoff (days 0 to +3 days) |
| HCDN streamflow | 71% | 55% | 52% | 38% | 34% | 25% |
| VIC-simulated runoff | 61% | 40% | 43% | 24% | 26% | 12% |
During the longer term Reanalysis period (
Figure 7), PE storm contributions have amounted to 13 to 17% of overall precipitation at most stations in central and northern California (
Table 3) with the fractions rising to as much at 22% at the most heavily influenced site, about half the fractions shown in
Figure 6. Recall, however, that there are 2.5 times as many ARs per season as PEs, in the chronologies used here, so that the contributions from ARs and PEs are roughly comparable on a storm-by-storm basis. Precipitation from the PE storms average about two to four times the long-term average precipitation rates. To verify the patterns shown, and to fill in spatially (based on the interpolation schemes of [
16]),
Figure 8 and
Table 3 show the corresponding contributions from ARs, water years 1998–2008, and PEs, 1948–2008, as indicated by the gridded VIC-input fields of precipitation. Overall,
Figure 6,
Figure 7 and
Figure 8 suggest that, whether ARs are considered for a short recent period or PEs for a longer historical period, similar contributions to overall precipitation, on a per-storm basis, are indicated. Thus the large contributions to overall precipitation associated with ARs in the recent (SSM/I) period are unlikely to be somehow unusual over the longer historical period.
Figure 7.
Same as
Figure 6, except for PE days from water year 1951–2008, based on
Table 2.
Figure 7.
Same as
Figure 6, except for PE days from water year 1951–2008, based on
Table 2.
Figure 8.
Same as main map in
Figure 6, except based on 1/8-degree gridded VIC-input precipitation fields, for
(a) AR events and
(b) PE events.
Figure 8.
Same as main map in
Figure 6, except based on 1/8-degree gridded VIC-input precipitation fields, for
(a) AR events and
(b) PE events.
Corresponding analyses of streamflow observations and simulations of runoff-plus-baseflow are presented in
Figure 9 and
Figure 10. The wet season along the West Coast is also the cool season so that much precipitation falls as snow, especially in the Sierra Nevada mountains along the eastern edge of Central California. This tendency towards snowfall rather than rain is important because the Sierra Nevada receives many of the highest precipitation totals in the State and generates many of the largest streamflow contributions. As a consequence, much of the precipitation associated with ARs and PEs is stored in seasonal snowpacks until much later in the year and thus it is not possible to catalog precisely as runoff what is from AR or non-AR events using the method here. As an alternative, Guan
et al. [
28] documented the contribution of AR days to snow water equivalent at snow pillow sites in the Sierra Nevada during WY 2004–2010. Their analysis concluded that on average 6–7 AR events occurred per year, and that they provided 40% of the total snow water equivalents over the 7 years studied, in broad agreement with the contributions to precipitation documented above. However, low- to mid-altitude parts of the Sierra Nevada and other mountain ranges of California often receive rain from cool-season ARs and from PEs so that significant runoff and streamflow signatures are nonetheless closely associated with AR and PE episodes in river basins across much of the State. When a few days of delay, following the AR or PE episodes, are allowed in calculations of streamflow contributed by these episodes, significant AR and PE contributions to overall streamflow are indicated, with 25–75% of the sites falling in the range of 34–52% of streamflow (
Table 3).
In the SSM/I period, ARs (and a three-day recovery period) are found to yield 50% and more of overall streamflow in coastal basins from California to the Canadian border (
Table 3;
Figure 9a). Farther inland, in the Central Sierra Nevada and in the Peninsular Range of southern California, some rivers yield 30 to 50% of overall streamflow, with most rivers in central and northern California yielding between from about 34 to 52% of overall streamflow during the AR-influenced periods. These AR streamflows average about two times the long-term mean streamflow rates from similar days of year (not shown). VIC-simulated runoff rates associated with AR episodes amount to broadly similar percentages of overall runoff in coastal basins and in the mountains of California (
Figure 10a). Significant fractional contributions to simulated runoff actually extend considerably father inland than in the observed streamflows. However, much of the area with the largest fractional contributions to overall runoff are areas that do not contribute much overall runoff at all, in absolute terms; AR runoff from such areas does not add to overall streamflows much and thus do not show up in the measured river discharges. Therefore, contributions mapped in
Figure 10 were masked to exclude all areas where average AR runoff generation was less than 0.5 mm/day. With this masking, comparisons between observed streamflow contributions from ARs and simulated runoff contributions are in good geographic agreement, although simulated fractional runoff contributions are generally smaller than their nearby streamflow counterparts.
Figure 9.
Contributions of streamflow during wet-season (November-April) episodes when atmospheric rivers (as ARs and PEs) made landfall on the West Coast to overall streamflow at HCDN streamflow gauging stations [
14] in the western US, for
(a) AR episodes, water years 1998–2008, based on
Table 1, and
(b) PE episodes, water years 1949–2008, based on
Table 2 (including streamflow in each case from PE-concurrent day and three following days). Inset map shows the ratio of average streamflow on the PE days (including concurrent day and 3 following days) to climatological means for the same combination of days. Concurrent and three following days are considered to allow for streamflow response times from the basins. Nonetheless, because higher-altitude mountains, such as the Sierra, capture large amounts of snow during AR or PE events that does not runoff until spring or summer, runoff percentages from these regions are expected to be underestimates overall.
Figure 9.
Contributions of streamflow during wet-season (November-April) episodes when atmospheric rivers (as ARs and PEs) made landfall on the West Coast to overall streamflow at HCDN streamflow gauging stations [
14] in the western US, for
(a) AR episodes, water years 1998–2008, based on
Table 1, and
(b) PE episodes, water years 1949–2008, based on
Table 2 (including streamflow in each case from PE-concurrent day and three following days). Inset map shows the ratio of average streamflow on the PE days (including concurrent day and 3 following days) to climatological means for the same combination of days. Concurrent and three following days are considered to allow for streamflow response times from the basins. Nonetheless, because higher-altitude mountains, such as the Sierra, capture large amounts of snow during AR or PE events that does not runoff until spring or summer, runoff percentages from these regions are expected to be underestimates overall.
Figure 10.
Same as main map in
Figure 8, except based on VIC-simulated runoff-plus-baseflow, and for
(a) AR episodes and
(b) PE episodes; both maps are masked to show AR- (or PE-) episode contributions to runoff-plus-baseflow in areas where average simulated runoff-plus-baseflow during episodes is >0.5 mm/day.
Figure 10.
Same as main map in
Figure 8, except based on VIC-simulated runoff-plus-baseflow, and for
(a) AR episodes and
(b) PE episodes; both maps are masked to show AR- (or PE-) episode contributions to runoff-plus-baseflow in areas where average simulated runoff-plus-baseflow during episodes is >0.5 mm/day.
On the longer term, streamflow yields associated with the PE episodes from water year 1949–2009 (
Figure 9b and
Table 3) are somewhat less than the AR contributions (
Figure 9a) in California, because there are about 2.5 times as many AR episodes per year as PE episodes. The streamflow contributions from PEs in the rivers of the Pacific Northwest are, similarly, about half as large as the (percentage) contributions from the AR episodes. Simulated runoff associated with PEs (
Figure 10b) approximates but is smaller than the AR contributions (
Figure 10a) in mountainous areas of northern California, and are much smaller in the Pacific Northwest.
Overall then, significant fractions of all precipitation and streamflow in the West Coast states derive from AR storms. This observation, together with the fact that a notably few storms (most often, AR storms) contribute most of California’s precipitation each year, on average, and that California’s year-to-year precipitation (and streamflow) variability is quite large, makes increased understanding of the details of how, where and when ARs arrive to support the State’s water resources crucial.
5.2. Year-To-Year Contributions
Contributions of PE events to water year precipitation (and streamflow) vary from year to year. Average PE contributions to water-year precipitation at cooperative weather stations in central and northern California are shown in
Figure 11, with contributions ranging from zero in several years to as much as 54% in 1986. To what extent do these variations (and variations elsewhere along the west coast) reflect large-scale climate modes either as concurrent or predictive associations?
Long-term, reproducible influences of large-scale Pacific climate influences on the occurrence of PEs, in general, remain difficult to discern or predict at present [
5], although on an event-by-event basis, strong ties to the tropical Pacific have been documented [
11,
29]. Nonetheless, year-to-year phases [
30] of the interannual El Nino-Southern Oscillation (ENSO) climate mode in the tropical Pacific [
31] and of the multidecadal Pacific Decadal Oscillation (PDO, [
32]) mode in the North Pacific climate are significantly associated with increases in the PE fractions of water-year precipitation (
Figure 12). The association indicated is such that, in El Nino and positive PDO (El Nino-like) years, PE fractions of water-year precipitation at stations in southern and south-central California, and, more spottily, in southern California and western Washington, respectively, are significantly larger than average. Comparison of
Figure 12 with
Figure 7 suggests that the correlations in
Figure 12 have their largest practical impacts in south central California and, perhaps, Washington, where both (a) the correlations with ENSO and PDO are significant and (b) the fractions of precipitation associated with PEs are also large. Notably, significant connections between ENSO or PDO and PE-precipitation contributions are not found in central and northern California from which so most of the State’s water resources are generated. Although such relations may eventually support improvements in long-lead forecasts of the role that PEs and floods will play in the overall water resources of southern California and western Washington, the simple (concurrent) relations in
Figure 12 could not be extended backwards in time into significant predictive associations between the status of ENSO or PDO in summer with PE-precipitation contributions at western weather stations.
Figure 11.
Averages of contributions to water-year total precipitation on PE days (plus day +1) at 202 cooperative weather stations in central and northern California (same area as defined in caption of
Table 3), 1951–2008.
Figure 11.
Averages of contributions to water-year total precipitation on PE days (plus day +1) at 202 cooperative weather stations in central and northern California (same area as defined in caption of
Table 3), 1951–2008.
Figure 12.
Correlations between PE contributions to water-year precipitation totals and concurrent water-year averages of
(a) the Nino3.4 [
31] sea-surface temperature (SST) index of the tropical ENSO climate mode, averaging SSTs in the region 5° N–5° S, 120° W–170° W, and
(b) the Pacific Decadal Oscillation SST-based index [
32]; colored dots indicate cooperative weather stations where the correlations are significantly different from zero at 95% confidence level and small gray dots are stations where the relations are not significant at this level.
Figure 12.
Correlations between PE contributions to water-year precipitation totals and concurrent water-year averages of
(a) the Nino3.4 [
31] sea-surface temperature (SST) index of the tropical ENSO climate mode, averaging SSTs in the region 5° N–5° S, 120° W–170° W, and
(b) the Pacific Decadal Oscillation SST-based index [
32]; colored dots indicate cooperative weather stations where the correlations are significantly different from zero at 95% confidence level and small gray dots are stations where the relations are not significant at this level.
As noted in
Figure 12, no reliable associations exist between ENSO or PDO status and PE-contributions to water-year precipitation in central and northern California, but this part of California lies along transition zones of many ENSO and PDO hydroclimatic influences [
33,
34], so that reliable teleconnections of those modes to northern California hydroclimatic conditions do not generally occur. Thus, in order to identify concurrent or predictive climate associations with PE-precipitation contributions in central and northern California, a wider range of possibilities must be investigated. In
Figure 13, rank correlations between the time series in
Figure 12 and concurrent (a) global sea-surface temperatures and (b) Pacific-North American 700-mbar heights are mapped. Above the planetary boundary layer, the large-scale atmospheric flow is nearly in geostrophic balance, and thus lines of equal height to the 700 mbar pressure surface approximate closely the stream lines followed by winds at the same level. Correlations with 700-mbar heights also indicate areas where the passage of low- and high-pressure weather systems are associated with the PE contributions.
Correlations are only mapped in
Figure 13 where they are significantly different from zero correlations with a simple student-t test. Thus the most significant correlations between winter sea-surface temperatures and PE-precipitation contributions in central and northern California (
Figure 13a) are found in the westernmost Pacific, with positive correlations (red) with sea-surface temperatures northeast of Japan and negative correlations (blue) in the tropical Pacific near the Philippines and Indonesia. A dipole of correlations is also found in the tropical and subtropical Atlantic basin north of the equator. The pattern of positive sea-surface temperature correlations in the western Pacific is located just north of the Kuroshio Extension Current and just north of the strongest part of the PDO sea-surface temperature pattern ([
32], and outlined in green in
Figure 13a). Thus the Pacific sea-surface temperature correlations with PE-precipitation contributions in central and northern California straddle (but do not parallel) the canonical PDO pattern and thus may reflect connections between large-than-average PE-precipitation contributions and concurrent southward displacements of the canonical positive-PDO sea-surface temperature pattern. Similarly, correlations between the PE-precipitation contributions and 700-mb heights (
Figure 13b) appear to indicate increased PE contributions in winters with lower-than-average 700-mbar heights or southward displacements of the climatological Aleutian Low pressure pattern (beneath the 700-mb center of action for PDO is indicated in green in
Figure 13b) [
33], which in turn reflects enhanced passage of low-pressure systems and storms across the North Pacific basin. Thus the 700-mb height correlations in
Figure 13b suggest a tendency for large PE-precipitation contributions in central and northern California to be associated with southern displacements of the storm tracks over the midlatitude Pacific and enhancement of the subtropical jet over the North Pacific basin (not shown). In particular, the structure of the low 700 mb height anomalies indicated by the negative corrections positioned north of Hawaii is consistent with atmospheric circulations that would favor northward advection of water vapor from the tropics near Hawaii—a classic characteristic of PE-type AR events [
5,
29].
Notably, no significant sea-surface temperature correlations are indicated in the equatorial central or eastern Pacific areas associated with tropical ENSO variability, and the only significant ties to equatorial sea-surface temperatures appear in the far westernmost parts of the Tropical Pacific “warm pool” around the Philippines. This warm pool is location of some of the warmest sea-surface temperatures on the planet, is a source of some of the warm waters associated with El Niños, and perhaps most importantly is a region that has seen important multi-decadal warming trends since the 1970s [
35]. The negative (blue) correlations indicated in this area would, on long-term average, suggest that warming of the Pacific warm pool since the 1970s may have contributed to the broad declines in PE contributions from the 1980s to 2008 in
Figure 11 (in agreement with the North American precipitation teleconnections found in [
35]), and that continued warming might be associated with decreasing contributions of PEs to California precipitation, if the historical associations continue.
Figure 13.
Rank correlations between average PE contributions to water-year precipitation totals at 202 cooperative weather stations in central and northern California (same area defined in
Table 3) and concurrent
(a) November-April sea-surface temperatures (SSTs), and
(b) November-April 700 mbar height anomalies [
12]; green hatched shapes in North Pacific surround primary centers of action (largest correlations) with respect to Pacific Decadal Oscillation [
31].
Figure 13.
Rank correlations between average PE contributions to water-year precipitation totals at 202 cooperative weather stations in central and northern California (same area defined in
Table 3) and concurrent
(a) November-April sea-surface temperatures (SSTs), and
(b) November-April 700 mbar height anomalies [
12]; green hatched shapes in North Pacific surround primary centers of action (largest correlations) with respect to Pacific Decadal Oscillation [
31].
Finally, historical relations between climatic conditions at the beginning of a water year and the PE-precipitation contributions in central and northern California during that water year are explored in
Figure 14. The correlations between sea-surface temperatures at the start of a water year and PE contributions (
Figure 14a) indicate that the connections between Pacific (and, in this case, Indian Ocean) warm pool and California PE-precipitation contributions may have some predictive elements. Cooler-than-normal warm pool sea-surface temperatures have historically tended to be associated with larger-than-normal PE contributions. As with the concurrent correlations in
Figure 13a, no other equatorial connection is apparent. Intriguingly, an impressive, if difficult to understand, historical connection appears, in the three panels of
Figure 14, that associates unusually wet conditions in the Pacific Northwest (14c), unusually low offshore pressures (14b) and underlying slightly cool sea-surface temperatures (14a) at the beginning of a water year (September–October) with higher than normal PE-precipitation contributions for central and northern California during the rest of the water year.
Figure 14.
Rank correlations between average PE contribution to water-year precipitation totals at 202 cooperative weather stations in central and northern California (same area defined in
Table 3) and
(a) preceding September-October sea-surface temperatures (SSTs),
(b) preceding September–October 700 mbar height anomalies, and (c) preceding September–October precipitation [
12].
Figure 14.
Rank correlations between average PE contribution to water-year precipitation totals at 202 cooperative weather stations in central and northern California (same area defined in
Table 3) and
(a) preceding September-October sea-surface temperatures (SSTs),
(b) preceding September–October 700 mbar height anomalies, and (c) preceding September–October precipitation [
12].
In principle, such relations might be used to condition the expectations of decision makers or to explicitly predict water years in which California will receive above-normal fractions of its overall precipitation and surface water supplies from PE storms. In such years, tradeoffs between managing reservoirs during major storms to focus on the hazardous aspects of these storms rather than their contributions to overall water resources might be particularly difficult, as the PE contributions are large and their forfeiture to flood-management practices particularly painful to resource managers. In years when PE storms will provide smaller fractions of the overall resources, more aggressive flood-management strategies during PE episodes might be implemented with greater confidence that the overall water resource will still fare well.
