Weather and Climate Inventory, Klamath Network, National Park Service, 2007
2.0. Climate Background
2.1. Climate and the KLMN Environment
Topography and coastal influences are instrumental in defining the climate of the KLMN (Odion et al. 2005). The Klamath-Siskiyou subregion contains REDW, ORCA, and WHIS. This subregion is characterized by extremely rugged topography. Topography works along with the proximity of the subregion to the Pacific Ocean to create exceptionally severe climatic gradients. The climate of the Klamath-Siskiyou subregion is typified by cool, wet winters and warm, dry summers. In winter, a strong low pressure area generally sets up in the Gulf of Alaska, with a relatively weak subtropical Pacific high-pressure system south of that. As a result, the prevailing westerlies that are common in temperate zones strengthen and move southward during the winter months, increasing the numbers of cyclonic storms (Bryson and Hare 1974; Miller 2002). These winter storms are responsible for the majority of the subregion’s annual precipitation. Topography also affects the distribution of precipitation during the winter months, with precipitation generally decreasing in the subregion from windward (west-facing) slopes and higher elevations to leeward (east facing) slopes and lower elevation areas (Miller 2002). Precipitation also tends to decrease from west to east across the Klamath-Siskiyou subregion. Despite deep, late-lying snowpacks, winters at high elevations in the Klamath-Siskiyou subregion are relatively mild and the ground rarely freezes.
In summer, the subtropical Pacific high-pressure system strengthens and the prevailing westerlies weaken and move northward. These shifts create dry conditions in the Klamath-Siskiyou subregion (Bryson and Hare 1974). When summer precipitation occurs, it usually comes in association with weak frontal disturbances and occasional thundershowers, especially at higher elevations. Lightning associated with thunderstorms commonly ignites fires in late summer and fall.
Although the Klamath-Siskiyou subregion is strongly moderated by the Pacific Ocean throughout the year, coastal influences are especially marked in summer. From June to September, warm, moist Pacific air is advected eastward by prevailing winds across the cold, upwelling coastal waters of the California current, creating a layer of moist and relatively cool air along the coast (Miller 2002). This moist, cool air is overlain by warmer, drier air, making this moist, marine layer relatively stable. The coastal mountains add to this stability by blocking the moist air from moving inland (Mitchell 1976), although occasionally a “marine push” can develop that will move cool, moist air from the Pacific Ocean over the Coast and Cascade mountain ranges into the interior (Mock 1996). The frequency and length of time a given site is under the influence of this maritime air plays a major role in the ecology of the Klamath-Siskiyou subregion. Maritime stratus and fogs decrease the amount of solar radiation that reaches the ground, lowering maximum temperatures and increasing the humidity during the otherwise dry summers. All these factors differentiate the maritime-influenced western slopes of the Klamath-Siskiyou subregion from the drier eastern slopes (Waring 1969). Coastal slopes and valleys that are favorably oriented to northwest summer winds are bathed in summer fogs and fog drip, vital sources of moisture for redwood trees (Burgess and Dawson 2004). The marine air masses effectively delimit the landward extent of the redwood biome.
The Cascades-Modoc subregion contains CRLA, LABE, and LAVO. This subregion is more isolated from the moderating climatic influence of the Pacific Ocean. At low to moderate elevations, summers are warm and dry and winters are cooler than along the coast. The western slopes of the Cascade Mountains receive abundant precipitation from winter storms, with the majority falling as snow at higher elevations. There is a significant increase in storm frequency with latitude in this subregion, such that CRLA headquarters receives nearly 50% percent more precipitation days through the year than LAVO headquarters. This precipitation difference reflects the latitudinal transition from the Mediterranean climate regime of California to the temperate maritime climate of the Pacific Northwest (Mitchell 1976). Above 2000 m elevation, snowpacks reach great depths and often cover the ground into summer (Redmond 2007). Snowfields currently persist year-round on Lassen Peak. The eastern slopes of the Cascades and adjacent Modoc Plateau are much drier, which is reflected in the open vegetation of these areas. During winter, cold continental air frequently invades the Modoc Plateau, but these cold air masses do not often reach the higher elevations of the Cascades or spill over onto the Cascades’ western slopes. Summer thunderstorms are frequent along the Cascades’ crest and eastern slopes in summer.
Future climate changes will likely have significant impacts for the KLMN. Although there is uncertainty as to the exact timing and magnitude of future climate change, there is a growing scientific consensus that climate change is occurring and that human activities are contributing to this change (NAST 2001). Estimates of global temperature increases for the next century range from 1.4° to 5.8°C, depending on the assumptions that are made about future greenhouse gas emissions, population growth, etc. (Albritton et al. 2001). For the western U.S., general circulation model simulations of future climate indicate that temperatures will likely increase in both winter and summer (Giorgi et al. 2001). Precipitation is also simulated to increase in winter, with changes in summer precipitation being less certain. Thus, the KLMN may experience warmer and wetter winters, and warmer summers in the future. Some modeling studies also suggest an increase in the strength of upwelling along the coast, which would help to maintain the coastal fogs that currently ameliorate coastal summer temperatures (Snyder et al. 2003). These fogs are considered important for maintaining appropriate climate conditions for the redwoods of REDW.
The many different potential impacts of climate change have significant management implications for KLMN park units. Shifts in the distributions of species attributed to recent climate change have already been identified (e.g., Parmesan and Yohe 2003) and these shifts will continue in the future. Of particular significance to biotic communities is the potential loss of winter freezing temperatures in the KLMN. Freezing temperatures control the distributions of a variety of plant and animal species. Loss of freezing temperatures would not only allow the expansion of certain native and non-native species in the region, but would also allow some insect pests to increase reproduction (Ayres and Lombardero 2000). Disturbance regimes, such as the frequency and magnitude of fire, will also be affected by climate change, with increased summer temperatures potentially increasing fire potential (Flannigan et al. 2000).
Climate change will also affect the hydrologic systems of the KLMN. Combined changes in temperature and precipitation will alter the amount, seasonal timing, and duration of snowpack and stream flows. These alterations affect both water quality and quantity. Mote (2003) evaluated snow data for the Pacific Northwest and found a decrease in snow water equivalent (i.e., the depth of water equivalent to the weight of the snowpack) related to increases in temperature for the period 1950-2000. A number of studies have also simulated future changes in snowpack and runoff, which indicate future decreases in snow (e.g., Leung et al. 2004) and changes in the timing of snowmelt runoff (e.g., Stewart et al. 2004) for the KLMN.
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