Search Results
Abstract
The northeastern Pacific Ocean is a preferential location for the formation of closed low pressure systems. These slow-moving, quasi-barotropic systems influence vertical stability and sustain a moist environment, giving them the potential to produce or affect sustained precipitation episodes along the west coast of the United States. They can remain motionless or change direction and speed more than once and thus often pose difficult forecast challenges. This study creates an objective climatological description of 500-hPa closed lows to assess their impacts on precipitation in the western United States and to explore interannual variability and preferred tracks. Geopotential height at 500 hPa from the NCEP–NCAR global reanalysis dataset was used at 6-h and 2.5° × 2.5° resolution for the period 1948–2011. Closed lows displayed seasonality and preferential durations. Time series for seasonal and annual event counts were found to exhibit strong interannual variability. Composites of the tracks of landfalling closed lows revealed preferential tracks as the features move inland over the western United States. Correlations of seasonal event totals for closed lows with ENSO indices, the Pacific decadal oscillation (PDO), and the Pacific–North American (PNA) pattern suggested an above-average number of events during the warm phase of ENSO and positive PDO and PNA phases. Precipitation at 30 U.S. Cooperative Observer stations was attributed to closed-low events, suggesting 20%–60% of annual precipitation along the West Coast may be associated with closed lows.
Abstract
The northeastern Pacific Ocean is a preferential location for the formation of closed low pressure systems. These slow-moving, quasi-barotropic systems influence vertical stability and sustain a moist environment, giving them the potential to produce or affect sustained precipitation episodes along the west coast of the United States. They can remain motionless or change direction and speed more than once and thus often pose difficult forecast challenges. This study creates an objective climatological description of 500-hPa closed lows to assess their impacts on precipitation in the western United States and to explore interannual variability and preferred tracks. Geopotential height at 500 hPa from the NCEP–NCAR global reanalysis dataset was used at 6-h and 2.5° × 2.5° resolution for the period 1948–2011. Closed lows displayed seasonality and preferential durations. Time series for seasonal and annual event counts were found to exhibit strong interannual variability. Composites of the tracks of landfalling closed lows revealed preferential tracks as the features move inland over the western United States. Correlations of seasonal event totals for closed lows with ENSO indices, the Pacific decadal oscillation (PDO), and the Pacific–North American (PNA) pattern suggested an above-average number of events during the warm phase of ENSO and positive PDO and PNA phases. Precipitation at 30 U.S. Cooperative Observer stations was attributed to closed-low events, suggesting 20%–60% of annual precipitation along the West Coast may be associated with closed lows.
Abstract
A novel approach is presented to objectively identify regional patterns of climate variability within the state of California using principal component analysis on monthly precipitation and temperature data from a network of 195 climate stations statewide and an ancillary gridded database. The confluence of large-scale circulation patterns and the complex geography of the state result in 11 regional modes of climate variability within the state. A comparison between the station and gridded analyses reveals that finescale spatial resolution is needed to adequately capture regional modes in complex orographic and coastal settings. Objectively identified regions can be employed not only in tracking regional climate signatures, but also in improving the understanding of mechanisms behind regional climate variability and climate change. The analysis has been incorporated into an operational tool called the California Climate Tracker.
Abstract
A novel approach is presented to objectively identify regional patterns of climate variability within the state of California using principal component analysis on monthly precipitation and temperature data from a network of 195 climate stations statewide and an ancillary gridded database. The confluence of large-scale circulation patterns and the complex geography of the state result in 11 regional modes of climate variability within the state. A comparison between the station and gridded analyses reveals that finescale spatial resolution is needed to adequately capture regional modes in complex orographic and coastal settings. Objectively identified regions can be employed not only in tracking regional climate signatures, but also in improving the understanding of mechanisms behind regional climate variability and climate change. The analysis has been incorporated into an operational tool called the California Climate Tracker.
