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Nicholas Siler and Dale Durran

Abstract

The partial reflection of mountain waves at the tropopause has been studied extensively for its contribution to downslope windstorms, but its impact on orographic precipitation has not been addressed. Here linear theory and numerical simulations are used to investigate how the tropopause affects the vertical structure of mountain waves and, in turn, orographic precipitation. Relative to the no-tropopause case, wave-induced ascent above the windward slope of a two-dimensional ridge is found to be enhanced or diminished depending on the ratio of the tropopause height to the vertical wavelength of the mountain waves—defined here as the “nondimensional tropopause height” . In idealized simulations of flow over both two-dimensional and three-dimensional ridges, variations in are found to modulate the precipitation rate by roughly a factor of 2 under typical atmospheric conditions. The sensitivity of precipitation to is related primarily to the depth of windward ascent but also to the location and strength of leeside descent, with significant impacts on the distribution of precipitation across the range (i.e., the rain-shadow effect). Using a modified version of Smith and Barstad’s orographic precipitation model, variations in are found to produce significant rain-shadow variability in the Washington Cascades, perhaps explaining some of the variability in rain-shadow strength observed among Cascade storms.

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Nicholas Siler and Dale Durran

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Recent studies have shown that weak rain shadows in the Cascade Mountains are associated with passing warm fronts, but the specific mechanisms responsible for this connection have eluded consensus. One theory holds that weak rain shadows are the result of enhanced precipitation over eastern slopes caused by easterly upslope flow; the other suggests that condensation is produced primarily over the western slopes, with enhanced east-slope precipitation occurring in dynamical regimes that minimize descent and evaporation east of the crest. Here these mechanisms are investigated through numerical simulations involving both real and idealized topography. Consistent with the second theory, storms with weak rain shadows are found to exhibit much weaker mountain waves in the lee of the Cascades than storms with strong rain shadows, with correspondingly weaker leeside evaporation. The muted wave activity during weak-rain-shadow storms is found to be caused by cold, zonally stagnant air at low levels in the lee, which precedes the warm front, and remains in place as the progression of the front is impeded by the mountains. As the front brings warmer air aloft, the static stability of the zonally stagnant layer increases, making it more resistant to erosion by the overlying flow. This in turn allows the weak rain shadow to persist long after the front has passed. If the midlatitude storm tracks shift poleward in a warmer climate, the results suggest there could be an increase in the strength of the rain shadow in mountainous regions astride the current storm tracks.

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Nicholas Siler, Gerard Roe, and Dale Durran

Abstract

Washington State’s Cascade Mountains exhibit a strong orographic rain shadow, with much wetter western slopes than eastern slopes due to prevailing westerly flow during the winter storm season. There is significant interannual variability in the magnitude of this rain-shadow effect, however, which has important consequences for water resource management, especially where water is a critically limited resource east of the crest. Here the influence of the large-scale circulation on the Cascade rain shadow is investigated using observations from the Snowfall Telemetry (SNOTEL) monitoring network, supplemented by stream gauge measurements. Two orthogonal indices are introduced as a basis set for representing variability in wintertime Cascade precipitation. First, the total precipitation index is a measure of regionwide precipitation and explains the majority of the variance in wintertime precipitation everywhere. Second, the rain-shadow index is a measure of the east–west precipitation gradient. It explains up to 31% of the variance west and east of the crest. A significant correlation is found between the winter-mean rain shadow and ENSO, with weak (strong) rain shadows associated with El Niño (La Niña). The analysis is supported by streamflow data from eastern and western watersheds. A preliminary review of individual storms suggests that the strongest rain shadows are associated with warm-sector events, while the weakest rain shadows occur during warm-frontal passages. This is consistent with known changes in storm tracks associated with ENSO, and a variety of mechanisms likely contribute.

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John Erich Christian, Nicholas Siler, Michelle Koutnik, and Gerard Roe

Abstract

Glacier mass balance provides a direct indicator of a glacier’s relationship with local climate, but internally generated variability in atmospheric circulation adds a significant degree of noise to mass-balance time series, making it difficult to correctly identify and interpret trends. This study applies “dynamical adjustment” to seasonal mass-balance records to identify and remove the component of variance in these time series that is associated with large-scale circulation fluctuations (dynamical adjustment refers here to a statistical method and not a glacier’s dynamical response to climate). Mass-balance records are investigated for three glaciers: Wolverine and Gulkana in Alaska and South Cascade in Washington. North Pacific sea level pressure and sea surface temperature fields perform comparably as predictors, each explaining 50%–60% of variance in winter balance and 25%–35% in summer balance for South Cascade and Wolverine Glaciers. Gulkana Glacier, located farther inland, is less closely linked to North Pacific climate variability, with the predictors explaining roughly 30% of variance in winter and summer balance. To investigate the degree to which this variability affects trends, adjusted mass-balance time series are compared to those in the raw data, with common results for all three glaciers; winter balance trends are not significant initially and do not gain robust significance after adjustment despite the large amount of circulation-related variability. However, the raw summer balance data have statistically significant negative trends that remain after dynamical adjustment. This indicates that these trends of increasing ablation in recent decades are not due to circulation anomalies and are consistent with anthropogenic warming.

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Nicholas Siler, Gerard H. Roe, and Kyle C. Armour

Abstract

Recent studies have shown that the change in poleward energy transport under global warming is well approximated by downgradient transport of near-surface moist static energy (MSE) modulated by the spatial pattern of radiative forcing, feedbacks, and ocean heat uptake. Here we explore the implications of downgradient MSE transport for changes in the vertically integrated moisture flux and thus the zonal-mean pattern of evaporation minus precipitation (E − P). Using a conventional energy balance model that we have modified to represent the Hadley cell, we find that downgradient MSE transport implies changes in E − P that mirror those simulated by comprehensive global climate models (GCMs), including a poleward expansion of the subtropical belt where E > P, and a poleward shift in the extratropical minimum of E − P associated with the storm tracks. The surface energy budget imposes further constraints on E and P independently: E increases almost everywhere, with relatively little spatial variability, while P must increase in the deep tropics, decrease in the subtropics, and increase in middle and high latitudes. Variations in the spatial pattern of radiative forcing, feedbacks, and ocean heat uptake across GCMs modulate these basic features, accounting for much of the model spread in the zonal-mean response of E and P to climate change. Thus, the principle of downgradient energy transport appears to provide a simple explanation for the basic structure of hydrologic cycle changes in GCM simulations of global warming.

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Kyle C. Armour, Nicholas Siler, Aaron Donohoe, and Gerard H. Roe

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Meridional atmospheric heat transport (AHT) has been investigated through three broad perspectives: a dynamic perspective, linking AHT to the poleward flux of moist static energy (MSE) by atmospheric motions; an energetic perspective, linking AHT to energy input to the atmosphere by top-of-atmosphere radiation and surface heat fluxes; and a diffusive perspective, representing AHT in terms downgradient energy transport. It is shown here that the three perspectives provide complementary diagnostics of meridional AHT and its changes under greenhouse gas forcing. When combined, the energetic and diffusive perspectives offer prognostic insights: anomalous AHT is constrained to satisfy the net energetic demands of radiative forcing, radiative feedbacks, and ocean heat uptake; in turn, the meridional pattern of warming must adjust to produce those AHT changes, and does so approximately according to diffusion of anomalous MSE. The relationship between temperature and MSE exerts strong constraints on the warming pattern, favoring polar amplification. These conclusions are supported by use of a diffusive moist energy balance model (EBM) that accurately predicts zonal-mean warming and AHT changes within comprehensive general circulation models (GCMs). A dry diffusive EBM predicts similar AHT changes in order to satisfy the same energetic constraints, but does so through tropically amplified warming—at odds with the GCMs’ polar-amplified warming pattern. The results suggest that polar-amplified warming is a near-inevitable consequence of a moist, diffusive atmosphere’s response to greenhouse gas forcing. In this view, atmospheric circulations must act to satisfy net AHT as constrained by energetics.

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Nicholas Siler, Yu Kosaka, Shang-Ping Xie, and Xichen Li

Abstract

The major El Niño of 2015/16 brought significantly less precipitation to California than previous events of comparable strength, much to the disappointment of residents suffering through the state’s fourth consecutive year of severe drought. Here, California’s weak precipitation in 2015/16 relative to previous major El Niño events is investigated within a 40-member ensemble of atmosphere-only simulations run with historical sea surface temperatures (SSTs) and constant radiative forcing. The simulations reveal significant differences in both California precipitation and the large-scale atmospheric circulation between 2015/16 and previous strong El Niño events, which are similar to (albeit weaker than) the differences found in observations. Principal component analysis indicates that these ensemble-mean differences were likely related to a pattern of tropical SST variability with a strong signal in the Indian Ocean and western Pacific and a weaker signal in the eastern equatorial Pacific and subtropical North Atlantic. This SST pattern was missed by the majority of forecast models, which could partly explain their erroneous predictions of above-average precipitation in California in 2015/16.

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