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Mi-Kyung Sung, Seok-Woo Son, Changhyun Yoo, Jaeyoung Hwang, and Soon-Il An

Abstract

In recent winters, there have been repeated observations of extreme warm and cold spells in the midlatitude countries. This has evoked questions regarding how winter temperature extremes are induced. In this study, we demonstrate that abnormally warm winter weather in East Asia can drive the onset of extremely cold weather in North America approximately one week forward. These seesawing extremes across the basin are mediated by the North Pacific Oscillation (NPO), one of the recurrent atmospheric patterns over the North Pacific. Budget analysis of the quasigeostrophic geopotential tendency equation shows that intense thermal advection over East Asia is able to trigger the growth of the NPO. Vorticity fluxes associated with the upper-level stationary trough then strengthen and maintain the NPO against thermal damping following the onset of the NPO. Differential diabatic heating accompanied by changes in circulation also positively contribute to the growth and maintenance of the NPO. These results imply that recurrent cold extremes, seemingly contrary to global warming, may be an inherent feature resulting from strengthening warm extremes.

Open access
Christopher J. Cardinale, Brian E. J. Rose, Andrea L. Lang, and Aaron Donohoe

Abstract

The flux of moist static energy into the polar regions plays a key role in the energy budget and climate of the polar regions. While usually studied from a vertically integrated perspective (F wall), this analysis examines its vertical structure, using the NASA-MERRA-2 reanalysis to compute climatological and anomalous fluxes of sensible, latent, and potential energy across 70°N and 65°S for the period 1980–2016. The vertical structure of the climatological flux is bimodal, with peaks in the middle to lower troposphere and middle to upper stratosphere. The near-zero flux at the tropopause defines the boundary between stratospheric (F strat) and tropospheric (F trop) contributions to F wall. Especially at 70°N, F strat is found to be important to the climatology and variability of F wall, contributing 20.9 W m−2 to F wall (19% of F wall) during the winter and explaining 23% of the variance of F wall. During winter, an anomalous poleward increase in F strat preceding a sudden stratospheric warming is followed by an increase in outgoing longwave radiation anomalies, with little influence on the surface energy budget of the Arctic. Conversely, a majority of the energy input by an anomalous poleward increase in F trop goes toward warming the Arctic surface. Overall, F trop is found to be a better metric than F wall for evaluating the influence of atmospheric circulations on the Arctic surface climate.

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Jed E. Lenetsky, Bruno Tremblay, Charles Brunette, and Gianluca Meneghello

Abstract

We use ocean observations and reanalyses to investigate the subseasonal predictability of summer and fall sea ice area (SIA) in the western Arctic Ocean associated with lateral ocean heat transport (OHT) through Bering Strait and vertical OHT along the Alaskan coastline from Ekman divergence and upwelling. Results show predictive skill of spring Bering Strait OHT anomalies in the Chukchi Sea and eastern East Siberian Sea for June and July SIA, followed by a sharp drop in predictive skill in August, September, and October and a resurgence of the correlation in November during freeze-up. Fall upwelling of Pacific Water along the Alaskan coastline—a mechanism that was proposed as a preconditioner for lower sea ice concentration (SIC) in the Beaufort Sea the following summer—shows minimal predictive strength on both local and regional scales for any months of the melt season. A statistical hindcast based on May Bering Strait OHT anomalies explains 77% of July Chukchi Sea SIA variance. Using OHT as a predictor of SIA anomalies in the Chukchi Sea improves hindcasts from the simple linear trend by 35% and predictions from spring sea ice thickness anomalies by 24%. This work highlights the importance of ocean heat anomalies for melt season sea ice prediction and provides observational evidence of subseasonal changes in forecast skill observed in model-based forecasts of the Chukchi Sea.

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Jennifer V. Lukovich, Julienne C. Stroeve, Alex Crawford, Lawrence Hamilton, Michel Tsamados, Harry Heorton, and François Massonnet

Abstract

In this study the impact of extreme cyclones on Arctic sea ice in summer is investigated. Examined in particular are relative thermodynamic and dynamic contributions to sea ice volume budgets in the vicinity of Arctic summer cyclones in 2012 and 2016. Results from this investigation illustrate that sea ice loss in the vicinity of the cyclone trajectories during each year was associated with different dominant processes: thermodynamic processes (melting) in the Pacific sector of the Arctic in 2012, and both thermodynamic and dynamic processes in the Pacific sector of the Arctic in 2016. Comparison of both years further suggests that the Arctic minimum sea ice extent is influenced by not only the strength of the cyclone, but also by the timing and location relative to the sea ice edge. Located near the sea ice edge in early August in 2012, and over the central Arctic later in August in 2016, extreme cyclones contributed to comparable sea ice area (SIA) loss, yet enhanced sea ice volume loss in 2012 relative to 2016. Central to a characterization of extreme cyclone impacts on Arctic sea ice from the perspective of thermodynamic and dynamic processes, we present an index describing relative thermodynamic and dynamic contributions to sea ice volume changes. This index helps to quantify and improve our understanding of initial sea ice state and dynamical responses to cyclones in a rapidly warming Arctic, with implications for seasonal ice forecasting, marine navigation, coastal community infrastructure, and designation of protected and ecologically sensitive marine zones.

Open access
Shubhi Agrawal, Craig R. Ferguson, Lance Bosart, and D. Alex Burrows

Abstract

A spectral analysis of Great Plains 850-hPa meridional winds (V850) from ECMWF’s coupled climate reanalysis of 1901–2010 (CERA-20C) reveals that their warm season (April–September) interannual variability peaks in May with 2–6-yr periodicity, suggestive of an underlying teleconnection influence on low-level jets (LLJs). Using an objective, dynamical jet classification framework based on 500-hPa wave activity, we pursue a large-scale teleconnection hypothesis separately for LLJs that are uncoupled (LLJUC) and coupled (LLJC) to the upper-level jet stream. Differentiating between jet types enables isolation of their respective sources of variability. In the U.S. south-central plains (SCP), May LLJCs account for nearly 1.6 times more precipitation and 1.5 times greater V850 compared to LLJUCs. Composite analyses of May 250-hPa geopotential height (Z250) conditioned on LLJC and LLJUC frequencies highlight a distinct planetary-scale Rossby wave pattern with wavenumber 5, indicative of an underlying circumglobal teleconnection (CGT). An index of May CGT is found to be significantly correlated with both LLJC (r = 0.62) and LLJUC (r = −0.48) frequencies. Additionally, a significant correlation is found between May LLJUC frequency and NAO (r = 0.33). Further analyses expose decadal-scale variations in the CGT–LLJC and CGT–LLJUC teleconnections that are linked to the PDO. Dynamically, these large-scale teleconnections impact LLJ class frequency and intensity via upper-level geopotential anomalies over the western United States that modulate near-surface geopotential and temperature gradients across the SCP.

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Hongyu Li, Qiang Zhang, Ping Yue, Liang Zhang, Xiaochen Niu, Hongli Zhang, Kaicheng Xing, Yuanyuan Jing, and Guofei Shang

Abstract

Investigating the response of land surface energy exchange to key climatic signals such as the East Asian summer monsoon (EASM) is essential for understanding the intensive interactions in the Earth system. This study focuses on the summer monsoon transition zone (SMTZ) in China, which has a climate rather sensitive to the EASM activity, and examined the response of land surface energy exchange over the SMTZ to summer monsoon activity. A flux evaluation of five reanalysis/modeling datasets indicates that JRA-55 (the Japanese 55-Year Reanalysis) reasonably represents interannual variations of surface heat fluxes over the SMTZ. The EASM activity is accurately identified in the SMTZ by introducing a monsoon temporal duration index (MTDI), which presents climate variations of summer rainfall and EASM activity better than commonly used summer monsoon indexes. Based on MTDI and long-term flux datasets, it was found that the interannual fluctuation of the EASM intensively controls surface energy partitioning and turbulent heat exchange but has a weak impact on radiative processes over the SMTZ. Furthermore, surface sensible and latent heat fluxes significantly responded to the influential period of the summer monsoon, exhibiting approximately quadratic/logarithmic relationships with the MTDI. More prominent interannual variabilities of turbulent heat fluxes were observed in weak summer monsoon years, during which an active interaction between surface energy exchange and a warming and drying climate occurred. An ensemble empirical mode decomposition (EEMD) analysis confirms that EASM activity dominates the quasi-biennial and multidecadal variations of turbulent heat fluxes over the SMTZ, which may be achieved by the transport of tropical quasi-biennial and Pacific decadal oscillation (PDO) signals to the midlatitudes of East Asia. The expected intensification of summer monsoon activity in the future may induce acceleration of energy and hydrological cycle and exert a substantial impact on the availability of water and the ecosystem stability over the SMTZ.

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Er Lu, Jiawei Hao, and Kexin Yang

Abstract

The temporal–spatial variations of the static stability of dry air and the relative importance of their influencing quantities are explored. Derivation shows that while it links to the vertical difference of temperature, static stability also relates to the temperature itself. The static stability is expressed as a nonlinear function of temperature and the vertical difference of temperature. The relative importance of the two influencing quantities is assessed with linear regression. Tests show that the linear fitting method is robust. The results of the dominance rely on the data examined, which include an interannual variation, a seasonal variation, and a spatial variation that consists of the grid points over the globe. It is revealed that in the lower troposphere, while the temporal variations of static stability are dominated by the vertical difference of temperature, the temperature itself may also have considerable influence, especially over the high latitudes of the two hemispheres. In the stratosphere, temperature tends to have more contributions. Over the Antarctic, temperature dominates the seasonal and interannual variations of the static stability. The spatial variation of the static stability of July is influenced by both temperature and its vertical difference before 1980, but after that it is dominated by temperature.

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Tao Zhu and Jing Yang

Abstract

Two types of mid-high-latitude low-frequency intraseasonal oscillations (LF-ISOs), featuring eastward and westward propagation, have been identified over the Eurasian continent in the past 37 summers (1982–2018). The eastward and westward propagating modes commonly have a dominant periodicity of 30–50 days near the Ural Mountains (UM) but have different origins and evolutions. The eastward propagating LF-ISO initiates over eastern North America, migrates northeastward across northeastern North America–western North Atlantic, central North Atlantic, western Europe, and the UM, then propagates southeastward to northwestern and eastern China, which is the Atlantic-Eurasian continental mode. In contrast, the westward propagating mode is quasi-circumpolar, initiating over the East Siberian Sea and moving southwestward across the UM and northern Europe and eventually reaching Greenland and the Canadian Arctic Archipelago. These two mid-high-latitude LF-ISOs are accompanied by significant tropical intraseasonal variations with evident tropical–extratropical interactions. Meanwhile, these two LF-ISOs have different decadal preferences before and after 2000, which are ascribed to the decadal change of both intraseasonal efficient kinetic energy obtained from the mean flow over their genesis region and their background flow associated with the North Atlantic Oscillation/Arctic Oscillation decadal change. This study deepens the understanding of subseasonal variations for mid-high latitudes and subseasonal prediction sources for low-latitude regions.

Open access
Julia Jeworrek, Gregory West, and Roland Stull

Abstract

Physics parameterizations in the Weather Research and Forecasting (WRF) Model are systematically varied to investigate precipitation forecast performance over the complex terrain of southwest British Columbia (BC). Comparing a full year of modeling data from over 100 WRF configurations to station observations reveals sensitivities of precipitation intensity, season, location, grid resolution, and accumulation window. The choice of cumulus and microphysics parameterizations is most important. The WSM5 microphysics scheme yields competitive verification scores when compared to more sophisticated and computationally expensive parameterizations. Although the scale-aware Grell–Freitas cumulus parameterization performs better for summertime convective precipitation, the conventional Kain–Fritsch parameterization better simulates wintertime frontal precipitation, which contributes to the majority of the annual precipitation in southwest BC. Finer grid spacings have lower relative biases and a more realistic spread in precipitation intensity distribution, yet higher relative standard deviations of their errors—they produce finer spatial differences and local extrema. Finer resolutions produce the best fraction of correct-to-incorrect forecasts across all precipitation intensities, whereas the coarser 27-km domain yields the highest hit rates and equitable threat scores. Verification metrics improve greatly with longer accumulation windows—hourly precipitation values are prone to double-penalty issues, while longer accumulation windows compensate for timing errors but lose information about short-term precipitation intensities. This study provides insights regarding WRF precipitation performance in complex terrain across a wide variety of configurations, using metrics important to a range of end users.

Open access
ARIANE MIDDEL, SAUD ALKHALED, FLORIAN A. SCHNEIDER, BJOERN HAGEN, and PAUL COSEO

Abstract

Cities increasingly recognize the importance of shade to reduce heat stress and adopt urban forestry plans with ambitious canopy goals. Yet, the implementation of tree and shade plans often faces maintenance, water use, and infrastructure challenges. Understanding the performance of natural and non-natural shade is critical to support active shade management in the built environment. We conducted hourly transects in Tempe, Arizona with the mobile human-biometeorological station MaRTy on hot summer days to quantify the efficacy of various shade types. We sampled sun-exposed reference locations and shade types grouped by urban form, lightweight/engineered shade, and tree species over multiple ground surfaces. We investigated shade performance during the day, at peak incoming solar, peak air temperature, and after sunset using three thermal metrics: the difference between a shaded and sun-exposed location in air temperature (ΔTa), surface temperature (ΔTs), and mean radiant temperature (ΔTMRT). ΔTa did not vary significantly between shade groups, but ΔTMRT spanned a 50°C range across observations. At daytime, shade from urban form most effectively reduced Ts and TMRT, followed by trees and lightweight structures. Shade from urban form performed differently with changing orientation. Tree shade performance varied widely; native and palm trees were least effective, while non-native trees were most effective. All shade types exhibited heat retention (positive ΔTMRT) after sunset. Based on the observations, we developed characteristic shade performance curves that will inform the City of Tempe’s design guidelines towards using “the right shade in the right place” and form the basis for the development of microclimate zones (MCSz).

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