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This study has investigated the relationship between temperature extremes and a subseasonal hemispheric teleconnection pattern over the Northern Hemisphere during boreal summer. By applying self-organizing map (SOM) analysis to 200-hPa geopotential fields from the European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim) for the period 1979–2012, a teleconnection pattern is identified that increased dramatically in its occurrence after the late 1990s. This pattern is characterized by a zonal wavenumber-5 pattern with anomalous high pressure cores over eastern Europe, northeastern Asia, the eastern North Pacific, the eastern United States, and Greenland. These high pressure centers coincide with regions of increasingly frequent temperature extremes in recent decades. To investigate the temporal evolution of the identified SOM pattern, time-lagged composites were performed relative to the days in which the 200-hPa geopotential field most closely resembled the SOM pattern. From day −10 to day 0, a wave train propagated from the central tropical Pacific to the Canadian Arctic Archipelago and Greenland. This poleward wave propagation was followed by the establishment of quasi-stationary high pressure centers over Greenland, Europe, and Asia. This study suggests that more frequent occurrence of the hemispheric teleconnection is linked to more severe and longer extreme weather events over the Northern Hemisphere since the late 1990s.

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This study investigates how the balance between wind and mass is treated in data assimilation, and how it affects the quality of the model states in an analysis–forecast cycle. This is done in terms of the dependence of balance on latitude and on the type of variable. The impact of the nonlinear balance equation is compared with regressed wind–mass balance in the 3‐Dimensional Variational data assimilation (3D‐Var) system of the Korea Institute of Atmospheric Prediction Systems (KIAPS). Its impact is significantly positive in temperature rather than in wind, despite the two‐way influence of the cross‐correlation between wind and mass, in terms of the root‐mean‐square difference (RMSD) of 6 h forecasts against the ERA‐Interim reanalysis data. This temperature effect is observed in the southern hemispheric (SH) polar jet of the mid‐troposphere, the SH midlatitudinal jet, and the mid/lower stratosphere in the Tropics, where there is strong zonal mean flow. Although the zonal wind forecast was harmed by application of the nonlinear balance, the temporal consistency of the damage is relatively weak compared to the improvement by the nonlinear balance in the temperature forecasts. In the SH midlatitudinal jet and the mid/lower stratosphere in the Tropics, the nonlinear balance equation, including the advection term, improves the quality of temperature RMSDs in the analysis–forecast cycle by imposing the proper balance in the initial conditions. However, in the SH polar jet of the mid‐troposphere, where the observation density is relatively low, the nonlinear balance equation achieves the same effect by reducing the analysis error (i.e. generating initial conditions more accurately). The nonlinear balance equation contributes to robustly improving the model states of the analysis–forecast cycles, depending on the dynamical activity and the observation density of the corresponding regions.

[1] In the mature season of El Niño, Rossby waves do not easily propagate into the polar region, and the seasonal climatology of sea ice is minimal. Austral summer is a barrier to the persistent Antarctic dipole pattern (ADP) in sea ice. The sea surface temperature (SST) anomaly of central Pacific type El Niño (CP-El Niño) in the central Pacific contributes to a strong Rossby wave response and weakening of the polar-front jet that yields strong ADP in austral spring just before the ADP barrier. The strong ADP produces intensive sea-ice-air feedback, which allows the ADP anomaly to breach the barrier. In the conventional El Niño (EP-El Niño) events, the upper-level structure cannot contribute to the strong anomalous high pressure. In EP-El Niño events, the anomalous high in the Bellingshausen Sea is replaced by an anomalous low after the austral autumn following the mature season, whereas the anomalous high pressure persists up to the austral winter in the CP-El Niño. In the CP-El Niño, the ADP persists until austral winter after the mature season of El Niño, whereas, in the EP-El Niño, it does not persist after austral summer. The central Pacific cold SST anomaly of La Niña together with the seasonal SST climatology prolongs the opposite phase of the ADP anomaly up to the austral winter. Consequently, the tropical climate anomaly is exported to extratropics at the central Pacific in the Southern Hemisphere.

The multimodel Global Land-Atmosphere Coupling Experiment (GLACE) identified the semiarid Southern Great Plains (SGP) as a hotspot for land-atmosphere (LA) coupling and, consequently, landderived temperature and precipitation predictability. The area including and surrounding the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) SGP Climate Research Facility has in particular been well studied in the context of LA coupling. Observation-based studies suggest a coupling signal that is much weaker than modeled, if not elusive. Using North American Regional Reanalysis and North American Land Data Assimilation System data, this study provides a 36-yr (1979-2014) climatology of coupling for ARM-SGP that 1) unifies prior interdisciplinary efforts and 2) isolates the origin of the (weak) coupling signal. Specifically, the climatology of a prominent convective triggering potential-low-level humidity index (CTP-HI low ) coupling classification is linked to corresponding synoptic-mesoscale weather and atmospheric moisture budget analyses. The CTP-HI low classification defines a dry-advantage regime for which convective triggering is preferentially favored over drier-than-average soils as well as a wet-advantage regime for which convective triggering is preferentially favored over wetter-than-average soils. This study shows that wet-advantage days are a result of horizontal moisture flux convergence over the region, and conversely, dryadvantage days are a result of zonal and vertical moisture flux divergence. In this context, the role of the land is nominal relative to that of atmospheric forcing. Surface flux partitioning, however, can play an important role in modulating diurnal precipitation cycle phase and amplitude and it is shown that soil moisture and sensible heat flux are significantly correlated with both occurrence and intensity of afternoon peak precipitation.

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Atmospheric numerical models using the spectral element method with cubed-sphere grids (CSGs) are highly scalable in terms of parallelization. However, there are no data assimilation systems for spectral element numerical models. The authors devised a spectral transformation method applicable to the model data on a CSG (STCS) for a three-dimensional variational data assimilation system (3DVAR). To evaluate the 3DVAR system based on the STCS, the authors conducted observing system simulation experiments (OSSEs) using Community Atmosphere Model with Spectral Element dynamical core (CAM-SE). They observed root-mean-squared error reductions: 24% and 34% for zonal and meridional winds (U and V), respectively; 20% for temperature (T); 4% for specific humidity (Q); and 57% for surface pressure (Ps) in analysis and 28% and 27% for U and V, respectively; 25% for T; 21% for Q; and 31% for Ps in 72-h forecast fields. In this paper, under the premise that the same number of grid points is set, the authors show that the use of a greater polynomial degree, np, produces better performance than use of a greater element count, ne, on equiangular coordinates in terms of the wave representation.