An interdecadal weakening in the North Atlantic storm track (NAST) and a poleward shift of the North Pacific storm track (NPST) are found during October–March for the period 1979–2015. A significant warming of surface air temperature (Ts) over northeastern North America and a La Niña–like change in the North Pacific under the background of Arctic amplification are found to be the contributors to the observed changes in the NAST and the NPST, respectively, via modulation of local baroclinicity. The interdecadal change in baroclinic energy conversion is consistent with changes in storm tracks with an energy loss from eddies to mean flow over the North Atlantic and an energy gain over the North Pacific. The analysis of simulations from the Community Earth System Model Large Ensemble project, although with some biases in storm-track and Ts simulations, supports the observed relationship between the NAST and Ts over northeastern North America, as well as the link between the NPST and El Niño–Southern Oscillation. The near-future projections of Ts and storm tracks are characterized by a warmer planet under the influence of increasing greenhouse gases and a significant weakening of both the NAST and the NPST. The potential role of the NAST in redistributing changes in Ts over the surrounding regions is also examined. The anomalous equatorward moisture flux associated with the weakening trend of the NAST would enhance the warming over its upstream region and hinder the warming over its downstream region via modulation of the downward infrared radiation.

This study investigates why OLR plays a small role in the Real-time Multivariate (Madden–Julian oscillation) MJO (RMM) index and how to improve it. The RMM index consists of the first two leading principal components (PCs) of a covariance matrix, which is constructed by combined daily anomalies of OLR and zonal winds at 850 (U850) and 200 hPa (U200) in the tropics after being normalized with their globally averaged standard deviations of 15.3 W m−2, 1.8 m s−1, and 4.9 m s−1, respectively. This covariance matrix is reasoned mathematically close to a correlation matrix. Both matrices substantially suppress the overall contribution of OLR and make the index more dynamical and nearly transparent to the convective initiation of the MJO. A covariance matrix that does not use normalized anomalies leads to the other extreme where OLR plays a dominant role while U850 and U200 are minor. Numerous tests indicate that a simple scaling of the anomalies (i.e., 2 W m−2, 1 m s−1, and 1 m s−1) can better balance the roles of OLR and winds. The revised PCs substantially enhance OLR over the eastern Indian and western Pacific Oceans and change it less notably in other locations, while they reduce U850 and U200 only slightly. Comparisons with the original RMM in spatial structure, power spectra, and standard deviation demonstrate improvements of the revised RMM index.

19The Madden-Julian Oscillation (MJO), the dominant mode of tropical intraseasonal 20 variability, influences weather and climate in the extratropics through atmospheric 21 teleconnection. In this study, two simulations using the Community Atmosphere Model version 22 5 (CAM5) -one with the default shallow and deep convection schemes and the other with the 23 Unified Convection scheme (UNICON) -are employed to examine the impacts of cumulus 24 parameterizations on the simulation of the boreal wintertime MJO teleconnection in the 25 Northern Hemisphere. We demonstrate that the UNICON substantially improves the MJO 26 teleconnection. When the UNICON is employed, the simulated circulation anomalies 27 associated with the MJO better resemble the observed counterpart, compared to the 28 simulation with the default convection schemes. Quantitatively, the pattern correlation for the 29 300-hPa geopotential height anomalies between the simulations and observation increases 30 from 0.07 for the default schemes to 0.54 for the UNICON. These circulation anomalies 31 associated with the MJO further help to enhance the surface air temperature and precipitation 32 anomalies over North America, although room for improvement is still evident. Initial value 33 calculations suggest that the realistic MJO teleconnection with the UNICON is not attributed to 34 the changes in the background wind, but primarily to the improved tropical convective heating 35 associated with the MJO. 36 Intercomparison Project Phase 3 (CMIP3), only 2 of the 14 models showed the spectral 61 characteristics of the MJO that were comparable to the observation. Most of the CMIP3 62 models lacked of pronounced peaks at the MJO time scales in their outgoing longwave 63 radiation (OLR) or 850-hPa zonal wind power spectra, and could not produce a realistic 64 eastward propagation of the MJO. Although the spectral characteristics of the MJO were 65 generally improved by the CMIP5 models, the improvement was incremental at best, and many 66 other aspects of the MJO still needed further improvements in order to be realistic (Hung et al. 67 2013). 68 Various attempts to improve MJO simulations by modifying a few particular aspects of 69 the cumulus parameterization often degrade the basic wind state despite its success in 70 capturing the MJO itself (Kim et al. 2011), which is known as the MJO paradox. Simulating 71 extratropical circulation response to the MJO is therefore extremely challenging because it 72 requires both realistic MJOs in the tropics which functions as a wave source and realistic mean 73 wind state as a wave guide. This MJO-mean state trade off could have hindered the use of 74 comprehensive models for an MJO teleconnection study. In terms of wave dynamics, the role 75 of the MJO and the mean state can be understood as follows; a realistic MJO simulation is 76 important as an ultimate wave source (although the wave source is also a function of wind), 77 and the background wind plays a critical role in wave excitation and propagation (Sardeshmukh 78...