Evolution characteristics of the Madden–Julian oscillation (MJO) during the eastern Pacific (EP) and central Pacific (CP) types of El Niño have been investigated. MJO activities are strengthened over the western Pacific during the predeveloping and developing phases of EP El Niño, but suppressed during the mature and decaying phases. In contrast, MJO activities do not show a clear relationship with CP El Niño before their occurrence over the western Pacific, but they increase over the central Pacific during the mature and decaying phases of CP El Niño. Lag correlation analyses further confirm that MJO activities over the western Pacific in boreal spring and early summer are closely related to EP El Niño up to 2–11 months later, but not for CP El Niño. EP El Niño tends to weaken the MJO and lead to a much shorter range of its eastward propagation. Anomalous descending motions over the Maritime Continent and western Pacific related to El Niño can suppress convection and moisture flux convergence there and weaken MJO activities over these regions during the mature phase of both types of El Niño. MJO activities over the western Pacific are much weaker in EP El Niño due to the stronger anomalous descending motions. Furthermore, the MJO propagates more continuously and farther eastward during CP El Niño because of robust moisture convergence over the central Pacific, which provides adequate moisture for the development of MJO convection.

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By employing a state‐of‐the‐art global atmospheric general circulation model from the Global Monsoons Model Inter‐comparison Project, this study investigates the mechanical and thermal impacts of the Tibetan‐Iranian Plateau (TIP) on the North Pacific storm track (NPST). When the TIP‐associated mechanical forcing is removed, the winter NPST strengthens remarkably on the northern flank of its climatological maximum and becomes inconspicuously separated from storm track activities over the Asian continent. The midwinter suppression of the NPST is almost inapparent but still exists. In contrast, when TIP‐associated thermal forcing is absent, the winter NPST amplitudes weaken prominently on the north and west sides, and the midwinter suppression remains obvious. The mechanical and thermal impact of the TIP on the NPST are robust not only in winter but also in other seasons. Without the TIP‐associated mechanical forcing, the intensified winter NPST may be dependent on the weakened and widened upper‐level jet, strengthened atmospheric baroclinicity on the north side of climatological winter upper‐level jet, enhanced baroclinic energy conversion, and attenuated East Asian trough. In contrast, without the TIP‐associated thermal forcing, the weakened winter NPST may be determined by the strengthened and narrowed upper‐level jet, reduced atmospheric baroclinicity on the northern flank of upper‐level jet, decreased baroclinic energy conversion, and intensified East Asian trough.

Abstract:We examine the capability of thirteen Coupled Model Intercomparison Project (CMIP) phase 5 (CMIP5) models in simulating climatology and interannual variability of Winter North Pacific Storm Track (WNPST). It is found that nearly half of the selected models can reproduce the spatial pattern of WNPST climatology. However, the strength and spatial variation of WNPST climatology are weak in most of the models. Most differences among the models are in the northeast of the simulated multi-model ensemble (MME) climatology, while it is more consistent in the south. The MME can reflect not only the center position, but also the strength and spatial distribution of interannual variation of the WNPST amplitude. Except for CNRM-CM5, the interannual standard deviations of simulated WNPST strength and spatial variation in all other models are weak. ACCESS1-3 and CanESM2 have a better capability in simulating the spatial modes of WNPST, while the simulated second and third modes in some models are in opposite order with those in NCEP (National Centers for Environmental Prediction) reanalysis. Only five models and MME can capture "midwinter suppression" feature in their simulations. Compared with NCEP reanalysis, the winter longitude index is larger and latitude index is smaller in most of the models, indicating the simulated storm track is further east and south. CNRM-CM5, MME and CMCC-CM could be used to evaluate interannual variation of strength index, longitude index and latitude index respectively. Nevertheless, only INM-CM4 and CNRM-CM5 can simulate southward drift of WNPST.

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