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Previous studies have shown that the Pacific Meridional Mode (PMM) has a crucial impact on the central Pacific El Niño–Southern Oscillation (ENSO). In this study, we redefined the PMM index used previously by focusing on the sea surface temperature in the subtropical northeastern Pacific (NEP), and found that the correlations between the PMM and ENSO are distinct on different timescales. We refer to this redefined index as PMM NEP. Results show that a positive PMM is weakly related to El Niño, while a negative PMM is more convincingly related to La Niña, specifically, the equatorial negative SSTAs possibly affecting negative PMM events. Moreover, the cause–effect relation between the PMM NEP and ENSO is much more significant on a decadal timescale than on an interannual timescale, and their interaction may be related to the equatorial zonal wind field. Our finding indicates that a La Niña-like decadal pattern is established through the combined actions of the PMM and ENSO, rather than an El Niño-like decadal pattern. • The PMM index defined in this study removes the signals of ENSO more effectively. • A positive PMM is weakly related to El Niño, while a negative PMM is more convincingly related to La Niña. • The relation between the PMM and ENSO is much more significant on a decadal timescale than on an interannual timescale. [ABSTRACT FROM AUTHOR]

Both the years 1998 and 2010 were the transitional years from El Niño to La Niña with record-low tropical cyclone (TC) genesis frequency over the western North Pacific (WNP). However, their rapid intensification (RI) magnitudes of TCs, defined as TC intensity change within 24 h during the RI stage, exhibited salient differences. RI magnitude in 1998 reached the third highest, but the lowest in 2010 since 1979. Results show that this remarkable difference in RI magnitude is related to the extremely favorable and unfavorable large-scale conditions, respectively. The extreme warming coupled with unstable atmosphere in key RI region provided favorable conditions for the high RI magnitude in 1998, while the extremely decreased low-level relative vorticity led to the very low RI magnitude in 2010. Further analyses show that this extreme warming in 1998 was driven by the enhanced net heat flux associated with the increase in downward short-wave radiation and latent heat flux, which was mainly caused by the less total cloud cover and reduced evaporation induced by the anomalous anticyclonic circulation, respectively. In sharp contrast, the unfavorable dynamic condition in 2010 mainly resulted from the combined impact of the La Niña events and the tropical east Indian Ocean warming. • TC RI magnitude over the WNP shows a salient difference in two strong La Niña years. • SST warming dominates the record-high in 1998. • Dynamic factors contribute to the record-low in 2010. • Large-scale conditions were modulated by the trans-basin SST changes. [ABSTRACT FROM AUTHOR]

Previous studies have documented the significant influences of the North Pacific subtropical sea surface temperature (SST) front (NPSTF) on the overlying atmospheric circulation. However, whether and how the NPSTF could affect remote circulations has not been well addressed. Here, we demonstrate that the enhanced NPSTF can induce a southward shift of the downstream North Atlantic westerly jet stream (NAWJS) in winter. A strong NPSTF with an increased meridional SST gradient induces a positive (negative) air temperature anomaly in its south (north) flank through the heat flux, leading to a direct thermal wind response and an indirect transient eddy feedback. The direct thermal wind response benefits a negative height anomaly south of Gulf of Alaska below 250 hPa and a positive one above 200 hPa, while the transient eddy activity could locally enhance and tilt northward with height. The kinetic energy significantly grows from the baroclinic energy conversion below 250 hPa over the NPSTF and from the barotropic energy conversion north of the NPSTF at the upper levels. This enhanced transient eddy activity gives rise to a negative height anomaly south of Gulf of Alaska through high-frequency transient eddy feedback forcing at all levels dominated by transient eddy vorticity flux. The anomalous low south of Gulf of Alaska favors an anomalous high over Canada and an anomalous low over the United States through Rossby energy propagation, causing the southward shift of the NAWJS. The numerical model further verifies the downstream influences of the NPSTF on the NAWJS in winter. • In addition to local effects, NPSTF can deliver its effect further downstream and modulate the meridional shift in NAWJS. • The height response south of Alaska Bay fosters downstream wind and height anomalies via Rossby energy propagation. • The transient eddy activity jointly induced by BTEC and BCEC dominates the local atmospheric response of NPSTF. [ABSTRACT FROM AUTHOR]