No abstract

This study revisits the characteristics and physical processes of the azimuthally asymmetric distribution of moist instability in the outer core of vertically sheared tropical cyclones (TCs) using a numerical model. The results indicate that a downshear–upshear contrast in outer-core conditional instability occurs in the weakly sheared TCs, while an enhanced downshear-left–downshear-right difference is found in strongly sheared storms. Specifically, lower (higher) conditional instability arises downshear left (right) in the strongly sheared TCs. Downward transports of low-entropy air by convective and mesoscale downdrafts in principal rainbands reduce the equivalent potential temperature (θe) in the downshear-left boundary layer, contributing to lower convective available potential energy. Positive horizontal advection of both potential temperature and water vapor by the asymmetric outflow leads to a midlevel maximum of θe in the same quadrant. Hence, a positive θe vertical gradient (thus potential stability) is present in the downshear-left outer core. In the downshear-right quadrant, a lack of convective downdrafts, together with surface fluxes, leads to higher θe in the boundary layer. A dry intrusion is found at the middle to upper levels in the downshear-right outer core, and significant negative horizontal advection of water vapor produces low θe near the midtroposphere. A negative vertical gradient of θe (thus potential instability) in the outer core arises below the downshear-right midtroposphere. The presence of azimuthally asymmetric moist instability is expected to play an important role in fostering and maintaining azimuthally asymmetric convective activity in the outer core of TCs.

Pacific Decadal Variability (PDV) has enormous influences on North American and Eurasian climate and Pacific ecosystems. Its change under anthropogenic warming is of prodigious societal concern and scientific dispute. We show evidence that the observed PDV has amplified in the northeast Pacific horseshoe‐like region while weakened over the South Pacific and Kuroshio‐Oyashio Extension (KOE) region since the beginning of the 21st century. Congruently, under the influence of PDV, precipitation anomalies have significantly amplified globally and reversed the sign over Africa and East Asia. Analysis of the Coupled Model Intercomparison Project Phase 6 (CMIP6) historical simulations and future projections suggests that greenhouse forcing weakens the PDV in the South and northwestern Pacific and shortens its periodicity. However, the observed PDV's enhancement over the North Pacific eastern boundary is likely due to internal variability. Our findings on the PDV's response to anthropogenic forcing and internal variability shed light on the PDV's future change and decadal prediction.

Idealized numerical experiments show that stronger shear is more favorable for tropical cyclone secondary eyewall formation (SEF) than weaker shear amid a moist‐tropical environment when the shear magnitude is smaller than 15 m s−1. In particular, the larger the shear, the earlier the SEF occurs. A stationary banding complex (SBC) prevails before SEF in the large shear experiments, suggesting that SBCs are helpful in the SEF. Furthermore, a well‐organized stratiform sector is associated with the SBC. With increasing shear, the shear‐forced outflow also increases in the upper layers, transporting more moisture and icy particles outward from the inner core and favoring the development of the stratiform sector. The diabatic cooling of the stratiform sector strengthens the underlying descending radial inflow, forcing updrafts immediately outside the inner core. The subsequent axisymmetrization of the updrafts leads to SEF.

North Pacific Gyre Oscillation (NPGO) is one of the important modes of decadal variability in the North Pacific sea surface temperature (SST) and sea surface height (SSH). This study investigated the potential influence of NPGO on spatial characteristics of the peak season (July to October) tropical cyclones (TCs) genesis (TCG) number over the western North Pacific (WNP) from 1965 to 2020. We show that NPGO is the first leading Empirical Orthogonal Function (EOF) mode of the North Pacific SST during the TC peak season in the recent 56 years. On the decadal time scale, NPGO has opposite impacts on TCG in the west and east WNP. The relatively weak positive correlation in the west of 140°E and the strong positive correlation in the east of 140°E result in an overall significant negative correlation between NPGO and WNP total TCG number (r = -0.49), which is much more robust than the relationship between Pacific Decadal Oscillation (PDO) and TCG. The critical factors of NPGO affecting TCG are the vertical motion in the west WNP and vertical wind shear (VWS) in the east WNP. The positive NPGO pattern could induce an anomalous off-equatorial vertical circulation, resulting in an upward motion and increased convective precipitation in the west WNP, favoring local TCG. The anomalous convective precipitation enhances the zonal gradient of the atmospheric heat source in the east WNP, increasing VWS. The North Pacific low-level anticyclonic and upper-level cyclonic associated with NPGO further enhance the VWS in the east WNP and lead to the negative low-level relative vorticity, inhibiting local TCG. This study emphasizes the importance of the NPGO’s climate impact in recent decades. The findings here have significant implications for the decadal prediction of WNP TCG change.