Previous studies of the barrier layer (BL) and tropical cyclone (TC) intensification show controversial results. Some studies suggest that the BL is favorable for TC intensification, whereas others demonstrate that the BL has little impact. The present paper reconciles these previous results by showing that the influence of the BL on TC intensification is complex and dependent on the stages, TC intensity, TC forcing time, and ocean stratification. The influence can be divided into three different stages. First, when a TC is weak or the TC‐induced forcing cannot break through the mixed layer (ML), the presence of the BL leads to a thin ML, results in a large ML cooling, and reduces the effective TC heat potential (ETCHP) by distributing the air‐sea heat loss to the thin ML, thus suppressing TC growth. Second, when TC strengthens or wind forcing increases, the forcing becomes strong enough to break through the ML and penetrates into the BL. The entrainment of BL water, which is warmer than the cooled ML, compensates the surface heat loss and leads to an increase of the ML temperature, supporting TC growth. Third, when the TC‐induced forcing is strong enough to break through the BL base and penetrates to the thermocline, the BL reduces the magnitudes of cooling temperature and of decreasing ETCHP compared to those without the BL, which is also favorable for TC intensification. Thus, whether the BL intensifies TCs depends on the conditions associated with the upper layer ocean and TCs.

Subsurface salinity anomalies propagating between mid- and low latitudes along isopycnal surfaces have been shown to play an important role in modulating ocean and climate variability. In this study, a sustained freshening and southwestward propagation of subsurface salinity anomalies in the northwest Pacific subtropical gyre and its dynamical mechanism are investigated using observations, numerical outputs, and a predictive model. Analyses of the observations show a pronounced subsurface freshening with salinity decreasing about 0.25 PSU near the 24.5-σθ surface in the northwest Pacific subtropical gyre during 2003–11. This freshening is found to be related to the surface forcing of salinity anomalies in the outcrop zone (25°–35°N, 130°–160°E). A predictive model based on the assumption of salinity conservation along the outcrop isopycnals is derived and used to examine this surface-forcing mechanism. The resemblance between the spatial structures of the subsurface salinity derived from the predictive model and from observations and numerical outputs suggests that subsurface salinity anomalies are ventilated over the outcrop zone. A salinity anomaly with an amplitude of about 0.25 PSU generated by the surface forcing is subducted in the outcrop zone and then propagates southwestward, accompanied by potential vorticity anomalies, to the east of Luzon Strait (~15°N) in roughly one year. When the anomalies reach 15°N, they turn and move gradually eastward toward the central Pacific, associated with an eastward countercurrent on the southern subtropical gyre.