Southward Events Research Articles

Propagation Diversity of 20–40‐Day Oscillation of Summer Precipitation in the Middle and Lower Reaches of the Yangtze River

AbstractThe summer precipitation in the middle and lower reaches of the Yangtze River (MLYR) is characterized by obvious intra‐seasonal oscillations. This study investigates the propagation diversity of the 20–40‐day summer precipitation oscillation in MLYR and its underlying mechanism. The 20–40‐day oscillation events manifest as three types: northward propagation, southward propagation, and local oscillation. For northward and southward events, the propagation of precipitation is accompanied by the movement of 20–40‐day low‐frequency cyclonic vortices in the lower troposphere. Further investigation into the mechanism of anomalous vorticity propagation accompanying low‐frequency vortices reveals that positive relative vorticity advection in the north of the vortex facilitates its northward propagation. Moreover, the positive advection is primarily reliant on the background southerly winds to transport the low‐frequency vorticity. Diabatic heating dominates the southward propagation of low‐frequency precipitation. The center of the latent heating is located in the south of the low‐frequency vortex, which steers its southward migration. Furthermore, the uneven spatial distribution of latent heating across the vortex may be attributed to the non‐uniform distribution of mean humidity. Based on the above results, sensitivity experiments are conducted using a regional climate model (RegCM4.6). The results demonstrate that when the southerly winds are increased (decreased) or the specific humidity gradient is decreased (increased), the southward (northward) events weaken and disappear, or transitioned into local oscillations or northward (southward) events. This further validates the physical processes through which the basic flow and humidity inhomogeneities affect the meridional propagation of the 20–40‐day oscillations of precipitation.

Phytoplankton pigment patterns and wind forcing off central California

We use a 4‐year time series of high‐resolution coastal zone color scanner imagery to study mesoscale variability in phytoplankton pigment (as a surrogate for biomass) distributions off central California during the spring‐summer upwelling season. We use empirical orthogonal functions to decompose the time series of spatial images into its dominant modes of variability. Similarly, we analyze wind fields derived from the Fleet Numerical Oceanography Center pressure fields. The results from these analyses are used to investigate the coupling between wind forcing of the upper ocean and phytoplankton distributions on mesoscales. Prior to the spring transition, wind stress is dominated by strong northward conditions, and pigment is low and relatively uniform throughout the California Current domain. The first strong southward wind event soon results in a bloom of phytoplankton that is also uniformly distributed offshore. Nearshore values fluctuate relatively little in response to variations in wind forcing. After the spring transition, the wind becomes more steady (southward) and the fluctuations are dominated by changes in the strength of the curl. During southward events, the curl tends to become more positive. Depending on the fluctuations and strength of these curl episodes, filaments begin to form at particular locations. The overall pigment concentration in the California Current drops as a larger fraction of the pigment is distributed within the filaments. The filaments also tend to become shorter during this period, which starts soon after the spring transition and usually reaches its maximum intensity in early summer. Winds (and curl) become much weaker in the mid‐July to August period, and the filaments become much less prominent in terms of concentration and their length increases. Isolated wind events occasionally reinvigorate the filaments. A final bloom in the fall occasionally appears with spatial structure similar to the spring bloom. We conclude that wind forcing, in particular the curl of the wind stress, plays an important role in the distribution of phytoplankton pigment in the California Current. Although the underlying dynamics, especially of the filaments, may be dominated by processes other than forcing by wind stress curl (such as eddies or topographically induced instabilities), it appears that curl may force the variability of the filaments and hence the pigment patterns.