Abstract
The possible effects of the Kuroshio current and associated sea surface temperature (SST) fronts on Southeastern China winter precipitation are examined using twin simulations of a regional atmospheric model. When the SST frontal features in the forcing data are artificially weakened, the simulated rainfall over Southeastern China, particularly heavy rainfall, is found to increase by a statistically significant amount, compared to the simulations with the observed SST fronts. This difference is attributed to an anomalous regional overturning circulation that interferes with the onshore moisture transport. With the presence of stronger frontal SSTs, an anomalous ascending motion is anchored along the warm current, with a descending branch to its western side, forming a circulation cell that strides across the coasts of Southern China. In days of heavy rainfall over the Southeastern China, the regional atmospheric circulation tunnels oceanic moisture onto land across the coasts, serving as the primary water vapor source for the precipitation. This process is weakened by the anomalous regional overturning circulation in that the imbalance between low-level offshore and upper-level onshore flux drains the land of moisture, and the up-lifted moisture import aloft creates unfavorable condition for land precipitation.
Highlights
Comparison with Tropical Rainfall Measuring Mission (TRMM) observation illustrates that the Weather Research and Forecasting (WRF) simulations captured reasonably well the major patterns of average (Fig. 1a, c) and heavy precipitation (Fig. 1b, d) over China and the Northwest Pacific (NWP) in winter, in both cases the WRF model tends to overestimate, over an diagonal band covering mid-eastern China, the Bohai Sea and the Sea of Japan in the heavy rainfall category (Fig. 1f)
Note that the region of Southeastern China (SEC) collocate with region of complex topography, where topographic effect plays an important role in creating rainfall, heavy rainfall
A winter SEC precipitation difference is observed in a set of twin WRF simulations with observed and weakened frontal Sea surface temperature (SST) forcing
Summary
Li et al (2016) documented a negative relationship between the long-term trends in spring time Southern China rainfall and South China Sea and the Philippine Sea SST, and highlighted the importance of tropical SSTs and regional convective circulation. The warming trend over the Pacific warm pool region has been associated with a drying trend over South Asia in boreal summer (Annamalai et al 2013), while the trend of the tropical Indian ocean has been found to contribute to a winter wetting in mid-eastern China (Li et al 2015). Given that the western boundary current systems, including the Kuroshio, are hot spots in the global ocean where the fastest warming trends are observed (Wu et al 2012), it is important to examine the potential role of SST fronts in modulating regional rainfall
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