Abstract
There were no TCs generated in July 2020 over the western North Pacific (WNP), which was the first time this had happened during since 1980. This study attempts to understand the cause of there having been no TCs generated in July 2020, and evaluates the prediction skill for the large-scale environmental conditions associated with the TC genesis number (TCGN). Results show that the main causes were the abnormal warming of sea surface temperature (SST) in the North Indian Ocean (NIO) and North Atlantic in July and the abnormal decrease in SST from April and May in the Niño4 region. The NIO SST can affect the large-scale environmental conditions via the SST–precipitation–wind feedback mechanism. Through the interaction between the tropical North Atlantic and the NIO, the abnormally warm North Atlantic SST further strengthened the impact of the NIO SST on the environmental conditions. The monthly difference (MD) of the Niño4 index from April to May is significantly correlated with the TCGN in July. Not only can the Niño4 MD in May affect the environmental conditions by affecting the WNP anticyclone, but it can also affect the NIO SST and precipitation anomalies through a shift in the position of the Walker circulation. Besides, the activity of the MJO also had a certain impact on the absence of TC genesis in July 2020. Although CFSv2 can successfully predict the local feedback affecting the July TCGN, it fails to forecast the large-scale environmental conditions associated with the absence of TC genesis over the WNP in July 2020.
Highlights
The western North Pacific (WNP), including the South China Sea (SCS), is the most active tropical cyclone (TC) basin on the globe (Chan, 2005)
The CFSv2 model can successfully predict the local feedback affecting the July TC genesis number (TCGN), it fails to forecast the large-scale environmental conditions associated with the absence of TC genesis over the WNP in July 2020
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author
Summary
The western North Pacific (WNP), including the South China Sea (SCS), is the most active tropical cyclone (TC) basin on the globe (Chan, 2005). Warm NIO SST anomalies in July can lead to positive SLP anomalies, anomalous downward motion, negative relative vorticity at 850 hPa, and positive VWS in the Maritime Continent, which suppress the July TC genesis over the WNP via the local feedback between the SST, precipitation and low-level zonal wind over the NIO. The results of EXP-July-NIO demonstrate that warmer NIO SSTs induce locally enhanced positive VWS anomalies over the Maritime Continent and decreases the 850 hPa-relative vorticity and SLP over the WNP, which are unfavorable to the WNP TC genesis in July (figure not shown) Both ENSO and tropical Indian Ocean SST anomalies play important roles in modulating the TC activity over the WNP (Du et al, 2011; Zhan et al, 2011; Tao et al, 2012). The CFSv2 model underestimated the intensity of the largest amplitude regional SST/precipitation/winds in the past 41 years, which led to the failure to predict the environmental conditions associated with the absence of TC genesis over the WNP in July 2020
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