Combined effect of the solar activity and ENSO on the tropical cyclone genesis frequency in the southeastern part of the western North Pacific

This study analyzes the frequency characteristics of tropical cyclone (TC) genesis in the southeastern part of the western North Pacific (SEWNP) during June-November from 1965 to 2019 and investigates the possible combined effect of the solar activity and El Nino-Southern Oscillation (ENSO). Results suggest that TCs generated in the SEWNP have the longest lifetime and greatest strength, and its frequency has apparent interannual and decadal variations, which is jointly affected by the solar activity and ENSO. In El Nino years during declining phases of solar cycle (1-3 years following the solar maximum), positive TC genesis frequency anomalies in the SEWNP are significantly strong and tend to occur in extremes. While the opposite is true for La Nina during ascending phases of solar cycle (1-3 years following the solar minimum). However, there exists no significant feature in the combined effect of La Nina (El Nino) and declining (ascending) phases of solar cycle. When declining (ascending) phases of solar cycle and El Nino (La Nina) are combined, the overlapping effect leads to apparently warmer (colder) sea surface temperature (SST) anomalies in the central equatorial Pacific and colder (warmer) SST anomalies in the western Pacific, so the SST anomalies gradient are stronger. It enhances low-level westerly (easterly) wind anomalies and upper-level easterly (westerly) wind anomalies, which is favorable for the further decrease (increase) of the vertical wind shear in the eastern (most) part of SEWNP. Moreover, the stronger and more westward low-level convergence (divergence) center appears in the Pacific, causing stronger low-level convergent (divergent) flow and upper-level divergent (convergent) flow anomalies, and strengthened (suppressed) ascending movement anomalies in the SEWNP. As a result, the TC genesis frequency in the SEWNP is much higher (lower). Further diagnoses show that absolute vorticity plays a leading role in El Nino years during declining phases of solar cycle, and its contribution is obviously much greater than other environmental factors. It is not perfectly symmetrical in La Nina years during ascending phases of solar cycle when the contribution of absolute vorticity is the greatest, but vertical wind shear is also important.

Automated summary

This study analyzes the frequency characteristics of tropical cyclone (TC) genesis in the southeastern part of the western North Pacific (SEWNP) from 1965 to 2019 and investigates the combined effect of solar activity and El Nino-Southern Oscillation (ENSO). Results show that TCs generated in the SEWNP have long lifetimes and high strength, and their frequency is influenced by solar activity and ENSO. El Nino years during declining solar cycle phases exhibit significantly strong TC genesis frequency anomalies in the SEWNP, while the opposite is true for La Nina during ascending solar cycle phases. However, there is no significant feature in the combined effect of La Nina/El Nino and declining/ascending solar cycle phases.