Abstract
The 2018 boreal summer in the Western North Pacific (WNP) is highlighted by 17 tropical cyclones (TC)—the highest record during the reported reliable years of TC observations. We contribute to the existing knowledge pool on this extreme TC frequency record by showing that the simultaneous highest recorded intensity of the WNP summer monsoon prompted the eastward extension of the monsoon trough and enhancement of tropical convective activities, which are both favorable for TC development. Such changes in the WNP summer monsoon environment led to the extreme TC frequency record during the 2018 boreal summer. Meanwhile, the highest record in TC frequency and the intensity of the WNP summer monsoon are both attributed with the combined increase in the anomalous westerlies originating from the cold tropical Indian Ocean sea surface temperature (SST) anomalies drawn towards the convective heat source that is associated with the warm central Pacific SST anomalies. Our results provide additional insights in characterizing above normal tropical cyclone and summer monsoon activities in the WNP in understanding seasonal predictable horizons in the WNP, and in support of disaster risk and impact reduction.
Highlights
Questions were expectedly raised on what caused such unprecedented increased tropical cyclones (TC) frequency in the Western North Pacific (WNP) it is not surprising that a number of reports have already contributed to the existing knowledge p ool[1,2,3,4]
The time-longitude diagram of 5-day running mean filtered daily 850 hPa zonal wind and outgoing longwave radiation (OLR) anomalies averaged from 10°S-10°N in JJA 2018 indicate enhanced anomalous westerlies and anomalous deep tropical convection throughout the WNP (Supplementary Fig. 3a,b)
The Madden–Julian Oscillation (MJO) is an intraseasonal tropical mode characterized by an eastward-propagating system of tropical convective activities that travels through its different phases; it is nearest to the maritime continent and WNP during its Phases 5–6
Summary
Questions were expectedly raised on what caused such unprecedented increased TC frequency in the WNP it is not surprising that a number of reports have already contributed to the existing knowledge p ool[1,2,3,4]. The third paper selected June–November[3] while the most recent report divided the WNP TC frequency into two seasons: JJA and September–October[4] These choices of regional scaling and seasonal clustering do not totally account for the variability that is inherent to each basin and season. The WNP TC frequency in September and September–November 2018 are both below normal and are not their respective season’s extreme records as well Such differences are largely due to the inherent seasonal changes in the WNP environment, which include the decay of the WNP summer monsoon, onset of the WNP boreal winter monsoon, and seasonal reversal related to the phase changes of the El Niño–Southern Oscillation (ENSO)[5]. Previous reports) to the existing knowledge pool in understanding the large-scale drivers behind above normal TC frequency and activity in the WNP
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