Promoting seasonal prediction capability of the early autumn tropical cyclone formation frequency over the western North Pacific: effect of Arctic sea ice

Abstract

Seasonal prediction of tropical cyclone (TC) activity has been a hot research theme in the past decades. Usually, the tropical sea surface temperatures (SSTs) provide considerable predictability sources for the western North Pacific (WNP) TC activity. Here, we emphasized that the Chukchi-Beaufort (C-B) and Greenland (GL) sea ice variability is closely linked to the year-to-year variations of the early autumn WNP TC formation frequency (TCF). Observational and numerical evidence proved that the excessive C-B and GL sea ice sustains from August to the following early autumn and triggers the southeastward propagation of the Rossby wave trains originating from the Arctic across Western Eurasia (Okhotsk Sea) to the WNP. The resultant anomalous low pressure over WNP provides suitable environmental conditions for TC formation―the enhancement of the lower-level relative vorticity and water moisture, and the decrease of vertical wind shear. For the reduced sea ice, an opposite situation tends to emerge. The persistent combined sea ice signal makes it a physically meaningful precursor for TCF prediction. The cross-validated hindcast and independent forecast based on both the tropical SST and the Arctic sea ice precursors present that the TCF index is predicted with much higher correlation coefficients than those of the empirical models with only the tropical SST predictors. The results demonstrate that the Arctic sea ice truly promotes the seasonal prediction capability of the WNP TCF.