A quasi-oscillatory semidecadal cycle in Northern Hemisphere seasonal networks

The relationships among tropical and extratropical climate modes in different seasons and the accumulative influences are not fully understood. After removing time-varying climatological averages, this study identified a semidecadal cycle in two tropical climate modes, namely the Indian Ocean Dipole (IOD) and El Niño-Southern Oscillation (ENSO). A regression of the nonseasonal convective activity of the Indo-Pacific warm pool on seasonal sea surface temperature gradients associated with the autumntime IOD and subsequent wintertime ENSO, indicated that the spring Arctic Oscillation (AO) and Asian-Pacific summer monsoons have a strong relationship before and 1 year after convection peaks. The seasonal Network on Decadal Echo (NODE) was noted to exhibit quasi-oscillatory characteristics, particularly during negative Atlantic Multidecadal Oscillation (AMO) phases. A semidecadal NODE is associated with a strong North Pacific center of action within the AO during a negative AMO phase. By contrast, during a positive AMO phase, the AO's influence on the NODE cycle weakens or disappears. During NODE cycles, a stronger monsoon Hadley cell following a negative spring AO is associated with a weaker oceanic trough in the western North Pacific (WNP) and suppressed tropical convection, favoring a negative IOD in autumn and a La Niña winter. Conversely, a positive AO leads to a deeper WNP trough and a weaker monsoon Hadley cell in summer, resulting in a positive IOD and an El Niño winter. The AO-monsoon Hadley cell-IOD and AO-WNP trough-ENSO relationships identified in this study provide process insights into mechanisms governing seasonal networks. Furthermore, multidecadal state-dependent oscillatory NODE cycles shed light on potential for hemispheric accumulation.