This storm had been in the making for 14 months after a double La Nina - the 2nd declared 6 mths after the first one finished. Wasn’t sure that had happened before but now that i’ve got the internet back on i see it has. El Ninos are one-off events but La Ninas can regenerate within the same cycle. (1998-2001 was the last multiple La Nina - no drama in Aus - you still need that alignment - though there were big floods in Taipei.)

BoM: La Niña occurs when equatorial trade winds become stronger, changing ocean surface currents and drawing cooler deep water up from below. This results in a cooling of the central and eastern tropical Pacific Ocean (while) piling up warm surface waters in the western Pacific and to the north of Australia. (causing increased evaporation causing increased rain)

The trade winds are the ones that blow through Taiwan - the notorious Dong Bei Ji Fong (NE autumn wind).

This is the fascinating thing about weather - the result is rain (East Aus) - but the cause, aetiologically, is wind in Asia.

So what causes wind?

Nat Geo: Wind is the movement of air caused by the uneven heating of the Earth by the sun. .. Warm equatorial air rises higher into the atmosphere and migrates toward the poles. This is a low-pressure system. At the same time, cooler, denser air moves over Earth’s surface toward the Equator to replace the heated air. This is a high-pressure system. The boundary between these two areas is called a front. The complex relationships between fronts cause different types of wind and weather patterns.

This is a trick of nature. The generating formula is simple - an interaction between roughly circular Highs and Lows which rotate in opposite directions - couldn’t be more basic - but when they brush up against each other - that’s where the magic happens. The weather in the last few decades does seem increasingly erratic - is this collision of hot and cold becoming a self-escalating chain reaction?

Recap: Uneven distributions of hot and cold caused big trade winds - twice - which caused ocean warming north of Australia which produced massive moisture, which fed a low pressure system which was blocked by a stationary high to the south.

As a result of this double La Nina, the ground was saturated before the first drop fell - in fighting parlance - you've softened your opponent up - now for the season-defining knock-out punch.

Edit: Read this morning that the La Nina, which had supposedly peaked and was receding, has now 'stalled'. Hope we're not in a for a third wave. There have been no more torrential downpours but it has continued to rain - almost every day.