The atmospheric memory that feeds billions of people: Monsoon rainfall mechanism

It's standard terminology. The point is that the threshold is different depending on whether you are coming from above or below. See here:

https://en.m.wikipedia.org/wiki/Hysteresis#Rate-independent

Which, at its core, is the point of science. There are plenty of things we know are real phenomena, that have important impacts, that we can't actually describe mechanistically. The entire idea of science is to be able to do that. And doing that well can be very hard.

Which is IMO why "science literacy" is so hard.

In an analogy: you can very easily point to chimps and humans and gorillas and say "those are similar, it's self evident" but it took a good few millennia for humans to be able to describe the hows and whys of the similarities in detail. Mechanistically.

As Niels Bohr's said: "Physics is not about how the world is, but about what we can say about the world,..."

Key sentence from the abstract: "The atmosphere’s fast mixing time scales were thought to inhibit the necessary memory effect for such multistability."

In other words, this is an important but fairly technical result.