As pointed, מִשְׁבָּר (billows or waves) and גַּל (waves, heap) in Jonah 2:3, they seem to have the same meaning according to the lexicon and also their usage. Is it indicating an emphasis or is there a difference?
The Hebrew expression at issue is וגליך משבריך.
The conceptual root of the first term is ŠBR, the second is GLL.
The concept behind these roots?
The first has the meaning of ‘to break in pieces’, and – happily – also in English we found the term ‘breaker’ to define an ‘heavy ocean wave’, or, more literally, ‘a wave that breaks itself against some objects (often, destroying them)’ > ‘a breaking wave’.
Interestingly, the Italian homologous term is ‘frangente’, which come from the Latin frango > fractum, ‘to break in pieces’ > ‘broken’ (compare the derived English terms ‘frangible’, ‘frangibility’, from the same Latin root).
Differently, the second conceptual root at issue, GLL, has a different meaning, instead pointing out the sense of ‘to be voluted’, ‘to roll’. ‘Volute’ is from Latin voluta, ‘a spiral scroll’, noun use of feminine past participle of volvere, ‘to turn around, roll’, from (supposed) Proto-Indo-European root wel- ‘to turn, revolve’.
So, the focus of this other Hebrew term is on the sea wave ‘voluted’ form.
In synthesis, the two terms – although refer both to sea waves – point to some different aspects of them.
When a man feels himself near the death, under similar to Jonah’s circumstances, the rush of adrenaline fixes in his memory every details belonging to the impending menace (please remember how the prophet did mention even the details of what in Hebrew was called SUP, that is a kind of sea-weeds [Jon 2:5]).
I cannot end this comment of mine without mention the beautiful faith-building expressions of Jonah toward God: “I went down to the land whose bars closed upon me forever; yet you brought up my life from the pit.” (Jon 2:6)
For all of people suffering several sicknesses – Covid-19 included - God will encourage all that are trusting in Him.