I highly recommend previous posts on this subject, mainly:
/u/qed1, who shows that the ancient and medieval science already theorized that seasons would be inverted without much evidence, and
/u/drylaw, who shows the work of mid-sixteenth century writer-scientist observing and explaining the phenomena
I myself have tried to show how did the realization of the inverted seasons evolve during European Age of Discovery in my post Did the first European explorers understand that the Southern Hemisphere experienced reversed seasons and why? What did they think about the so called Torrid zone?
I don't think it is really possible to answer who was the first to realize it and when, but as /u/qed1 demonstrate it was certainly deduced theoretically to be the case by the ancient Greeks and Romans
Thanks to u/terminus-trantor for the shoutout! I actually revised my earlier answer over here after reading more on the topic, and further added to it below to hopefully make it clearer.
I can think of one example of a Spanish writer mentioning differences in climate regarding South America. José de Acosta was a Jesuit who spent 15 years traveling in South and Middle America (esp. Peru). Upon his return to Spain he wrote his Natural and Moral History of the Indies published in 1590, often described as very „advanced“ for its time in its description of natural phenomena (including geophysics) and containing elements of proto-ethnology. I'm not sure if Acosta was the „first“ writer to give quite detailed explanations for the reversal of the seasons, but he was certainly influential, with his „Historia“ being among the most widely-read works on the Americas at that time.
In addition Acosta had himself experienced many of the climatic phenomena he describes this is quite a departure from the earliest Spanish chroniclers like for example Peter Martyr, who had never been to the Americas. I should add that the Jesuits had by that time in the 16th c. built an early global network of information exchange, that was centered on Rome but included reports from Jesuit missions from the Americas, increasingly Asia and other regions.
Among the possible explanations José Acosta gives for the reversal of the seasons are: The strong rains ( connected to the sun), the shortness of the day (to explain the surprinslgly moderate temperatures in many parts), as well as other meterological explanations focusing on the ocean and winds that would cool the temperature. Overall we see here a rather "proto-scientific" mentality at work, focusing on experiences and concrete examples, as well as comparing with similar phenomena in Europe and Africa.
It's important to note again though, that as a Jesuit and someone living in the 16th c., it's not surprising that Acosta intersperses his accounts with mentions of how this climate shows the workings of God or a divine spirit. And while he directly challenges many ideas of classical philosophers incl. Aristotle, he also ackowledges that they are still right on other points.
I'll now give some longer but hopefully interesting quotes from Acosta's Historia natural giving the most important of his explanations, starting with the influence of the rains - Books II and III of the Historia focus on meteorological observations. (note that I'm taking from an early 17th c. English translation because I unfortunately didn't find a newer one online):
Considering with my selfe often times what should cause the Equinoctial to be so moist, as I have said, to refute the opinion of the Ancients, I finde no other reason but the great force of the sunne in those partes, whereby it drawes unto it a great aboundance of vapors from out of the Ocean, which in those parts is very great and spatious; and having drawne unto it this great aboundance of vapours, doth suddenly dissolve them into raine, and it approoved by many tryed experiences, that the raine and great stormes from heaven proceed from the violent heat of the Sunne; [....] but when the Sun retyres, the heat is moderate, and then there falls no raine; whereby we may conclude that the force and heat of the Sunne is the cause of raine in those countries. [...] [T]he great draught which the Ancients held to be in the middle region, which they call the burning Zone, is nothing at all; but, contrariwise, there is great humiditie, and then it raines most when the sunne is nearest.
Note how the last sentence directly contradicts ancient Greek views of the „torrid zone“ as an inhabitable place – Acosta himself had experienced parts of South America as very habitable. In this chapter Acosta notes the moderate temperatures in most parts of South America, despite them lying under the equinox. Here they are connected again with rain but also the shortness of the days:
This presupposed, if any one demaund of me, why under the Equinoctiall Line the heat is not so violent in summer as in some other Regions (as in Andalusia in the moneths of Iuly and August), I will aswere, that in Andalusia the dayes are longer and the nights shorter; and as the day being hot, inflames and causeth heat, so the nights being cold and moist, give a refreshing. [...] And although the burning Zone be nearer the Sunne then all other Regions, yet doth not the heate continue there so long. [...] He therefore that shal put these two properties of the Zone in one ballance, that it is most rainie in the time of greatest heate, and that the dayes are shortest there, he shall perchance finde them to equall the other two contrarieties, which bee, that the Sunne is neerer and more directly over them then in other Regions.
Further reasons for differences in temperature between parts of "Old" and New World" that lie on a similiar altitude include for Acosta:
The first is the Ocean, the second the situation of the land, and the third, the nature and propertie of many and sundry windes. [...] I have first placed the Sea, for without doubt, the neerenesse thereof doth helpe to temper and coole the heat; and commonly Countries lying neere the sea are cooler then those that are farther off. Caeteris paribus, as I have said, even so the greatest part of the new world, lying very neere the Ocean, wee may with reason say, although it bee under the burning Zone, yet doth it receive a great benefite from the sea to temper the heat.
He then goes into further discussions of the incluence of cold winds and the nature of the land to explain the temperate climate in parts of South America. As noted, there's an overall clear tendency of focusing on experience to explain such weather and climate phenomena, all the while praising God for these workings.
Another interesting part comes in the chapter titled „Of Aristotle’s Opinion of the New World and What It Was that Caused Him to Deny It“ where Acosta discusses differences between the phenoma he describes with classic ancient authors in more detail, esp. with Aristotle who was reverred as „the Philosopher“ in Europe for centuries. One focus is the already-mentioned torrid zone:
In addition to the reasons I have mentioned there was another that moved the ancients to believe that it was impossible for men to pass from there to this New World; and they said that in addition to the immensity of the Ocean the heat of the region that they call torrid, or burnt, was so extreme that it would not allow men—no matter how daring—to cross it either by land or sea, from one pole to the other.
For even those philosophers who affirmed that the world was round, as indeed it is, and that there was habitable land near the two poles, denied in spite of this that human beings could live in the middle region that includes the two tropics, which is the greatest of the five zones or regions into which the cosmographers and astronomers divided the world. The reason they gave as to why this Torrid Zone was uninhabitable was the burning heat of the sun, which is always so close overhead and scorches that whole region and hence causes it to lack water and vegetation. Aristotle was of this opinion, and, though a great philosopher, he was mistaken in this...
Other disagreements with Aristotle include Acosta's analysis of the trade winds. Acosta also discussed the variety of climate in tropical regions which varied according to its location near the coast, or in the highlands. According to him, different climates are found in the same latitude because of the proximity of the ocean, the influence of rains and winds, and the properties of the land. So we get a disagreement with traditional authorities like Aristotle, who had described these regions as uninhabitable following European climate patterns. Acosta's views can be seen as adding new perspectives while still holding to the geocentric theory – seeing how Copernican views were not yet widely accepted at that point in time.
As mentioned Acosta was a member of the Jesuits, a religious order that would continue to focus strongly on scientific methods, and also use European scientific developments to aid in conversion campaigns, in order to showcase the Christian god's supremacy (e.g. in China or Korea). Last but not least, I should note that Acosta's Historia Natural (published in the late 16th c.) circulated widely and proved to be a major source for other authors on the Americas throughout the 17th century at least.
I'll close this part with the last title of part two of his Historia, which sums up many of his arguments quite nicely:
Ch. XIV. - That they which inhabite under the Equinoctiall, live a sweete and pleasant life.