Question

how are the limits of a climatic region fixed

Answer 1

because of the ANGLE at which sunrays fall on the surface of the Earth

Answer 2

Some define the climate of a region in terms of its average weather over a large number of years the mean values of temperature, cloudiness, rainfall and so forth. This notion of climate is not adequate because the variability of weather is another essential aspect of the climate of a region
The average global temperature declined from about 1940 to 1975. From 1975 it started increasing. Global warming alarmists wanted to construe all of the increase in average global temperature as being due to the increase in CO2 in the atmosphere. The problem was that about the year 2000 the average global stopped increasing and remained steady for more than a decade, despite the fact that the CO2 content of the atmosphere continued to increase. The recent increases in AGT are due to ENSO rather than the increase in CO2. Some alarmists looked for ways to revise the procedure for estimate the average temperature of the Earth's surface. With 70 percent of the Earth's surface covered with oceans the global average is sensitive to the limited temperature readings of ships, there is a lot of room for revisions that would show an increase in global temperature since 2000. But the readings of atmospheric temperature from satellites do not show such an increase.
Ironically if the global warming alarmists accept the Pacific Multidecadal Oscillation then they would have an explanation for the absence of global temperature increases since 2000 and furthermore a stronger justification for the case they are trying to make. That is that the effect of increased CO2 has increased in recent years and if it had not the average global temperature would have started declining after about 2005. Thus by being dishonest about the Pacific Multidecadal Oscillation they ended up weakening their case.

i hope its hlp you