Why does Hebrew New Year begins in spring?
The answer is simple. 1st Month of the Torah year is a reminder to us about our Exodus from Mizraim, therefore, God specifically made it first, because it is the first month in our story.
Alternatively, as you know, most cereal grass begin to flower in spring and the first bread happens also in spring. Therefore, we celebrate beginning of the New Year in spring.
Usually, if the first bread is plentiful it would mean plentiful fruit harvest, therefore, once again, we celebrate New Year in spring.
There is also a parallel with Genesis. God created herbs first, then trees. Since herbs could not grow in the middle of winter, we have no choice but to admit that herbs and trees were created in spring time (when the Sun gets closer to Earth).
And of'course there is a very explicit statement of Exodus 12:2
YLT Ex 12:2 'This month is to you the chief of months--it is the first to you of the months of the year;
This is how we know that our 1st month, Abib, or new year (rosh chodashim) happens in spring, during flowering stage of the herbs, when cereal grass is ready for harvest, when the day is 12 hours or more.