Warm Climate Merlot
Merlot grapes are fleshy and plump so in hot/warm climates the flesh inside holds a lot of sugar and flavour while the skins develop and ripen quickly keeping tannin low (tannin is in the skin and seeds of grapes, the thicker the skin, the more tannin).
Sugar turns to alcohol during the fermentation process so grapes have to be harvested before they have too much sugar or the wine will be "hot" (a term used to describe a lot of alcohol in wine).
Wines from warm climates tend to have a higher alcohol content that cool climate Merlots.
As the grapes develop relatively quickly, tannin is minimalized allowing the skins to have fruitier flavours.
In a Nutshell - Warm Climate Merlots are often bottled as 100% merlot, have loads of fruit flavour, less tannin and are what wine-geeks call silky and generous.
What's not to like about that?
Cool Climate Merlot
By contrast cool climate Merlots take longer to ripen allowing the skins to thicken and produce more tannin.
Sugar levels are lower so alcohol levels are lower and sometimes a longer fermentation process.
Because the grapes spend longer times on the vines they pick up other flavours from the surrounding environment (a term called "terroir") so along with their fruity flavours there are often nuances of mineral, licorice, pepper and tobacco.
To make cool climate more drinkable it is often blended with other varietals, most commonly Cabernet Sauvignon, Cabernet Franc, Petit Verdot and Malbec (making up the classic "Bordeaux Blend")
In a Nutshell - Cool Climate Merlots are often more structured (meaning there are many different layers of flavour and texture to work through) with tougher tannins and can take time to become silky and smooth.
Also these can be difficult to drink for those looking for "fruity and fleshy" as they rarely are.
Given time though and a cool climate Merlot is incredible and an unrivaled treat.