Question:
I have noticed a pattern in Dublin weather in the last 4 years in which a cold snap arrives in mid and late November but then late December is very very mild. What might cause this?
Answer:
Ireland’s weather is influenced by a number of large-scale atmospheric and oceanic patterns—particularly the jet stream and the North Atlantic Oscillation (NAO)—along with high-pressure “blocking” systems.
While a four-year pattern on its own is too short to confirm a true climate trend (30 years minimum of data are considered climate), there are a few reasons why Dublin (and Ireland in general) might experience cold snaps in November followed by milder spells around late December:
1. Position of the Jet Stream
Early Winter Cold in November
- During mid-to-late autumn (around November), the jet stream can shift southward or buckle, allowing cold Arctic air to flow toward Ireland and the UK. –> often leads to short-lived cold snaps with frosty mornings and even occasional snowfall.
Late December Milder Flows
- By late December the jet stream may lift northward or flatten out, allowing milder Atlantic air to sweep across Ireland from the southwest. This tends to bring wetter but milder conditions.
2. Influence of the North Atlantic Oscillation (NAO)
NAO Basics:
- The NAO is a large-scale pattern that measures the relative pressure difference between the Azores High and the Icelandic Low.
Negative NAO Phase (Colder Air Invasion):
- A negative NAO phase often corresponds to a weaker westerly (winds FROM the west) airflow and/or a “blocked” pattern, allowing cold Arctic air to sink farther south.
- This scenario can bring chilly weather to northwest Europe in early winter.
Positive NAO Phase (Milder Atlantic Air):
- A positive NAO phase ramps up strong westerlies off the Atlantic, ushering in milder, wetter conditions.
- If the NAO transitions from negative to positive around December, Ireland sees a shift from cool, high-pressure conditions to milder temperatures and rain.
While the consistency over four years is interesting, it fits within the natural variability of Ireland’s highly changeable maritime climate.
