The Roaring Forties and New Zealand's weather
The near-permanent westerly winds that put the wind at New Zealand's back - and the fronts on its doorstep.
The Roaring Forties are the powerful, near-permanent westerly winds that blow between roughly 40 and 50 degrees south latitude. New Zealand lies squarely inside this belt, which is why the country's prevailing wind is from the west and why weather systems march in relentlessly from the Tasman Sea. The winds owe their strength to a Southern Hemisphere that is almost free of land to slow them down.
What are the Roaring Forties?
The name dates from the age of sail, when mariners described the strong and steady westerlies of the "forties" latitudes as roaring. They are not a storm or a single event but a persistent circulation feature: a broad band of west-to-east wind that circles the globe in the Southern Hemisphere's mid-latitudes. Most of New Zealand - from Auckland at about 37 degrees south down to Stewart Island near 47 degrees - sits within or on the edge of this belt, so the westerlies are the country's default weather driver.
Why are they so strong in the Southern Hemisphere?
Westerlies exist in both hemispheres, but they are far stronger and steadier in the south. The reason is simple geography: between 40 and 60 degrees south there is almost no land - just the open Southern Ocean encircling Antarctica. In the Northern Hemisphere, continents like Asia and North America disrupt the flow and create big north-south meanders. With little land to create friction or block the flow, Southern Hemisphere westerlies build to a sustained strength that has no real northern equivalent. The temperature contrast between the warm subtropics and frozen Antarctica keeps the pressure gradient - and therefore the wind - powering along.
Wind ultimately flows from high pressure towards low pressure, and in the Southern Ocean that gradient runs almost unbroken all the way around the globe. Because the flow is not forced to detour around mountain ranges or heat up over deserts, it stays organised and fast. This is why a yacht in the Southern Ocean, or a weather buoy off the bottom of the South Island, can record gale-force westerlies for days on end while the equivalent latitude in the north is far more variable.
Forties, Fifties, Sixties: the latitude ladder
Sailors gave each band of intensifying westerlies its own nickname as they pushed further south:
| Belt | Latitude | Character |
|---|---|---|
| Roaring Forties | 40-50 S | Strong, steady westerlies; New Zealand sits here |
| Furious Fifties | 50-60 S | Stronger and stormier; south of the mainland |
| Screaming Sixties | 60-70 S | Fiercest of all; approaching Antarctica |
New Zealand's main islands catch the Roaring Forties, while its subantarctic islands and the Southern Ocean shipping and sailing routes reach into the Furious Fifties.
How the Roaring Forties shape New Zealand's climate
Because the prevailing wind is westerly, the western side of the country bears the brunt of incoming moisture. The West Coast of the South Island and the western North Island are markedly wetter than the east. The westerlies also deliver the steady procession of fronts and depressions that make the weather so changeable: as each system rides the westerly flow across the Tasman, New Zealand experiences the familiar cycle of a north-westerly airstream, a front, and a cooler south-westerly change.
The strength of the westerlies varies with large-scale climate patterns. NIWA tracks drivers such as the Southern Annular Mode, which describes whether the belt of westerlies is contracted towards Antarctica or expanded northward over New Zealand - a shift that can tip whole seasons towards wetter-and-windier or calmer-and-drier.
The nor'wester and the rain shadow
The Roaring Forties are the engine behind one of New Zealand's signature weather experiences. When the westerly flow strengthens ahead of a front, it becomes a moist north-westerly that piles rain onto the West Coast, then descends the eastern flank of the Southern Alps as the hot, dry, blustery nor'wester. The same mechanism carves out the eastern rain shadow, keeping Canterbury and Central Otago far drier than the drenched West Coast just over the divide.
History: sailing and the clipper route
The Roaring Forties were the highway of the great sailing era. Clipper ships bound from Europe to Australia and New Zealand rode these westerlies eastward across the Southern Ocean to make fast passages, then continued east around the globe rather than beating back against the wind. Those same winds that sped the clippers along are the winds that still set the rhythm of New Zealand's weather today. Ports such as Lyttelton and Bluff were shaped by this trade, and the westerly swell the Roaring Forties generate still rolls onto the country's western and southern coasts, driving the surf and the long-period swells that reach even sheltered harbours.
Reading westerly weather with model comparison
The relentless westerly flow means the key forecasting question in New Zealand is usually about timing: when will the next front arrive, and how strong will the nor'wester be ahead of it? Global models can differ by several hours on exactly that. checkweather lines up ECMWF, GFS and ICON and converts their level of agreement into a confidence score, grading an accuracy scoreboard against what actually happened - so when a front is racing in on the Roaring Forties, you can see at a glance whether the models are locked in on the timing or still hedging.