Now let's look at the air masses and circulation systems that produce New Zealand's weather. The first thing to note is that in the Southern Hemisphere, cyclones (low pressure systems) flow clockwise, and anticyclones (high pressure systems) flow counter-clockwise.
Wind Map:
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We're not in Kansas anymore! This map of surface winds shows mid-latitude cyclonic patterns in the Southern Ocean and the Tasman Sea on April 5, 2015. The Coriolis effect deflects airflow significantly to the left in the latitudes of the Westerlies (~30-60 degrees South) where the polar and tropical air masses meet, producing the cyclonic patterns shown. Weather can get severe this time of year, due to the contrast between the lingering high ocean temperatures and the potential for Polar air masses to "break out" of their wave pattern as the sun leaves the Antarctic.Arrows added in Illustrator to a screen capture from http://earth.nullschool.net/#current/wind/surface/level/orthographic=172.44,-52.75,270 |
Upper Level Circulation:
Upper-level airflow in the Southern Ocean. I won't go into the physics of the zonal and meridional wind components depicted in color here (because I don't understand it). Basically, the undulations roughly correspond to the waves of the Antarctic Jet. And isn't it neat to see what Earth would look like as a lava lamp? Science is so awesome.
Video created by Matt Owens for Fairfax Climate Watch, from
http://www.fairfaxclimatewatch.com/blog/2013/11/the-rise-and-fall-of-the-westerlies.html
So those are some of the main large-scale air circulation patterns. Now we'll look at how the air masses come into play. Unlike in the U.S. (and the Northern Hemisphere in general), there isn't much land down there to create large temperature differences, so New Zealand doesn't see our strong tornadoes and surface-heat-driven mesoscale convective complexes. However, it is in the latitudes between the subpolar low and the subtropical high, so it gets plenty of midlatitude cyclones and their associated thunderstorms.
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Another view of the Antarctic wave - blue are troughs and pink are ridges. The fourteen-year-old in me just can't resist calling attention to the title of this photo (top left, by the date). Those kinky Kiwis!!Image from MetService weather blog at http://blog.metservice.com/2011/07/a-winter-storm/ |
These maps from Earth.nullschool.net depict wind flow as a still image, making it easier to see the smaller patterns of convergence, divergence and mixing:
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This image of 500-millibar level winds shows the Antarctic Jet really well. There is an intensified low to the east of New Zealand at the boundary between cold and warm air masses - what they call a "polar outbreak". More about that below.Screen capture from http://earth.nullschool.net |
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Surface winds and temperatures. Lots of confrontation between polar and tropical air masses, in latitutes with high vorticity, promotes cyclonogenesis with support from the upper level Jet Stream.Screen capture from http://earth.nullschool.net |
New Zealand is affected primarily by mP and mT air masses throughout the year. The cold Antarctic air starts out as cAA, but it takes on enough heat and moisture crossing the ocean that it's mP air when it reaches New Zealand. For its part, the warm tropical air is just... warm tropical air, heading south to cool off. Thanks to some help from the Coriolis effect and the wave flow aloft, New Zealand gets the cold-type occluded fronts that supply Christchurch's winter moisture.
Some weather maps will show us how this plays out.
Surface Pressure, Sunday April 5 at Noon:
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Here we can see a broad high pressure system covering New Zealand, and lots of cyclonic lows and mini-fronts buzzing around the ocean to the south. The low at 1008 in the upper middle of the frame, just off the east coast of Australia, is going to deepen and dump a bunch of moisture on the Southern Alps in the next 48 hours, as the warm front gets occluded by the polar air and develops precipitation.New Zealand highlighted in Photoshop on a surface pressure chart from MetService New Zealand http://www.metservice.com/maps-radar/maps/tasman-sea-nz |
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24 hours later, the high has moved to the southeast with the zonal flow aloft. The pressure gradient has increased sharply, the low has deepened to 992 millibars, and it's now being occluded by the cold air continuing to flow up and around from the west with the Jet stream. There's a fair amount of moisture in this system, and it's piling up against the Southern Alps along the northwestern edge of the South Island. (Notice the pressure ridge that defines the mountain range and hints at the orographic effect.)New Zealand highlighted in Photoshop on a surface pressure chart by MetService New Zealand: http://www.metservice.com/maps-radar/maps/tasman-sea-nz |
Rain:
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This map has both the upper level winds and the surface pressures, so we can see the system in action. At noon on Monday, April 6, the low is off the west coast. Warmer colors depict areas of higher precipitation. The low is 988 millibars with heavy rain over the ocean. According to MetService's 30-day weather history, surface winds were hitting Queenstown, which is roughly equivalent to Evergreen in terms of aspect and elevation, from the northeast at 50km/hour, and brought only 1.2 mm of rain. Christchurch had similar wind speed and direction, and received no precipitation.Map from 5-Day Rain Forecast page at MetService New Zealand http://www.metservice.com/maps-radar/rain-forecast/rain-forecast-5-day |
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Now at 12 AM on April 7, the "head" of the system has moved around the southern tip of the South Island, and orographic precipitation is occurring along its "tail" on the western side of the Alps. The cold occluded front has developed what we will assume are cumulus and cumulonimbus clouds. According to Weather Underground's history for station 93800, the southwestern tip of the island got half an inch of rain. And according to MetService, Christchurch got zero, because the precipitation all got wrung out over the Alps just as it does over the Rockies before it reaches Denver.Map from 5-Day Rain Forecast page at MetService New Zealand http://www.metservice.com/maps-radar/rain-forecast/rain-forecast-5-day |
New Zealand also sees standard cold-front cloud formation and precipitation patterns, as well as Polar Outbreaks like we get in the US (only theirs are maritime, not continental).
Surface Pressure & Infrared:
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Surface pressure and fronts (top) and infrared (bottom) for 6 AM Sunday, April 12, which at the time of this writing, hasn't happened yet. Except it has, in New Zealand, which is at UTC +13. If we're at UTC -6, then they've already had nineteen hours of our tomorrow happen. OK, I'm confused now... Anyway, check out how well the pressure map and the cloud map line up! So illustrative. The low pressure system off the southeast coast is attached to a cold front that has some lovely cloud formation along its spine. But... this is not your normal, everyday fall cold front; this is a "Polar Blast" in the making! (See below.)New Zealand highlighted in Photoshop on maps from MetService New Zealand http://www.metservice.com/maps-radar/satellite/tasman-sea-nz-infrared |
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Oh Dear God! Can you believe this crap?? Actually, if you're anything like me, you're probably like "ummm okaaaaay, nice weather map." But no!! This is the beginning of a Serious. Polar. Blast. (AKA Polar Outbreak.) You know when American TV weatherpeople are all like "OMG a Polar Vortex is coming!!" like there's more than one lurking up there? The Kiwis are smarter than that; they just call it like it is (which doesn't make it any less exciting). A bunch of damn cold air is about to blast up from Antarctica and snow all over their lovely islands. I would feel sorry for them, but I'm all "sorried out" after sympathizing with my New England family and friends all winter.Image from MetService Weather Blog http://blog.metservice.com/2011/07/a-winter-storm/ |
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Look at all that COLD mPk air just waltzing up from the Antarctic. It's getting warmed from below enough to create instability, and bringing some unseasonably low temperatures, snow showers, and high unruly seas to the region. The worst swells are predicted directly under the low.Image from http://www.metservice.com/maps-radar/rain-forecast/rain-forecast-5-day |
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Wow, that's a serious blob of cold air pushing its way north. Low temperatures are blue and green, at the 500-millibar level, and windspeeds at the 250-millibar level are denoted with numbers too small for us to see or worry about.Image from http://blog.metservice.com/2015/04/winter-is-coming-for-a-bit-updated-friday-10-april-2015/ |
Occasionally, extra-tropical cyclones develop into hurricanes that affect the region, especially in the late summer and early fall. The most recent was Hurricane Pam, a category 5 cyclone, which devastated the South Pacific islands of Vanuatu with a central pressure of 896 millibars and 250 km/h winds, but largely tracked northeast of New Zealand as detailed in MetService New Zealand's summary.
That's all I've got to say about how New Zealand's air masses and the South Island's mountains affect Christchurch's weather. Let's leave them to enjoy their winter prequel.
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Lose something, guys?Image from http://designingsound.org/2010/09/david-farmer-special-the-lord-of-the-rings-exclusive-interview/ |
