Spring rollercoaster slowly winds down

By Dan Corbett

A very active system brought strong to severe gales and rain to much of New Zealand during the second week of October.

Spring has certainly lived up to the name, as the active period of transition between the winter and summer. A broad upper level trough and some very active westerlies in the upper levels of the atmosphere were the main driving force of the often unsettled weather.

With some good upper level dynamics in place during recent weeks, conditions were ideal for some active lows to develop.

A very active system brought strong to severe gales and rain to much of New Zealand during the second week of October. A low developed in the Tasman Sea, then rapidly deepened as it moved toward the west coast of the South Island on October 12.

The northerlies ahead of the low reached severe gale at times and a second area of strong south-westerlies followed behind a new low which developed just east of Canterbury later in the day. The pressure in the new low dropped below 976mb at its peak.

The weather system brought strong to severe gales from the west coast of Northland to the east coast of Southland.

Manakau Heads, to the west of Auckland, had gusts of more than 80 knots (140km/h) during the middle of the day and severe gales prevented passengers from reboarding a docked cruise ship at Akaroa.

The strong spring westerlies eased for a time and the end of October brought more settled weather.

With fine weather and lighter winds, afternoon highs climbed into the middle 20s across the Far North and interior parts of the South Island.

Heading toward summer

The ocean

The Pacific Ocean plays a major part in influencing our weather patterns and the critical area of sea-surface temperatures to watch is along the equator, between the date line, 180 degrees longitude, and Central America. When this area is cooler than normal, the corresponding pattern is called La Nina. This gives strong trade winds and encourages the high-pressure regions near New Zealand to be further south than normal. The opposite pattern is an El Nino - it has weak trade winds, and causes the westerlies of the "roaring 40s" to shift northward over New Zealand.

Last summer was a La Nina, but for the past few months the sea surface temperatures in this critical area have ranged between normal and just above normal. This is a neutral/borderline weak El Nino.

The consensus for the next several months is we should remain in this scenario. When the Pacific has a neutral influence on our weather pattern, we can expect our weather to become more varied with wider extremes of wind, rain and temperature. This also allows other factors to come into play to influence our day-to-day weather.

The atmosphere

Atmospheric weather patterns have not responded much to the El Nino direction. The Southern Oscillation index, which gives a snapshot of the weather patterns between Tahiti and Darwin, has been oscillating and remains close to zero. This shows the atmosphere has still not picked up on the ocean's weak El Nino signal. The fluctuations may continue during the next couple of mouths as we ease through this borderline neutral/weak El Nino state.

When it comes to seasonal outlooks, it is enticing to try comparing the coming season with a close analogy from the recent past. Do not be guided by what happened during the early summer of 2011 because it went on to become a La Nina summer.

In 2009, we had a weak El Nino developing so that might be a rough guide but bear in mind we may stay in a neutral state and not all El Ninos behave in exactly the same way.

On the border; neutral/weak El NinoThe current borderline neutral El Nino episode seems to have helped lows deepen around the Chatham Islands, with highs lingering in the Tasman Sea. This pattern may apply at times during the next few months. Overall, air temperatures may be about to just above normal and rainfall is expected to be near normal, except perhaps just above in some northern areas.

Anticyclones are likely to build in the Tasmania area and then fade away as they move northeastward across the Tasman Sea and over Northland, bringing periods of settled weather. The anticyclones are expected to become more noticeable during this month and bring extended periods of dry weather, especially in the north.

Troughs and fronts are likely to move on to New Zealand from the south, with lows forming in the troughs and deepening as they move off to east.

There may also be periods of enhanced westerlies across New Zealand. Watch for these episodes as they may be accompanied by boisterous and thundery fronts.

Passing lows in the Southern Ocean may bring brief bursts of northwesterlies to New Zealand.

Watch for any periods of blocking which can cause the weather patterns to get stuck in place.

The incoming cyclone season for the tropical South Pacific is considered about normal. There is a chance a cyclone may move from the tropics toward northern New Zealand, most likely from January to March.

- Hamilton News

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