Professional Skipper - Free Sample issue (July/Aug 2011)

WHAT'S AHEAD – WEATHERWISE A BLOCKING HIGH AFFECTS OUR WEATHER BY BOB MCDAVITT The surface weather map across Australia/New Zealand/South Pacific for April 27 I region of high pressure moved over southern New Zealand, and for the following two weeks this high stalled out to our east over the South Pacific Ocean. As an anticyclone intensifies, its isobars spread outwards and thus bunch closer together around the rim. This enhances the wind speeds of fronts on its western flank. This helps to explain the extremely strong down-slope winds experienced in New Zealand on April 26, as shown on the map. Converging surface winds managed to focus the rain in Hawke's Bay into a zone between Cape Kidnappers and Porangahau. Trade winds on the northern side of a large high are also enhanced. The Fiji Meteorological Service maintained a marine gale warning on the trade winds about the southern Cook Islands for the week starting May 4. This episode is a good example of a blocking anticyclone which extends vertically so the normal westerly airflow aloft is redirected around the system. Winds aloft act as a steering field, and this normally helps anticyclones in our part of the world to migrate eastwards across our weather maps. When these are disrupted the surface high stalls, and the high is then called a "blocking high". Blocking is a reasonably regular feature in the open sea east of New Zealand and can sometimes modify the seasonal weather patterns, as in this case. Troughs and lows moving onto New Zealand from the Tasman Sea encountered this block near the 180 degree longitude and thus stalled and dumped abnormal amounts of rain. Some of the troughs had enhanced instability, such as the one that produced severe thunderstorms over Auckland on May 3. One of these spawned the Albany tornado. The counter-clockwise flow around this particular anticyclone fed a flow from the northeast into the New Zealand region for a few weeks. As this air moved over warmer seas north of New Zealand it loaded up with water vapour, and this fuel helped form cloud, wind and rain in regions of lowering pressure, such as the troughs and fronts over New Zealand and the Tasman Sea. On May 13, a disturbed sou'westerly flow followed a cold 54 Professional Skipper July/August 2011 r The surface weather map across Australia/New Zealand/South Pacific for May 8 front across New Zealand and restored westerly winds aloft, so the blocking high was finally knocked away. This brought a noticeable change in New Zealand's weather pattern, with an outbreak of strong nor'west then sou'west winds that lasted for almost a week. Huge swells formed in the Tasman Sea and washed onto Fiji's southern coast by May 20. Now for the winter outlook. Evaporation takes place faster over a warmer sea, and water vapour is conveyed by global winds from the equator to the pole, fueling the weather. This weather engine is driven by the temperatures where the sea is hottest, as is found along the equatorial Pacific. The sea temperatures there were below normal over our summer, giving us La Nina conditions. However, they are now returning to normal and a transition to neutral (neither La Nina nor El Nino) conditions is taking place in the ocean. The impact of a La Nina or El Nino pattern on our daily weather maps is assessed by a parameter called the Southern Oscillation Index, or SOI. During April the SOI remained highly positive, indicating La Nina weather patterns are still forming and circulating around. In particular, anticyclones have recently been more intense than normal to south of Tahiti. After that blocking high of early May, the SOI has quickly dropped to nearly zero, and this indicates La Nina-like features are now fading from our weather maps. We are now seeing a swing from La Nina to neutral. A neutral balance between La Nina and El Nino may well result in extra variability in our weather, with only a weak controlling influence on our weather patterns from the Pacific Ocean. Over the next few months, the Tasman Sea is expected to become a breeding ground for low-pressure systems. It is here that moist winds from the subtropics mix with cooling winds from the south, resulting sometimes in large depressions with several days of wind and rain. These troughs or lows may be accompanied by several days of enhanced nor'east or northerly winds. Disturbed cool west to sou'west winds from the Southern Ocean are expected to burst across New Zealand at times, bringing a noticeable polar outbreak and squally showers.

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