Now that the 2015 Weather Almanac for NZ has been released to bookshops we can précis what next year's weather may bring. We can also make comparisons with this year. Firstly, 2015 sees a return of closer lunar perigees, with the moon averagely closer to earth for the first time in 4 years, and to be not as close again until 2020. This is a factor that will cause increased tropical cyclone activity.
Due to the tilt of the earth the moon moves monthly north and south of the equator creating its declination cycle, and 2015 happens to be the year of minimum declination. It means that in September 2015 the moon only reaches 18deg latitude north and south of the equator every 27.3-days, its shortest monthly transit range in 18.61 years. In years of maximum declination (2006 and 1987) the monthly range was 28 degrees north and south.
Minimum declination creates a slower moon speed relative to the earth which brings changes to weather patterns. During minimum years the monthly moon does not reach the latitudes of the tropics. Weather events are slower to develop and to leave, and cold spells and summery days hang on for relatively longer periods. This can bring higher rainfall and more days with recordable rain than in years of higher declination, or longer dry or windy spells, according to the season.
Previous minimum declination years have been 1997, 1978/9, and 1959/60, all years that like 2015 saw winter perigees accompanying full moons. These were years of milder winter temperatures. 1979 saw a mild wet winter and closer perigees in late winter, much like 2015. The 2014/15 summer may be cooler than average but dry weather in the South Island may stretch longer into March. We can expect warmer than average temperatures between March and June for the North Island, and between March and August for the South Island.
We predicted there would be no widespread drought to start this year nor after summer, despite the national climate office saying otherwise and calling for an El Nino-driven drought. But only during the past week has the SOI index jumped from neutral to positive. This needs to be exceed 0.4C for at least 5 months to be called El Nino, which will, if it persists, take us well past this coming summer.
This December will be drier than average for the South Island, although wetter for the North Island, with the third week the driest for the whole country. January rain may be disappointing for campers and visiting tourists.
February brings a fortnight of clear weather for all. There will be enough widespread dry conditions to again bring talk of drought but this dryness is not likely to persist. We may see an El Nino but only after summer has well gone. Hopes of continuing dry weather will be dashed by the formation of rain-bearing tropical cyclones in February.
Tropical cyclones affect us in the last 10 days of both February and March. The highest tides of the year occur between February and April and will bring many extreme events, including storms, floods and chances of seismic activity. As well as brief heat waves at the start and end of February and start of March, we can also expect stormy conditions in the last part of March.
This year it was drier than average for parts of Northland for the first 4 or 5 months of the year, and most remaining areas were relieved by the anticipated April downpours. In 2015, Northland fares well in January with above average rain amounts, but February is worryingly drier. March for Northland is again wetter, and April and May drier, so any drought fears will not be founded. In fact the alternating wet and dry months together with seasonal warmth will bring welcome pasture growth for the colder months.
Showers just before Waitangi Day in 2015 for all districts should ease around the day itself. Because it is near full moon there may be also be unusually warm daytime temperatures and associated fire risks. Expect wet weather also in the lead up to Easter weekend, with Good Friday seeing the last of lingering showers, leaving most of the long weekend skies relatively clear except for Hawkes Bay and Canterbury which may stay unsettled.
June 2014 saw an intense tropical depression, wintry polar blasts and good snow base for the opening of ski fields. In 2015 early snow in April and May will again prematurely excite snow bunnies, but the dryness of May will allow snow to disappear. Ruapehu saw no skiing until the third week of July in 2014 and will be similar in 2015. Despite winter being cooler than average for the North Island, significant precipitation causing ski-able snow may have to wait until mid season.
Christchurch was affected by flooding and cold temperatures this year, and the North Island winter was milder than average compared to South island cold. In 2015 it will be warmer than average for most of the country from March to June and it may be the turn of the South Island to experience a comparatively mild winter. Then follows a coolish spring with a lack of good sunshine hours. Gales and rain arrive in the last week of September which may damage fruit.
Overall, 2015 will be a drier than average year than 2014, with El Nino in the news around mid year. Tropical cyclones do not normally occur during an El Nino period, and an active cyclone season will be in the news. Most monsoonal activity across the north of Australia will be between February and April.
Heightened earthquake risk times in the Pacific region are around the ends of January, May, July, September and November, and in the second week of April. Christchurch should not be affected as much as Central NZ and the shake-prone eastern regions further north.
Ken Ring of www.predictweather.com is the author of the Weather Almanac for NZ for 2015