Welcome to another season of Daylight saving.

Our vernal (spring) equinox (Latin for equal night) was at lunchtime on Wed 23 September.  That’s a special moment in the earth’s orbit, and it fittingly picked by many to mark the start of the Southern hemisphere SPRING. Daylight saving kicks in the following Sunday.

The best definition of the equinox is when the sun, as seen from earth, is directly overhead the equator.

As viewed from earth the sun is directly overhead a different latitude every day of the year. The difference between this latitude and the equator is called the sun’s declination, as all navigators know already.

Although equinox comes from Latin for qual night, it is only a 12 hour day if you measure sunrise and sunset using the MID-POINT of the sun. Sunrise and sunset tables actually use the top limb of the sun so that, plus atmospheric refraction, adds an extra 5 to 8 minutes of (rather dull) sunlight. The name given to the day with 12 hours exactly sunlight/darkness is the equilux – in New Zealand that’s between19 and 20 Sept.

From now on the days will lengthen. But the rate is not linear—Auckland’s dawn is getting earlier at the rate of 10 minutes per week at present, but dusk is only getting later at the rate of 6 minutes per week. At the equinox the delimiter (line on the earth dividing day and night) is shaped like a square wave and during the next few months or so it slowly reshapes into a sine wave. You can watch this by downloading the program Home Planet from http://www.fourmilab.ch/homeplanet/ and watching this in the animate mode.

There is a lag between the change in increasing sunlight and the spot with the warmest temperatures on earth. The lag in the ocean is a few weeks more than in the land. Currently the warmest SST band in the open part of the Pacific is at around 8 deg N.

Note that the 28C isotherm in the South Pacific already extends as far as between Samoa and Fiji. It is extending southwards at a steady rate. This is the time of the year that the subtropical ridge—that zone which divides the trade winds from the roaring 40s – starts to drift southwards. As it drifts south, the isobars on its southern side may bunch closer together in places, and that increases the disturbed westerly winds in the  roaring 40s. This period, known as equinoctial gales, is likely to last until mid-November. A strong El Nino encourages these gales.    

Bob McDavitt

YES the oceans are breaking records!

This diagram, sourced from https://climatereanalyzer.org/clim/sst_daily/, provides a time-series view of daily mean sea-surface temperature (SST) using NOAA’s Optimum Interpolation SST (OISST) version 2.1 dataset. It offers a useful snapshot of how ocean temperatures are changing over time and helps place present conditions in a longer-term context.

OISST is a global, gridded dataset with a spatial resolution of 0.25° by 0.25°. Its temperature estimates blend observations from satellites, ships and ocean buoys, helping to provide consistent coverage across areas where direct measurements may be sparse. The dataset begins on 1 September 1981 and extends to the present, usually with a delay of one or two days. The most recent two weeks of data are preliminary until NOAA posts the finalised product. Daily values should therefore be treated as estimates, and any apparent record high or low in OISST should be checked against other datasets before firm conclusions are drawn.
The diagram indicates that the average temperature of the world’s oceans is now about 1°C above the long-term normal and roughly half a degree warmer than in 2023, the previous recent El Niño year. This matters because oceans store enormous amounts of heat and release it gradually into the atmosphere. Warmer seas can influence rainfall, wind patterns, humidity and the energy available to developing weather systems. El Niño is not the only factor affecting global ocean warmth, but it can reinforce unusual temperature patterns across the tropical Pacific and alter atmospheric circulation well beyond the tropics.

For New Zealand, the spring outlook suggests a period of variable and sometimes vigorous weather. A strong El Niño often favours more frequent westerly or southwesterly airflows. Several active fronts may cross the country, followed by gusty west-to-southwest winds and bursts of heavy southerly swell. These conditions can create challenging periods for coastal communities, marine activities, transport and exposed farming areas. Rainfall may be higher than normal in the west and southwest, where moist air is lifted over the ranges, while the north and east may experience drier-than-normal conditions because they sit in the lee of the main mountain chains.

However, El Niño does not determine the weather on every day or in every location. Individual storms, local sea temperatures and shorter-term climate patterns can still produce significant departures from the seasonal outlook. A drier regional forecast, for example, does not rule out isolated heavy rain, just as a wetter outlook does not mean continuous rainfall. Communities should therefore use the outlook as a guide to increased likelihood rather than as a fixed prediction.

The practical message is that unusually warm oceans deserve close attention. Farmers may need to plan for uneven rainfall, water demand and wind exposure; coastal users should monitor swell forecasts; and households should stay alert to rapidly changing conditions. The Climate Reanalyzer graphic is valuable because it makes the broad trend visible, but it is best interpreted alongside official forecasts and other independent climate datasets. Taken together, these sources point to a spring shaped by elevated ocean heat, a strong El Niño influence and a greater chance of sharp contrasts between wetter western areas and drier northern and eastern districts.

Basically, because the ocean is breaking temperature records, we are moving into some weather patterns which we have not seen before, and so should take extra care to monitor the daily weather forecasts before sailing this spring.

Bob McDavitt