Equinox and Equilux

Bob McDavitt's ideas for sailing around the South Pacific. Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world. Compiled 20 September 2026 THE EQUINOX Our vernal (spring) equinox (Latin for equal night) is at 00:05UTC on Wed 23 September. That's 12:05pm in NZ . That's a special moment in the earth's orbit of our sun, and it fittingly picked by many to mark the start of the Southern hemisphere SPRING. Many in NZ, for practical reasons, such as climatologists who measures averages, etc. on a monthly cycle, take the MONTHS of September, October and November to be Spring, but astronomers, for logical reasons, use the four 'corners' of the earth's orbit as markers (equinoxes and solstices). 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 equal 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 between 19 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. The Delimiter is a square wave at the equinox At this time of the year the days in NZ are getting longer at the rate of around an hour per month. It is thus a good time to do a switch to Daylight saving, and indeed in NZ we SPRING our clocks ahead an hour at 2am next Sunday 27 Sep. This rate starts easing from now onwards, but the extra sunlight is now affecting the weather patterns. Our spin axis is NOT at a right angle to our motion around the sun: the 23 deg tilt influences our heating cycle. OK it nods the southern hemisphere closer to the sun than the northern hemisphere for the next six months - and we are now approaching the flatter side of our elliptical orbit for the next six months, but the MAIN FACTOR is to do with the concentration of sun beams over a smaller area (and, in the northern hemisphere, the spreading out over a wider area. December Solstice, with the southern hemisphere hogging the sunlight. 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. Sea surface temperatures. The warmest isotherms are coming south. 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 parts of the roaring 40s. This period, known as equinoctial gales, is likely to last until mid-November. A strong El Nino encourages these gales. TROPICS The Atlantic basin so far has not produced a hurricane this year. This is mainly due to stronger than normal upper winds ripping developing tropical systems apart. . Tropical Storm Dujuan formed near the Northern Mariana Islands and travelled north. . Tropical Storm Norbert passed over the open waters of the Pacific, followed by depression FIFTEEN and storm ODALYS. . Depression FAY travelled north in North Atlantic. WEATHER ZONES SPCZ is weak and mainly over Tuvalu and Tokelau . A new convergence zone is expected to develop this week between Vanuatu and Fiji There is a HIGH 1023hPa tonight in north Tasman Sea and a Low 985 L1 near Chatham Islands. Between these systems are strong southwest winds and rough seas. The High is expected to fade away and the low to travel off to the east allowing another low L2 to come in from the southwest and follow it. There is another HIGH H1 in the Aussie Bight that is expected to cross Tasmania on Tuesday and then travel NE across the Tasman Sea by Saturday and then fade away. Low L3 is a broad trough that is expected to come in from the southern Ocean and onto NZ on Sat 26 and Sun 27 Sept. There is expected to be strong to gale winds with the frontal zones in the system as isobars get squashed between L3 and H1. Resulting rough SW swells may reach Minerva om Mon /Tues 28/29 Sept. This seems to be a repeating pattern, with a burst of strong winds affecting northern NZ around the weekend. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> If you would like more details about your voyage, check metbob.com Or Facebook at /www.facebook.com/metbobnz/ Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom). Weathergram archive (with translator) is at weathergram.blogspot.co.nz. Contact is bob@metbob.com or text 64277762212. >>>>>>>>>>>>>>>>

Weathergram

Bob McDavitt's ideas for sailing around the South Pacific. Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world. Compiled 13 September 2026 The PDO and how it mitigates a strong El Nino. Ask the salmon. they know about climate cycles. Salmon are cold water fish. When the sea is cold (<14C) they grow well, and when the sea is warm they get underweight. Also, they are also anadromous, meaning they spawn in fresh water In NW America people depending on salmon have over the year observed several patterns in the good years and the bad years. (Sadly, salmon populations there have plummeted this century). The strong El Nino in 1957-58 triggered a symposium that linked a bad-salmon year to the weather and sea-surface temperature. Then years later Bjerknes published the first description on ENSO. El Nino favours a warmer and drier winter in the Pacific NW along with warmer ocean near the coast, so is bad news for their salmon with coastal SE winds. La Nina, with a NW coastal wind, is usually good news for their salmon. Cooler and wetter spring/winters are best for Californian Salmon. But there was NOT a good correlation between ENSO and Californian rainfall. When Steven Hare was investigating the good-year/bad-year patterns in the 1990s he came across a paper plotting pink salmon catches in the Gulf of Alaska with air and sea temperature measurements from 1925 to 1985. The three timeseries rose and fell together - high until the mid-40s, then low to mid 70s, then high again. This hinted to another climate pattern sometimes mitigating and sometimes reinforcing the ENSO. Some warm or cold phases of this pattern are lasting 20 to 30 years. Steven joined forces with others (Mantua and Zhang) and coined the term Pacific Decadal Oscillation or PDO in a paper that appeared in 1997. Now in September2026 the current El Nino is strong and getting stronger. However, we are having a NEGATIVE PDO. This is mitigating the resulting weather around California. However, the Walker circulation is clearly located now over the central Pacific, produced many low-pressure centres near the equator. The MetService Streamline even had a Low directly on the equator in a recent streamline analysis: TROPICS . Hurricane Lowell caused extensive damage and power failures across the western Hawaiian Islands. . Hurricanes Marie and Karina passed well to the west of Baja California. . Japan was drenched by Tropical Storm Krovanh. WEATHER ZONES SPCZ is active from Solomons to northern Fiji/Tonga. High H1 in Tasman Sea is moving to the NE and decaying. The isobars over Coral Sea and near Fiji have been squeezed between this high and the low pressures over the equator to north of Fiji= a windy start to Musket Cove regatta week. However, winds there are expected to weaken by Friday. L1 is the broad and deep trough east of NZ which left NZ last Friday. On its northern side is an area of light winds making its way eastwards across the tropics. There is a convergence zone in this area with a line of showers/drizzle crossing the southern Cooks next few days. Low L2 is a broad trough that is expected to cross southern NZ on Wednesday, central NZ on Thursday and then deepen into a Low over northern NZ on Friday. An accompanying zone of strong SW wind and swell NZ is expected to travel north as far as Fiji on Saturday. Uncomfortable High H2 is expected to travel NE from Aussie Bight to Brisbane area by end of the week and then might stall in the mid Tasman Sea after that. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> If you would like more details about your voyage, check metbob.com Or Facebook at /www.facebook.com/metbobnz/ Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom). Weathergram archive (with translator) is at weathergram.blogspot.co.nz. Contact is bob@metbob.com or text 64277762212. >>>>>>>>>>>>>>>>

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Bob McDavitt's ideas for sailing around the South Pacific. Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world. Compiled 06 September 2026 A review of last month's weather Here is a link to a YouTube clip giving an animated loop of the isobars and streamlines For last month is at youtu.be/yc2pJnX6RVU Most of the month brought anticyclones along 30 to 40south but passing troughs with lows crossed New Zealand on 9, 14-15 and 28-29 August The fascinating feature of the month was the number of lows forming near the equator, a facet of this strengthening El Nino. In an El Nino the Walker Circulation over the Pacific equator shifts so that its rising pillar gets close to 180deg/ Kirimati. In an El Nino the Walker Circulation over the Pacific equator shifts so that its rising pillar gets close to 180deg/ Kiribati. Sea Surface temperature anomalies from psl.noaa.gov/map/clim/sst.shtml El Nino 's warmth in the eastern equatorial pacific continues to intensify , and that cooler patch on its southwest side over Southwest pacific is bringing dry conditions . A time/longitude plot shows anomalies now getting over +4 degrees C This is record breaking. Average isobars for past month The subtropical ridge has weakened in north Pacific. In the south Pacific it has resumed between New Zealand and South America and weakened over Australia. Pressure anomalies for past two months From www.psl.noaa.gov/map/images/fnl/slp_30b.fnl.html The anomaly pressure pattern for last month shows well above normal pressures between Queensland and northern New Zealand, surrounded by a moat of lower than normal pressures. The 1020 isobar has shifted north across Tasmania and extended eastwards along 30S to east of New Zealand. TROPICS Tropical cyclone activity increased sharply during the week, with Tropical Storm Edouard lashing the U.S. Gulf Coast with strong winds and dumping nearly 2 feet of rain on portions of eastern Texas. . Former Typhoon Saidel triggered evacuations and transport disruptions in China's Zhejiang province with tropical storm force. . Several other tropical cyclones spun up over the open waters of the Atlantic, and the eastern, central and western Pacific Ocean Hawaii is now under threat from both LOWELL and KARINA. WEATHER ZONES High H1 south of Tahiti at30S is moving off to the southeast. Low L1 and L2 belong to a trough crossing New Zealand on Monday then moving off to the east A zone of light winds on the north side of this system is travelling eastwards in tandem along 20S, fading the trade winds. Another low from the Southern Ocean and associated trough is expected to cross southern New Zealand on Thursday and Friday. Avoid. High H2 in Aussie bight by Wednesday is expected to travel into central Tasman Sea by end of the week. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> If you would like more details about your voyage, check metbob.com Or Facebook at /www.facebook.com/metbobnz/ Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom). Weathergram archive (with translator) is at weathergram.blogspot.co.nz. Contact is bob@metbob.com or text 64277762212. >>>>>>>>>>>>>>>>