Bobgram 16 Feb

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 16 February2025

Over 50 years of MJO

Fifty years ago, I started forecasting at Wellington Airport. In 1976 I started a 3-year period of forecasting at Nadi Airport and in 1979 I returned there another stint. During that summer in my spare time, I did some (unpublished) research investigating the extent and persistence of the South Pacific convergence zone SPCZ by compiling daily a cloudiness measure from the past 10 days of satellite imagery (thanks to then Director of Fiji Met Service Ram Krishna).


The story is that when we analysed the tropical weather maps in Fiji we would write the 24 hour pressure change next to each observation. Red for falling and blue for rising. We even had special red/blue pencils for this. I noticed that once a month or so we would go thru a period with a pulse of falling pressure travelling east across our map, followed by a pulse of blue, then a few weeks of relative quiet. In my stint starting 1979 I designed my cloudiness experiment to check the impact of this pulse on the SPCZ. You can see an animation of these maps here https://www.youtube.com/watch?v=dyXXHMA0_ro Very poor quality and jerky animation, and it didn't seem to show much, so I never bothered publishing anything. I wasn't aware of it at that time, but Madden and Julian had already published THEIR paper on this phenomenon

"Detection of a 40-50 day oscillation in the zonal wind in the Tropical Pacific."
https://journals.ametsoc.org/view/journals/atsc/28/5/1520-0469_1971_028_0702_doadoi_2_0_co_2.xml

Back in the early 70s, Roland A. Madden and Paul R. Julian were working for NCAR in Boulder Colorado, studying the QBO (Quasi -biennial Oscillation) n almost two-yearly switch of the zonal wind sin the stratosphere, and its impact on cyclone formation (one year more have increased risk, then be followed by a year with less risk). They wanted to find equatorial waves, so they examined 10 years of radiosonde (balloon flight data) from Canto Island. And found this 40-50day wave, now called the MJO.


The MJO is a pulse of extra convection that goes eastwards around the tropics, mainly noticeable from Indian Ocean, across the Indonesian to "top of Australia" region and the across the pacific. It is easy enough to track and thus worth following for it is indeed a forecasting tool.

In the following 50 years this mechanism has been studied by many, bringing with it many new terms to meteorology such mesoscale convective systems (MCSs), and convectively coupled equatorial waves (CCEWs). Papers have been written about how it triggers cyclones, extreme weather (even outside the tropics), and the onset of monsoons.

To use this forecasting tool in your own cruise planning, I recommend a trip to

https://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/foregfs.shtml
and to help you read the imagery, remember my mnemonic
BLUE is BUBBLY .................. YELLOW is MELLOW

The forecast shows an initial position and date, and then the trend for the following fortnight. Try to avoid the bubbly blue areas.

The website also shows recent data as a time /Longitude plot centred on the 180 meridian.


This graph shows the cyclic nature of the MJO and how it mainly manifests just from near 0 to near 180, and each pulse seems to have its own pace and intensity.

Anyway, today's data shows that the active phase of the MJO that has been affecting North Australia, and the South Pacific convergence zone is now slowly exiting, stage right. However, the activity near Tonga by midweek might deepen rapidly near southern Cooks late this week. Avoid.

This means that over the next few weeks of February there should be a period of relatively quiet weather with the SPCZ.

It also means that the next MJO active pulse is likely to move across the north end of Australia early in March and then into the Pacific around mid-March. Avoid.

TROPICS
Category-5 Cyclone Zelia took aim on a coastal stretch of northwestern Australia, halting mining and port operations in the remote region. * Cyclones Vince and Taliah lost force over the central Indian Ocean.

WEATHER ZONES

Rain accumulation this week from Windy.com shows an intense SPCZ with heavy rain especially from Rotuma to the Niua (N Tonga) to Niue to Southern Cooks associated with passing lows.
The wind accumulation shows wind zones associated with lows L2, L3 and L4.
Light winds this week for Vanuatu, after all that thundery weather there last week.

LOWS and HIGHS
The series of tropical lows L1, L2 and L3 are all expected to be steered slowly to south or southeast. L3 might grow to be a tropical cyclone affecting Southern Coos area late this week. Take note and check.

There is a Low approaching North Island on Monday from the north, and another deepening in Tasman Sea by Tuesday. These should cross NZ next few days and combine as L4 to south of South Island by late Wednesday, followed by a front and a southerly change Thu/Fri.

High H1 is expected to travel from Southern Bight to central Tasman Sea on Friday 21 Feb. Then this High H1 is expected to cross NZ on Sat/Sun 22/23 February.

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