Compiled Sun 21 January 2018
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.
The state of the ENSO = La Nina now relaxing
El Nino and La Nina are opposite ends of an identifiable tropical influence on our seasonal weather: the ‘La Nina’ is related to cooler than normal seas along the equatorial eastern pacific. It shifts the subtropical ridge away from the equator. The ‘El Nino’, with its warmer than normal seas, draws the subtropical ridge closer to the equator. Their comings and goings can last several months, maybe over a year, and so their status can be used to forecast the weather for the coming season.
The main parameter we watch from the atmosphere is the Southern Oscillation Index SOI (30 day running mean) as it sums up the whole weather pattern over the South Pacific in one number. It is based on the normalized difference in the barometer readings between Tahiti and Darwin, in other words the placement of isobars on the weather map. When the SOI is more than plus one (standard deviation from its mean) for more than a month we call it a LA NINA event, and when it stays more than minus one we call it an EL NINO event.
In late 2017 there was a surge of the SOI to over plus one and into LA NINA territory in October and November. But during December and January it has relaxed and is now very near normal. Maybe this means that the extremes weather events we have seen in the past few months will now also relax.
A relaxing LA NINA is seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi&p=weekly
(Note that in this graph on the vertical axis 10= 1 standard deviation)
NINO3.4 is a region in the eastern equatorial Pacific Ocean that acts as a heat storage area during an El Nino, or becomes cooler than normal during a La Nina. This plays with the heat budget of the atmosphere and thus with the weather patterns.
At the farmonline web site we can see the trend in the sea surface temperature in the NINO3.4 area. The diagram shows the weekly temperature anomalies since Jan 2015, with the El Nino of 2015 looking like a hump on a camel. Since then there has been a cool period late 2016/early2017, and then a warm period around mid-2017. We are now in a cool period again. And in the past few months it has been as much as 1 deg C below normal—a LA NINA, still at work in the ocean.
A Continuing La Nina in the ocean as seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=nino34&p=monthly
Looking at the sea temperature anomalies along a transect of the equatorial Pacific shows that there is a plume of warmer sea at depth in the western Pacific travelling eastwards and warming the cooler surface waters of the eastern Pacific from below. This transect may be seen at www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/figure04.gif
The International Research Institute of the Climate Prediction Centre compiles data from several ENSO prediction models. The early January combo probability forecast issued early January shows that the likelihood of La Nina (red bars in the graph) is expected to drop below 50% by MAM = March April May, and the outlook is for a period of neutral influence from the tropics (similar to background climatology as given by the grey line). See
CPC/IRI predictions from iri.columbia.edu/our-expertise/climate/forecasts/enso/current/
Latest SST anomaly map shows a weakening in the extent and intensity of the cooler eddies along the equator. Note the warmer than normal Gulf stream of NE America, north of 35S (but cooler than normal further south). In the NZ region the warmer than normal central Tasman Sea is a potential source of energy that may help activate mid-latitude depressions over the next month of two. Sea surface temperatures for 18 Jan may be seen at www.ospo.noaa.gov/Products/ocean/sst/anomaly/index.html
TC BERGUITTA is fading away near 30S 50E to south of Madagascar. There are a few tropical depressions around but nothing more serious at this stage.
There is an MJO period of expected extra convection that is expected to travel across northern Australia this week and into the western pacific in early February. This may trigger some tropical developments near Cocos Islands rom Wednesday, and in the heat trough of northern Australia by the end of this week, and perhaps in the Coral Sea next week. Associated with this MGO thee is a zone of near equatorial westerly winds already in place as seen on windyty.com ...keep an eye of these winds as they can easily help trigger tropical depressions.
Looking at the weekly rain maps from last week at trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif, and the week before, we can see that the main convective rain over the past week has been in the Indian Ocean and stretching to Northern Australia and around Indonesia. This cluster of extra convection is expected to make its way eastwards and start appearing in the Coral Sea area around early February.
SPCZ=South Pacific Convergence zone.
The SPCZ is hovering from Coral Sea to northern Vanuatu to Samoa then to Southern Cooks, and is of increasing intensity this week, There may be a tropical low developing in western Coral Sea early next week.
Subtropical ridge (STR)
HIGH to east of North island is expected to continue further off to the east along 35 to 40 South.
Another HIGH is expected to form in central Tasman Sea by end of Tuesday, and then hover there until early next week.
Around Tasman Sea
The remains of a trough are expected to travel onto northern NZ next few days, and fade away, but they still have some maritime Polar airmass and some cooler-than-normal air aloft, so are still able to produce isolated downpours until Tuesday with light variable winds and reasonable high isobars. A typical passing summer trough.
From Tuesday, expect a generally northerly to northeast wind-flow along the Australian eastern seaboard this week, and light variable winds/or sea breezes around NZ.
“We may have all come on different ships, but we’re in the same boat now.”
Martin Luther King, Jr.
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