La Niña weather events: how do they impact Tiritiri Matangi?

NIWA, the National Institute of Water and Atmospheric Research, forecast the La Niña weather event for the 2022/3 season almost a year ago, making four in a row for us here in the south-west Pacific region. However, no one predicted the destructive strength of the cyclones that were to hit earlier this year. So what is La Niña and what effect does it have on our wildlife on Tiritiri Matangi? John Sibley explains.

Broadly speaking, in a La Niña year, our weather systems here in the North Island tend to arrive predominantly from a northeasterly direction, bringing warm, moist weather. Conversely, the south-western end of the South Island experiences cooler, drier conditions on average.

La Niña weather events are generated far away in the eastern Pacific region, when easterly equatorial trade winds move warm surface water across to the western Pacific, causing an upwelling of cold water off South America. The warm water pushed ahead of it generally brings rain-laden north-easterlies down to Aotearoa/ New Zealand.

In this article, the term “weather” refers to short-term atmospheric conditions, whereas “climate” is the weather of a specific region averaged over a long period of time, 30 years or more. “Climate change” refers to the average of long-term changes.

Last winter, NIWA reported the trade winds had been up to 60% stronger than normal, which did not bode well for the coming summer season.

El Niño is essentially the opposite situation, where the equatorial easterly trade winds weaken and can even reverse direction, resulting in cooler sea temperatures here in the western Pacific region. Broadly speaking, the westerlies prevail and cold southerlies bring cooler winters.

These two conditions can alternate in different years, giving the El Niño Southern Oscillation (ENSO). It is extremely likely that climate change has made them more extreme in recent years.

The words “broadly” and “generally” have been chosen carefully here, because La Niña events usually influence the overall weather in Aotearoa by less than 25%, the rest being down to the interaction of other factors, so any future outcome is particularly hard to predict with any precision.

An important indicator of climate change is the increased frequency of marine heat waves detected off the country in recent years, with sea surface temperatures averaging 2°C to 3°C above normal. These affect the distribution of plankton and small bait fish, which sustain the seven species of seabirds with breeding populations on Tiritiri Matangi. Kororā/little penguins and kuaka/ common diving petrels are a particular concern at this time.

The effect of warmer seas on global climate change is crucial. Ocean currents convey vast quantities of heat energy around the planet and for millennia have created stable temperate climates in higher latitudes that would otherwise experience Arctic conditions.

This astonishing ability of water to absorb and store heat energy is one of the reasons why conditions on our planet are stable enough to support life. Water can store over 4500 times more heat energy than an equivalent volume of air, which is why it has such a powerful influence on world climate.

Although the heat energy carried by ocean currents brings benefits to some, the warmer they get, the more energy they impart to fuel extreme weather events, which are becoming much more frequent and damaging. Worryingly, several key deep ocean currents transporting heat around the globe are now faltering and slowing down as climate tipping points are reached. quite noticeable extremes from month to month, with some very dry months followed by periods of heavy rain.

Tiritiri Matangi has always had a dryish climate by virtue of its position in the Gulf. The diagram at left shows how the prevailing westerly airstream over Aotearoa causes rainfall to diminish from west to east. Eastern Whangapāraoa and Tiritiri Matangi receive about 75% (or less) of the rain falling to the west. (Figures are in 100’s of mm per year).

The table below shows monthly rainfall figures on the Island over seven years. Colours indicate rainfall deficits or excesses relative to the MetService monthly rainfall data, averaged over a 30-year period for the Auckland region.

These wet and dry extremes over several years seem to have badly affected many mature trees on Tiritiri Matangi. For example, the mature tawa tree on the Kawerau Track is slowly dying, and many good-sized kōwhai, māhoe and karo have succumbed recently.

Above: Tiritiri Matangi weather data (modified). White: little difference (figures +/- 20% of Auckland regional average).

Orange: moderate drought (between 20-40% less than regional average).

Pink: severe drought (more than 40% less than regional average).

Green: moderate rainfall (between 20-40% more than regional average).

Blue: extreme rainfall (more than 40% more than regional average).

From these figures, 50% of the time there is either drought (14%), or extreme drought conditions (36%) prevailing on Tiritiri Matangi. For 19% of the year, there is either moderately high rainfall (5%), or extremely high rainfall (14%). Extreme rainfall is about three times more likely than steady moderate rainfall. For 31% of the year the Island has rainfall conditions comparable to the rest of the Auckland region.

The summers of 2019/20 were particularly dry on the Island and, although the total for 2021/22 was back on track, there were still

Māhoe, in particular, seems to be vulnerable to both extreme drought and excessively wet soil. The pikirangi/New Zealand mistletoe plants on the Wattle Track and on Graham’s Road have also died, and nearly all the ancient Psilotum or whisk ferns on the Island have disappeared in the past four years.

The 2019 and 2020 summers were drier than usual for La Niña years. The wet northeasters in 2020/21 missed us, but hit Eastern Australia instead, causing record flooding. The rainfall total in 2021 looked normal, but had several sharply fluctuating wet and dry monthly variations. The summer cyclone season in 2022/23 saw the arrival of two tropical cyclones, Hale and Gabrielle in January and February, between them giving a combined rainfall of just under half the average for the entire year.

The intense low-pressure systems associated with these cyclones caused the sea level to rise half a metre above normal high-tide levels. Plus, 130-140km/h winds created waves over 10m-high along the Northland coast. Later the wind swung round to the southwest, causing the Hobbs Beach Track to be badly eroded.

Ray Walter reported that the extremely destructive surge caused by tropical cyclone Gabrielle was higher than anything he has ever seen in all the years he has been on the Island. As climate change progresses we are more and more likely to see these “100-year events” happening with much greater frequency.

Climate change is not new, and over the millennia it has been the driver of many an ecological upheaval. Humans are also part of the ecological web of life on Earth and are just as vulnerable as the rest of the living world. Evidence suggests the past 5000 years or so of human history have been unusually stable climatically, compared to earlier pre-historic times. This has allowed human civilisations to rise and flourish. When cultures did decline and disappear, as in Central America and parts of the Middle East, the cause is thought to have been largely due to shifting rainfall patterns causing crop failure, famine and mass migrations of people.

What lessons can be applied to Tiritiri Matangi? Change is in the air, so to speak; if the plant species making up our distinctive coastal broadleaf forest biome become stressed, then the birds and other animals that live there will be affected too. As with any ecosystem, breeding success for many animals might falter. Over time the species composition of the ecosystem is therefore likely to subtly change as it adapts and morphs to reach a new equilibrium in the changed climatic conditions. Species that cannot adapt will move away or die and other species may take their place.

In the sea around Tiritiri Matangi the recent marine heat waves seem to be causing phytoplankton and zooplankton populations to change. Small bait fish that feed on this plankton are highly mobile and will follow their food supply to the cooler distant waters of the outer Gulf.

Kororā/little penguins are known to hunt within a 20km radius of the Island and, if their fish prey moves away, they will starve as we have seen recently in Northland regions where kororā in their hundreds have died of starvation.

After little penguins finish breeding, they need to put on weight again before beginning their annual moult, which requires them to stay ashore in burrows for around three weeks without feeding. This begins as early as January, and starvation is a real possibility if the birds cannot put on sufficient weight beforehand. Kuaka/diving petrels similarly rely on being able to catch their food within 20km of shore, in order to raise their chicks during the breeding season.

Recent sea surface temperatures measured off Tiritiri Matangi would seem to indicate the biggest temperature changes are happening in winter, when minimum temperatures are presently around 1.5-2°C higher now than 60 years ago. Present summer maximum temperatures seem to be relatively unchanged, and further monitoring is needed to confirm this trend.

The diagram (opposite page, top left) shows two populations of kuaka in the Gulf with their respective foraging ranges. In the outer Gulf they feed mainly on krill, which is abundant there. Krill is a shrimp-like oceanic crustacean, and is much less abundant in the coastal waters of the inner Gulf, so the diving petrels breeding on Tiritiri Matangi feed mainly on small bait fish, such as mohimohi/ pilchards, korowhāwhā/anchovies and hautere/jack mackerel. Likewise, the kororā on the Island also feed on these small fish and squid. If the fish leave the inner Gulf due to marine heat waves then these birds will be forced to move away or starve.

Left: Foraging effort of kuaka/ common diving petrel is relative to prey abundance. Above: Penilia – a key plankton species off Tiritiri Matangi. Below, right: A kākāriki chick that failed to thrive.

More than 30 plankton species have been sampled and analysed regularly off the wharf for over two years now. Findings indicate the main prey species that supports the bait fish population around Tiritiri Matangi is the crustacean Penilia avirostris.

Anecdotally, P. avirostris was much more abundant prior to 2017. At that time they occurred in numbers that would on occasions completely clog a plankton sampling net. Plankton was not systematically counted and recorded until late 2019, by which time the numbers of P. avirostris had dropped dramatically.

Encouragingly, the summer of 2022/23 delivered a significant improvement in the numbers of P. avirostris present (see spring/summer results on graph, below left), so perhaps we are looking at a short-term population fluctuation after all – time will tell. The abundance scale on the graphs is logarithmic, so a score of five represents 10,000 individuals per m3 of seawater and a six would be 100,000.

Other major food sources for small fish and birds around Tiritiri Matangi are collectively known as “jellies”. This group is composed of the many and varied cnidarian “jellyfish” species, plus the jelly-bodied salps (see graph).

P. avirostris is a seasonal species, as can be seen in the graph, dying back in winter, leaving resistant eggs which hatch in spring. Fish must find an alternative food source when numbers drop. Copepod crustaceans seldom reach these numbers, and cnidarians and salps seem to form the next most important stable food source for small bait fish during winter.

Like the P. avirostris, the 2022/23 statistics for the cnidarians/ salps also show a recent marked improvement (see graph for March 2023). It would also be a mistake to underestimate the resilience of the bird populations. “Boom and bust” trends are features of many wild animal populations, and past experience shows us the Island's birds are capable of bouncing back after periods of environmental stress.

Above all, we can see that Tiritiri Matangi is not just an island cut off from the rest of the world, but is part of a bigger marine and global environment which is presently undergoing rapid and profound changes.

This young kākāriki is a poignant reminder of the struggle that all of the birds on the

Island face – the role that climate change plays in their quest for food, which dominates their lives and dictates their breeding success. This young bird came so close to fledging, but died at the last moment, probably due to lack of food as the dry weather in January 2022 took its toll. Despite the recent destructive cyclones, the latest breeding season has seen modest improvements in breeding success for many bird species on Tiritiri Matangi, probably due to increased rainfall boosting food supply (see p. 11). Early MetService predictions are favouring an El Niño weather pattern emerging later in the year, which could cause a colder winter and a more south-westerly influence on the weather ahead. It remains to be seen what effect this has on the bird life on Tiritiri Matangi.

Supporters of Tiritiri Matangi

The Supporters of Tiritiri Matangi (SoTM) is a volunteer Incorporated Society that works closely with the Department of Conservation to make the most of the wonderful conservation restoration project that is Tiritiri Matangi. Every year volunteers put thousands of hours into the project and raise funds through donations, guiding and our island-based gift shop. If you’d like to share in this exciting project, membership is just $25 for a single adult, family or corporate; $30 if you are overseas; and $13 for children or students. Dawn Chorus, our magazine, is sent out to members every quarter. See www.tiritirimatangi. org.nz or contact PO Box 90-814 Victoria St West, Auckland.

SoTM Committee

Chairperson: Carl Hayson chairperson@tiritirimatangi.org.nz, 027 3397105

Secretary: Gloria Nash secretary@tiritirimatangi.org.nz

Treasurer: Peter Lee treasurer@tiritirimatangi.org.nz

Committee: Stacey Balich, Hester Cooper, Barry Fraser, Rachel Goddard, Val Lee, Jane Thompson, Ray Walter, Michael Watson, Janet Petricevich

Operations manager: Debbie Chapman opsmanager@tiritirimatangi.org.nz

Acting guiding and shop manager: Debbie Chapman guiding@tiritirimatangi.org.nz, 09 476 0010

Membership: Rose Coveny membership@tiritirimatangi.org.nz

Educator: Barbara Hughes

Assistant educator: Liz Maire educator@tiritirimatangi.org.nz

Fundraiser: Juliet Hawkeswood fundraiser@tiritirimatangi.org.nz

Social Media: Stacey Balich socialmedia@tiritirimatangi.org.nz

Dawn Chorus editor: Lyn Barnes editor@tiritirimatangi.org.nz, 021 407 820

Island ranger: Talia Hochwimmer tiritirimatangi@doc.govt.nz, 09 476 0920

Coming Events

King’s Birthday: Working Weekend Saturday 3 –Monday 5 June

Tiritiri Matangi Talk Monday 12 June (see p.5 for details)

“Secrets and surprises from our island ecosystems”

Guest speaker: David Towns

7.30pm Unitec, Carrington Rd

Matariki: Friday 14 July

Supporters’ Weekend: Saturday 15 July-Sunday 16 July

Supporters’ Weekend: Saturday 2 Sept-Sunday 3 Sept

Supporters’ AGM: Monday 18 Sept

Guides’ Day Out: Sunday 17 Sept and Saturday 23 Sept

For the latest information on events on the Island, visit the SoTM website www.tiritirimatangi.org.nz

Visiting Tiritiri Matangi Island for education or recreation

Day trips: Weather permitting, Explore runs a return ferry service Wednesday to Sunday from Auckland Viaduct and the Gulf Harbour Marina. Bookings essential. Phone 0800 397 567 or visit https://www.exploregroup.co.nz/tiritiri-matangi-island/tiritirimatangi-island-ferry/. Call 09 916 2241 after 7am on the day to confirm the vessel is running.

School and tertiary institution visits: The Tiritiri education programme covers from pre-school (3-4 year-olds), to Year 13 (17-18-year-olds), along with tertiary students. The focus in primary and secondary areas is on delivering the required Nature of Science and Living World objectives from the NZ Science Curriculum. At the senior biology level there are a number of NCEA Achievement Standards where support material and presentations are available. For senior students the Sustainability (EFS) Achievement Standards are available on the NZQA website. There is huge potential in that these standards relate directly to Tiritiri in various subject areas: science, economics, tourism, geography, religious education, marketing, health and physical education. The Island also provides a superb environment for creative writing, photography and art workshops.

Tertiary students have the opportunity to learn about the history of Tiritiri and tools of conservation as well as to familiarise themselves with population genetics, evolution and speciation. Groups wishing to visit should go to www.tiritirimatangi.org.nz/schoolvisits.htm or contact schoolbooking@tiritirimatangi.org.nz. Bookings are essential.

Overnight visits: Camping is not permitted and there is limited bunkhouse accommodation at $20 a night for members ($40 for non-members). Bookings essential. For further information: www.doc.govt.nz/tiritiribunkhouse or ph: 09 379 6476.

Supporters’ discount: Volunteers doing official SoTM work get free accommodation but this must be booked through the Guiding and Shop Manager at guiding@tiritirimatangi.org.nz or 09 476 0010. SoTM members visiting privately can get a 50% discount but must first book and pay online. Then email aucklandvc@doc. govt.nz giving the booking number and SoTM membership number (which is found on the address label of Dawn Chorus or on the email for your digital copy). DOC will then refund the discount to your credit card.