02/04/2022
Our climate outlook for April-June has been released and here's what you need to know.......
➡️ The next week
- In the North Island, low rainfall totals are expected over the next week along with a continuation of above average temperatures. Weekly rainfall will likely be less than 10 mm in most places, possibly with the exception of parts of Taranaki and South Waikato.
- Soil moisture levels may remain the same or decrease slightly in most places, with hotspots in the Hauraki Plains and Kaipara continuing.
- In the South Island, a few fronts may bring modest rainfall to the driest areas in the south and west, with about a 50% chance for more than 10 mm in Southland and Stewart Island. Weekly totals of 25-50 mm look likely along the West Coast. Amounts of 10 mm or less are likely in the east and north.
- Soil moisture levels may remain the same or increase slightly in the driest areas. Even if a small improvement were to occur, it will take several soaking rainfall events to ease the dryness, particularly in Southland. Hotspots in Otago, Southland, and Stewart Island, while unlikely to ease, might not grow or intensify over the week ahead.
➡️ The next season
- La Niña conditions are forecast to continue over the next three months (65% chance).
- Air pressure is forecast to be higher than normal to the east of the country and lower than normal to the northwest, causing more easterly quarter winds than normal.
- Rainfall is about equally likely to be near normal or below normal in the west of the North Island and north and west of the South Island, near or above normal in the north and east of the North Island, and most likely to be near normal in the east of the South Island.
- Seasonal rainfall may be influenced by a risk for ex-tropical cyclone activity or atmospheric rivers around New Zealand during the first half of April and again in early-to-mid May. These systems can cause heavy rainfall and flooding.
- Temperatures are very likely to be warmer than average in the north and west of the North Island and about equally likely to be near average or above average elsewhere.
- Marine heatwave (MHW) conditions continue offshore of all regions except the east of the North Island. This is expected to continue to influence periods of above average temperatures and delay the seasonal transition to cooler temperatures and frosts.
- Around the country, sea surface temperatures (SSTs) ranged from 0.3˚C to 3.3˚C above average during March. A severe marine heatwave developed in the western South Island with the largest SST anomaly in at least the last four decades. For more information, see the background.
- Soil moisture levels are about equally likely to be near normal or below normal in the west of the South Island and most likely to be near normal in all other regions.
- River flows are about equally likely to be near normal or above normal in the east of the North Island, near normal or below normal in the west of both islands, and most likely to be near normal in all other regions.
➡️ Nerd alert 🤓
The NINO3.4 Index anomaly (in the central Pacific) over the last month (through 27 March) was -0.79˚C, near the La Niña threshold. The March monthly Southern Oscillation Index (SOI) was +1.5, well within the La Niña range.
In the subsurface equatorial Pacific, the warm pool that was present during February surfaced in the east during March. Cooler subsurface waters redeveloped in the central and west. As a result, upper-oceanic heat content decreased substantially in the central and western Pacific but continued to increase in the east. The overall signature was reflective of a central Pacific La Niña.
Importantly, trade winds across the central and western equatorial Pacific were stronger than normal during March, preventing the La Niña event from weakening.
Trades were weaker than normal in the east, allowing for the development of an area of warmer than average ocean water off the coast of Ecuador. Stronger than normal trades are expected to continue in the central and west during April with reduced trades or westerlies in the east, reinforcing the current pattern.
This now means that La Niña conditions are favoured to continue during April-June (65% chance). Between July-September, there is a 50% chance for El Niño Southern Oscillation Neutral (ENSO) neutral conditions and a 40% chance for La Niña. During October-December, ENSO neutral and La Niña are about equally likely at a 35-40% chance each. Since 1950, there have only been two other “triple-dip” La Niña events: 1998-2000 and 1973-1975.
During March, convective forcing focused over the eastern Indian Ocean, Maritime Continent, and western Pacific, like it did during February. Suppressed convection was favoured over the eastern Pacific, consistent with the continuation of La Niña.
During the first two to three weeks of April, the Madden-Julian Oscillation (MJO) will be active in the central and eastern Pacific, focusing over the region of warm SSTs off the coast of Ecuador and over northern South America (resemblant of phase 8). This may promote a pattern that is favourable to tropical cyclone development in the Southwest Pacific and could influence New Zealand’s weather patterns. Historically, phase 8 has been associated with lower than normal air pressure and unsettled conditions in the New Zealand region.
During late April, convective forcing may re-emerge over the Indian Ocean and Maritime Continent (phases 3-4). Historically, phase 3 favoured high pressure over New Zealand and drier than normal conditions. Phase 4 favoured drier conditions in the North Island and eastern South Island with wetter weather in the west. Phases 3 and 4 also favoured warmer than average temperatures. The MJO may return to the phase 8 area by mid-May, historically associated with lower air pressure and wetter conditions in the west of both islands.
The Southern Annular Mode (SAM) displayed a bit more variability during March than in recent months, although it still spent more time in its positive phase. Some variability is likely during April, although the SAM will still likely spend more time in its positive phase than negative phase, likely continuing to limit the number of southerly fronts.
New Zealand’s coastal water temperatures remained above average during March, with the western South Island posting its largest monthly anomaly in at least the last four decades (since at least September 1981). This would also become the largest monthly SST anomaly observed in any of the country’s six climate regions. At the end of March, marine heatwave (MHW) conditions were occurring offshore of all regions of the country except the eastern North Island. Model guidance continues to suggest the persistence of warmer than average coastal waters through the rest of autumn into winter, consistent with the continuation of La Niña-like conditions. Warmer than average coastal sea temperatures will likely delay the seasonal transition to cooler temperatures around the country. Enhanced baroclinicity in the New Zealand region, as a result of southerly air masses clashing with the remnant MHW, may result in more frequent cyclogenesis, or developing low pressure and unsettled weather in the coming months.
NZ coastal SST anomalies
(through 29 March)
North NI 1.39˚C
West NI 1.02˚C
East NI 0.30˚C
North SI 1.48˚C
West SI 3.25˚C
East SI 1.32˚C
