4 minute read
A distinct lack of westerlies
Georgina Griffiths : Meteorologist, MetService
The year 2022 so far – a lack of westerlies
Farmers and growers are astute watchers of the weather because life and livelihood depend on it. January to May 2022 has been unusual in that Highs have favoured southern latitudes (Figure 1), while easterlies have prevailed across Aotearoa New Zealand.
Figure 1 shows the year-to-date weather map compared to normal. It indicates differences compared to climatology, with red colours showing areas with more Highs than usual, and blue colours indicating more Lows than usual. It is obvious that Highs have favoured southern latitudes (the area south of, and over, the South Island). In contrast, the region near Fiji has experienced more frequent Lows. In between, there has been a prevailing easterly wind regime – or in other words, a lack of westerlies.
Figure 1: The mean sea level pressure anomaly (deviation from normal) for the year 2022 so far (1 January – 16 May 2022). Red colours indicate higher than normal pressures, while blue colours indicate lower than usual pressures (more lows). Map produced courtesy of NOAA/ESRL Physical Sciences Division
And while the ongoing La Niña conditions may have taken the blame, credit must also be given to a persistently quiet Southern Ocean, also known as a prevailing positive SAM (Southern Annular Mode), Figure 2. In other words, a lack of Southern Ocean storms washing up and over the country. The day-to-day weather maps during the positive phase often show high pressure south of the South Island (as well as over the Chatham Islands).
Figure 2: A plot of the SAM from 1 January 2021 to 30 May 2002. Note the persistence of the positive (quiet) phase during summer 2021–2022, and then a prevailing positive regime through most of early 2022. The lighter (washed out) bars from mid-May to end of May indicate the forecast state of the SAM, predicting the first decent cold and stormy Southern Ocean outbreak this year
The knockout combination of lows to the north and easterlies (La Niña) and a largely quiet Southern Ocean (positive Southern Annular Mode) led to an extended period of very low rainfall in Southland through the first quarter of 2022. A medium-scale adverse drought event was declared in Southland, Clutha and Queenstown Lakes on 1 April, acknowledging the extended dry conditions earlier in 2022 in these regions (Figure 3).
In contrast, areas exposed to easterly rainfall, such as Northland, Gisborne and Christchurch have run wetter than usual (Figures 4, 5 and 6), while Bay of Plenty and Nelson are sitting closer to normal (Figures 7 and 8). Looking towards winter, all eyes are on the Tasman Sea, where an active subtropical jet is forecast to spawn frequent Tasman Lows, and a wetter and warmer than normal winter is predicted for most regions of the North Island and upper South Island.
Figure 3: Queenstown Figure 4: Whangarei Figure 5: Napier
Figure 6: Christchurch Figure 7: Te Puke Figure 8: Nelson
Figure 3: Queenstown annual rainfall accumulation (mm) for the last five years (2018 to 2022). The annual average rainfall accumulation is shown in black. The year-to-date rain accumulation as at time of writing (mid-May) was 70 percent of normal Figure 4: Whangarei annual rainfall accumulation (mm) for the last five years (2018 to 2022). The annual average rainfall accumulation is shown in black. The year-to-date rain accumulation as at time of writing (mid-May) was 128 percent of normal Figure 5: Napier annual rainfall accumulation (mm) for the last five years (2018 to 2022). The annual average rainfall accumulation is shown in black. The year-to-date rain accumulation as at time of writing (mid-May) was 176 percent of normal Figure 6: Christchurch annual rainfall accumulation (mm) for the last five years (2018 to 2022). The annual average rainfall accumulation is shown in black. The year-to-date rain accumulation as at time of writing (mid-May) was 135 percent of normal Figure 7: Te Puke annual rainfall accumulation (mm) for the last five years (2018 to 2022). The annual average rainfall accumulation is shown in black. The year-to-date rain accumulation as at time of writing (mid-May) was 92 percent of normal Figure 8: Nelson annual rainfall accumulation (mm) for the last five years (2018 to 2022). The annual average rainfall accumulation is shown in black. The year-to-date rain accumulation as at time of writing (mid-May) was 87 percent of normal As always, you should keep up to date with the MetService long-range forecast at http://metservice.com/rural/monthlyoutlook, or ask us questions on the MetService Facebook or Twitter feeds.
