Prepare for heat & humidity
For New Zealand this means a clear shift in the odds, favouring above average summer temperatures for most regions, bar the eastern South Island. In the case of the north and east of the North Island, there is also a clear signal for higher humidity.
Below average sea temperatures were evident across much of the central and eastern equatorial Pacific Ocean by late October (the classic “cold tongue”), and sea surface temperatures had cooled sufficiently to meet La Nina thresholds by mid November.
Below average sea surface temperatures, in a narrow band along the equator, are the primary hallmark of La Nina (Figure 1). Other indicators, related to the atmospheric component of this weather pattern, are enhanced easterly trade winds, suppressed convection (rainfall) in the tropics near the Dateline (180), and a sustained positive Southern Oscillation Index (SOI).
The 2017 La Nina is a late-developer. Usually, La Nina conditions will form during the southern hemisphere autumn, strengthen during winter and spring, and peak around Christmas time. A typical La Nina event will continue in the first few months of the new calendar year, before dying out again during the autumn period. This La Nina was very slow to form, and is currently relatively weak. Figure 2 shows a time series of previous La Nina events, as measured by the NINO3.4 Index.
WHAT DOES THIS MEAN FOR NEW ZEALAND?
New Zealand weather maps don’t always play out to the “standard” La Nina recipe during weak or late-developing events. Sometimes, we see the New Zealand region being influenced by several competing influences. Historically, late and/or weak La Nina events have historically had mixed (variable) results on New Zealand summer (December – February) rainfall.
To illustrate, Christchurch summer (December – February) rainfall totals, as a percentage of summer normal, are shown below (Table 1). It is evident that, in Canterbury at least, La Nina conditions are no guarantee for a clearcut recipe for summer rainfall: