La Niña and hydro-electricity supply, summer 2010/2011
The latest seasonal climate outlook from the NIWA National Climate Centre states that “a moderate to strong La Niña in the tropical Pacific is expected to persist through the summer of 2010/11”.
La Niña conditions are likely to continue through to autumn of 2011 and then to ease. Such La Niña conditions have been indicated by NIWA since mid-2010.
La Niña conditions tend to be associated with below-normal inflows into the main hydro-electricity generating lakes. The graphic below shows the range of total summer inflows (in terms of generation capacity in MW) for non-La Niña and La Niña years.
Figure 1: Box plots of New Zealand-wide summertime (Dec-Feb) inflows (expressed as generation capacity in MW) for non-La Niña and La Niña summers. The rectangle indicates the inter-quartile range (25 to 75 percentile) of the historical distribution, and the line in the rectangle indicates the median. The “whiskers” show the minimum and maximum flows. The asterisk indicates an extreme outlier, the 1957/58 season. [Courtesy of A. McKerchar, based on data from NZX]
In the summer of 2007/08, La Niña conditions prevailed, much as they do now, and the total inflow was almost exactly the median value indicated for La Niña years in Figure 1.
As outlined in the NIWA seasonal climate outlook, there is a significant risk of below-normal inflows over the summer, especially for the South Island alpine region. The outlook states: seasonal rainfall is likely to be below normal in the western South Island [including the Southern Alps and the headwaters of the main South Island rivers]. River flows and soil moisture levels are very likely to be below normal in the west and south of the South Island.
