Expressed as 11-year running means (1945- 2008), winter headwater flows in the Waitaki River (New Zealand) and the IPO index show similar patterns in the way they vary over time. However, this effect is not evident in the other seasons. A possible explanation is the IPO in its positive phase is associated with warmer air temperatures in winter, but not in other seasons. Analysis of Mt Cook (Hermitage station) daily rainfalls suggests that warmer winter air temperatures on precipitation days are associated with higher mean daily precipitation, so the positive IPO may result in greater amounts of winter precipitation falling as rain rather than snow. This could provide the link between the winter IPO and headwater discharge. On the other hand, temperature is only weakly associated with precipitation magnitudes in other seasons, when higher average temperatures do not provide the same restricting effect on precipitation as in winter.