Interannual Extremes in New Zealand Precipitation Linked to Modes of Southern Hemisphere Climate Variability

Abstract

Interannual extremes in New Zealand rainfall and their modulation by modes of Southern Hemisphere climate variability are examined in observations and a coupled climate model. North Island extreme dry (wet) years are characterized by locally increased (reduced) sea level pressure (SLP), cold (warm) sea surface temperature (SST) anomalies in the southern Tasman Sea and to the north of the island, and coinciding reduced (enhanced) evaporation upstream of the mean southwesterly airflow. During extreme dry (wet) years in South Island precipitation, an enhanced (reduced) meridional SLP gradient occurs, with circumpolar strengthened (weakened) subpolar westerlies and an easterly (westerly) anomaly in zonal wind in the subtropics. As a result, via Ekman transport, anomalously cold (warm) SST appears under the subpolar westerlies, while anomalies of the opposite sign occur farther north. The phase and magnitude of the resulting SST and evaporation anomalies cannot account for the rainfall extremes over the South Island, suggesting a purely atmospheric mode of variability as the driving factor, in this case the Southern Annular Mode (SAM). New Zealand rainfall variability is predominantly modulated by two Southern Hemisphere climate modes, namely, the El Niño-Southern Oscillation (ENSO) and the SAM, with a latitudinal gradation in influence of the respective phenomena, and a notable interaction with orographic features. While this heterogeneity is apparent both latitudinally and as a result of orographic effects, climate modes can force local rainfall anomalies with considerable variations across both islands. North Island precipitation is for the most part regulated by both local air-sea heat fluxes and circulation changes associated with the tropical ENSO mode. In contrast, for the South Island the influence of the large-scale general atmospheric circulation dominates, especially via the strength and position of the subpolar westerlies, which are modulated by the extratropical SAM.

Components of precipitation and temperature anomalies and change associated with modes of the Southern Hemisphere

“…The reduction in rainfall over western Australia in Mk3 is too strong. Nevertheless there is considerable realism in the pattern over South America, and some also for southern Africa and NZ (Ummenhofer and England, 2007).…”

Cloud effects on surface energy and mass balance in the ablation area of Brewster Glacier, New Zealand

“…Advances in automatic weather station (AWS) deployment on glacier surfaces (Mölg et al, 2009b), the availability of high-quality radiation mea-J. P. Conway and N. J. Cullen: Cloud effects on surface energy and mass balance, Brewster Glacier surements (van den Broeke et al, 2004), and development of methods to extract information about cloud cover in data sparse areas , have allowed the variation of SEB and SMB with cloud cover to be characterized in many areas. Sicart et al (2010) show that clouds dominate day-to-day variations in LW↓ in mountainous areas while numerous studies detail the fundamental changes in SEB with cloudiness that are often coincident with changes in glacier surface boundary layer (SBL) properties (van den Broeke et al, 2006;Giesen et al, 2008;Gillett and Cullen, 2011).…”

“…P. Conway and N. J. Cullen: Cloud effects on surface energy and mass balance, Brewster Glacier surements (van den Broeke et al, 2004), and development of methods to extract information about cloud cover in data sparse areas , have allowed the variation of SEB and SMB with cloud cover to be characterized in many areas. Sicart et al (2010) show that clouds dominate day-to-day variations in LW↓ in mountainous areas while numerous studies detail the fundamental changes in SEB with cloudiness that are often coincident with changes in glacier surface boundary layer (SBL) properties (van den Broeke et al, 2006;Giesen et al, 2008;Gillett and Cullen, 2011). Given their strong control on the SEB, and coincidence with changes in SBL properties it is vital that the role of clouds in altering the sensitivity of SMB to changes in atmospheric state variables (especially air temperature) be assessed.…”

“…The glaciers of the Southern Alps of New Zealand occupy a unique position in the westerly wind belt of the Southern Ocean, a region dominated by mid-latitude atmospheric circulation (Tait and Fitzharris, 1998;Ummenhofer and England, 2007). The large barrier the Southern Alps poses to the prevailing winds creates a high precipitation environment, which, coupled to the relatively low elevation of glacier termini (Hoelzle et al, 2007), creates high mass turnover glaciers that have shown high sensitivity to climatic variations in temperature-index glacier modelling studies (Anderson et al, 2006;Oerlemans, 2010).…”

Spatial and temporal variability of annual glacier equilibrium line altitudes in the Southern Alps, New Zealand

“…This freezing level is often restricted in mid-winter to high altitudes but, during summer, can occur at altitudes as low as 500 m ). As precipitation is relatively uniform in space (Ummenhofer & England 2007) and over a year (Oerlemans 1994;Fitzharris et al 1999), snow can be deposited at high altitudes all year round. However, due to the mid-latitude maritime location of the glaciers and the high levels of summer solar radiation receipt, ablation rates are high Evans & Cox 2006), creating a high-flux system.…”