GeoLog

Southern Alps

Imaggeo on Mondays: Nor’Wester in the Southern Alps of New Zealand

Imaggeo on Mondays: Nor’Wester in the Southern Alps of New Zealand

Stephan Winkler’s 2017 Imaggeo Photo Contest finalist photo showcases an unusual weather phenomenon…

The image shows a typical weather situation in the Southern Alps of New Zealand with a moist, westerly airflow pushing over the Main Divide [which separates the water catchments of the more heavily populated eastern side of the island from those on the west coast] to create a typical foehn wind [dry and warm winds which form on the downside of a mountain range] pattern (locally called Nor’Wester) in the region. Immediately west of this Main Divide, annual precipitation of up to 15,000 mm has been estimated.

The upper part of Tasman Glacier, as other glaciers around and immediately east of the Main Divide, receive impressive amounts of snow due to an overspill effect and can still be regarded as maritime.

In the image, however, the situation is displayed when right at the Main Divide the clouds disappear due to increasing temperatures when flowing over the Divide. The foehn wind developing with such weather pattern can be very strong. However, the image nicely shows how the glaciation of the central Southern Alps is influence by the availability of moisture and the dynamic character of the regional climate.

Description by Stephan Winkler (Senior Lecturer in Quaternary Geology and Palaeoclimatology at the University of Canterbury), as published previously on imaggeo.egu.eu

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

 

Imaggeo on Mondays: Foehn clouds

This week’s post is brought to you by Stefan Winkler, a Senior Lecturer in Quaternary Geology & Palaeoclimatology, who explains how the mountain tops of the Southern Alps become decorated by beautiful blanket-like cloud formations.

The Sothern Alps of New Zealand are a geoscientifically dynamic environment in all aspects. They are arguably one of the youngest high mountain ranges in the world formed at the plate tectonic boundary between the Australian and the Pacific Plate. Their dominating tectonic structure, the Alpine Fault running some 600 km mainly parallel to the mountain ranges of New Zealand’s South Island, caused not only an impressive horizontal displacement of rock formations, but also an overall vertical uplift of estimated c. 20 km during the past 10 – 15 Million years. Aoraki/Mt.Cook visible in the left background on the image with its height of ‘only’ 3724 m a.s.l. is the highest peak of the mountain range that is currently uplifted by 4 – 5 mm per year. Together with reconstructed uplift rates of up to 10 mm per year for the centre of the Southern Alps this indication how efficient and important weathering and erosion processes are in this region.

Foehn clouds over Aoraki/Mt.Cook. Credit: Stefan Winkler (distributed via imaggeo.egu.eu)

Foehn clouds over Aoraki/Mt.Cook. Credit: Stefan Winkler (distributed via imaggeo.egu.eu)

The ranges of the Southern Alps rise just 10 – 15 km inland the West Coast of the South Island as a wall parallel to the coast line up to 3,000 metres and more. They are a major topographic obstacle for the predominantly westerly airflow and provide a classic example of how ‘föhn’ winds are generated along mountain ranges perpendicular to an air flow. Föhn winds are dry and warm, forming on the downside of a mountain range. On the western slopes of the Southern Alps, orographic precipitation amounts to impressive 5,000 mm at the base and 10,000 mm + on in the high-lying accumulation areas of the mountain glaciers concentrating around the Main Divide. At and east of the Main Divide this locally named ‘Nor’wester’ creates impressive foehn clouds (altocumulus lenticularis, hogback clouds, seen in this week’s Imaggeo on Mondays image) that form in waves parallel to the Main Divide and are often streamlined by the high wind speed. The frequent occurrence of strong and warm Nor’westers contributes to the sharp decline of precipitation immediately east of the Main Divide.

The foreground of the image displays another aspect of this dynamic environment: the current wastage and retreat of glaciers in New Zealand. The section of the proglacial lake with its sediment-laden greyish water colour on the image would still have been covered by the debris-covered lower glacier tongue of Mueller Glacier only 15 years ago. Now, the terminus has retread to a position to the left outside the image. The lake is bounded by the glacier’s lateral moraine – unconsolidated accumulations of rock and soil debris resulting from weathering of the rock walks surrounding a glacire – that are more than 120 m high from base to top (or crest, to give it its technical name) and were last overtopped during the so-called ‘Little Ice Age’ when the glacier surface reached higher than its crest. At this glacier, the maximum of this Little Ice Age has been dated to 1720/30, but as late as during the late 20th century it remained close to its frontal maximum position and had only shrunk vertically. Today the lateral moraines are heavily reworked and eroded by paraglacial processes following the latest vertical and horizontal ice retreat. In some places on Mueller Glacier’s foreland the crest of lateral moraines retreat up to 1 m per year back and give again evidence of a very dynamic geo-ecosystem.

By Stefan Winkler, Senior Lecturer in Quaternary Geology and Palaeoclimatology at the Univeristy of Canterbury.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

Imaggeo on Mondays: A solitary floating island

With 2014 officially named the hottest year on record, there is evidence of the effects of rising global temperatures across the globe. The solitary, shimmering iceberg in today’s Imaggeo on Mondays photograph is a reminder that one of the best places to look for evidence of change is in glaciers. Daniela Domeisen tells the story of this lonely frozen block of ancient ice.

Iceberg on Tasman glacier lake. Credit: Daniela Domeisen (distributed via imaggeo.egu.eu)

Iceberg on Tasman glacier lake. Credit: Daniela Domeisen (distributed via imaggeo.egu.eu)

The picture shows an iceberg on Tasman glacier lake in the Southern Alps of New Zealand, in the centre of Aoraki / Mount Cook National Park. The lake consists of melt water from the Tasman glacier, which calves into the lake at its far end. The glacier is one of the largest in New Zealand and flows along New Zealand’s highest peaks, Mt Tasman and Mt Cook.

As most glaciers on Earth, the glaciers in Aoraki / Mount Cook National Park are retreating at a fast pace. The lower parts of the Tasman glacier are at less than 1000m above sea level and are therefore melting especially fast. The Tasman glacier lake has formed over the past two to three decades and has in the meantime reached a length of several kilometers. It is projected to almost double in size as the glacier retreats further.

Icebergs constantly calve from the Tasman glacier into the lake and drift down the lake, driven by a weak current towards the lake’s outflow while melting in the process. The ice contained in the icebergs is several thousand years old, beautifully transparent and clean when looking at a single piece of it.

The pictured iceberg was about 10 meters wide. From its shape, and melting pattern, it is likely that it had turned to its side after calving into the lake. With some force it was possible to tip the smaller icebergs and see a shiny blue surface which had been beautifully polished by the water.

On the lake, everything was completely peaceful and quiet, except for the distant sound of a continuous rippling and trickling coming from the moraines on the sides of the lake, as pictured in the background of the photo. Stones and rocks of various sizes slid down and fell into the lake as the ice inside the moraines melted in the bright, sunny and warm January weather.

The changes which are observed in most places as a result of the changing climate are often either too slow to be observed or invisible to the naked eye. The glacier, its lake and icebergs, however, are continuously changing, and a couple of hours spent on the water give a lively impression of a quiet place where things are changing fast enough to be able to observe a notable difference between the time one enters and leaves the place. The beauty of the glacier and its lake with the glittering icebergs provide a spectacular glimpse of a transient place.

By Daniela Domeisen, Research Analyst, MarexSpectron, London

If you pre-register for the 2015 General Assembly (Vienna, 12 – 17 April), you can take part in our annual photo competition! From 1 February up until 1 March, every participant pre-registered for the General Assembly can submit up three original photos and one moving image related to the Earth, planetary, and space sciences in competition for free registration to next year’s General Assembly!  These can include fantastic field photos, a stunning shot of your favourite thin section, what you’ve captured out on holiday or under the electron microscope – if it’s geoscientific, it fits the bill. Find out more about how to take part at http://imaggeo.egu.eu/photo-contest/information/.