Thongchai Thailand

ANTARCTICA HEAT WAVE OF 2020

Posted on: February 26, 2020

IMAGE#1: MAP OF THE ANTARCTIC PENINSULA SHOWING EAGLE ISLAND

bandicam 2020-02-26 14-32-17-900

 

 

IMAGE#2: NASA EARTH OBSERVATORY SHOWS ICE MELT ON EAGLE ISLAND 

ANTARCTICA-HEAT-WAVE

 

IMAGE#3: GEOLOGY OF ANTARCTICA 

 

IMAGE#4: MARTOS ETAL 2017: GEOTHERMAL HEAT FLUX MAP

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YASMINA-2

 

IMAGE#5: DZIADEK 2017: GEOTHERMAL HEAT FLUX MAP 

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IMAGE#6: FOEHN AND CHINOOK WINDS

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RELATED POSTS: [LINK][LINK] [LINK]

 

 

THIS POST IS A CRITICAL REVIEW OF AN ONLINE ARTICLE [LINK] CLAIMING THAT A Heat Wave in Antarctica Has Melted 20% of an Island’s Snow in 9 Days . IT IS PRESENTED AS A SERIES OF CLAIMS MADE IN THE ARTICLE FOLLOWED BY RESPONSES TO THOSE CLAIMS.

 

TWO RELEVANT BIBLIOGRAPHIES ARE PROVIDED AT THE END OF THE POST. THE FIRST BIBLIOGRAPHY INCLUDES TEN STUDIES ON GEOTHERMAL FLUX DATA IN ANTARCTICA. THE SECOND BIBLIOGRAPHY INCLUDES EIGHT STUDIES ON FOEHN AND CHINOOK WIND WARMING EVENTS IN ANTARCTICA. 

 

CLAIM#1: Antarctic temperatures recently hit record highs twice in the space of one week. RESPONSE#1: These isolated temperature events were constrained both in space and time in a remote tip of the Antarctic Peninsula on specific days. They have no interpretation in terms of AGW climate change – a theory about the effect of fossil fuel emissions on long term warming trends in global mean temperature. In a related posts it is shown that a more rational interpretation of such isolated temperature events in a geologically active region is provided by the known geothermal heal fluxes in these locations [LINK]

CLAIM#2:  A heat wave melted around 20% of the snow of one of its islands, as illustrated by new NASA Earth Observatory images (Image#2 above). During the heat wave, around 4 inches of snow covering Eagle Island melted as well as the highest ever temperature recorded on the continent reaching 64.9 degrees Fahrenheit (18.3 degrees Celsius), the same as temperatures in Los Angeles on the same day.  RESPONSE#2: AGW climate change is a theory about a long term warming trend in global mean temperature. Isolated temperature events of this nature in a remote corner of Antarctica have no interpretation in this context and it has not been shown that the temperature events in the extreme North of the Antarctic Peninsula are a response to atmospheric forcing. The mean surface temperature of Antarctica does not show evidence of global warming [LINK] . A more likely source of the energy in such isolated events is geothermal heat particularly so when the location of the heat event is a geologically active area as seen in the geothermal heat flux maps in Image #4 and Image #5. Yet another possibility is the warming and snow melt effects of by Foehn and Chinook winds (Image#6) that are known to occur in this region [LINK] [LINK]

CLAIM#3: The images of Eagle Island, captured by NASA’s Landsat-8 satellite on February 4th and February 13th, reveal the startling difference nine days of record-breaking temperatures can make on the planet’s coldest continent. While the usually anomalous high temperatures are a cause for alarm, the picture of a snow-covered island transformed into one with melt-pools and exposed rocky terrain has researchers concerned about the effects climate change is having on the region. You see these kinds of melt events in Alaska and Greenland, but not usually in Antarctica. RESPONSE#3: Geothermal heat induced ice and snow melt is common in the geologically active areas of Antarctica such as the tip of the Antarctic Peninsula. Consider also that the assumed causation by global warming is not supported by the temperature data as shown in a related post [LINK] . The geothermal heat flux maps in Images 4&5 and Foehn and Chinook wind events  (Image#6) above provide a better explanation for such isolated and evanescent heat events than AGW global warming. These phenomena cannot be generalized across Antarctica because they are location specific and the the specific locations of these melt events coincide with known locations of geological activity.

CLAIM#4: Such dramatic snowmelt is the result of increased temperatures over a sustained period of time which is thought to be the result of overall global temperatures rising. However, other conditions also influenced the sudden heat wave in the Antarctic climate including unusually weak winds which prevented a warm surge moving southwards from Chile and penetrating the continent.  RESPONSE#4: As shown in a related post, there is no evidence of an AGW climate change warming trend in Antarctica [LINK] . As shown in a related post, [LINK] , West Antarctica, and particularly so the Antarctic Peninsula, is a geologically active area where geothermal heat from the West Antarctic Rift and the Marie Byrd Mantle Plume and Foehn and Chinook wind events  (Image#6) are more likely explanations of isolated and brief heat and melt events. Nearby to the events described is Deception Island where an extreme volcanic eruption had created a large hot spring lake now popular with tourists as seen in the image below.

bandicam 2020-02-09 12-19-23-705

 

 

 

GEOTHERMAL HEAT FLUX BIBLIOGRAPHY

  1. Scambos, Ted A., et al. “The link between climate warming and break-up of ice shelves in the Antarctic Peninsula.” Journal of Glaciology 46.154 (2000): 516-530.  A review of in situ and remote-sensing data covering the ice shelves of the Antarctic Peninsula provides a series of characteristics closely associated with rapid shelf retreat: deeply embayed ice fronts; calving of myriad small elongate bergs in punctuated events; increasing flow speed; and the presence of melt ponds on the ice-shelf surface in the vicinity of the break-ups. As climate has warmed in the Antarctic Peninsula region, melt-season duration and the extent of ponding have increased. Most break-up events have occurred during longer melt seasons, suggesting that meltwater itself, not just warming, is responsible. Regions that show melting without pond formation are relatively unchanged. Melt ponds thus appear to be a robust harbinger of ice-shelf retreat. We use these observations to guide a model of ice-shelf flow and the effects of meltwater. Crevasses present in a region of surface ponding will likely fill to the brim with water. We hypothesize (building on Weertman (1973), Hughes (1983) and Van der Veen (1998)) that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat. A thermodynamic finite-element model is used to evaluate ice flow and the strain field, and simple extensions of this model are used to investigate crack propagation by meltwater. The model results support the hypothesis.
  2. Convey, P., et al. “The flora of the South Sandwich Islands, with particular reference to the influence of geothermal heating.” Journal of Biogeography 27.6 (2000): 1279-1295.  Data obtained in 1997 are combined with updated records from the only previous survey (in 1964) to provide a baseline description of the flora of the archipelago, which currently includes 1 phanerogam, 38 mosses, 11 liverworts, 5 basidiomycete fungi, 41 lichenised fungi and 16 diatoms with, additionally, several taxa identified only to genus. Major elements of the moss and liverwort floras are composed of South American taxa (32% and 73%, respectively), with a further 45% of mosses having bipolar or cosmopolitan distributions. These two groups show low levels of Antarctic endemicity (11% and 18%, respectively). In contrast, 52% of lichens and 80% of basidiomycete fungi are endemic to the Antarctic. A further 36% of lichens are bipolar/cosmopolitan, with only 5% of South American origin. The flora of the South Sandwich Islands is clearly derived from those of other Antarctic zones. The flora of unheated ground is closely related to that of the maritime Antarctic, although with a very limited number of species represented. That of heated ground contains both maritime and sub‐Antarctic elements, confirming the importance of geothermal heating for successful colonisation of the latter group. The occurrence of several maritime Antarctic species only on heated ground confirms the extreme severity of the archipelago’s climate in comparison with well‐studied sites much further south in this biogeographical zone.
  3. Smith, RI Lewis. “The bryophyte flora of geothermal habitats on Deception Island, Antarctica.” The Journal of the Hattori Botanical Laboratory 97 (2005): 233-248.  Deception Island is one of the most volcanically active sites south of 60°S. Between 1967 and 1970 three major eruptions devastated large expanses of the landscape and its predominantly cryptogamic vegetation. Since 1970 extensive recolonisation has occurred on the more stable surfaces. Unheated ground supports several bryophyte and lichen communities typical of much of the maritime Antarctic, but geothermal habitats possess remarkable associations of bryophytes, many of the species being unknown or very rare elsewhere in the Antarctic. Nine geothermal sites were located and their vegetation investigated in detail. Communities associated with more transient sites have disappeared when the geothermal activity ceased. Mosses and liverworts occur to within a few centimetres of fumarole vents where temperatures reach 90-95℃, while temperatures within adjacent moss turf can reach 35-50℃ or more and remain consistently between 25 and 45℃. Most of the bryoflora has a Patagonian-Fuegian provenance and it is presumed that, unlike most species, the thermophiles are not pre-adapted to the Antarctic environment, being able to colonise only where the warm and humid conditions prevail.
  4. Vieira, Gonçalo, et al. “Geomorphological observations of permafrost and ground-ice degradation on Deception and Livingston Islands, Maritime Antarctica.” (2008): 1939-1844. The Antarctic Peninsula is experiencing one of the fastest increases in mean annual air temperatures (ca. 2.5oC in the last 50 years) on Earth. If the observed warming trend continues as indicated by climate models, the region could suffer widespread permafrost degradation. This paper presents field observations of geomorphological features linked to permafrost and ground-ice degradation at two study areas: northwest Hurd Peninsula (Livingston Island) and Deception Island along the Antarctic Peninsula. These observations include thermokarst features, debris flows, active-layer detachment slides, and rockfalls. The processes observed may be linked not only to an increase in temperature, but also to increased rainfall, which can trigger debris flows and other processes. On Deception Island some thermokarst (holes in the ground produced by the selective melting of permafrost)  features may be related to anomalous geothermal heat flux from volcanic activity.
  5. Mulvaney, Robert, et al. “Recent Antarctic Peninsula warming relative to Holocene climate and ice-shelf history.” Nature 489.7414 (2012): 141-144Rapid warming over the past 50 years on the Antarctic Peninsula is associated with the collapse of a number of ice shelves and accelerating glacier mass loss1,2,3,4,5,6,7. In contrast, warming has been comparatively modest over West Antarctica and significant changes have not been observed over most of East Antarctica8,9, suggesting that the ice-core palaeoclimate records available from these areas may not be representative of the climate history of the Antarctic Peninsula. Here we show that the Antarctic Peninsula experienced an early-Holocene warm period followed by stable temperatures, from about 9,200 to 2,500 years ago, that were similar to modern-day levels. Our temperature estimates are based on an ice-core record of deuterium variations from James Ross Island, off the northeastern tip of the Antarctic Peninsula. We find that the late-Holocene development of ice shelves near James Ross Island was coincident with pronounced cooling from 2,500 to 600 years ago. This cooling was part of a millennial-scale climate excursion with opposing anomalies on the eastern and western sides of the Antarctic Peninsula. Although warming of the northeastern Antarctic Peninsula began around 600 years ago, the high rate of warming over the past century is unusual (but not unprecedented) in the context of natural climate variability over the past two millennia. The connection shown here between past temperature and ice-shelf stability suggests that warming for several centuries rendered ice shelves on the northeastern Antarctic Peninsula vulnerable to collapse. Continued warming to temperatures that now exceed the stable conditions of most of the Holocene epoch is likely to cause ice-shelf instability to encroach farther southward along the Antarctic Peninsula.
  6. Fraser, Ceridwen I., et al. “Geothermal activity helps life survive glacial cycles.” Proceedings of the National Academy of Sciences 111.15 (2014): 5634-5639.  The evolution and maintenance of diversity through cycles of past climate change have hinged largely on the availability of refugia (places where life can survive through a period of unfavorable conditions such as glaciation). Geothermal refugia may have been particularly important for survival through past glaciations. Our spatial modeling of Antarctic biodiversity indicates that some terrestrial groups likely survived throughout intense glacial cycles on ice-free land or in sub-ice caves associated with areas of geothermal activity, from which recolonization of the rest of the continent took place. These results provide unexpected insights into the responses of various species to past climate change and the importance of geothermal regions in promoting biodiversity. Furthermore, they indicate the likely locations of biodiversity “hotspots” in Antarctica, suggesting a critical focus for future conservation efforts.
  7. An, Meijian, et al. “Temperature, lithosphere‐asthenosphere boundary, and heat flux beneath the Antarctic Plate inferred from seismic velocities.” Journal of Geophysical Research: Solid Earth 120.12 (2015): 8720-8742.  We estimate the upper mantle temperature of the Antarctic Plate based on the thermoelastic properties of mantle minerals and S velocities using a new 3‐D shear velocity model, AN1‐S. Crustal temperatures and surface heat fluxes are then calculated from the upper mantle temperature assuming steady state thermal conduction. The temperature at the top of the asthenosphere beneath the oceanic region and West Antarctica is higher than the dry mantle solidus, indicating the presence of melt. From the temperature values, we generate depth maps of the lithosphere‐asthenosphere boundary and the Curie temperature isotherm. The maps show that East Antarctica has a thick lithosphere similar to that of other stable cratons, with the thickest lithosphere (~250 km) between Domes A and C. The thin crust and lithosphere beneath West Antarctica are similar to those of modern subduction‐related rift systems in East Asia. A cold region beneath the Antarctic Peninsula is similar in spatial extent to that of a flat‐subducted slab beneath the southern Andes, indicating a possible remnant of the Phoenix Plate, which was subducted prior to 10 Ma. The oceanic lithosphere generally thickens with increasing age, and the age‐thickness correlation depends on the spreading rate of the ridge that formed the lithosphere. Significant flattening of the age‐thickness curves is not observed for the mature oceanic lithosphere of the Antarctic Plate.
  8. Dziadek, Ricarda, et al. “Geothermal heat flux in the Amundsen Sea sector of West Antarctica: New insights from temperature measurements, depth to the bottom of the magnetic source estimation, and thermal modeling.” Geochemistry, Geophysics, Geosystems 18.7 (2017): 2657-2672[FULL TEXT]  Focused research on the Pine Island and Thwaites glaciers, which drain the West Antarctic Ice Shelf (WAIS) into the Amundsen Sea Embayment (ASE), revealed strong signs of instability in recent decades that result from variety of reasons, such as inflow of warmer ocean currents and reverse bedrock topography, and has been established as the Marine Ice Sheet Instability hypothesis. Geothermal heat flux (GHF) is a poorly constrained parameter in Antarctica and suspected to affect basal conditions of ice sheets, i.e., basal melting and subglacial hydrology. Thermomechanical models demonstrate the influential boundary condition of geothermal heat flux for (paleo) ice sheet stability. Due to a complex tectonic and magmatic history of West Antarctica, the region is suspected to exhibit strong heterogeneous geothermal heat flux variations. We present an approach to investigate ranges of realistic heat fluxes in the ASE by different methods, discuss direct observations, and 3‐D numerical models that incorporate boundary conditions derived from various geophysical studies, including our new Depth to the Bottom of the Magnetic Source (DBMS) estimates. Our in situ temperature measurements at 26 sites in the ASE more than triples the number of direct GHF observations in West Antarctica. We demonstrate by our numerical 3‐D models that GHF spatially varies from 68 up to 110 mW m−2.
  9. Martos, Yasmina M., et al. “Heat flux distribution of Antarctica unveiled.” Geophysical Research Letters 44.22 (2017): 11-417[FULL TEXT]  Antarctica is the largest reservoir of ice on Earth. Understanding its ice sheet dynamics is crucial to unraveling past global climate change and making robust climatic and sea level predictions. Of the basic parameters that shape and control ice flow, the most poorly known is geothermal heat flux. Direct observations of heat flux are difficult to obtain in Antarctica, and until now continent‐wide heat flux maps have only been derived from low‐resolution satellite magnetic and seismological data. We present a high‐resolution heat flux map and associated uncertainty derived from spectral analysis of the most advanced continental compilation of airborne magnetic data. Small‐scale spatial variability and features consistent with known geology are better reproduced than in previous models, between 36% and 50%. Our high‐resolution heat flux map and its uncertainty distribution provide an important new boundary condition to be used in studies on future subglacial hydrology, ice sheet dynamics, and sea level change.
  10. Burton‐Johnson, Alex, et al. “A new heat flux model for the Antarctic Peninsula incorporating spatially variable upper crustal radiogenic heat production.” Geophysical Research Letters 44.11 (2017): 5436-5446.  A new method for modeling heat flux shows that the upper crust contributes up to 70% of the Antarctic Peninsula’s subglacial heat flux and that heat flux values are more variable at smaller spatial resolutions than geophysical methods can resolve. Results indicate a higher heat flux on the east and south of the Peninsula (mean 81 mW m−2) where silicic rocks predominate, than on the west and north (mean 67 mW m−2) where volcanic arc and quartzose sediments are dominant. While the data supports the contribution of heat‐producing element‐enriched granitic rocks to high heat flux values, sedimentary rocks can be of comparative importance dependent on their provenance and petrography. Models of subglacial heat flux must utilize a heterogeneous upper crust with variable radioactive heat production if they are to accurately predict basal conditions of the ice sheet. Our new methodology and data set facilitate improved numerical model simulations of ice sheet dynamics.
     

 

FOEHN AND CHINOOK WINDS

  1. Nylen, Thomas H., Andrew G. Fountain, and Peter T. Doran. “Climatology of katabatic winds in the McMurdo dry valleys, southern Victoria Land, Antarctica.” Journal of Geophysical Research: Atmospheres 109.D3 (2004)Katabatic winds dramatically affect the climate of the McMurdo dry valleys, Antarctica. Winter wind events can increase local air temperatures by 30°C. The frequency of katabatic winds largely controls winter (June to August) temperatures, increasing 1°C per 1% increase in katabatic frequency, and it overwhelms the effect of topographic elevation (lapse rate). Summer katabatic winds are important, but their influence on summer temperature is less. The spatial distribution of katabatic winds varies significantly. Winter events increase by 14% for every 10 km up valley toward the ice sheet, and summer events increase by 3%. The spatial distribution of katabatic frequency seems to be partly controlled by inversions. The relatively slow propagation speed of a katabatic front compared to its wind speed suggests a highly turbulent flow. The apparent wind skip (down‐valley stations can be affected before up‐valley ones) may be caused by flow deflection in the complex topography and by flow over inversions, which eventually break down. A strong return flow occurs at down‐valley stations prior to onset of the katabatic winds and after they dissipate. Although the onset and termination of the katabatic winds are typically abrupt, elevated air temperatures remain for days afterward. We estimate that current frequencies of katabatic winds increase annual average temperatures by 0.7° to 2.2°C, depending on location. Seasonally, they increase (decrease) winter average temperatures (relative humidity) by 0.8° to 4.2° (−1.8 to −8.5%) and summer temperatures by 0.1° to 0.4°C (−0.9% to −4.1%). Long‐term changes of dry valley air temperatures cannot be understood without knowledge of changes in katabatic winds.
  2. Walker, Virginia K., Gerald R. Palmer, and Gerrit Voordouw. “Freeze-thaw tolerance and clues to the winter survival of a soil community.” Appl. Environ. Microbiol. 72.3 (2006): 1784-1792.  Although efforts have been made to sample microorganisms from polar regions and to investigate a few of the properties that facilitate survival at freezing or subzero temperatures, soil communities that overwinter in areas exposed to alternate freezing and thawing caused by Foehn or Chinook winds have been largely overlooked. We designed and constructed a cryocycler to automatically subject soil cultures to alternating freeze-thaw cycles. After 48 freeze-thaw cycles, control Escherichia coli and Pseudomonas chlororaphis isolates were no longer viable. Mixed cultures derived from soil samples collected from a Chinook zone showed that the population complexity and viability were reduced after 48 cycles. However, when bacteria that were still viable after the freeze-thaw treatments were used to obtain selected cultures, these cultures proved to be >1,000-fold more freeze-thaw tolerant than the original consortium. Single-colony isolates obtained from survivors after an additional 48 freeze-thaw cycles were putatively identified by 16S RNA gene fragment sequencing. Five different genera were recognized, and one of the cultures, Chryseobacterium sp. strain C14, inhibited ice recrystallization, a property characteristic of antifreeze proteins that prevents the growth of large, potentially damaging ice crystals at temperatures close to the melting temperature. This strain was also notable since cell-free medium derived from cultures of it appeared to enhance the multiple freeze-thaw survival of another isolate, Enterococcus sp. strain C8. The results of this study and the development of a cryocycler should allow further investigations into the biochemical and soil community adaptations to the rigors of a Chinook environment.
  3. Speirs, Johanna C., et al. “Foehn winds in the McMurdo Dry Valleys, Antarctica: The origin of extreme warming events.” Journal of Climate 23.13 (2010): 3577-3598 Foehn winds resulting from topographic modification of airflow in the lee of mountain barriers are frequently experienced in the McMurdo Dry Valleys (MDVs) of Antarctica. Strong foehn winds in the MDVs cause dramatic warming at onset and have significant effects on landscape forming processes; however, no detailed scientific investigation of foehn in the MDVs has been conducted. As a result, they are often misinterpreted as adiabatically warmed katabatic winds draining from the polar plateau. Herein observations from surface weather stations and numerical model output from the Antarctic Mesoscale Prediction System (AMPS) during foehn events in the MDVs are presented. Results show that foehn winds in the MDVs are caused by topographic modification of south-southwesterly airflow, which is channeled into the valleys from higher levels. Modeling of a winter foehn event identifies mountain wave activity similar to that associated with midlatitude foehn winds. These events are found to be caused by strong pressure gradients over the mountain ranges of the MDVs related to synoptic-scale cyclones positioned off the coast of Marie Byrd Land. Analysis of meteorological records for 2006 and 2007 finds an increase of 10% in the frequency of foehn events in 2007 compared to 2006, which corresponds to stronger pressure gradients in the Ross Sea region. It is postulated that the intra- and interannual frequency and intensity of foehn events in the MDVs may therefore vary in response to the position and frequency of cyclones in the Ross Sea region.
  4. Steinhoff, Daniel Frederick. Dynamics and Variability of Foehn Winds in the McMurdo Dry Valleys Antarctica. Diss. The Ohio State University, 2011.  The McMurdo Dry Valleys (“MDVs”) are the largest ice-free region in Antarctica, featuring perennially ice-covered lakes that are fed by ephemeral melt streams in the summer. The MDVs have been an NSF-funded Long-Term Ecological Research (LTER) site since 1993, and LTER research has shown that the hydrology and biology of the MDVs are extremely sensitive to small climatic fluctuations, especially during summer when temperatures episodically rise above freezing. However, the atmospheric processes that control MDVs summer climate, namely the westerly foehn and easterly sea-breeze regimes, are not well understood. The goals of this study are to (i) produce a coherent physical mechanism for the development and spatial extent of foehn winds in the MDVs, and (ii) determine aspects of large-scale climate variability responsible for intraseasonal and interannual differences in MDVs temperature. Polar WRF simulations are run for a prominent foehn case study at 500 m horizontal grid spacing to study the mesoscale components of foehn events, and 15 summers at 2 km horizontal grid spacing to analyze event and temporal variability. The Polar WRF simulations have been tailored for use in the MDVs through modifications to the input soil conditions, snow cover, land use, and sea ice. An objective foehn identification method is used to identify and categorize events, as well as validate the model against LTER AWS observations. The MDVs foehn mechanism consists of a gap wind through a topographic constriction south of the MDVs, forced by pressure differences on each side of the gap and typically set up by cyclonic flow over the Ross and Amundsen Seas. Significant mountain wave activity over the gap modulates the flow response over the MDVs themselves, and pressure-driven channeling drives foehn flow down-valley. During strongly forced events, mass accumulation east of the MDVs from flow around Ross Island is responsible for easterly intrusions, and not a thermally forced sea breeze as previously thought. A variety of ambient flow directions and associated synoptic-scale patterns can result in MDVs foehn, but adequate forcing is necessary to activate the foehn mechanism. The warmest foehn events are associated with amplified circulation patterns that are not associated with particular interannual modes of variability, but instead related to intraseasonal variability forced by the extratropical response to a stagnant MJO. Implications of the findings upon current MDVs paleoclimate theories on the existence of huge melt lakes at the LGM are also presented.
  5. Elvidge, Andrew. Polar föhn winds and warming over the Larsen C Ice Shelf, Antarctica. Diss. University of East Anglia, 2013. Recent hypotheses that the foehn effect is partly responsible for warming to the east of the Antarctic Peninsula (AP) and enhanced melt rates on the Larsen C Ice Shelf are supported in a study combining the analysis of observational and high resolution model data. Leeside warming and drying during foehn events is observed in new aircraft, radiosonde and automatic weather station data and simulated by the UK Met Office Unified Model at ~1.5 km grid spacing (MetUM 1.5 km). Three contrasting cases are investigated. In Case A relatively weak southwesterly flow induces a nonlinear foehn event. Strongly accelerated flow above and a hydraulic jump immediately downwind of the lee slopes lead to high amplitude warming in the immediate lee of the AP, downwind of which the warming effect diminishes rapidly due to the upward ‘rebound’ of the foehn flow. Case C defines a relatively linear case associated with strong northwesterly winds. The lack of a hydraulic jump enables foehn flow to flood across the entire ice shelf at low levels. Melt rates are high due to a combination of large radiative heat flux, due to dry, clear leeside conditions, and sensible heat flux downward from the warm, well-mixed foehn flow. Climatological work suggests that such strong northwesterly cases are often responsible for high Larsen C melt rates. Case B describes a weak, relatively non-linear foehn event associated with insignificant daytime melt rates.

    Previously unknown jets – named polar foehn jets – emanating from the mouths of leeside inlets are identified as a type of gap flow. They are cool and moist relative to adjacent calmer regions, due to lower-altitude upwind source regions, and are characterised by larger turbulent heat fluxes both within the air column and at the surface. The relative importance of the three mechanisms deemed to induce leeside foehn warming (isentropic drawdown, latent heating and sensible heating) are quantified using a novel method analysing back trajectories and MetUM 1.5 km model output. It is shown that, depending on the linearity of the flow regime and the humidity of the air mass, each mechanism can dominate. This implies that there is no dominant foehn warming mechanism, contrary to the conclusions of previous work.

  6. Steinhoff, Daniel F., David H. Bromwich, and Andrew Monaghan. “Dynamics of the foehn mechanism in the McMurdo Dry Valleys of Antarctica from Polar WRF.” Quarterly Journal of the Royal Meteorological Society 139.675 (2013): 1615-1631.  Foehn events over the McMurdo Dry Valleys (MDVs), the largest ice‐free region of Antarctica, promote glacial melt that supports biological activity in the lakes, streams, rocks and soils. Although MDVs foehn events are known to depend upon the synoptic‐scale circulation, the physical processes responsible for foehn events are unknown. A polar‐optimized version of the Weather Research and Forecasting model (Polar WRF) is used for a case study of a representative summer foehn event from 29 December 2006 to 1 January 2007 in order to identify and explain the MDVs foehn mechanism. Pressure differences across an elevated mountain gap upstream of the MDVs provide forcing for southerly flow into the western, upvalley entrance of the MDVs. Complex terrain over the elevated gap and the MDVs leads to mountain wave effects such as leeside acceleration, hydraulic jumps, wave breaking and critical layers. These mountain wave effects depend on the ambient (geostrophic) wind direction. Pressure‐driven channelling then brings the warm, dry foehn air downvalley to eastern MDV sites. Brief easterly intrusions of maritime air into the eastern MDVs during foehn events previously have been attributed to either a sea‐breeze effect in summer or local cold‐pooling effects in winter. In this particular case, the easterly intrusions result from blocking effects of nearby Ross Island and the adjacent Antarctic coast. Temperature variability during the summer foehn event, which is important for meltwater production and biological activity when it exceeds 0°C, primarily depends on the source airmass rather than differences in foehn dynamics.
  7. Cape, M. R., et al. “Foehn winds link climate‐driven warming to ice shelf evolution in Antarctica.” Journal of Geophysical Research: Atmospheres 120.21 (2015): 11-037Rapid warming of the Antarctic Peninsula over the past several decades has led to extensive surface melting on its eastern side, and the disintegration of the Prince Gustav, Larsen A, and Larsen B ice shelves. The warming trend has been attributed to strengthening of circumpolar westerlies resulting from a positive trend in the Southern Annular Mode (SAM), which is thought to promote more frequent warm, dry, downsloping foehn winds along the lee, or eastern side, of the peninsula. We examined variability in foehn frequency and its relationship to temperature and patterns of synoptic‐scale circulation using a multidecadal meteorological record from the Argentine station Matienzo, located between the Larsen A and B embayments. This record was further augmented with a network of six weather stations installed under the U.S. NSF LARsen Ice Shelf System, Antarctica, project. Significant warming was observed in all seasons at Matienzo, with the largest seasonal increase occurring in austral winter (+3.71°C between 1962–1972 and 1999–2010). Frequency and duration of foehn events were found to strongly influence regional temperature variability over hourly to seasonal time scales. Surface temperature and foehn winds were also sensitive to climate variability, with both variables exhibiting strong, positive correlations with the SAM index. Concomitant positive trends in foehn frequency, temperature, and SAM are present during austral summer, with sustained foehn events consistently associated with surface melting across the ice sheet and ice shelves. These observations support the notion that increased foehn frequency played a critical role in precipitating the collapse of the Larsen B ice shelf.
  8. Elvidge, Andrew D., and Ian A. Renfrew. “The causes of foehn warming in the lee of mountains.” Bulletin of the American Meteorological Society 97.3 (2016): 455-466.  The foehn effect is well known as the warming, drying, and cloud clearance experienced on the lee side of mountain ranges during “flow over” conditions. Foehn flows were first described more than a century ago when two mechanisms for this warming effect were postulated: an isentropic drawdown mechanism, where potentially warmer air from aloft is brought down adiabatically, and a latent heating and precipitation mechanism, where air cools less on ascent—owing to condensation and latent heat release—than on its dry descent on the lee side. Here, for the first time, the direct quantitative contribution of these and other foehn warming mechanisms is shown. The results suggest a new paradigm is required after it is demonstrated that a third mechanism, mechanical mixing of the foehn flow by turbulence, is significant. In fact, depending on the flow dynamics, any of the three warming mechanisms can dominate. A novel Lagrangian heat budget model, back trajectories, high-resolution numerical model output, and aircraft observations are all employed. The study focuses on a unique natural laboratory—one that allows unambiguous quantification of the leeside warming—namely, the Antarctic Peninsula and Larsen C Ice Shelf. The demonstration that three foehn warming mechanisms are important has ramifications for weather forecasting in mountainous areas and associated hazards such as ice shelf melt and wildfires.

 

 

 

 

 

 

 

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