Seasonal Retreat Research Articles

Arctic Continental-Shelf Sediment Dynamics.

Sediments covering Arctic continental shelves are uniquely impacted by ice processes. Delivery of sediments is generally limited to the summer, when rivers are ice free, permafrost bluffs are thawing, and sea ice is undergoing its seasonal retreat. Once delivered to the coastal zone, sediments follow complex pathways to their final depocenters-for example, fluvial sediments may experience enhanced seaward advection in the spring due to routing under nearshore sea ice; during the open-water season, boundary-layer transport may be altered by strong stratification in the ocean due to ice melt; during the fall storm season, sediments may be entrained into sea ice through the production of anchor ice and frazil; and in the winter, large ice keels more than 20 m tall plow the seafloor (sometimes to seabed depths of 1-2 m), creating a type of physical mixing that dwarfs the decimeter-scale mixing from bioturbation observed in lower-latitude shelf systems. This review summarizes the work done on subtidal sediment dynamics over the last 50 years in Arctic shelf systems backed by soft-sediment coastlines and suggests directions for future sediment studies in a changing Arctic. Reduced sea ice, increased wave energy, and increased sediment supply from bluffs (and possibly rivers) will likely alter marine sediment dynamics in the Arctic now and into the future.

Tracking the south polar seasonal cap retreat of Mars using computer vision

We use daily polar stereographic mosaics obtained from the Mars Color Imager (MARCI) to autonomously track the recession of the South Polar Seasonal Cap (SPSC) during Mars Years (MYs) 28 to 31, for the period Ls = 178° to Ls = 345°. Utilizing an autonomous approach, we can delineate the SPSC by fitting an ellipse, offering the advantage of faster processing. This allows us to monitor the recession with enhanced time sampling capabilities. The SPSC recession is divided into two segments separated by a discontinuity at Ls = ∼260°, corresponding to the detachment of the Mountains of Mitchel seasonal cap from the main cap. Before the discontinuity, the SPSC exhibits variability where the recession rate can vary by up to 50%, with the fastest recession occurring at Ls = ∼235° and the slowest at Ls = ∼191°. After the discontinuity, the SPSC experiences reduced variability and undergoes a monotonic deceleration until Ls = ∼319°, gradually approaching the stable latitude of ∼86° S, consistent with the South Polar Residual Cap. MY 28 demonstrates distinct behavior in response to the Global Dust Storm (GDS), as the SPSC was smaller and sublimated faster before the onset of the storm compared to the 4-year average. Plain language summaryThe seasonal frost layer on Mars, consisting of frozen carbon dioxide, covers vast expanses during the winters. This frost forms as carbon dioxide gas in the atmosphere turns into solid ice on the surface, impacting the overall atmospheric pressure. We studied daily images captured by the Mars Color Imager (MARCI) to automatically monitor the retreat of the South Polar Seasonal Cap (SPSC) as it returns to the atmosphere during the spring over a span of four years. To achieve this, we employed a computer programming language called Python and a publicly available software library designed for visual analysis. By fitting an ellipse that most closely resembles the contour of the cap, we were able to trace and measure the change in the SPSC over time and determine how quickly SPSC shrinks. We discovered that our strategy produces results that are similar to those of previously published approaches, but in a lot less time and with increasing accuracy. We noticed new patterns in the seasonal retreat of the seasonal cap, including brief periods of faster shrinking, likely influenced by wind patterns. Additionally, we investigated a prominent icy area known as the Mountains of Mitchel seasonal cap and found that its behavior differed before and after a significant global dust storm in Mars Year 28.

Glacier-specific factors drive differing seasonal and interannual dynamics of Nunatakassaap Sermia and Illullip Sermia, Greenland

ABSTRACT Accelerated ice discharge from marine-terminating outlet glaciers accounted for ~48 percent of ice loss from Greenland between 1992 and 2018, and the northwest has been the largest source of dynamic ice loss. Here, we assess the dynamics of two neighboring northwest Greenland glaciers, Nunatakassaap Sermia (NS) and Illullip Sermia (IS), for 2000 to 2020. Retreat rates at NS far exceeded those at IS, and NS accelerated and thinned substantially following the loss of its ice tongue. This was initially driven by loss of buttressing, followed by feedbacks between thinning, surface slope, and effective pressure, as NS retreated into a deeper and wider section of its fjord. At IS, acceleration and thinning were limited due to its location on a bedrock ridge. At NS, net retreat occurred when seasonal retreat persisted through the winter, whereas at IS it resulted from higher summer retreat rates. The timing of seasonal cycles in frontal positions and ice velocities differed markedly between NS and IS: we suggest that ice velocities respond to seasonal meltwater availability at IS and terminus position variations at NS. Overall, our results demonstrate that the dynamic behavior of NS and IS differs markedly at seasonal to decadal timescales and highlight the importance of glacier-specific factors.

Weekly to monthly terminus variability of Greenland's marine-terminating outlet glaciers

Abstract. Seasonal terminus-position variability of Greenland's marine-terminating outlet glaciers is superimposed on multidecadal trends of glacier retreat. To characterize this seasonal variability, we manually digitized terminus positions for 219 marine-terminating glaciers in Greenland from January 2015 through December 2021 using Sentinel-1 synthetic aperture radar (SAR) mosaics. We digitized at a monthly frequency for 199 glaciers and at a 6 d frequency for 20 glaciers. We found that nearly 80 % of glacier termini in Greenland vary significantly on a seasonal basis. For these seasonally varying glaciers, on average, seasonal retreat typically begins in mid-May, and seasonal advance generally commences in early October. The timing of the initiation of the retreat period may be related to the timing of the onset of ice-sheet surface melt. The rate of retreat events peaks in late summer and reaches a minimum in late winter and early spring. The median magnitude of terminus-position seasonality, the difference between glacier length at the dates of peak advance and retreat, is about 220 m. We find a stronger correlation between this magnitude and glacier velocity than between magnitude and glacier width. Terminus-position seasonality can influence longer-term glacier dynamics and, consequently, ice-sheet mass balance. This study contributes to our understanding of terminus-position seasonality for individual glaciers and collectively for glaciers around the entire Greenland Ice Sheet.

Evidence of phytoplankton blooms under Antarctic sea ice

Areas covered in compact sea ice were often assumed to prohibit upper-ocean photosynthesis. Yet, under-ice phytoplankton blooms (UIBs) have increasingly been observed in the Arctic, driven by anthropogenic changes to the optical properties of Arctic sea ice. Here, we show evidence that the Southern Ocean may also support widespread UIBs. We compile 77 time series of water column samples from biogeochemical Argo floats that profiled under compact (80%–100% concentration) sea ice in austral spring–summer since 2014. We find that that nearly all (88%) such measurements recorded increasing phytoplankton biomass before the seasonal retreat of sea ice. A significant fraction (26%) met a observationally determined threshold for an under-ice bloom, with an average maximum chlorophyll-a measurement of 1.13 mg/m3. We perform a supporting analysis of joint light, sea ice, and ocean conditions from ICESat-2 laser altimetry and climate model contributions to CMIP6, finding that from 3 to 5 million square kilometers of the compact-ice-covered Southern Ocean has sufficient conditions to support light-limited UIBs. Comparisons between the frequency of bloom observations and modeled bloom predictions invite future work into mechanisms sustaining or limiting under-ice phytoplankton blooms in the Southern Hemisphere.

The Relationship Between Submarine Melt and Subglacial Discharge From Observations at a Tidewater Glacier

AbstractAt tidewater glacier termini, ocean‐glacier interactions hinge on two sources of freshwater—submarine melt and subglacial discharge—yet these freshwater fluxes are often unconstrained in their magnitude, seasonality, and relationship. With measurements of ocean velocity, temperature and salinity, fjord budgets can be evaluated to partition the freshwater flux into submarine melt and subglacial discharge. We apply these methods to calculate the freshwater fluxes at LeConte Glacier, Alaska, across a wide range of oceanic and atmospheric conditions during six surveys in 2016–2018. We compare these ocean‐derived fluxes with an estimate of subglacial discharge from a surface mass balance model and with estimates of submarine melt from multibeam sonar and autonomous kayaks, finding relatively good agreement between these independent estimates. Across spring, summer, and fall, the relationship between subglacial discharge and submarine melt follows a scaling law predicted by standard theory (melt ∼ discharge1/3), although the total magnitude of melt is an order of magnitude larger than theoretical estimates. Subglacial discharge is the dominant driver of variability in melt, while the dependence of melt on fjord properties is not discernible. A comparison of oceanic budgets with glacier records indicates that submarine melt removes 33%–49% of the ice flux into the terminus across spring, summer, and fall periods. Thus, melt is a significant component of the glacier's mass balance, and we find that melt correlates with seasonal retreat; however, melt does not appear to directly amplify calving.

Exploring the Pacific Arctic Seasonal Ice Zone With Saildrone USVs

More high-quality, in situ observations of essential marine variables are needed over the seasonal ice zone to better understand Arctic (or Antarctic) weather, climate, and ecosystems. To better assess the potential for arrays of uncrewed surface vehicles (USVs) to provide such observations, five wind-driven and solar-powered saildrones were sailed into the Chukchi and Beaufort Seas following the 2019 seasonal retreat of sea ice. They were equipped to observe the surface oceanic and atmospheric variables required to estimate air-sea fluxes of heat, momentum and carbon dioxide. Some of these variables were made available to weather forecast centers in real time. Our objective here is to analyze the effectiveness of existing remote ice navigation products and highlight the challenges and opportunities for improving remote ice navigation strategies with USVs. We examine the sources of navigational sea-ice distribution information based on post-mission tabulation of the sea-ice conditions encountered by the vehicles. The satellite-based ice-concentration analyses consulted during the mission exhibited large disagreements when the sea ice was retreating fastest (e.g., the 10% concentration contours differed between analyses by up to ∼175 km). Attempts to use saildrone observations to detect the ice edge revealed that in situ temperature and salinity measurements varied sufficiently in ice bands and open water that it is difficult to use these variables alone as a reliable ice-edge indicator. Devising robust strategies for remote ice zone navigation may depend on developing the capability to recognize sea ice and initiate navigational maneuvers with cameras and processing capability onboard the vehicles.

Exploring the Pacific Arctic Seasonal Ice Zone With Saildrone USVs.

More high-quality, in situ observations of essential marine variables are needed over the seasonal ice zone to better understand Arctic (or Antarctic) weather, climate, and ecosystems. To better assess the potential for arrays of uncrewed surface vehicles (USVs) to provide such observations, five wind-driven and solar-powered saildrones were sailed into the Chukchi and Beaufort Seas following the 2019 seasonal retreat of sea ice. They were equipped to observe the surface oceanic and atmospheric variables required to estimate air-sea fluxes of heat, momentum and carbon dioxide. Some of these variables were made available to weather forecast centers in real time. Our objective here is to analyze the effectiveness of existing remote ice navigation products and highlight the challenges and opportunities for improving remote ice navigation strategies with USVs. We examine the sources of navigational sea-ice distribution information based on post-mission tabulation of the sea-ice conditions encountered by the vehicles. The satellite-based ice-concentration analyses consulted during the mission exhibited large disagreements when the sea ice was retreating fastest (e.g., the 10% concentration contours differed between analyses by up to ~175 km). Attempts to use saildrone observations to detect the ice edge revealed that in situ temperature and salinity measurements varied sufficiently in ice bands and open water that it is difficult to use these variables alone as a reliable ice-edge indicator. Devising robust strategies for remote ice zone navigation may depend on developing the capability to recognize sea ice and initiate navigational maneuvers with cameras and processing capability onboard the vehicles.

Distinct Frontal Ablation Processes Drive Heterogeneous Submarine Terminus Morphology

AbstractCalving and submarine melt drive frontal ablation and sculpt the ice face of marine‐terminating glaciers. However, there are sparse observations of submarine termini, which limit estimates of spatially varying submarine melt. Here we present a detailed survey of a west Greenland glacier to reveal heterogeneity in submarine terminus morphology. We find that the majority of the terminus (~77%) is undercut, driven by calving in the upper water column and submarine melting at depth. The remaining ~23% of the terminus is overcut, driven by calving alone. We use observations of six subglacial discharge outlets, combined with a plume model, to estimate spatially varying discharge fluxes. While small discharge fluxes (<43 m3/s) feed numerous, deeply undercut outlets with subsurface plumes, ~70% of the net subglacial flux emerges at the terminus center, producing a vigorous, surface‐reaching plume. This primary outlet drives large, localized seasonal retreat that exceeds calving rates at secondary outlets.

MEDITATION IN TAI NUEA LAY BUDDHIST PRACTICE

ABSTRACT The Tai Nuea ethnolinguistic group is found on the periphery of Theravāda Buddhist influence in parts of southwestern China, northern Myanmar, and in small communities in northwestern Laos. Their relative isolation from mainstream reform movements indicates that they may have much to contribute to the understanding of pre-modern local, and especially lay, Buddhist practices in mainland Southeast Asia. This article focuses on weekly days of lay practice during the annual rainy season retreat in a Tai Nuea village in Mueang Sing, northwestern Laos. The practice is undertaken with an awareness of ageing and approaching death by both women and men who are mainly over 50 years of age. It is distinctly lay oriented and takes place with minimal input from the monastic community. There is extensive use of litany and Pāli phrases to request and to take leave of specific activities throughout the day, and during formal meditation small kammaṭṭhāna (meditation) manuals are worn on the head and the entire body is covered with a white cloth. Within a holistic framework of devotion to the Triple Gem and the practices of generosity and morality, meditation is seen as one important component of meritorious activity rather than as a tool for personal transformation.

Geodetic measurements reveal short-term changes of glacial mass near Jakobshavn Isbræ (Greenland) from 2007 to 2017

The Global Positioning System (GPS) and Gravity Recovery and Climate Experiment (GRACE) provide important geodetic datasets to study glacial mass change. Applying the multichannel singular spectral analysis to the GPS-measured vertical and horizonal crustal displacement and GRACE-derived vertical displacement near Jakobshavn Isbræ (JI) in western Greenland from 2007 to 2017, we reconstruct the short-term loading displacements due to ice mass changes. Both the vertical and east displacements show strong seasonal variability. They also reveal three episodes of transient displacements: downward and eastward motion from late 2007 to around 2010, sustained upward and westward motion from 2010 to early 2013, and downward and eastward motion till late 2016. We also forward model the seasonal and transient displacements caused by surface mass balance (SMB) and glacier dynamics. Our model agrees well with the geodetic observations and provides quantitative insights into the contribution from SMB and ice dynamics to the ice mass changes. We find that SMB is the dominant contributor to the seasonal and transient displacements at three out of four GPS sites (AASI, ILUL, and QEQE). While, at the fourth GPS site (KAGA) that is closest to the glacier, the contributions to the transient displacements from SMB and glacier dynamics are comparable. The forward modeling also suggests that the dynamic mass change in the JI catchment underwent strong seasonal variations and these variations correlated more with the seasonal retreat and advance of the calving front than with the changes of glacial velocities. Our altimetry results reveal that the frontal portion of JI catchment lost 34 Gt in 2012 and this loss of ice declined to only 11 Gt in 2016 due to widespread thickening along the main flowline.

A two-dimensional glacier–fjord coupled model applied to estimate submarine melt rates and front position changes of Hansbreen, Svalbard

ABSTRACTWe have developed a two-dimensional coupled glacier–fjord model, which runs automatically using Elmer/Ice and MITgcm software packages, to investigate the magnitude of submarine melting along a vertical glacier front and its potential influence on glacier calving and front position changes. We apply this model to simulate the Hansbreen glacier–Hansbukta proglacial–fjord system, Southwestern Svalbard, during the summer of 2010. The limited size of this system allows us to resolve some of the small-scale processes occurring at the ice–ocean interface in the fjord model, using a 0.5 s time step and a 1 m grid resolution near the glacier front. We use a rich set of field data spanning the period April–August 2010 to constrain, calibrate and validate the model. We adjust circulation patterns in the fjord by tuning subglacial discharge inputs that best match observed temperature while maintaining a compromise with observed salinity, suggesting a convectively driven circulation in Hansbukta. The results of our model simulations suggest that both submarine melting and crevasse hydrofracturing exert important controls on seasonal frontal ablation, with submarine melting alone not being sufficient for reproducing the observed patterns of seasonal retreat. Both submarine melt and calving rates accumulated along the entire simulation period are of the same order of magnitude, ~100 m. The model results also indicate that changes in submarine melting lag meltwater production by 4–5 weeks, which suggests that it may take up to a month for meltwater to traverse the englacial and subglacial drainage network.

Do pelagic grazers benefit from sea ice? Insights from the Antarctic sea ice proxy IPSO<sub>25</sub>

Abstract. Sea ice affects primary production in polar regions in multiple ways. It can dampen water column productivity by reducing light or nutrient supply, provide a habitat for ice algae and condition the marginal ice zone (MIZ) for phytoplankton blooms on its seasonal retreat. The relative importance of three different carbon sources (sea ice derived, sea ice conditioned, non-sea-ice associated) for the polar food web is not well understood, partly due to the lack of methods that enable their unambiguous distinction. Here we analysed two highly branched isoprenoid (HBI) biomarkers to trace sea-ice-derived and sea-ice-conditioned carbon in Antarctic krill (Euphausia superba) and relate their concentrations to the grazers' body reserves, growth and recruitment. During our sampling in January–February 2003, the proxy for sea ice diatoms (a di-unsaturated HBI termed IPSO25, δ13C = −12.5 ± 3.3 ‰) occurred in open waters of the western Scotia Sea, where seasonal ice retreat was slow. In suspended matter from surface waters, IPSO25 was present at a few stations close to the ice edge, but in krill the marker was widespread. Even at stations that had been ice-free for several weeks, IPSO25 was found in krill stomachs, suggesting that they gathered the ice-derived algae from below the upper mixed layer. Peak abundances of the proxy for MIZ diatoms (a tri-unsaturated HBI termed HBI III, δ13C = −42.2 ± 2.4 ‰) occurred in regions of fast sea ice retreat and persistent salinity-driven stratification in the eastern Scotia Sea. Krill sampled in the area defined by the ice edge bloom likewise contained high amounts of HBI III. As indicators for the grazer's performance we used the mass–length ratio, size of digestive gland and growth rate for krill, and recruitment for the biomass-dominant calanoid copepods Calanoides acutus and Calanus propinquus. These indices consistently point to blooms in the MIZ as an important feeding ground for pelagic grazers. Even though ice-conditioned blooms are of much shorter duration than blooms downstream of the permanently sea-ice-free South Georgia, they enabled fast growth and offspring development. Our study shows two rarely considered ways that pelagic grazers may benefit from sea ice: firstly, after their release from sea ice, suspended or sinking ice algae can supplement the grazers' diet if phytoplankton concentrations are low. Secondly, conditioning effects of seasonal sea ice can promote pelagic primary production and therefore food availability in spring and summer.

The blindness of hindsight: Irish and British poets look back on early fascist Italy

ABSTRACTIn the interwar period, the small town of Rapallo, Italy, was the year-round home of Ezra and Dorothy Pound and a seasonal retreat for W. B. Yeats and George Yeats. The promise of good company, the hope of good weather, and the potential for poetic collaboration drew to Rapallo a number of poets who were influential in shaping twentieth-century poetry. However, Pound’s virulent fascism and the Pact of Friendship and Alliance between Germany and Italy (1939) meant that writers were loathe to recognise the degree to which Rapallo was instrumental to late modernist networks. For the most part, biographers have followed suit. This essay attends to memoirs written by Nancy Cunard, H. D., Richard Aldington, and Thomas MacGreevy to illustrate post-war aversions to acknowledging the importance of Rapallo and to demonstrate how writers negotiated their relationship to Pound in constructing their own literary biographies in the shadow of the Second World War.

Decadal variability in coastal phytoplankton community composition in a changing West Antarctic Peninsula

The coastal waters of the West Antarctic Peninsula (WAP) are associated with large phytoplankton blooms dominated by large (>20µm) diatoms however, nanoplankton (<20µm) are also an important component of the food web. The dominant nanoflagellates in the WAP are cryptomonad algae. Using a twenty-year time series collected by the Palmer Long Term Ecological Research program at the United States Palmer Research Station, we assessed long-term patterns and stability in the coastal phytoplankton communities in the WAP. There was significant interannual variability in the integrated water column chlorophyll a (chl-a) concentrations, which varied by a factor of 5 over the 20-year time series. There has been a significant positive increase in the seasonally integrated concentration of chl-a over the time series. The dominant phytoplankton were diatoms, with cryptophytes the second most abundant. Mixed flagellates also constituted a significant fraction of the chl-a but showed less interannual variability than diatoms and cryophytes. Peak phytoplankton biomass was observed in summer months, when monthly averaged wind speed was lower than in the fall and autumn. Cryptophytes were most abundant during the summer months (December-January) after the seasonal retreat of sea ice. While diatoms were observed over the full range of observed salinities 32–34.5) as well as over the full range of in situ temperatures (−1.5 to 2.5°C), the cryptophyte populations were observed in locations with lower salinity 32.5–33.75) and colder water (−1 to 1°C). Environmental factors that favored a shallower seasonal mixed layer resulted in larger diatom blooms compared to the other phytoplankton taxa. During summer with lower phytoplankton biomass, a larger proportion of the chlorophyll a was associated with cryptophytes. These results demonstrate that continued temperature changes along the West Antarctic Peninsula will result in changes in phytoplankton concentration and community composition, which has significant ramifications for the food web.

Variability, trends, and predictability of seasonal sea ice retreat and advance in the Chukchi Sea

AbstractAs assessed over the period 1979–2014, the date that sea ice retreats to the shelf break (150 m contour) of the Chukchi Sea has a linear trend of −0.7 days per year. The date of seasonal ice advance back to the shelf break has a steeper trend of about +1.5 days per year, together yielding an increase in the open water period of 80 days. Based on detrended time series, we ask how interannual variability in advance and retreat dates relate to various forcing parameters including radiation fluxes, temperature and wind (from numerical reanalyses), and the oceanic heat inflow through the Bering Strait (from in situ moorings). Of all variables considered, the retreat date is most strongly correlated (r ∼ 0.8) with the April through June Bering Strait heat inflow. After testing a suite of statistical linear models using several potential predictors, the best model for predicting the date of retreat includes only the April through June Bering Strait heat inflow, which explains 68% of retreat date variance. The best model predicting the ice advance date includes the July through September inflow and the date of retreat, explaining 67% of advance date variance. We address these relationships by discussing heat balances within the Chukchi Sea, and the hypothesis of oceanic heat transport triggering ocean heat uptake and ice‐albedo feedback. Developing an operational prediction scheme for seasonal retreat and advance would require timely acquisition of Bering Strait heat inflow data. Predictability will likely always be limited by the chaotic nature of atmospheric circulation patterns.

Characteristics of colored dissolved organic material in first year landfast sea ice and the underlying water column in the Canadian Arctic in the early spring

The absorption of solar energy by colored dissolved organic material (CDOM) in Arctic surface waters during the springtime is increasingly recognized as having profound impacts on seasonal retreat of ice from the Arctic Ocean. This study observed the production of CDOM within first year landfast sea ice, its contribution to light absorption within the water column and its photoreactivity, at a location near Ellef Ringnes Island in the Canadian Archipelago during the spring of 2011. CDOM was the optically dominant component of non-water absorption, increasing the energy absorption and heating potential in the top 8m by 30% over pure seawater. CDOM absorption within sea ice brine channels increased with solar-stimulated ice algae production. This ice algal mediated production of CDOM exceeded photooxidation rates, potentiating a net delivery of CDOM to the immediately underlying water column from brine channels in the ice. The rates of production and loss of CDOM in Arctic waters during the early spring quantified here will help constrain a major uncertainty in modeling light absorption by surface waters that contributes to sea ice loss in the Arctic.

Regional change in snow water equivalent–surface air temperature relationship over Eurasia during boreal spring

Present study investigates local relationship between surface air temperature and snow water equivalent (SWE) change over mid- and high-latitudes of Eurasia during boreal spring. Positive correlation is generally observed around the periphery of snow covered region, indicative of an effect of snow on surface temperature change. In contrast, negative correlation is usually found over large snow amount area, implying a response of snow change to wind-induced surface temperature anomalies. With the seasonal retreat of snow covered region, region of positive correlation between SWE and surface air temperature shifts northeastward from March to May. A diagnosis of surface heat flux anomalies in April suggests that the snow impact on surface air temperature is dominant in east Europe and west Siberia through modulating surface shortwave radiation. In contrast, atmospheric effect on SWE is important in Siberia and Russia Far East through wind-induced surface sensible heat flux change. Further analysis reveals that atmospheric circulation anomalies in association with snowmelt over east Siberia may be partly attributed to sea surface temperature anomalies in the North Atlantic and the atmospheric circulation anomaly pattern associated with snowmelt over Russia Far East has a close association with the Arctic Oscillation.

Aeolian processes as drivers of landform evolution at the South Pole of Mars

We combine observations of surface morphology, topography, subsurface stratigraphy, and near surface clouds with mesoscale simulations of south polar winds and temperature to investigate processes governing the evolution of spiral troughs on the South Pole of Mars. In general we find that the south polar troughs are cyclic steps that all formed during an erosional period, contrary to the troughs at the North Pole, which are constructional features. The Shallow Radar instrument (SHARAD) onboard Mars Reconnaissance Orbiter detects subsurface stratigraphy indicating relatively recent accumulation that occurred post trough formation in many locations. Using optical instruments, especially the Thermal Emission Imaging System (THEMIS), we find low altitude trough clouds in over 500 images spanning 6 Mars years. The locations of detected clouds correspond to where recent accumulation is detected by SHARAD, and offers clues about surface evolution. The clouds migrate by season, moving poleward from 71° S at ~Ls 200° until Ls 318°, when the last cloud is detected. Our atmospheric simulations find that the fastest winds on the pole are found roughly near the external boundary of the seasonal CO2 ice cap. Thus, we find that the migration of clouds (and katabatic jumps) corresponds spatially to the retreat of the CO2 seasonal ice as detected by Titus (2005) and that trough morphology, through recent accumulation, is integrally related to this seasonal retreat.

Spatiotemporal Variability and Predictability of Relative Humidity over West African Monsoon Region

Abstract Spatial and temporal variability of relative humidity over the West African monsoon (WAM) region is investigated. In particular, the variability during the onset and retreat periods of the monsoon is considered. A K-means cluster analysis was performed to identify spatially coherent regions of relative humidity variability during the two periods. The cluster average of the relative humidity provides a robust representative index of the strength and timing of the transition periods between the dry and wet periods. Correlating the cluster indices with large-scale circulation and sea surface temperatures indicates that the land–ocean temperature gradient and the corresponding circulation, tropical Atlantic sea surface temperatures (SSTs), and to a somewhat lesser extent tropical Pacific SSTs all play a role in modulating the timing of the monsoon season relative humidity onset and retreat. These connections to large-scale climate features were also found to be persistent over interseasonal time scales, and thus best linear predictive models were developed to enable skillful forecasts of relative humidity during the two periods at 15–75-day lead times. The public health risks due to meningitis epidemics are of grave concern to the population in this region, and these risks are strongly tied to regional humidity levels. Because of this linkage, the understanding and predictability of relative humidity variability is of use in meningitis epidemic risk mitigation, which motivated this research.