Continental patterns of bird migration linked to climate variability

For ˜100 years, the continental patterns of avian migration in North America have been described in the context of three or four primary flyways. This spatial compartmentalization often fails to adequately reflect a critical characterization of migration — phenology. This shortcoming has been partly due to the lack of reliable continental-scale data, a gap filled by our current study. Here, we leveraged unique radar-based data quantifying migration phenology and used an objective regionalization approach to introduce a new spatial framework that reflects interannual variability. Therefore, the resulting spatial classification is intrinsically different from the “flyway concept”. We identified two regions with distinct interannual variability of spring migration across the contiguous U.S. This data-driven framework enabled us to explore the climatic cues affecting the interannual variability of migration phenology, “specific to each region” across North America. For example, our “two-region” approach allowed us to identify an east-west dipole pattern in migratory behavior linked to atmospheric Rossby waves. Also, we revealed that migration movements over the western U.S. was inversely related to interannual and low-frequency variability of regional temperature. A similar link but weaker and only for interannual variability was evident for the eastern region. However, this region was more strongly tied to climate teleconnections, particularly to the East Pacific-North Pacific (EP-NP) pattern. The results suggest that oceanic forcing in the tropical Pacific—through a chain of processes including Rossby wave trains—controls the climatic conditions, associated with bird migration over the eastern U.S. Our spatial platform would facilitate better understanding of the mechanisms responsible for broad-scale migration phenology and its potential future changes.

Variability and changes in Pearl River Delta water level: oceanic and atmospheric forcing perspectives

Understanding water level (WL) fluctuations in river deltas is of importance for managing water resources and minimizing the impacts of floods and droughts. Here, we demonstrate the competing effects of atmospheric and oceanic forcing on multi-timescale variability and changes in the Pearl River Delta (PRD) WLs in southern China, using 52 years (1961–2012) of in-situ observations at 13 hydrological stations. PRD WL presents significant seasonal to decadal variations, with large amplitudes upstream related to strong variability of southern China rainfall, and with relatively small amplitudes at the coastal stations determined by sea level (SL) fluctuations of the northern South China Sea. We find that the strengths of atmospheric and oceanic forcing in PRD are not mutually independent, leading to a distinct contrast of WL–forcing relationships at upstream and coastal stations. In the transition zone, because of counteracts of atmospheric and oceanic forcing, no robust relationships are identified between WL and either of the forcing. We further show that in the drought season of the warm ENSO and PDO epochs, the effect of atmospheric (oceanic) forcing on PRD WL is largely enhanced (weakened), due to increased southern China rainfall and negative SL anomalies. Over the observation period, WL significantly decreased at upstream stations, by up to 28–42 mm/year for flood season, contrasting with the upward trends of <4.3 mm/year at coastal stations across all seasons. Southern China rainfall explains little of the observed WL trends, whilst SL rise is mostly responsible for the WL trends at coastal stations.

Using Acoustic Travel Time to Monitor the Heat Variability of Glacial Fjords

Monitoring the heat content variability of glacial fjords is crucial to understanding the effects of oceanic forcing on marine-terminating glaciers. A Pressure-sensor equipped Inverted Echo Sounder (PIES) was deployed mid-fjord in Sermilik Fjord in southeast Greenland from August 2011 to September 2012 alongside a moored array of instruments recording temperature, conductivity and velocity. Historical hydrography is used to quantify the relationship between acoustic travel time and the vertically-averaged heat content, and a new method is developed for filtering acoustic return echoes in an ice-influenced environment. We show that PIES measurements, combined with a knowledge of the fjord’s two-layer density structure, can be used to reconstruct the thickness and temperature of the inflowing water. Additionally, we find that fjord-shelf exchange events are identifiable in the travel time record implying the PIES can be used to monitor fjord circulation. Finally, we show that PIES data can be combined with moored temperature records to derive the heat content of the upper layer of the fjord where moored instruments are at great risk of being damaged by transiting icebergs.

Global and monthly glacier mass balance from radar altimetry from 2010 to 2020

&lt;p&gt;Glaciers are currently experiencing the largest land-ice imbalance and are the largest contributor to sea level rise after ocean thermal expansion, contributing ~30% to sea level budget. Global monitoring of these regions remains a challenging task since global estimates rely on a variety of observations and models to achieve the required spatial and temporal coverage, and significant differences remain between current estimates. Here we report, for the first time, the application of radar altimetry to retrieve spatially and temporally resolved elevation and mass changes of glaciers on a global scale. We apply interferometric swath altimetry to CryoSat-2 data acquired between 2010 and 2020 over all large mountain glacier regions and provide monthly and annual time series of glacier mass loss for each region, together with linear mass losses. We report ubiquitous and sustained ice loss ranging from 82.3 &amp;#177; 6.3 Gt yr&lt;sup&gt;&amp;#8722;1&lt;/sup&gt; in Alaska, to 3.4 &amp;#177; 2.5 Gt yr&lt;sup&gt;&amp;#8722;1&lt;/sup&gt; for the Antarctica Periphery. While there is a considerable spatial and temporal variability in imbalance, reflecting the complexity of regional atmospheric and oceanic forcing and of glacier forcing, the global glacier trend is remarkably sustained over this period. Globally, glaciers have lost a combined mean of 275 &amp;#177; 15 Gt yr&lt;sup&gt;&amp;#8722;1&lt;/sup&gt; between 2010 and 2020 contributing 0.76 &amp;#177; 0.5 mm yr&lt;sup&gt;&amp;#8722;1 &lt;/sup&gt;to global Sea Level Rise.&lt;/p&gt;

Contrasting retreat patterns of east Greenland tidewater glaciers

&lt;p&gt;Between 2000 and 2010, glaciers on Greenland&amp;#8217;s east coast were shown to have distinct contrasts in patterns and rates of ice front retreat north and south of 69&amp;#176;N latitude. The correspondence of this transition zone with the northern limit of subtropical waters carried by the Irminger Current has led to the hypothesis that variability in coastal heat transport is the dominant mechanisms causing this regional difference (e.g. Seale et al. 2011). However, whether these regional differences exist for recent glacier change is unknown. Here we examine seasonal and interannual variability in Landsat-8 derived ice-front positions with respect to atmospheric and oceanic forcings for 24 east Greenland outlet glaciers between 2013 and 2017.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;We find that all glaciers exhibit seasonal advance and retreat cycles proportional to glacier width and velocity, though there is a distinct difference between the interannual trends of glacier termini north and south of 69&lt;sup&gt;o&lt;/sup&gt;N throughout our study period. Glaciers above this latitude showed either limited or gradual terminus change over time that was mostly linear on annual timescales. This contrasts with glaciers south of 69&amp;#176;N where step-wise retreat was observed between 2016 and 2017, following a period of relative stability between 2013 and 2016. We find that retreat south of 69&amp;#176;N during 2016 was coincident with periods of anomalously warm atmospheric and subsurface oceanic temperatures, and a marked decline in sea ice/m&amp;#233;lange. Warm atmospheric conditions were also experienced north of 69&amp;#176;N, though subsurface oceanic temperature increases and changes in m&amp;#233;lange cover were not as marked. Our work supports the hypothesis that differences in the terminus response of glaciers either side of 69&amp;#176;N can be explained by contrasting oceanic forcing regimes above and below this latitude.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;References:&amp;#160;Seale, A., Christoffersen, P., Mugford, R. I. and O&amp;#8217;Leary, M. (2011) Ocean forcing of the Greenland Ice Sheet: Calving fronts and patterns of retreat identified by automatic satellite monitoring of eastern outlet glaciers. Journal of Geophysical Research Letters, &lt;strong&gt;116&lt;/strong&gt;, doi: 10.1029/2010JF001847.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

The contrasting response of outlet glaciers to interior and ocean forcing

The dynamics of marine-terminating outlet glaciers are of fundamental interest in glaciology and affect mass loss from ice sheets in a warming climate. In this study, we analyze the response of outlet glaciers to different sources of climate forcing. We find that outlet glaciers have a characteristically different transient response to surface-mass-balance forcing applied over the interior than to oceanic forcing applied at the grounding line. A recently developed reduced model represents outlet-glacier dynamics via two widely separated response timescales: a fast response associated with grounding-zone dynamics and a slow response of interior ice. The reduced model is shown to emulate the behavior of a more complex numerical model of ice flow. Together, these models demonstrate that ocean forcing first engages the fast, local response and then the slow adjustment of interior ice, whereas surface-mass-balance forcing is dominated by the slow interior adjustment. We also demonstrate the importance of the timescales of stochastic forcing for assessing the natural variability in outlet glaciers, highlighting that decadal persistence in ocean variability can affect the behavior of outlet glaciers on centennial and longer timescales. Finally, we show that these transient responses have important implications for attributing observed glacier changes to natural or anthropogenic influences; the future change already committed by past forcing; and the impact of past climate changes on the preindustrial glacier state, against which current and future anthropogenic influences are assessed.

Deglacial sea ice variability at the continental margin off western Dronning Maud Land

&lt;p&gt;Reconstructions of sea ice conditions proximal to the Antarctic coast are often hampered by a limited preservation potential of diatoms in these areas. While silica frustules are affected by opal dissolution, specific organic molecules, highly branched isoprenoids (HBIs) produced by diatoms, are well preserved in continental margin and shelf sediments and may help to overcome this gap. Here, we present biomarker and geochemical data obtained from a very well &lt;sup&gt;14&lt;/sup&gt;C-dated gravity core from the continental slope off Atka Bay in the northeastern part of the Weddell Sea. HBIs, the HBI-based PIPSO&lt;sub&gt;25&lt;/sub&gt; index (Vorrath et al., 2019), glycerol dialkyl glycerol tetraether (GDGT) proxies and phytosterols reveal highly variable sea ice conditions and water temperatures as well as primary productivity changes over the last deglacial. These biomarker records are compared to ice core data and further complemented by physical property and XRF scanning data to estimate potential linkages between oceanic forcing and ice-shelf dynamics.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;References&lt;/p&gt;&lt;p&gt;Vorrath, M.E., M&amp;#252;ller, J., Esper, O., Mollenhauer, G., Haas, C., Schefu&amp;#223;, E., and Fahl, K., 2019. Highly branched isoprenoids for Southern Ocean sea ice reconstructions: a pilot study from the Western Antarctic Peninsula. Biogeosciences, v. 16, no. 15, p. 2961-2981.&lt;/p&gt;

Reflection Seismic Interpretation of Topography and Acoustic Impedance beneath Thwaites Glacier, West Antarctica

&lt;p&gt;Thwaites Glacier (TG), West Antarctica, is losing mass in response to oceanic forcing. Future evolution could lead to deglaciation of the marine basins of the West Antarctic ice sheet, depending on ongoing and future climate forcings, but also on basal topography/bathymetry, basal properties, and physical processes operating within the grounding zone. Hence, it is important to know the actual distribution of bed types of TG&amp;#8217;s interior and grounding zone, and to incorporate them accurately in models to improve estimates of retreat rates and stability. Here we determine bed reflectivity and acoustic impedance via amplitude analysis of reflection seismic data. We report on the results from two lines &amp;#8211; a longitudinal (L-Line) and a transverse (N-Line) &amp;#8211; on a central flowline of TG 100 km inland from the grounding zone. There is considerable variability in bed forms and properties, both within this dataset and in-comparison with nearby work. Notably, we find the same hard (bedrock) stoss and soft (till) lee pattern observed elsewhere on TG in prior work. Physical understanding indicates the basal flow law describing motion over different regions of TG&amp;#8217;s bed likely varies from nearly viscous over the hard bedrock regions to nearly plastic over soft till regions, providing a template for modeling.&lt;/p&gt;