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A century of variation in the dependence of Greenland iceberg calving on ice sheet surface mass balance and regional climate change.

Bigg GR, Wei HL, Wilton DJ, Zhao Y, Billings SA, Hanna E, Kadirkamanathan V - Proc. Math. Phys. Eng. Sci. (2014)

Bottom Line: A century-long record of Greenland icebergs comes from the International Ice Patrol's record of icebergs (I48N) passing latitude 48° N, off Newfoundland.I48N exhibits strong interannual variability, with a significant increase in amplitude over recent decades.We also suggest that GrIS calving discharge is episodic on at least a regional scale and has recently been increasing significantly, largely as a result of west Greenland sources.

View Article: PubMed Central - PubMed

Affiliation: Department of Geography , University of Sheffield , Sheffield S10 2TN, UK.

ABSTRACT
Iceberg calving is a major component of the total mass balance of the Greenland ice sheet (GrIS). A century-long record of Greenland icebergs comes from the International Ice Patrol's record of icebergs (I48N) passing latitude 48° N, off Newfoundland. I48N exhibits strong interannual variability, with a significant increase in amplitude over recent decades. In this study, we show, through a combination of nonlinear system identification and coupled ocean-iceberg modelling, that I48N's variability is predominantly caused by fluctuation in GrIS calving discharge rather than open ocean iceberg melting. We also demonstrate that the episodic variation in iceberg discharge is strongly linked to a nonlinear combination of recent changes in the surface mass balance (SMB) of the GrIS and regional atmospheric and oceanic climate variability, on the scale of the previous 1-3 years, with the dominant causal mechanism shifting between glaciological (SMB) and climatic (ocean temperature) over time. We suggest that this is a change in whether glacial run-off or under-ice melting is dominant, respectively. We also suggest that GrIS calving discharge is episodic on at least a regional scale and has recently been increasing significantly, largely as a result of west Greenland sources.

No MeSH data available.


Related in: MedlinePlus

Iceberg numbers at 48° N. Yearly totals of the observed iceberg number (in black; I48N), the ocean model's iceberg flux (unvarying discharge and discharge scaled as equation (2.1)) and the NARMAX model's fit to I48N. Right axis shows GrIS calving discharge with I48N used as proxy (in Gt yr−1 or km3 yr−1). The inset shows I48N's mean annual cycle.
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RSPA20130662F2: Iceberg numbers at 48° N. Yearly totals of the observed iceberg number (in black; I48N), the ocean model's iceberg flux (unvarying discharge and discharge scaled as equation (2.1)) and the NARMAX model's fit to I48N. Right axis shows GrIS calving discharge with I48N used as proxy (in Gt yr−1 or km3 yr−1). The inset shows I48N's mean annual cycle.

Mentions: Monthly, counts of iceberg numbers observed south of 48° N in the western North Atlantic have been compiled by the United States Coast Guard since 1913, absorbing previous reports to the US Hydrographic Service back to 1900. While these numbers have always been acknowledged as estimates [9] and come from a range of platforms evolving over time (ships to aircraft, radar, the use of models and more recently satellites [10]), the series (I48N) is the only long-term estimate of iceberg numbers in the northwest Atlantic available. I48N has a strong, and regular, seasonal cycle (figure 2), with a pronounced peak in numbers in spring to early summer. This is likely to be due to a combination of seasonal peaks in discharge [11], a delay effect from the release of icebergs being trapped in winter sea ice [12] and varying travel paths [13]. However, interannual variability in I48N will be due to a combination of variations in the calving flux, overwhelmingly from Greenland [12], as well as climate fluctuations over the northwest Atlantic. It is known that the former can be very variable from year to year for individual glaciers [8,14,15]. However, the Labrador Sea climate can also change very quickly [16]. Therefore, in this study we explore the long-term variability in iceberg discharge from the GrIS through considering I48N using a combination of ocean–iceberg and nonlinear auto-regressive moving average with exogenous input (NARMAX) system identification modelling both to separate the discharge signal in I48N from the mean ocean–atmospheric interactions following calving and to understand the processes and time scales involved in its variation. The NARMAX approach is described below in the Material and methods, but, briefly, it is a modelling framework which allows the user to construct linear or nonlinear dynamic models between inputs (exogenous variables) and outputs (auto-regressive variables) in the presence of coloured and nonlinear noise.Figure 2.


A century of variation in the dependence of Greenland iceberg calving on ice sheet surface mass balance and regional climate change.

Bigg GR, Wei HL, Wilton DJ, Zhao Y, Billings SA, Hanna E, Kadirkamanathan V - Proc. Math. Phys. Eng. Sci. (2014)

Iceberg numbers at 48° N. Yearly totals of the observed iceberg number (in black; I48N), the ocean model's iceberg flux (unvarying discharge and discharge scaled as equation (2.1)) and the NARMAX model's fit to I48N. Right axis shows GrIS calving discharge with I48N used as proxy (in Gt yr−1 or km3 yr−1). The inset shows I48N's mean annual cycle.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4042714&req=5

RSPA20130662F2: Iceberg numbers at 48° N. Yearly totals of the observed iceberg number (in black; I48N), the ocean model's iceberg flux (unvarying discharge and discharge scaled as equation (2.1)) and the NARMAX model's fit to I48N. Right axis shows GrIS calving discharge with I48N used as proxy (in Gt yr−1 or km3 yr−1). The inset shows I48N's mean annual cycle.
Mentions: Monthly, counts of iceberg numbers observed south of 48° N in the western North Atlantic have been compiled by the United States Coast Guard since 1913, absorbing previous reports to the US Hydrographic Service back to 1900. While these numbers have always been acknowledged as estimates [9] and come from a range of platforms evolving over time (ships to aircraft, radar, the use of models and more recently satellites [10]), the series (I48N) is the only long-term estimate of iceberg numbers in the northwest Atlantic available. I48N has a strong, and regular, seasonal cycle (figure 2), with a pronounced peak in numbers in spring to early summer. This is likely to be due to a combination of seasonal peaks in discharge [11], a delay effect from the release of icebergs being trapped in winter sea ice [12] and varying travel paths [13]. However, interannual variability in I48N will be due to a combination of variations in the calving flux, overwhelmingly from Greenland [12], as well as climate fluctuations over the northwest Atlantic. It is known that the former can be very variable from year to year for individual glaciers [8,14,15]. However, the Labrador Sea climate can also change very quickly [16]. Therefore, in this study we explore the long-term variability in iceberg discharge from the GrIS through considering I48N using a combination of ocean–iceberg and nonlinear auto-regressive moving average with exogenous input (NARMAX) system identification modelling both to separate the discharge signal in I48N from the mean ocean–atmospheric interactions following calving and to understand the processes and time scales involved in its variation. The NARMAX approach is described below in the Material and methods, but, briefly, it is a modelling framework which allows the user to construct linear or nonlinear dynamic models between inputs (exogenous variables) and outputs (auto-regressive variables) in the presence of coloured and nonlinear noise.Figure 2.

Bottom Line: A century-long record of Greenland icebergs comes from the International Ice Patrol's record of icebergs (I48N) passing latitude 48° N, off Newfoundland.I48N exhibits strong interannual variability, with a significant increase in amplitude over recent decades.We also suggest that GrIS calving discharge is episodic on at least a regional scale and has recently been increasing significantly, largely as a result of west Greenland sources.

View Article: PubMed Central - PubMed

Affiliation: Department of Geography , University of Sheffield , Sheffield S10 2TN, UK.

ABSTRACT
Iceberg calving is a major component of the total mass balance of the Greenland ice sheet (GrIS). A century-long record of Greenland icebergs comes from the International Ice Patrol's record of icebergs (I48N) passing latitude 48° N, off Newfoundland. I48N exhibits strong interannual variability, with a significant increase in amplitude over recent decades. In this study, we show, through a combination of nonlinear system identification and coupled ocean-iceberg modelling, that I48N's variability is predominantly caused by fluctuation in GrIS calving discharge rather than open ocean iceberg melting. We also demonstrate that the episodic variation in iceberg discharge is strongly linked to a nonlinear combination of recent changes in the surface mass balance (SMB) of the GrIS and regional atmospheric and oceanic climate variability, on the scale of the previous 1-3 years, with the dominant causal mechanism shifting between glaciological (SMB) and climatic (ocean temperature) over time. We suggest that this is a change in whether glacial run-off or under-ice melting is dominant, respectively. We also suggest that GrIS calving discharge is episodic on at least a regional scale and has recently been increasing significantly, largely as a result of west Greenland sources.

No MeSH data available.


Related in: MedlinePlus