http://nsidc.org/arcticseaicenews/
資料來源：美國國家冰雪資料中心
最後更新日期：2017/10/5
Arctic sea ice 2017: Tapping the brakes in September
北極海冰2017現況：融冰速度踩煞車的九月
After setting a record low seasonal maximum in early March, Arctic sea ice
extent continued to track low through July. However, the rate of ice loss
slowed in August and September. The daily minimum extent, reached on
September 13, was the eighth lowest on record, while the monthly average
extent was seventh lowest. In Antarctica, sea ice extent may have reached its
annual winter maximum.
北極海冰在三月初達到破紀錄新低的最大面積後，開始穩定縮減至七月。然而在八到九月
北極海冰消融的速度放緩，在9/13達到最小面積（此為歷史第八低，而九月平均面積為歷
史第七低）。在南極，海冰面積可能已達到冬天最大值。
Overview of conditions　現況概述
Arctic sea ice extent for September 2017 averaged 4.87 million square
kilometers (1.88 million square miles), the seventh lowest in the 1979 to
2017 satellite record. This was 1.67 million square kilometers (645,000
square miles) below the 1981 to 2010 average, and 1.24 million square
kilometers (479,000 square miles) above the record low September set in 2012.
北極海冰面積九月平均為487萬平方公里（188萬平方英里），為自1979年以來歷史第七
低。和1981至2010平均相比少了167萬平方公里（64.5萬平方英里），而比歷史最低的2012
年九月多了124萬平方公里（47.9萬平方英里）。
After reaching the minimum on September 13 (eighth lowest on record), extent
initially increased slowly (about 20,000 square kilometers, or 8,000 square
miles, per day). However, starting September 26 and persisting through the
end of the month, ice growth rates increased to about 60,000 square
kilometers (23,000 square miles) per day. During the second half of the
month, extent increased in all sectors except in the Beaufort Sea, where some
local ice retreat persisted. The most rapid growth occurred along the
Siberian side of the Arctic Ocean, where the ice edge advanced as much as 150
kilometers (90 miles) over the latter half of September. At the end of
September, the ice edge in the Beaufort and Chukchi Seas remained
considerably further north than is typical.
在9/13達到（歷史第八低的）最小面積後，北極海冰面積開始（以約每天2萬平方公里，
8000平方英里速度）緩慢增加。但是從9/26開始至月底，速度加快至每天6萬平方公里（
2.3萬平方英里）。在後半月，除了波佛特海仍有冰層消退，其餘地方結冰面積皆開始增
加。西伯利亞是增加最快的地方，冰層邊緣長度在後半月增加將近150公里（90英里）。
在九月底，波特佛海和楚科奇海的冰層邊緣離正常位置仍北了許多。
https://goo.gl/kEwJr5
Near-record open water reported in Beaufort and Chukchi seas
波佛特海和楚科奇海的未結冰水域面積已逼近歷史紀錄
http://nsidc.org/arcticseaicenews/files/2017/10/n_extn_hires.png
Figure 1. Arctic sea ice extent for September 2017 was 4.87 million square
kilometers (1.88 million square miles). The magenta line shows the 1981 to
2010 average extent for that month. Sea Ice Index data. About the data
圖1. 北極海冰面積在九月為487萬平方公里（188萬平方英里）。紅紫色為1981至2010九月
同期的平均面積。
Conditions in context　詳細現況
September air temperatures at the 925 hPa level (approximately 2,500 feet
above sea level) were above average over much of the Arctic Ocean.
Temperatures ranged from 5 degrees Celsius (9 degrees Fahrenheit) above the
1981 to 2010 long term average in the far northern Atlantic east of
Greenland, to 1 to 2 degrees Celsius (2 to 4 degrees Fahrenheit) above the
reference period in the western Arctic. Cooler conditions (1 degree Celsius
or 2 degrees Fahrenheit below average) were present in Baffin Bay. Part of
the above average temperatures over the coastal areas of the Arctic Ocean and
in the northern North Atlantic likely results from heat fluxes from open
water.
北極海多數地方九月925百帕層（約海拔2500英呎）的溫度皆高於平均。格陵蘭東北方海域
比1981至2010年平均高了攝氏5度（華氏9度），北極海西部高了攝氏1~2度（華氏2~4
度）。巴芬灣則為負距平（低了攝氏1度，華氏2度）。在北極海與北大西洋北方部分區域
的正距平造成熱通量進而導致仍有未結凍水域的產生。
Looking back at this past summer (June through August), air temperatures at
the 925 hPa level averaged for June through August were near or below the
1981 to 2010 average over much of the Arctic Ocean, notably along the
Siberian side centered over the Laptev Sea (1 degree Celsius or 1.8 degrees
Fahrenheit below the 1981 to 2010 average). By contrast, temperatures were
slightly above average over much of the East Siberian, Chukchi and Beaufort
Seas (1 degree Celsius, or 1.8 degrees Fahrenheit above average).
六至八月，925百帕層的溫度將近或低於1981至2010年平均，特別是在西伯利亞的拉普捷夫
海（低於攝氏1度，華氏1.8度）。相對地，東西伯利亞、波佛特海與楚科奇海則略高於平
均（攝氏1度，華氏1.8度）。
Like 2016, the summer of 2017 was characterized by persistently stormy
patterns over the central Arctic Ocean, reflected in the summer average sea
level pressure field (Figure 2b) as an area of low pressure centered just
south of the North Pole in the Siberian sector of the Arctic. As has been
shown in past studies, low pressure systems found over the central Arctic
Ocean in summer are typically “cold cored.” This helps to explain the cool
summer temperatures noted above. The cyclonic (counterclockwise) winds
associated with the stormy pattern also tend to spread out the sea ice. Both
processes likely helped to slow sea ice loss this summer.
如同2016年，2017年夏天北極海中心為暴風型，反映出夏天平均環境場為低壓中心，除了
北極海南部的西伯利亞扇形區域。過去研究顯示，低壓環境場在夏天常為典型冷源。這幫
助解釋涼夏的原因。逆時針環流除了和暴風型態有關也有助於擴張海冰。兩者都可能幫助
減緩夏天海冰消融速度。
http://nsidc.org/arcticseaicenews/files/1954/10/asina_N_iqr_timeseries.png
Figure 2a. The graph above shows Arctic sea ice extent as of October 4, 2017,
along with daily ice extent data for five previous years. 2017 is shown in
blue, 2016 in green, 2015 in orange, 2014 in brown, 2013 in purple, and 2012
in dotted brown. The 1981 to 2010 median is in dark gray. The gray areas
around the median line show the interquartile and interdecile ranges of the
data. Sea Ice Index data.
圖2a. 至2017/10/4與過去五年、1980至2010年平均北極海冰面積曲線。灰色區域為四分位
距與十分位距。
http://nsidc.org/arcticseaicenews/files/2017/10/PressAnomSL_JJA2017.png
Figure 2b. This image shows the departure from average sea level pressure in
millibars over the Arctic for June, July, and August in 2017. Yellows and
reds indicate higher than average pressures; blues and purples indicate lower
than average pressures.
圖2b. 六至八月海平面氣壓距平分布，黃色與紅色為正距平；藍色與紫色為負距平。
September 2017 compared to previous years　2017年九月與過去比較
The linear rate of sea ice decline for September is 86,100 square kilometers
(33,200 square miles) per year, or 13.2 percent per decade relative to the
1981 to 2010 average. For comparison, the decline rate was calculated at 13.7
percent after the 2013 minimum, and 13.4 percent in 2016. Although sea ice
shows significant year-to-year variability, the overall trend of decline
remains strong.
1981至2010年九月海冰以每年8.61萬平方公里（3.32萬平方英里）速度，每十年13.2%比例
遞減。此比例數字在2013年為13.7%，2016年為13.4%。雖然每年可能有變異，但大體趨勢
是非常明顯的。
http://nsidc.org/arcticseaicenews/files/2017/10/monthly_ice_09_NH_v2.1.png
Figure 3. Monthly September ice extent for 1979 to 2017 shows a decline of
13.2 percent per decade.
圖3. 1979至2017年九月海冰面積呈現每十年13.2%比例遞減。
Thickness and age trends in Arctic sea ice from models and data
北極海冰厚度與年齡
According to estimates from the University of Washington Polar Science Center
’s PIOMAS, which assimilates observational data into a coupled ice-ocean
model, sea ice volume was at record low levels from January through June of
2017. However, the generally cool summer conditions slowed the rate of ice
melt, and the ice volume for September ended up fourth lowest in the PIOMAS
record, above 2010, 2011, and 2012.
根據華盛頓大學極區科學中心PIOMAS模式，將過去海冰觀測資料模型同步比較，海冰體積
在2017年一至六月創下歷史新低。但涼夏環境減少消融速度，九月結算為歷史第四低，僅
次於2010、2011、2012三年。
Another way to assess the volume of the ice, at least in a qualitative sense,
is through tracking sea ice age (Figure 4b). Older ice is generally thicker
ice. Over the satellite record, there has been a significant decline in
coverage of the oldest, thickest ice. While this year’s minimum sea ice
extent is higher than in 2016, the marginal gain can be largely attributed to
younger ice types: first-year ice (0 to 1 years old) and second-year ice (1
to 2 years old). The oldest ice, that which is over 4 years old, is only
slightly higher than last year and remains almost non-existent within the
Arctic. At the minimum this year, ice older than 4 years constituted only
~150,000 square kilometers (~58,000 square miles), compared to over 2 million
square kilometers (~770,000 square miles) during the mid-1980s.
另一個至少在定性層面推估海冰體積的方式為海冰年齡追蹤。老冰通常較厚。根據衛星紀
錄，老冰和厚冰呈現顯著下降。雖然今年海冰最小面積高於去年，大多數歸因於年輕的
冰：1歲的冰（0~1年）和2歲的冰（1~2年）。超過四年的老冰僅略微高於去年且並不在北
極海。今年最小面積時，超過四年的老冰僅有15萬平方公里（5.8萬平方英里），而1980年
中期為超過200萬平方公里（77萬平方英里）。
http://nsidc.org/arcticseaicenews/files/2017/10/figure4-2.png
Figure 4a. This image from the Pan-Arctic Ice Ocean Modeling and Assimilation
System (PIOMAS) shows Arctic sea ice thickness departures from average
(anomaly) in meters for September 2017, relative to the 2000 to 2015 average.
Reds indicate thicker than average ice; blues indicate thinner than average
ice.
圖4a. PIOMAS顯示與2000至2015年平均相比，2017年九月厚冰已偏離北極海。紅色為厚於
平均；藍色為薄於平均。
http://nsidc.org/arcticseaicenews/files/1954/10/figure4b.png
Figure 4b. Sea ice age distribution at the annual minimum extent for 1985
(upper left) and 2017 (upper right). Time series (bottom) of different age
categories the minimum extent for 1985 to 2017. Note that the ice age product
does not include ice in the Canadian Archipelago. Data from Tschudi et al.,
EASE-Grid Sea Ice Age, Version 3
圖4b. 1985年（左上）與2017年（右上）最小面積時海冰年齡分布。1985年至2017年海冰
年齡分布的時間序列（下圖）。須注意此圖不包含北極群島的冰。
Antarctic maximum extent　南極海冰達到最大值
Antarctic sea ice may have reached its maximum extent on September 15, at
17.98 million square kilometers (6.94 million square miles), among the
earliest maxima on record. If this date and extent hold, it will be the
second-lowest daily maximum in the satellite record, 20,000 square kilometers
(7,700 square miles) above 1986. Antarctic sea ice extent has been at record
or near-record lows since September 2016. A series of recent studies have
explored causes of the sudden decline in extent that occurred in austral late
winter and spring of 2016. Most studies conclude that an unusual period of
strong meridional winds—consistent with a very pronounced negative phase of
the Southern Annular Mode index, coupled with a significant ‘wave-3 pattern’
in the atmospheric circulation—were the cause. A ‘wave-3 pattern’ refers
to a tendency for circulation around the southern continent to resemble a
three-leaf clover, rather than the more typical near-zonal (along lines of
longitude) pattern.
南極海冰可能已在9/15達到最大面積，1798萬平方公里（694萬平方英里）。如果未來沒有
特殊變化，將會是歷史第二低，比1986年多了2萬平方公里（7700平方英里）。過了9/16，
海冰面積一直將近或已創下同期新低。一系列的研究顯示可能是由於2016年澳洲較晚入冬
與入春導致。
http://nsidc.org/arcticseaicenews/files/1954/10/chart2.png
Figure 5. The graph above shows Antarctic sea ice extent as of October 4,
2017, along with daily ice extent data for 2017 (aqua), 2016 (red), 2013
(dotted green), and 1986 (yellow). The 1981 to 2010 median is in dark gray.
The gray areas around the median line show the interquartile and interdecile
ranges of the data. Sea Ice Index data.
圖5. 至2017/10/4與過去五年、1980至2010年平均南極海冰面積曲線。灰色區域為四分位
距與十分位距。
P.S. 後面的部分牽涉到不少專有名詞，為了避免翻譯謬誤誤導板友，故僅附上原文，
　　 歡迎各位本科系的板友對後面的內容進行解釋。
The Maud Rise polynya, discussed in our last post, continues to grow and is
now at about 35,000 square kilometers (14,000 square miles). A recent study
(see Further reading, below) discusses how its formation is related to
climate patterns and natural variability, and that the recent reappearance
supports a forecast by an updated climate model.
Driftwood and long-term changes in Arctic ice movement
While the satellite record has been key in documenting large declines in the
Arctic sea ice cover during the past four decades, the data record is still
relatively short. To provide a longer record, scientists turn to the geologic
record and proxy data. One approach is to analyze the age, transport, and
deposition of driftwood. Driftwood distribution depends strongly on past sea
ice conditions and ocean currents. New research using 913 driftwood samples
collected across the western Arctic (Figure 6) has shed new insight on sea
ice changes during the Holocene, between 12,000 years ago to present. During
the early Holocene (12,000 to 8,000 years ago), the analysis suggests that
the clockwise Beaufort Gyre dominated Arctic Ocean circulation, allowing more
sea ice to stay within the Arctic Ocean. In the mid-Holocene (8,000 to 4,000
years ago), temperatures were higher and the Transpolar Drift dominated,
leading to more ice export out of the Arctic Ocean through Fram Strait and
less sea ice in the Arctic Ocean. In the late Holocene (4,000 years ago to
present), the Beaufort Gyre once again strengthened as temperatures slowly
cooled until the most recent several decades.
https://goo.gl/GudGtA
Figure 6. The maps show two modes of wintertime Arctic sea ice circulation
patterns. (a) shows the Low Arctic Oscillation (AO) index has a strong
Beaufort Gyre which supports ice re-circulation within the Arctic. (b) shows
the High AO index in which the Beaufort Gyre is weak and the Transpolar Drift
expands, leading to Arctic ice exported in a shorter time interval. Bold
numbers show the average time in years for ice starting from various
locations to be exported through Fram Strait under the illustrated patterns.
The red dashed lines encircle the region of ice recirculation and persistence
(Rigor et al., 2002). Over continents, light blue lines show watersheds with
named major rivers (shown as bold blue lines) that export driftwood into the
Arctic Ocean. Green letters indicate driftwood sample regions: CAA, Canadian
Arctic Archipelago; EG, East Greenland; JM, Jan Mayen; NG, North Greenland;
FJL, Franz Josef Land; NZ, Novaya Zemlya; SB, Svalbard. Circulation patterns
compiled and modified from Rigor et al. (2002).