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Wkejts Rwilliamsa
Wkejts Rwilliamsa


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Sea ice covers the ocean surrounding Antarctica, forming a key component of the coupled ocean-atmosphere-cryosphere system in southern high latitudes that helps regulate climate, ocean circulation, and marine ecosystems. The extent of Antarctic sea ice varies greatly from year to year, but 40 years of satellite records show a long-term trend. Although some Antarctic regions have experienced reductions in sea ice extent, the overall trend since 1979 shows increased ice.

One novel approach to identify the mechanisms behind observed trends is to use a nudging technique. In this technique, one or several variables in a climate model are relaxed, or nudged, toward an observed value (over either the whole model domain or a part of it) while the rest of the model is left unchanged (i.e., free-running). A comparison of three recent studies in which winds, ice drift, and sea surface temperatures were nudged individually or in combination yields valuable insights into the influence of these factors on Antarctic sea ice extent.

Antarctic sea ice is strongly coupled to the overlying atmosphere through winds, air temperatures, and other factors. Because sea ice drift is strongly controlled by surface winds, trends in surface winds can create corresponding trends in sea ice drift [e.g., Holland and Kwok, 2012]. Wind can also indirectly affect sea ice across different seasons through its influence on the open ocean (e.g., by affecting mixing of the upper ocean layers and the heat fluxes between the ocean and atmosphere). For example, spring winds that lead to summer cooling of sea surface temperatures (SSTs) may enhance sea ice formation in the subsequent fall freeze-up.

Over the past 40 years, significant trends have been observed in atmospheric circulation over Antarctica. These trends, which are important for sea ice, include a strengthening of the westerlies and a deepening of the cyclonic Amundsen Low. The trends are a result of complex interactions between natural (internal) variability and external forcings such as changes in atmospheric ozone and greenhouse gases. Yet climate models tend to underestimate coupling between winds and sea ice as well as these decades-long atmospheric circulation trends [e.g., Holland et al., 2017].

To investigate the impact of wind-driven ice motion on sea ice expansion from another angle, Sun and Eisenman [2021] ran separate experiments with CESM1 that directly nudged sea ice drift in the model to match satellite-derived drift observations from 1992 to 2015. This approach eliminated possible model biases introduced by simulating the coupling between winds and sea ice drift and thus had the advantage of isolating the effects of ice motion alone. However, it had the disadvantage of not accounting for potential indirect effects of winds on sea ice (such as by summer winds over open ocean), thus omitting processes that could drive ice expansion. In their results, Sun and Eisenman show that their simulations reproduce a significant portion of the observed sea ice trend and also capture regional patterns.

Antarctic sea ice is bordered by the ice-free Southern Ocean, which itself has experienced cooling SSTs over recent decades. This cooling trend is partly due to natural variability and partly due to upwelling currents that bring to the surface deep waters that have not yet warmed because of anthropogenic climate change [e.g., Armour et al., 2016].

Zhang et al. [2021] addressed this question through ensemble modeling with CESM1 that nudged Southern Ocean SST anomalies to match the observed anomalies from 1979 to 2013. In these experiments, the ensemble mean Antarctic sea ice trend over this period is near zero, meaning there was no change in extent. This trend compares with a calculated loss of 0.36 million square kilometers of sea ice per decade in the free-running CESM1-LENS model and an actual observed gain of 0.23 million square kilometers per decade. Thus, the SST nudging compensates for 60% of the model bias seen with CESM1-LENS (0.36 million out of 0.59 million square kilometers per decade).

To further assess the role of Southern Ocean SSTs, Blanchard-Wrigglesworth et al. [2021] ran another set of experiments in which both winds and Southern Ocean SST anomalies were nudged to match observations from 1979 to 2018. In these experiments, the overall sea ice trend was essentially zero, compared with the 0.4 million square kilometers lost per decade calculated in CESM1-LENS from 1979 to 2018 and with the observed trend of 0.1 million square kilometers added per decade. Thus, these experiments in which both winds and SSTs were nudged captured 80% of the discrepancy between the observed trend and that simulated by CESM1-LENS. The regional sea ice trends in these experiments were only slightly improved compared to the regional sea ice trends in the winds-only nudged experiments, indicating that winds are more important than remote SSTs for regional sea ice trends.

While Nazi Germany introduced a series of improvements to the Enigma over the years, and these hampered decryption efforts, they did not prevent Poland from cracking the machine as early as December 1932 and reading messages prior to and into the war. Poland's sharing of her achievements enabled the western Allies to exploit Enigma-enciphered messages as a major source of intelligence.[2] Many commentators say the flow of Ultra communications intelligence from the decrypting of Enigma, Lorenz, and other ciphers shortened the war substantially and may even have altered its outcome.[3]

The Enigma machine was invented by German engineer Arthur Scherbius at the end of World War I.[4] The German firm Scherbius & Ritter, co-founded by Scherbius, patented ideas for a cipher machine in 1918 and began marketing the finished product under the brand name Enigma in 1923, initially targeted at commercial markets.[5] Early models were used commercially from the early 1920s, and adopted by military and government services of several countries, most notably Nazi Germany before and during World War II.[6]

Each rotor can be set to one of 26 possible starting positions when placed in an Enigma machine. After insertion, a rotor can be turned to the correct position by hand, using the grooved finger-wheel which protrudes from the internal Enigma cover when closed. In order for the operator to know the rotor's position, each has an alphabet tyre (or letter ring) attached to the outside of the rotor disc, with 26 characters (typically letters); one of these is visible through the window for that slot in the cover, thus indicating the rotational position of the rotor. In early models, the alphabet ring was fixed to the rotor disc. A later improvement was the ability to adjust the alphabet ring relative to the rotor disc. The position of the ring was known as the Ringstellung ("ring setting"), and that setting was a part of the initial setup needed prior to an operating session. In modern terms it was a part of the initialization vector.

The Naval version of the Wehrmacht Enigma had always been issued with more rotors than the other services: At first six, then seven, and finally eight. The additional rotors were marked VI, VII and VIII, all with different wiring, and had two notches, resulting in more frequent turnover. The four-rotor Naval Enigma (M4) machine accommodated an extra rotor in the same space as the three-rotor version. This was accomplished by replacing the original reflector with a thinner one and by adding a thin fourth rotor. That fourth rotor was one of two types, Beta or Gamma, and never stepped, but could be manually set to any of 26 positions. One of the 26 made the machine perform identically to the three-rotor machine.

The advancement of a rotor other than the left-hand one was called a turnover by the British. This was achieved by a ratchet and pawl mechanism. Each rotor had a ratchet with 26 teeth and every time a key was pressed, the set of spring-loaded pawls moved forward in unison, trying to engage with a ratchet. The alphabet ring of the rotor to the right normally prevented this. As this ring rotated with its rotor, a notch machined into it would eventually align itself with the pawl, allowing it to engage with the ratchet, and advance the rotor on its left. The right-hand pawl, having no rotor and ring to its right, stepped its rotor with every key depression.[19] For a single-notch rotor in the right-hand position, the middle rotor stepped once for every 26 steps of the right-hand rotor. Similarly for rotors two and three. For a two-notch rotor, the rotor to its left would turn over twice for each rotation.

To make room for the Naval fourth rotors, the reflector was made much thinner. The fourth rotor fitted into the space made available. No other changes were made, which eased the changeover. Since there were only three pawls, the fourth rotor never stepped, but could be manually set into one of 26 possible positions.

Some M4 Enigmas used the Schreibmax, a small printer that could print the 26 letters on a narrow paper ribbon. This eliminated the need for a second operator to read the lamps and transcribe the letters. The Schreibmax was placed on top of the Enigma machine and was connected to the lamp panel. To install the printer, the lamp cover and light bulbs had to be removed. It improved both convenience and operational security; the printer could be installed remotely such that the signal officer operating the machine no longer had to see the decrypted plaintext.

Here the enciphering begins trivially with the first "mapping" representing the keyboard (which has no effect), followed by the plugboard, configured as AE.BF.CM.DQ.HU.JN.LX.PR.SZ.VW which has no effect on 'G', followed by the VIII rotor in the 03 position, which maps G to A, then the VI rotor in the 17 position, which maps A to N, ..., and finally the plugboard again, which maps B to F, producing the overall mapping indicated at the final step: G to F. 59ce067264


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