Coupled Evolution of Ice Shelf and Ocean in the Amundsen Sea Sector of Antarctica

随着我们的星球变暖，冰盖缩小，这是一个将水从陆地转移到海洋从而提高海平面的过程。这一结果最终可能使全球海平面上升10米，这在直觉上似乎是显而易见的。然而，在南极冰盖的情况下，起作用的过程并不明显。冰盖上方的大气太冷，不能促使大量融化，因此所有落在内陆的雪都会以冰的形式返回海洋，只有在漂浮后才会融化。寒冷的大气层产生了冰冷的表层水域，因此融化冰层的大部分热量来自海洋内部深处。随着浮冰从下面融化，所谓的冰架变薄，使冰更快地从陆地上流出，从而抬高海平面。所以，潜在的过程是清楚的，但为什么随着气候变暖，它会导致南极洲的冰流失？融化冰的水在海洋中太深，感觉不到大气变暖。然而，随着大气变暖，环流模式发生了变化，影响了驱动洋流的风，从而将更多的深层温暖的水输送到冰层。理解这些过程是如何运作的一直是具有挑战性的。目前还不清楚为什么风的变化会给冰层带来更多而不是更少的温暖水。然而，观察和建模给了我们一个一致的答案，随着我们将研究重点放在关键未知上，我们对过程的理解也在增长。然而，还有一个谜题到目前为止受到的关注要少得多。更多的温水导致冰架融化得更快，冰架变薄，冰流离开陆地的速度加快。这种流动的加速将更多的冰输送到冰架上，因此冰架应该开始增长，或者至少变薄得不那么快，除非海洋热量输送继续增长。直到最近，人们一直认为这正是正在发生的事情，但随着我们对海洋观测记录的延长，我们已经看到了十年一遇的变暖和变冷周期。那么，为什么冰架要继续变薄呢？答案肯定在于冰架变薄本身对融化速度的影响。同样，目前还不清楚为什么冰的变化应该增加而不是减少融化。然而，在这种情况下，观测关键过程是格外困难的，因为它们发生在100米甚至1000米厚的冰下。这是我们将通过这个项目解决的挑战，派遣一艘自动潜水艇在冰下进行关键的海洋测量，包括水温和洋流。这些对冰层下海洋的直接观测将使我们能够验证我们用来模拟过程的海洋模型是正确的，或者如果它们不正确的话，我们可以改进它们。这将不是第一次进行这样的测量，但新的观测将在两个重要方面与过去极少数的观测有所不同。其中一些将是早期测量的重复，因此我们将在冰架范围发生重大变化之前和之后进行观测。因此，我们可以直接回答伴随着冰盖形状变化而发生的海洋环流变化的问题。其他观测将针对直到最近冰层搁浅的地区。因为雷达信号能穿透冰层，但不能穿透海水，所以只有当冰层停留在陆地上，而不是漂浮在水面上时，我们才能绘制出地形。因此，自相矛盾的是，我们对新形成的海洋洞穴的几何形状的了解，比我们对人类首次探索南极地区以来一直存在的洞穴的了解要准确得多。因此，我们理解空洞几何形状和海洋环流之间联系的能力在新开放的空洞中得到了加强，这些空洞是我们实地行动的目标之一。

As our planet warms the ice cover shrinks, a process that transfers water from land to ocean and thereby raises sea level. The result, which could ultimately raise global sea level by 10s of metres, seems intuitively obvious. However, in the case of the Antarctic Ice Sheet, the processes at work are less than obvious. The atmosphere over the ice sheet is too cold to drive significant melting, so all the snow that falls in the interior is returned to the ocean as ice that only melts once it is afloat. The cold atmosphere creates cold surface waters, so most of the heat that melts the ice comes from deep within the ocean's interior. As it melts the floating ice from underneath, the thinning of the so-called ice shelves allows ice to flow off the land more rapidly, hence raising sea level.So, the underlying process is clear, but why should it drive a loss of ice from Antarctica as the climate warms? The waters that melt the ice are too deep in the ocean to feel atmospheric warming. However, as the atmosphere warms the circulation patterns change, influencing the winds that drive the ocean currents, and that delivers more of the deep warm water to the ice. Understanding how the processes work has been challenging. It is not immediately obvious why a change in the winds should deliver more, rather than less, warm water to the ice. Nevertheless, observation and modelling give us a consistent answer and our understanding of the processes grows as we focus our research on key unknowns.However, there is another puzzle that has received much less attention to date. More warm water leads to more rapid melting of the ice shelves, they thin and the flow of ice off the land accelerates. That acceleration of the flow delivers more ice to the ice shelves, and they should therefore start to grow, or at least thin less rapidly, unless the ocean heat delivery continues to grow. Until recently it was assumed that that is exactly what was happening, but as our record of ocean observations has lengthened, we have seen decadal cycles of warming and cooling. Why then should the ice shelves continue to thin?The answer must lie in the way in which the thinning of the ice shelves themselves affects the melt rate. Again, it is not immediately clear why the change in the ice should increase rather than decrease the melt. However, in this case observation of the key processes is exceptionally difficult because they take place beneath 100s or even 1000s of metres of ice.That is the challenge we will address with this project, by sending an autonomous submarine beneath the ice to make the critical measurements of the ocean, including the temperature of the water and the currents. Those direct observations of the ocean beneath the ice will allow us to verify that the ocean models we use to simulate the processes are correct, or to improve them if they are not.This will not be the first time such measurements have been made, but the new observations will differ in two important respects from the very few that have been made in the past. Some will be repeats of earlier measurements, so we will have observations from before and after a significant change in the extent of the ice shelf. Thus, we can directly answer the question of what change in the ocean circulation accompanied the change in shape of the ice cover. Other observations will target regions where the ice was grounded until recently. Because radar signals penetrate ice, but not seawater, we are able to map the topography only when the ice rests on the land and not when it is afloat. Thus, we paradoxically know the geometry of newly formed ocean cavities with much greater accuracy than we do the cavities that have been there since humans first explored the south polar regions. Our ability to understand the links between cavity geometry and ocean circulation is therefore enhanced in the newly opened cavities that are among the targets of our field campaign.