High resolution global modelling of ocean tides

潮汐通常被认为是海面高度的有规律的变化。我们所认为的“潮汐”是由许多不同的潮汐信号组成的--一些是由太阳驱动的，一些是由月球驱动的--其中最大的潮汐信号在大约12或24小时内上下振荡。这些潮汐或多或少地独立运作，有时会相互加强，产生异常大的潮差（大潮），有时会部分抵消，产生异常小的潮差（小潮）。在大多数沿海地区，12小时潮族给出了主要响应（如英国周围），但在某些地方（如太平洋），24小时潮汐族较大。这个项目的主要目的是制作一个整个地球仪的海洋潮汐的计算机模型--但不使用任何关于潮汐本身的数据。这样的模型将简单地使用来自太阳和月球的作用力以及海洋的深度和形状的信息。这是一个重要的目标，因为在我们能够建立这样一个模型之前，我们不能确定我们理解了形成潮汐的所有过程。这种模式的一个主要障碍是所谓的内部潮流的重要性。这是一种发生在海洋内部的独特潮汐，从海洋表面几乎看不到。它的发生是因为海洋底部有冷（重）水，顶部有暖（轻）水，允许波浪在两者之间产生。内潮与我们所熟悉的地表潮汐密切相关，而且在决定它们的过程中确实起着重要的作用。因此，为了确定表面的潮汐，我们还必须在计算机模型中包括内部潮汐。然而，内潮的水平波长很短，从5公里到150公里不等，需要大量的计算资源来模拟整个地球仪上的这些波。最新的计算机模型试图做到这一点，但效率相当低，需要在大型计算机上进行长时间的模拟（通常还需要进行一些其他近似）。我们将以一种完全不同的方式来构建我们的计算机模型，它保证比其他模型快得多，并且将包括内部潮汐的所有重要细节。这种方法的第一个输出将是一个模型，该模型将在台式计算机上运行，在几个小时内产生八个主要潮汐信号的全球预测。然而，我们将通过开发一个在超级计算机上运行的替代版本来扩展这种方法，以查看更精细的细节。这将需要开发新的算法来处理大量的未知数。该模型的输出将用于帮助我们了解潮汐能如何在海洋中传输。这种计算机模型的主要好处之一是，它们可以用来预测过去（或未来）的潮汐。所需要的只是海平面如何随时间变化的记录（或预测）。有可靠的记录表明，自大约26，000年前的最后一个冰河时代以来，海平面发生了怎样的变化，当时的海平面比现在低了大约120米。我们将利用这些记录来详细研究这段时间内潮汐的变化。这些变化的细节对海洋学家（他们研究能量在海洋周围转移的方式，以及这如何帮助确定全球气候）和气候科学家（因为大的冰河时代潮汐可能与漂浮的冰架发生破坏性相互作用，导致快速气候变化）很感兴趣。

Tides are usually thought of as regular variations in the height of the sea surface. What we think of as `the' tide is made up of many distinct tidal signals - some driven by the Sun and some by the Moon - the largest of which oscillate up and down over about 12 or 24 hours. These tides operate more or less independently, and sometimes reinforce each other to give an unusually large tidal range (spring tides) and sometimes partially cancel each other to give an unusually small tidal range (neap tides). At most coastal locations the 12 hour family of tides give the dominant response (such as around the U.K.), but the 24 hour family of tides is larger in some locations (such as in the Pacific). The main aim of this project is to produce a computer model of ocean tides over the whole globe - but without using any data about the tides themselves. Such a model will simply use information about the forcing from the Sun and Moon, and the depth and shape of the oceans. This is an important goal, since until we can produce such a model we cannot be sure that we understand all the processes which shape the tides. A major obstacle to such models is the importance of a so-called internal tide. This is a distinct tide which occurs inside the ocean, and is barely visible from the ocean surface. It occurs because the ocean has cold (heavy) water at the bottom and warm (light) water at the top, allowing waves to be generated in between. The internal tides go hand-in-hand with the familiar tides at the surface, and indeed play an important role in determining them. So, to determine the tides at the surface we must also include the internal tides in our computer model. However, the internal tides have a short horizontal wavelength, ranging from about 5km to 150km, and significant computing resources are required to model these waves over the entire globe. The newest computer models attempt to do this, but in a rather inefficient way, requiring long simulations on large computers (and typically making some other approximations, too). We will construct our computer model in a completely different way, which promises to be much faster than other models, and will include the all-important details of the internal tides. The first output of this approach will be a model which will run on a desktop computer, producing global predictions of the eight main tidal signals in a few hours. However, we will extend this approach to look at finer details by developing an alternative version to run on a supercomputer. This will require the development of new algorithms to deal with the huge number of unknowns. The output of this model will be used to help us understand how tidal energy is transferred within the ocean. One of the main benefits of such computer models is that they can be used to predict tides in the past (or future). All that is required is a record (or prediction) of how sea level changes over time. There are reliable records of how the sea level has changed since the height of the last Ice Age, about 26,000 years ago, when sea levels were about 120 metres lower than at present. We will use these records to make a detailed examination of how tides have changed over this period of time. The details of these changes are interesting to oceanographers (who study the way in which energy is transferred around the ocean, and how this helps determine the global climate), and climate scientists (since it is possible that large Ice Age tides destructively interacted with floating ice shelves leading to rapid climate change).