Response of Arctic sea level and hydrography to hydrological regime change over boreal catchments (RASLyBoCa)

流入北冰洋的淡水的变化是北极水域区域动态和海平面变化的关键驱动因素。在北极，新鲜的地表水维持着强烈的分层。这个盐跃层在很大程度上保护了寒冷的极地地表水和海冰，使其不受下面来自大西洋的温暖水域的影响，从而抑制了垂直热通量。淡水含量的变化可能会影响区域立体海平面，但海洋动力学的改变也可能通过北极内部的物质运输改变海平面。河流径流的水文状况似乎是不稳定的。然而，排放到北冰洋的观测记录仍然太少，无法解决有关北极海平面和气候的长期行为和发展的重要科学问题。除了原位观测和水文模型之外，卫星重力测量（GRACE）和卫星测高为观测大型集水区的水文开辟了额外的新途径，即通过测量集水区的储水量和河流的水位，这些水可以通过几种方法转化为排放。对于海冰-海洋模型，径流信息的稀疏性（在北极）通常要求假定径流的季节周期是平稳的。在这里，我们建议取消这一假设。我们建议使用海冰-海洋环流模型来研究年际变化径流对北极环流和淡水收支的影响。提出的研究的主要目的是评估和量化北极海平面和水文地形对北方集水区水文制度变化的响应。该项目的目标有助于1889年优先研究方案的战略：1)通过空间测量方法（时变重力测量、卫星测高）改进北方流域水文参数的观测记录。2)海冰和海洋动力学对淡水强迫（包括径流）变化的敏感性分析。为此，将配置一个泛北极模式，并将前向扰动方法（固定与年际变化径流）与伴随方法相结合，其中通过单个伴随模式模拟评估全球目标函数（例如全球或区域淡水含量或海平面）对所有强迫参数的敏感性。3) 1990年以来，模式输出与大陆径流变化的关系与观测到的北冰洋和北大西洋液态淡水含量（以及由此产生的空间高度变化）的变异性的比较。这些比较不仅可以作为模式评估，而且还有助于解释相对稀疏的海洋原位记录。

Changes in freshwater influx into the Arctic Ocean are a key driver of regional dynamics and sea level change in the Arctic waters. Fresh surface waters maintain a strong stratification in the Arctic. This halocline largely shields the cool polar surface water and sea ice from the warmer waters of Atlantic origin below and hence inhibits vertical heat fluxes. Changes in the freshwater content are likely to affect regional steric sea level, but also the modified ocean dynamics may change sea level through mass transports within the Arctic. The hydrological regime of river runoff appears to be non-stationary. However, the observational record of discharge into the Arctic Ocean is still too sparse to address important science questions into the long-term behaviour and development of Arctic sea level and climate. Apart from in-situ observations and hydrological models, satellite gravimetry (GRACE) and satellite altimetry open up additional new avenues of observing the hydrology of large catchments literally by measuring the water storage over catchments and water level along rivers, which can be transformed into discharge through several methods. For sea ice-ocean models, the sparsity of runoff information (in the Arctic) generally requires that the seasonal cycle of runoff is assumed to the stationary. Here we propose to lift this assumption. We propose to use a sea ice-ocean general circulation model to study the impact of interannually varying runoff on the Arctic circulation and freshwater budget. The main objective of the proposed study is to assess and quantify the response of Arctic sea level and hydrography to hydrological regime change over boreal catchments. The project objectives contribute to the strategy of the Priority Research Programme 1889 by1) Improving the observational record of hydrological parameters over boreal catchments by geodetic spaceborne methods (time varying gravimetry, satellite altimetry). Achieving long and dense time series of runoff for all major catchments draining into the Arctic Ocean. 2) Sensitivity analysis of sea ice and ocean dynamics to changes in freshwater forcing (including runoff). To this end a pan-Arctic model will be configured and methods of forward perturbations (stationary vs. interannually varying runoff) will be combined with adjoint methods, where the sensitivity of global objective functions, e.g. global or regional freshwater content or sea level, to all forcing parameters is assessed through single adjoint model simulations. 3) Comparison of model output in relation to changes in continental runoff with observed variability of liquid freshwater content (and resulting steric height changes) in the Arctic Ocean and the North Atlantic since 1990. Not only do these comparisons serve as model evaluation, but they will also help interpreting comparatively sparse oceanographic in-situ records.