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沃茨区域动态和海平面变化的主要驱动因素。北极的新鲜表层沃茨保持着强烈的分层现象。这种盐跃层在很大程度上保护了寒冷的极地表面水和海冰，使其免受下面来自大西洋的温暖沃茨的影响，从而抑制了垂直热通量。淡水含量的变化可能会影响区域空间海平面，但改变海洋动力学也可能通过北极内部的物质输送改变海平面。河川径流的水文情势呈现非平稳性。然而，关于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。除了现场观测和水文模型外，卫星重力测量和卫星测高还开辟了观测大型集水区水文的新途径，即测量集水区的蓄水量和沿着的水位，并可通过几种方法将其转化为流量。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循环是平稳的。在这里，我们建议解除这一假设。我们建议使用一个海冰-海洋环流模式来研究年际变化的径流对北极环流和淡水收支的影响。拟议研究的主要目标是评估和量化北极海平面和水文地理对北方集水区水文状况变化的反应。该项目的目标有助于1889年优先研究方案的战略，方法是：1）通过大地测量空间方法（时变重力测量、卫星测高）改进北方集水区水文参数的观测记录。实现所有主要集水区流入北冰洋径流的长时间密集时间序列。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（固定径流与年际变化径流）与伴随方法相结合，通过单一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年以来北冰洋和北大西洋大陆径流变化与观测到的液态淡水含量变化（以及由此产生的空间高度变化）的模式输出比较。这些比较不仅作为模型评估，但他们也将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.