Study Of Characteristics Of Water In The Overflow Source Area In The Nordic Seas And Its Transport Mechanism

As a contact between the North Atlantic and the Arctic Ocean, the Nordic Seas play an important role in the global climate system. The Atlantic water with high temperature and salinity is affected by topography and air-sea interaction, sinks to the intermediate or deep layers through convection in the Greenland Sea and forms denser water which flows southward. This water spills to the deep layers of the North Atlantic and can supply the North Atlantic Deep Water (NADW) at the Greenland-Scotland Ridge as overflows, which has a significant impact on the global climate system and the large-scale ocean circulation. Studying of this process comprehensively is of great importance to predicting the Arctic Mediterraneans’s impact on the climate change and response to it.Studies about the deep convection and overflow process have a long history, but it is still not clear for the link between the two. This is the key subject of this thesis. We form a theoretical framework about mechanisms about the Greenland -Scotland Ridge (GSR) overflows water’s formation in the Nordic Seas, based on the high spatial resolution data obtained during an expedition of the Nordic Seas in 2014. We bring up the concept of "first- stage"overflows of the Greenland Sea, and study how it affects the GSR overflows. We can use temperature and salinity data to deduce flows, but it’s hard to separate the density-driven flow and wind-driven flow completely. To overcome this shortcoming, the steric height in the Nordic Seas is also studied, and compared with the sea surface height data, which gives us hints about the density-driven flow.Greenland Sea water has higher density, and its associated potential energy drives this water spreading to other areas, in the form similar to Greenland Scotland Ridge overflows, so we can assume that there are two stages about overflows, and the outflow of the Greenland Sea can be taken as the first stage which lies in the interior Nordic Seas. The "first-stage" overflow can only occur in a particular location because of the constraints of topography, and the Jan Mayen Channel is its main passage, while there are also intermittent overflows in the gaps of the Mohn Ridge. The flow contributes to the GSR overflow in two ways. The intermediate water of the Greenland Sea with slightly high temperature and salinity passes Jan Mayen Channel and flows southward along the Jan Mayen Ridge, which can contribute to the GSR overflow directly; the water with low temperature and salinity in the upper layers gathers in the northern Norwegian Basin after passing the Mohn Ridge, and forms a small "cold water reservoir" there. The water from upper 200 meters of "cold reservoir" transforms gradually, and might eventually join the Atlantic water in the Norwegian Sea. The water of 200-300 meters subducts in the vicinity of the front, and sink to the 300-600 meters of the Norwegian Sea, forming a huge reservoir of cold water. The cold reservoir drives a large quantity of water southwards, and forms the major part of the GSR overflows. The formation and maintain of this cold reservoir is owing to the distributions of the fronts in northern Norwegian Sea. The Jan Mayen Front (JMF) located west of the Norwegian Basin is limited to the upper 300 meters or less, while the Norwegian Current Front (NCF), which separates the Atlantic Water of the Norwegian Basin and the Lofoten Basin, mainly locates in 300-600 meters depth. This frontal distribution favors the colder and fresher water in the Greenland Sea flowing into the interior Norwegian Sea through isopycnal mixing, and forms a cold reservoir. The water below 600 meters is too heavy to overflow directly because of its high density, and this water exchanges between the basins under dynamic conditions as the base of the overflow waters.The steric height in the Nordic Seas during the period of 1993-2010 is calculated using the EN3 dataset.The average stetic height is low in the central part and high along the coast of Norway, which is highly correlated with the distribution of the flow field. By comparing the steric height and sea surface height anomaly derived from the satellite altimeter, we find that there is a high correlation between the two in the Lofoten Basin and northern Norwegian Basin, corresponding to the cold reservoir in the 2014 Nordic Seas expedition, which indicates that the region has a strong baroclinic effect. In addition, the amplitudes of the Nordic Seas area-mean sea surface height anomalies and steric hight anomalies are highly consistent in 1993-2002. But the amplitude of steric hight anomaly is significantly lower than that of sea surface height anomaly, especially after 2006, the former can only explain less than 50% of the latter.