MULTIDECADAL VARIABILITY IN HIGH-LATITUDE OCEAN
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One of the key findings of this study is striking coherent pattern of long-term variations of the key components of the Arctic Ocean freshwater and heat budgets: upper Arctic Ocean freshwater content (FWC) and heat content of the intermediate Atlantic Water (AW) of the Arctic Ocean.
Upper Arctic Ocean FWC: Observational data demonstrate that over the 20th century the central Arctic Ocean became increasingly saltier. In contrast, long-term FWC trends over the Siberian shelf show general freshening tendency. These FWC trends are modulated by strong MDV with sustained and widespread patterns. Associated with this variability, the FWC record shows two periods in the 1920-30s and in recent decades when the central Arctic Ocean was saltier and two periods in the earlier century and in the 1940-70s when it was fresher. Our analysis of potential causes for the recent central Arctic Ocean salinification suggests that the FWC anomalies generated on arctic shelves (including anomalies resulting from river discharge inputs) and those caused by net atmospheric precipitation were too small to trigger long-term FWC variations in the central Arctic Ocean; to the contrary, they tend to moderate the observed long-term central-basin FWC changes. Variability of the intermediate Atlantic Water did not have apparent impact on changes of the upper Arctic Ocean water masses. Our estimates suggest that ice production and sustained draining of freshwater from the Arctic Ocean in response to winds are the key contributors to the salinification of the upper Arctic Ocean over recent decades. Strength of the export of arctic ice and water controls the supply of Arctic fresh water to sub-polar basins while the intensity of the Arctic Ocean FWC anomalies is of less importance.
AW heat content: Over the 20th century Atlantic Water variability was dominated by MDV. Associated with this variability, the Atlantic Water temperature record shows two warm periods in the 1930-40s and in recent decades and two cold periods in the earlier century and in the 1960-70s. Over recent decades, the data show a warming and salinification of the Atlantic layer, accompanied by its shoaling and, probably, thinning. Our estimate of the Atlantic Water temperature variability shows a general warming trend; however, over the 100-year record there are periods (including the recent decades) with short-term trends strongly amplified by multi-decadal variations. Our hydrographic data support a negative feedback mechanism through which changes of density act to moderate the inflow of Atlantic Water to the Arctic Ocean, consistent with the decrease of positive Atlantic Water temperature anomalies in the late 1990s.
Finally, since the high-latitude heat/freshwater budgets play a crucial role in establishing and regulating global thermohaline circulation, the long-term variations of the freshwater content discussed here should be considered when assessing climate change and variability.