Steric sea level change: An arctic perspective

Is the Arctic Ocean getting fresher in response to an accelerated hydrologic cycle? Is it warming like most of the rest of the World Ocean? And what is the combined effect of heat and freshwater changes on steric sea level? These are the questions that we address in this talk. We have analyzed hydrographic (temperature and salinity) data from the World Ocean Database (WOD'01, published by the National Oceanographic Data Center), from the Arctic Ocean Database (AOA, published by the Environmental Working Group), and from selected individual cruises over the North Pacific, North Atlantic, and Arctic Oceans. The period of analysis is 1950-2000. The data are integrated over 0-1000 m depth and presented as freshwater content (halosteric height), heat content (thermosteric height), and dynamic topography (total steric height). First, we present the long-term mean of these quantities from both polar stereographic and sector zonal average viewpoints. These clearly illustrate the steric height minimum in the Nordic Seas that drives 2 circulation branches: (i) the northern limb of the Atlantic Ocean Meridional Overturning Circulation and (ii) the "arctic backdoor" i.e., the flow of freshwater across the Arctic Ocean from the Pacific to the Atlantic Ocean. In terms of change, we analyze annual anomalies, and find in the North Atlantic subpolar gyre freshening, cooling, and steric height decrease in the late 1960s, presumably in response to Arctic Ocean sea ice export during the Great Salinity Anomaly. About five years later, steric height dropped precipitously in the Arctic Ocean, and the Nordic Seas responded with a freshening and steric height rise. These trends generally continued through the 1980s, but started to reverse during the 1990s. Meanwhile, the North Pacific Ocean warmed and steric height rose during 1975-1995, with weak freshwater trends, and a reversal after 1995. Comparing steric height trends over the 1960s-1990s, we see little change in the steric height gradient between the North Pacific Ocean and the Nordic Sea, but some evidence for an increase in this gradient between the North Pacific Ocean and the Labrador Sea. The implications for circulation will be discussed.