Variability and Trends of the Alaska Gyre From Argo and Satellite Altimetry

The interannual variability and trends of the Alaska Gyre and Gulf of Alaska (GOA) circulation are examined using meridional geostrophic transport from Argo temperature and salinity (2004-2017) and altimetric sea surface height (1993-2017). More than half of the top 1,500 m meridional transport variability in the Alaska Gyre is accounted for by a statistical mode strongly correlated with the Pacific Decadal Oscillation (PDO) index, consistent with the PDO exerting a major influence on North Pacific sea surface temperature variability. During a positive phase of the PDO, the zero-transport streamline separating the subtropical from the Alaska Gyre is shifted to the south from its mean position, while more transport is diverted northward, associated with a stronger and larger Alaska Gyre. Additionally, over the 25-year altimetric record there is a linear, increasing trend in strength of the Alaska Gyre (but not in areal extent), accompanied by an increasing trend for the incoming North Pacific Current. The effect of the PDO transport mode on GOA circulation is weak. Temperature and salinity volume averaged for the GOA covary with the PDO index, with warmer and fresher waters during a positive phase. Despite correlated anomalies for temperature, salinity, and northward transport into the GOA, however, geostrophic advection from the south contributes only minimally to the interannual variations of water properties in the GOA. An exception was the marine heat wave of 2013/2014 and its aftermath when temperature advection from the south played a more appreciable role for warming and subsequent cooling of the GOA. Plain Language Summary Variability of the ocean circulation in the Alaska Gyre is one of the factors influencing the rich ecosystem in the Gulf of Alaska. Improved understanding of the variability and trends of the Alaska Gyre is thus of prime concern, given its biologic and economic implications. Using data from the Global Argo program and satellite altimetry covering the 25-year period from 1993 to 2017, we diagnose the changes in Alaska Gyre structure and transport and how they fit in the larger picture of North Pacific global climate modes, such as the Pacific Decadal Oscillation (PDO). During a positive phase of the PDO, more transport is diverted northward, associated with a stronger and larger Alaska Gyre. Additionally, over the 25-year record the Alaska Gyre has continuously increased in strength (but not in size). While temperature and salinity in the Gulf of Alaska vary with the PDO, with warmer and fresher waters during a positive phase, advection from the south contributes only minimally to the year-to-year variations of water properties in the Gulf of Alaska, suggesting that these variations are mostly determined by local processes instead.