Martinson and Iannuzzi: Weddell Gyre and Global Climate

We examine the spatiotemporal variability of the upper ocean-sea ice system of the Atlantic sector of the Southern Ocean subpolar seas (Weddell gyre), and the nature of its covariability with extrapolar climate, identifying teleconnections and their mechanisms. To systematically evaluate the sporadic and sparse distribution of subpolar data we employed an optimal analysis involving Empirical Orthogonal Functions (EOFs). The EOFs reveal that the spatial pattern of coherent spatial covariability of Weddell gyre characteristics is dominated by high interannual variability near the northern (circumpolar) rim of the gyre and lesser variability (10-20% of the variance) in the gyre's central core region. We find considerable, statistically-significant teleconnections between subpolar and extrapolar climate. The dominant link is with ENSO over its broad region of influence, whereby the subpolar upper ocean response is enhanced winter-average cyclonic forcing during tropical warm events (El Niño); the opposite occurs for cold events (La Niña). During El Niño the subpolar gyre contracts so the pycnocline shallows in the gyre center and deepens at the northern rim; sea ice expands northward leading to enhanced surface freshwater in the northern rim. This regional subpolar response is consistent with recent GCM modeling analyses showing that equatorial warm anomalies trigger an increase in the Pacific equator-pole meridional temperature gradient which shifts the subtropical jet equatorward, and farther from the available potential energy of the Antarctic, decreasing the cyclone activity and climatological forcing of the Pacific subpolar gyres. The Pacific equatorial warming also perturbs the Walker cell circulation leading to the opposite response in the Atlantic, resulting in increased cyclonic forcing in the Atlantic's subpolar gyre. We also find that the Weddell gyre interior OAI characteristics covary with sea ice extent in the Atlantic region, and inversely with the sea ice extent in the Amundsen/Bellingshausen regions, reflecting a strong Antarctic Dipole.