The Ross Sea Response to Evolving Ocean-Ice Interactions in a Changing Climate

Abstract

Early 1990s to late 2000s freshening (ΔS ≈ -0.001–0.002) and warming (Δθ ≈ 0.02°C–0.035°C) of bottom waters was detected in the southern Pacific Ocean, and Ross Sea source waters progressively freshened during the past four decades. This study investigates potential freshwater anomaly sources and quantifies their effect.

Glacial melt water inputs to the GCT increased by 1.3 km^3 per decade (1976– 2007), more rapidly so after 2000 (6.8 km^3 per decade), freshening local Shelf Water by 0.0004 per decade. Lighter basal melt inputs to the LAT started in 1994 and also picked up after 2000 to 14.9 km^3 per decade, lowering the local Antarctic Surface Water salinity by -0.017 per decade. Upstream in the Amundsen Sea surface water freshened by -0.03 per decade (1994–2007) mostly (50%) from larger melt water inputs from the Pine Island (17.7 km^3 per decade) and Dotson (14.8 km^3 per decade) glaciers.

Two decades of steady (1978-2000) strengthening of sea ice productivity (200 km^3 per decade) within the Ross Sea Polynya suddenly reversed to weakening (-98.6 km^3 per decade) and resulted in Shelf Water freshening (-0.02 per decade) thereafter. To fully account for the observed variability in Ross Sea waters, the progressive (1992- 2011) adjustment of the density field and induced advective contributions are estimated based on a simplified three-layer stratification. Eastern (western) inflow (outflow) of light surface (dense shelf) water increased by 28% (15%) to 1.11 Sv (1.01 Sv) by 2011; whereas a sluggish intermediate inflow (0.02 Sv) of Modified Circumpolar Deep Water turned into outflow after 2007, thus contributing 0.09 Sv by 2011 to the ventilation of deep waters farther offshore.

The estimated evolution of overturning and advective salt fluxes in the Ross Sea yield overall freshening of water masses similar to those derived from observations. Volumetric mean salinities declined at -0.07 per decade for Antarctic Surface Water, -0.05 per decade for Modified Circumpolar Water, and -0.03 per decade for Shelf Water. Outflow intensification of Shelf Water mixtures is also consistent with bottom water property changes (freshening and warming) measured farther downstream in the southern Pacific Ocean.