How northern freshwater input can stabilise thermohaline circulation

Abstract

A B S T R A C TThe North Atlantic thermohaline circulation (THC) carries heat and salt towards the Arctic. This circulation is partly sustained by buoyancy loss and is generally believed to be inhibited by northern freshwater input as indicated by the 'box-model ' of Stommel (1961). The inferred freshwater-sensitivity of the THC, however, varies considerably between studies, both quantitatively and qualitatively. The northernmost branch of the Atlantic THC, which forms a double estuarine circulation in the Arctic Mediterranean, is one example where both buoyancy loss and buoyancy gain facilitate circulation. We have built on Stommel's original concept to examine the freshwater-sensitivity of a double estuarine circulation. The net inflow into the double estuary is found to be more sensitive to a change in the distribution of freshwater than to a change in the total freshwater input. A double estuarine circulation is more stable than a single overturning, requiring a larger amount and more localised freshwater input into regions of buoyancy loss to induce a thermohaline 'collapse'. For the Arctic Mediterranean, these findings imply that the Atlantic inflow may be relatively insensitive to increased freshwater input. Complementing Stommel's thermal and haline flow regimes, the double estuarine circulation allows for a third: the throughflow regime. In this regime, a THC with warm poleward surface flow can be sustained without production of dense water; a decrease in high-latitude dense water formation does therefore not necessarily affect regional surface conditions as strongly as generally thought.

Understanding Arctic Ocean Circulation: A Review of Ocean Dynamics in a Changing Climate

“…These general ideas have been extended further by considering the Arctic Mediterranean to be a double estuary (Eldevik & Nilsen, ; Lambert et al, ). This conceptualizes cooling and dense‐water formation in the Nordic Seas as a negative estuary and positive buoyancy forcing (freshwater input) in the Arctic Ocean (i.e., a positive estuary).…”

“…Heat loss in the Nordic Seas drives an overturning circulation there (Mauritzen, ) while the freshwater input to the north drives an estuarine circulation with the Atlantic Water layer. Lambert et al () find that because of the Arctic estuary circulation, an Atlantic Water inflow to the Arctic can persist even in the absence of deep convection in the Nordic Seas. This is an important point in the context of discussions related to Atlantic Water heat entering the Arctic being influenced by the strength of the Atlantic Meridional Overturning Circulation (AMOC).…”

Holocene Hydrographic Variations From the Baltic‐North Sea Transitional Area (IODP Site M0059)

“…The salinity changes in the Baltic depend on the irregular inflow of North Sea water through the Danish Straits and the amount of freshwater runoff, which are in turn controlled by climatic factors in the Atlantic (Hänninen et al, 2000). The Baltic freshwater outflow into the Nordic Seas has a large potential influence on water mass transformation (Lambert et al, 2016(Lambert et al, , 2018Winsor et al, 2001), by enhancing Polar Water outflow and suppressing Deep Water outflow (Lambert et al, 2018).…”

Tracing the Imprint of River Runoff Variability on Arctic Water Mass Transformation

“…The Arctic is characterized by a significant modification of circulating water masses due to freshwater input and by cooling due to surface heat loss (Carmack et al, ; Rudels et al, ). This water mass transformation maintains a steady state circulation of inflowing warm, saline water, and two branches of outflow: one carrying cold fresh water and the other carrying cold dense water (Eldevik & Nilsen, ; Lambert et al, ; Rudels, ; Stigebrandt, ). Besides this steady state circulation, the Arctic exhibits a strong seasonality due to solar irradiance, the formation and melt of sea ice, and river runoff.…”

“…Furthermore, Jahn, Tremblay, Mysak, et al () found an increase in the northward ocean heat transport with increased Arctic freshwater export. The impact of long‐term changes in Arctic freshwater input may have consequences beyond the Arctic domain, as Lambert et al () suggested that increased Arctic runoff can stabilize the northern branch of the Atlantic thermohaline circulation. Although knowledge of these long‐term impacts is important in light of the projected increase in Arctic river runoff, little is known of the short‐term impacts of runoff variability.…”