Stochastically-driven multidecadal variability of the Atlantic meridional overturning circulation in CCSM3

Abstract

The Atlantic meridional overturning circulation (AMOC) in the last 250 years of the 700-yearlong present-day control integration of the Community Climate System Model version 3 with T85 atmospheric resolution exhibits a red noise-like irregular multi-decadal variability with a persistence longer than 10 years, which markedly contrasts with the preceding ~300 years of very regular and stronger AMOC variability with ~20 year periodicity. The red noise-like multidecadal AMOC variability is primarily forced by the surface fluxes associated with stochastic changes in the North Atlantic Oscillation (NAO) that intensify and shift northward the deep convection in the Labrador Sea. However, the persistence of the AMOC and the associated oceanic anomalies that are directly forced by the NAO forcing does not exceed about 5 years.The additional persistence originates from anomalous horizontal advection and vertical mixing, which generate density anomalies on the continental shelf along the eastern boundary of the subpolar gyre. These anomalies are subsequently advected by the mean boundary current into the northern part of the Labrador Sea convection region, reinforcing the density changes directly forced by the NAO. As no evidence was found of a clear two-way coupling with the atmosphere, the multi-decadal AMOC variability in the last 250 years of the integration is an ocean-only response to stochastic NAO forcing with a delayed positive feedback caused by the changes in the horizontal ocean circulation.

The Influence of the AMOC Variability on the Atmosphere in CCSM3

“…In the oscillatory regime, the covariability between the AMOC and SLP is stronger, reflecting the stronger AMOC variability during that period, but the MCA patterns are nearly undistinguishable (not shown). As the AMOC response has been extensively discussed in Danabasoglu (2008) and Kwon and Frankignoul (2012), we focus on the relation between SLP and prior AMOC variations (lag . 0), which likely reflects an oceanic influence on the large-scale atmospheric circulation.…”

“…with Fig. 3 of Kwon and Frankignoul 2012), typically reaching 1.2 Sv near 458N. The SLP response resembles a positive NAO, except that the Icelandic low is 50% deeper than the Azores high compared to SLP EOF1, reaching about 1.1 hPa north of Iceland for a 0.5-hPa high off the Iberian Peninsula that corresponds to 20%-30% of the typical amplitude of the NAO in JFM (after 1 /4-½-1 /4 smoothing).…”

“…The AMOC was first in an oscillatory regime with a dominant period of about 20 yr and an amplitude of 4 Sv (years 150-449), which was investigated by Danabasoglu (2008). The regime then changed abruptly, showing weaker and red noise-like fluctuations of about 2 Sv in the last 250 yr (years 450-699), with a dominant time scale (period) of 30-40 yr, which was investigated by Kwon and Frankignoul (2012).…”

“…The AMOC was in a quasi-oscillatory regime for 300 yr with a 20-yr dominant period, as investigated by Danabasoglu (2008) and Tulloch and Marshall (2012). The regime then changed abruptly, showing weaker and slower red noise-like fluctuations in the last 250 yr, as analyzed by Kwon and Frankignoul (2012). Danabasoglu (2008) has suggested that the 20-yr cycle in the oscillatory regime may reflect an atmosphere-ocean coupled mode, while Kwon and Frankignoul (2012) argued that the AMOC variability in the red noise regime was dominated by a damped ocean mode driven by stochastic NAO forcing.…”

“…The regime then changed abruptly, showing weaker and slower red noise-like fluctuations in the last 250 yr, as analyzed by Kwon and Frankignoul (2012). Danabasoglu (2008) has suggested that the 20-yr cycle in the oscillatory regime may reflect an atmosphere-ocean coupled mode, while Kwon and Frankignoul (2012) argued that the AMOC variability in the red noise regime was dominated by a damped ocean mode driven by stochastic NAO forcing. Here, we show that in each regime the AMOC variability has a significant influence on the atmospheric circulation, albeit with different and perhaps opposite features.…”