Regional Patterns of Sea Level Change Related to Interannual Variability and Multidecadal Trends in the Atlantic Meridional Overturning Circulation*

Abstract

Some studies of ocean climate model experiments suggest that regional changes in dynamic sea level could provide a valuable indicator of trends in the strength of the Atlantic meridional overturning circulation (MOC). This paper describes the use of a sequence of global ocean–ice model experiments to show that the diagnosed patterns of sea surface height (SSH) anomalies associated with changes in the MOC in the North Atlantic (NA) depend critically on the time scales of interest. Model hindcast simulations for 1958–2004 reproduce the observed pattern of SSH variability with extrema occurring along the Gulf Stream (GS) and in the subpolar gyre (SPG), but they also show that the pattern is primarily related to the wind-driven variability of MOC and gyre circulation on interannual time scales; it is reflected also in the leading EOF of SSH variability over the NA Ocean, as described in previous studies. The pattern, however, is not useful as a “fingerprint” of longer-term changes in the MOC: as shown with a companion experiment, a multidecadal, gradual decline in the MOC [of 5 Sv (1 Sv ≡ 106 m3 s−1) over 5 decades] induces a much broader, basin-scale SSH rise over the mid-to-high-latitude NA, with amplitudes of 20 cm. The detectability of such a trend is low along the GS since low-frequency SSH changes are effectively masked here by strong variability on shorter time scales. More favorable signal-to-noise ratios are found in the SPG and the eastern NA, where a MOC trend of 0.1 Sv yr−1 would leave a significant imprint in SSH already after about 20 years.

Decadal Variability of Eddy Kinetic Energy in the South Pacific Subtropical Countercurrent in an Ocean General Circulation Model

“…ORCA025 configurations have been widely used in studies investigating ocean variability mechanisms in different areas (e.g., Lübbecke et al 2008;Lorbacher et al 2010;Rühs et al 2015), as well as influences of large-scale atmospheric variability on the ocean (e.g., Feng et al 2011;Ummenhofer et al 2013;Barrier et al 2015) and intrinsic oceanic variability on different time scales (Penduff et al 2010;Sérazin et al 2015).…”

A Comparative Stability Analysis of Atlantic and Pacific Niño Modes*

“…For a dataset in which all variables are available and related to each other in a dynamically consistent way, we use output from the Nucleus for European Modeling of the Ocean (NEMO) global ocean circulation model with a horizontal resolution of 0.58 (NEMO-ORCA05). The hindcast simulation used in this study is described in more detail in Lorbacher et al (2010). NEMO-ORCA05 is forced by the interannually varying Co-ordinated Ocean-Ice Reference Experiments (CORE) (Griffies et al 2009) atmospheric forcing for the time period 1958 to 2004.…”

Changes in extreme regional sea surface height due to an abrupt weakening of the Atlantic meridional overturning circulation

“…However, regional sea level changes have been very inhomogeneous over this period and are affected by changes in atmospheric wind stresses, and surface heat and freshwater fluxes (Ishii et al, 2003;Volkov et al, 2003;Antonov et al, 2005;Bindoff et al, 2007;Berge-Nguyen et al, 2008;Church et al, 2008;Woodworth et al, 2011a). Hence, regional sea level change is often associated with variations in ocean circulation (Landerer et al, 2007;Levermann et al, 2005;Flückiger et al, 2006;Stammer, 2008;Stammer et al, 2011;Lorbacher et al, 2010;Hu et al, 2011). Changes in the amount of water in the oceans (barystatic), as well as changes in land water mass distribution, and their gravitational, elastic and rotational effects (changes in the static equilibrium) may also affect regional sea level (Farrell and Clark, 1976;Clark et al, 1978;Mitrovica et al, 2001Mitrovica et al, , 2011Kopp et al, 2010).…”