Pacific Meridional Overturning Circulation Research Articles

The role of the PMOC in modulating the deglacial shift of the ITCZ

Based on an energy-constraint framework, the effect of the Pacific meridional overturning circulation (PMOC) has been investigated on the meridional shift of the intertropical convergence zone (ITCZ). During Heinrich Event 1, the global ITCZ displaces southward in response to a shutdown of the Atlantic meridional overturning circulation (AMOC). However, the PMOC resumes and induces an enhanced northward meridional heat transport (MHT). Further analyses show that the resumption of the PMOC and its associated MHT can relieve the decrease of oceanic cross-equatorial heat transport by the collapsed AMOC and lower the compensating decline of atmospheric cross-equatorial meridional heat transport by 0.09PW, leading to an offset of the southward shift of the global ITCZ by 0.5°. Regionally the developed PMOC does not affect the southward movement of the Atlantic ITCZ much since the latter is dominated by the collapsed AMOC that induces the change of Atlantic MHT. In contrast, the PMOC-induced northward heat transport largely abates the hemispheric temperature contrast in the tropical Pacific and reduces the southward ITCZ shift by as much as 5° in the eastern Pacific.

Variability in North Pacific intermediate and deep water ventilation during Heinrich events in two coupled climate models

The responses of North Pacific intermediate and deep water ventilation and ocean biogeochemical properties to northern North Atlantic glacial freshwater perturbations are evaluated with a coupled atmosphere–ocean general circulation model MIROC and an earth system model of intermediate complexity LOVECLIM. When the Atlantic meridional overturning circulation (AMOC) is weakened as a result of the North Atlantic freshwater discharge, both models simulate subthermocline and intermediate water warming in the Pacific Ocean. The sensitivities of the Pacific meridional overturning circulation (PMOC) to AMOC weakening differ significantly between the two models. MIROC simulates a small enhancement of the deep sinking branch of the PMOC in the North Pacific. On the contrary, the LOVECLIM freshwater experiment exhibits intensified deep water formation in the North Pacific, associated with a maximum transport change of 19Sv. Despite the significant differences in ocean circulation response, both models successfully reproduce high-oxygen and low-nutrient conditions of intermediate and deep waters, in accordance with sediment core based paleoproxy reconstructions from the North Pacific and Bering Sea during Heinrich event 1. Emergence of younger intermediate and deep water in the North Pacific can be partly attributed to an overall enhanced mixing as well as intensified overturning circulation of the subpolar North Pacific. Our models simulate broad features observed in several paleoproxy data of the Pacific Ocean: biological production decrease in northern Japan, cooling in the western North Pacific Ocean, and the southward shift of the Pacific intertropical convergence zone.

Removing the North Pacific halocline: Effects on global climate, ocean circulation and the carbon cycle

A well-pronounced halocline is a key feature of today's subarctic North Pacific. There is indirect paleo-evidences from the last glacial termination as well as from the early and middle Pliocene that this halocline has not always been there. To study the effects of North Pacific salinity on global climate, ocean circulation and the marine carbon cycle, we perform idealized experiments using an Earth system model of intermediate complexity (LOVECLIM). Imposing a negative freshwater flux in the northern North Pacific, the halocline vanishes and a deep Pacific meridional overturning circulation (PMOC) establishes. The associated increase of meridional heat transport in the Pacific leads to a bipolar seesaw response in temperature, with warming in the North Pacific and over North America and cooling in the Southern Ocean. As a result of the formation of North Pacific deep water (NPDW), the surface branch of the global conveyor belt circulation weakens. Transport through the Indonesian Seas decreases by 50% as the warm and saline waters of the equatorial Pacific are diverted into the North Pacific.In our idealized experiments, the enhanced global deep water formation is balanced by an increase in diapycnal mixing. As a result nutrient concentrations in the euphotic zone increase by about 25% globally, leading to a 20% increase in global export production. The effect of greater export production on atmospheric pCO2 is, however, compensated by the enhanced transport of dissolved inorganic carbon (DIC) to the surface. As a result, the atmospheric CO2 concentration increases by only 5ppmv. Our results further suggest that the absence of the subarctic halocline for instance during Heinrich event 1 and the Pliocene may have exerted a strong influence on global climate and the carbon cycle.

Scaling of the strength of the meridional overturning with vertical diffusivity in an idealized global geometry

An important expression of the non-linear character of the ocean’s meridional overturning circulation (MOC) is the scaling of its amplitude with the magnitude of the vertical mixing coefficient (diffusivity) of heat and salt. This paper extends recent work that indicated that the Atlantic and Pacific MOC exhibit different scaling behaviour. An idealized two-basin model configuration is used to study the meridional overturning circulation under restoring boundary conditions. In particular, the effects of wind forcing and the choice of the parametrization of lateral mixing are examined.Without wind, the scaling is similar in the two basins and consistent with theoretical predictions, provided that the diffusivity is small enough. Towards higher diffusivities the scaling of the overturning diverges.With non-zero wind, the sensitivity is strongly determined by the choice of the lateral mixing scheme. In case of traditional horizontal diffusion, the scaling behaviour is asymmetric due to spurious diapycnal mixing.With the Gent-McWilliams parametrization, the scaling is symmetric for the lower range of diffusivities, where results agree partly with theoretical scaling relations. In all cases considered, the pycnocline depth has the same sensitivity to diffusivity in both basins, implying that there is no universally valid relation between overturning strength and pycnocline depth.

Comments on “Decadal Changes in the North Atlantic and Pacific Meridional Overturning Circulation and Heat Flux”

Abstract It is demonstrated that current level models, both with and without assimilation, generate too shallow an overturning in the North Atlantic (just like their predecessors) because they do not reproduce the descent of plumes of cold water from the Greenland and Norwegian Seas. Consequently, the prediction of decadal change from one such model reported by C. Wunsch and P. Heimbach is queried.

The Atlantic–Pacific Seesaw

Abstract A global, oceanic teleconnection of salinity, meridional overturning circulation (MOC), and climate of the North Atlantic and North Pacific is proposed. Simulations with a global climate model show that an extraction of freshwater from the Pacific results not only in an increase of salinity there, but also in a decrease of salinity in the Atlantic. As a result, a Pacific MOC develops while the Atlantic MOC collapses without freshwater perturbation in the Atlantic. Similarly, an input of freshwater to the Atlantic leads not only to a decrease of salinity there, but also to an increase of salinity in the Pacific. The Atlantic MOC collapses, whereas the Pacific MOC develops without freshwater perturbation in the Pacific. The mechanism behind this antiphase Atlantic– Pacific relationship is the positive feedback between ocean circulation and salinity contrasts, originally proposed by Stommel to operate between low and high latitudes. Here the authors show that the same mechanism operates on the Atlan...