Analyzing Of MOC Variability Under Different Radioactive Forcing Background

This article uses some ocean data from a newly developed climate numericalmodel ECHAM5/MPIOM, which Max-Planck-Institute for Meteorology developedfor the forth assessment report of Intergovernmental Panel on Climate Change (IPCC),to discuss variations of Atlantic Meridional Overturning Circulation (MOC) and itsmechanism. Especially, the article does some research on variability of Atlantic MOCindex, oscillation period and leading EOF of MOC etc. under four differentradioactive forcing backgrounds. As a branch of the fifth Phase of Coupled ModelIntercomparison Project (CMIP5), this study not only contributes on studies of MOCvariations’ mechanism, but also offers data support for comparison, verification andmodification between different numerical models in CMIP5. Besides, the study ofAtlantic MOC responds to different radioactive forcing scenarios may help the IPCCto reveal future climate change trend and possible version, which governments coulduse to draw up CO2emission limited policies.First, the article uses ocean data, which from ECHAM5/MPIOM model underHistorical scenario, to analyzing Atlantic MOC oscillation area, oscillation period andthe leading EOF of MOC by some research methods such as Empirical OrthogonalFunctions Decomposition (EOF), wavelet analysis, lag correlation analysis andregression analysis, and describes a complete oscillation process. The meanconclusions are as follows:1. MOC maximum anomaly happens at0°-50°N in North Atlantic. The dominantperiod of interannual variability is1year, while the dominant period of interdecadalvariability is30years and the dominant period of multi-interdecadal variability is50years.2. The dynamic mechanism of the variations of MOC is: The stream function ofsurface Northern flow and deep level Southern flow increase, so heat transport fromlow latitude to high latitude increase which leads to a high temperature in Atlantic high latitude area. Thus, the sea surface density decline and sink depth of downwelling decrease. The decreased of North Atlantic Deep Water finally leads to thedecrease of MOC strength. It causes the decline of surface Northern flow and deeplevel Southern flow stream function and then high latitude sea water cooler and sinkwater deeper, the MOC strength increase again. Besides, when sea surfacetemperature (SST) goes up and sea surface density (SSD) falls, sea surface height(SSH) will increase. The sea level raises results an anticyclone circulation whichrestrains North Atlantic warm circulation and the heat transports to high latitudedecreases. A completely oscillation period is52years.Second, the article discuses patterns of global mean SST and sea surface salinity(SSS), heat transport between ocean and atmosphere and MOC stream functionanomaly under different radioactive forcing backgrounds. Furthermore, four teamocean data under different radioactive forcing scenarios are used to analyze MOCcirculation strength, oscillation period and the variance of first leading EOF in eachscenario. A series of meaningful conclusions are drawn, including:1. In Rcp26, Rcp45and Rcp85scenario, SST raises0.5℃,0.95℃and2.2℃;SST raising area are North Atlantic high level area, North Atlantic high level area andequator area and the whole global. The global SSS stay the same while SSS indifferent region change. LAB Sea gains more heat form atmosphere while GIN Searelease more heat along with radioactive forcing increase.2. Oscillation region of MOC meridional stream function presents as a rhombuswhich means the anomaly transport from North Atlantic high latitude to south. Itproves that North Atlantic high latitude is region of MOC oscillation. MOCmeridional stream function anomaly, which presents the oscillation energy, increasesalong with the radioactive forcing. MOC index goes down to16Sv, about70%inRcp85scenario while changes little in other two scenarios.3. EOF decomposition of MOC meridional stream function anomaly shows thatthe first leading contribution raise a lot along with the radioactive forcing increase, itis29%,32%and82%in Rcp26, Rcp45and Rcp85.Atlantic MOC down-wellings, which are important engines to the whole MOC circulation, locate in high latitude of North Atlantic: Greenland–Iceland–Norway Seas(GIN) and Labrador Sea (LAB). Sea surface factors like SST and SSS in these regionsare discussed to show their variances under different radioactive forcing scenarios.Study suggests that:1. Mixed layer depth in LAB Sea area is more sensitive to radioactive forcingchanges. First the maximum MOC anomaly happens near the North Atlantic LAB Seawhile the MOC anomaly in GIN Sea is very weak. The MOC anomaly transport from50°N in North Atlantic to south, so we consider the LAB Sea is region of oscillationsignal. Second, the Greenland-Scotland sea mountain, which located near the60°N,presents as a signal barrier leads to the insensitive to radioactive forcing in GIN Sea.2. Along with the increase of radioactive forcing, mixed layer depth anomalydecreases, meaning the mixed layer trends stable. SSS affects mixed layer depth morethan SST in LAB Sea while the two factors have the same effect in GIN Sea. In thesetwo sea area, SST has a long time affects like decadal to mixed layer depth while SSShas a quicker affect.