Lu Jian

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  • Article
    Understanding the Drivers of Atlantic Multidecadal Variability Using a Stochastic Model Hierarchy
    (American Meteorological Society, 2023-01-23) Liu, Glenn ; Kwon, Young-Oh ; Frankignoul, Claude ; Lu, Jian
    The relative importance of ocean and atmospheric dynamics in generating Atlantic multidecadal variability (AMV) remains an open question. Comparisons between climate models with a slab ocean (SLAB) and fully dynamic ocean components (FULL) are often used to explore this question, but cannot reveal how individual ocean processes generate these differences. We build a hierarchy of physically interpretable stochastic models to investigate the contribution of two upper-ocean processes to AMV: the role of seasonal variation and mixed-layer entrainment. This interpretability arises from the stochastic model’s simplified representation of sea surface temperature (SST), considering only the local upper-ocean response to white-noise atmospheric forcing and its impact on surface heat exchange. We focus on understanding differences between SLAB and FULL non-eddy-resolving preindustrial control simulations of the Community Earth System Model 1 (CESM), and estimate the stochastic model parameters from each respective simulation. Despite its simplicity, the stochastic model reproduces temporal characteristics of SST variability in the SPG, including reemergence, seasonal-to-interannual persistence, and power spectra. Furthermore, the unrealistically persistent SST of the CESM-SLAB ocean simulation is reproduced in the equivalent stochastic model configuration where the mixed-layer depth (MLD) is constant. The stochastic model also reveals that vertical entrainment primarily damps SST variability, thus explaining why SLAB exhibits larger SST variance than FULL. The stochastic model driven by temporally stochastic, spatially coherent forcing patterns reproduces the canonical AMV pattern. However, the amplitude of low-frequency variability remains underestimated, suggesting a role for ocean dynamics beyond entrainment.
  • Article
    Freshwater Flux Variability Lengthens the Period of the Low‐Frequency AMOC Variability
    (American Geophysical Union, 2022-10-18) Liu, Fukai ; Lu, Jian ; Kwon, Young‐Oh ; Frankignoul, Claude ; Luo, Yiyong
    Atlantic Meridional Overturning Circulation (AMOC) exhibits interdecadal to multidecadal variability, yet the role of surface freshwater flux (FWF) variability in this AMOC variability remains unclear. This study isolates the contribution of FWF variability in modulating AMOC through a partially coupled experiment, in which the effect of the interactive FWF is disabled. It is demonstrated that the impact of the coupled FWF variability enhances the persistence of density and deep convection anomalies in the Labrador Sea (LS), thus lengthening the period of the AMOC oscillation on multidecadal timescale and suppressing its ∼30‐year periodicity. Further lead‐lag regressions illuminate that the more persistent LS density anomalies are maintained by two mechanisms: (a) The local temperature‐salinity coupling through the evaporation and (b) a downstream propagation along the East Greenland Current of the extra salinity anomaly due to the sea ice melting changes associated with an atmosphere forcing over the southern Greenland tip.