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Interannual Variation of Mixed-layer Heat Balance: |
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contrasting different climate phenemena and the roles of linear |
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versus nonlinear effects |
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|
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Tong Lee, Ichiro Fukumori, Benyang Tang, and Dimitris Menemenlis |
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|
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Variation of mixed-layer heat balance are studied using an ECCO |
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assimilation product (http://www.ecco-group.org). The analysis |
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focuses on the similarity and difference associated with El |
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Nino-Southern Oscillation (ENSO), Indian-Ocean Dipole (OID), and |
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Pacific Decadal Oscillation (PDO), and the relative importance of |
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linear versus nonlinear advective tendencies. In the eastern and central |
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equatorial Pacific, the warming and cooling are caused by the combined |
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effect of oceanic advection and diffusion. Surface heat flux opposes |
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the change. The opposite is true for the western equatorial Pacific. |
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The relative contribution of different components of oceanic advection |
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is spatially dependent. Zonal advection is dominant in the central |
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equatorial Pacific where zonal temperature gradient associated with |
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the warm-pool edge is large. Vertical advection is more important in |
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the eastern equatorial Pacific where upwelling and vertical |
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temperature gradient are large. Diffusion has a smaller magnitude |
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than advection, but is comparable to the temporal change of |
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mixed-layer temperature (MLT). Advection of anomalous temperature |
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gradient by mean flow and advection of mean temperature gradient by |
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anomalous flow have comparable magnitude, both being in-phase with the |
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change of MLT. Nonlinear tendency (advection of anomalous temperature |
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gradient by anomalous flow) is sizable and counteracts the two linear |
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tendencies above. Different from the tropical Pacific, nonlinear effect |
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in mid-latitude Pacific is very small; the advection of mean temperature |
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gradient by anomalous flow has the largest contribution to the total |
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advection of heat. The role of total oceanic advection in MLT balance is |
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similar for ENSO, IOD, and PDO. However, that of surface heat flux is |
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different. |
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