Natural Variability And Externally Forced Responses Of Sea Level And Ocean Heat Content

Sea level and ocean heat content are key indicators for the behaviors of climate system,and are of great socioeconomical importance for the coastal community.Natural variability and externally forced responses are two fundamental elements in governing the spatio-temporal evolution of sea le-vel and heat content.In this thesis,based on a novel ocean reanalysis Estimating the Circulation and Climate of the Ocean version 4(ECCOv4)and results from Coupled Model Intercomparison Project Phase 5(CMIP5),we study the roles of different ocean dynamical processes in the natural variability and forced responses of sea level and heat content.In ECCOv4,the interannual fingerprints,decadal fingerprints,and linear trends of sea level in the Indo-Pacific region are dominated by the thermosteric component.The halosteric component exhibits non-negligible contributions to the decadal fingerprints and linear trends in broad regions of the Pacific Ocean,and tends to compensate the thermosteric counterpart.The mass sea level is important for interannual and decadal sea level variability in coastal regions,but its signal is mostly weak in open oceans.A comparison of mass sea level between the gravity satellite observations and two ocean reanalyses shows a good agreement in its variability,but some discrepancies in its linear trends.The interannual fingerprints,decadal fingerprints,and linear trends of steric sea level are mostly due to variations of the upper thermocline water,while the mode water plays a role in mid and high latitude expressions of those signals.The positive and negative steric sea level anomalies are related to the deepening and shoaling of isopycnal surfaces,respectively.Determined by the background temperature and salinity stratification of the ocean,isopycnal heaving generates stronger thermosteric signals than halosteric signals in most of the upper ocean.The decadal fingerprints and linear trends of thermosteric and halosteric sea level are partly affected by changes of water mass properties(i.e.,density compensated temperature and salinity anomalies),which explains the counteracting behaviors between the thermosteric and halosteric sea level.The global ocean loses heat during the decaying phase of El Nino with a magnitude of 120 TW(1 TW = 1012 W)per unit Nino 3.4 change in ECCOv4.At the same time,heat redistributions within the global ocean during ENSO events are observed.During the developing and peak phases of El Nino,heat is fluxed upward to 0-100 m in both tropics and higher latitudes,and then redistributed toward the tropics.This process tends to trigger a lagged surface heat flux response locally,and leads to a net heat loss of the ocean during the decaying phase of El Nino,which is partly compensated by ocean heat gains in higher latitudes via the surface heat flux.Importantly,these results demonstrate that the ENSO-related heat redistribution,both meridional and vertical,is an important mechanism for ENSO's modulation of the global integrated ocean heat content.The anomalous heat transport associated with ENSO is mainly driven by anomalous advection of mean temperature,except for the tropics of 100 m where the anomalous vertical heat exchange is controlled by multiple processes.The anomalous heat advection contains contributions from both the nonmass-balanced and mass-balanced components.During ENSO events,the former dominates the upper 100 m integral of heat redistribution between the tropics and higher latitudes,and the latter is important for vertical heat exchanges across 100 and 440 m via its zonal and meridional cell components in the tropics and higher latitudes,respectively.Temporal changes in patterns of Sea Level Rise(SLR)and Ocean Heat Uptake(OHU)are studied using 18 CMIP5 models in their 21st century projections under aggressive mitigation(RCP2.6),medium mitigation(RCP4.5),and high emission(RCP8.5)scenarios.In mitigation scenarios(RCP2.6 and RCP4.5),the SLR and OHU patterns exhibit non-negligible changes during the 21st century.These pattern changes are anti-correlated with the patterns in the early 21st century,i.e.,pattern changes tend to oppose initial patterns.In contrast,little pattern change is identified for high emission scenario RCP8.5 during the 21st century.The SLR and OHU pattern changes in mitigation scenarios can be interpreted by considering the spatially varying response timescales of the ocean.The timing of the radiative forcing stabilization differs across scenarios,which leads to the scenario-dependency of the pattern changes.During the 21st century,the OHU pattern changes lead to 6-8%reduction in the thermal expansion efficiency of the ocean(?)in RCP2.6 and RCP4.5,which is a factor of 2-4 compared to the impact of the ocean warming.The impacts of the OHU pattern changes and the ocean warming to ? cancel out each other in RCP8.5.