Bimodality and internal variability of the Gulf Stream and the Kuroshio

Abstract

The large-scale ocean circulation is driven both by windstress forcing and by fluxes of heat and freshwater at the ocean-atmosphere surface. At midlatitudes, the most prominent features of the mainly wind-driven surface circulation are the large coherent circulation patterns (gyres) (chapter 1: Fig. 1.1). In the North Atlantic and North Pacific a relatively strong anticyclonic (subtropical) gyre is seen and a relatively weak cyclonic (subpolar) gyre. Along the western boundaries of the ocean basins strong currents exist. These currents are referred to as western boundary currents and the poleward-flowing currents carry warm water from the tropics to midlatitudes. Hence, they play an important role in the climate system. The major western boundary currents in the North Atlantic and North Pacific are called the Gulf Stream and Kuroshio, respectively. For a long time, observations have been pointing out that the Kuroshio displays bimodal meandering behavior off the southern coast of Japan (chapter 1: Fig. 1.4). For the Gulf Stream, weakly and strongly deflected paths near the coast of South Carolina have been observed (chapter 1: Fig. 1.3). This suggests that bimodal behavior may occur in the Gulf Stream as well, although less pronounced than in the Kuroshio. Apart from this bimodality, both currents show dominant variability on intermonthly timescales. Within a hierarchy of equivalent barotropic models on a ß-plane (chapter 1), it is found that multiple mean flows are dynamically possible for the North Atlantic wind-driven circulation. Besides, the modes of intermonthly variability arising as instabilities on the mean states are closely linked within this hierarchy of models. Therefore, the two main questions addressed in this thesis (chapters 3-5) are: Can the observed (possible) bimodal behavior of the Kuroshio (Gulf Stream) be explained by transitions between multiple steady states? Can the observed variability of the Gulf Stream and the Kuroshio on intermonthly timescales be explained by instabilities of the wind-driven ocean circulation? The approach that is followed in this thesis to answer these questions is to combine data analysis of observations and Ocean General CirculationModels, and dynamical systems analysis of intermediate complexity models. This approach is new, and therefore the connection between both types of analyses is studied in this thesis as well. The associated question is to what extent the statistical techniques are capable to extract dynamically relevant modes from the datasets. This question is addressed in both chapter 2 and chapter 6, where it is shown, in the context of intermediate ocean models with(out) additive noise, that the dominant (and/or statistically significant) statistical modes have dynamical relevance. In chapter 3 evidence is given from a high resolution simulation of the Parallel Ocean Climate Model (POCM) and from intermediate complexity models to support the hypothesis that multiple mean paths of both the Kuroshio and the Gulf Stream are dynamically possible. These paths are found as multiple steady states in a barotropic shallow-water model using techniques of numerical bifurcation theory. In POCM, transitions between similar mean paths are found, with the patterns having similarity to the ones in the observations as well. To study whether atmospheric noise can induce transitions between the multiple steady states, a stochastic component is added to the annual mean wind stress forcing in the barotropic shallow-water model and differences between the transition behavior in the Gulf Stream and Kuroshio are considered. In chapter 4 dominant patterns of variability are determined using multivariate time series analyses of nonseasonal altimeter data and sea surface temperature observations of the North Atlantic, and more specific the Gulf Stream region. A statistically significant propagating mode of variability with a timescale close to 9 months is found, the latter timescale corresponding to dominant variability found in earlier studies. In addition, output from POCM is analyzed, which also displays variability on a timescale of 9 months, although not statistically significant at the 95% confidence level. The vertical structure of this 9-month mode turns out to be approximately equivalent barotropic. Following the idea that this mode is due to internal ocean dynamics, instabilities on the mean states are determined within the same barotropic shallow-water model as used in chapter 3. Within this model, the three different mean flow paths of the Gulf Stream all become unstable to oscillatory modes. For reasonable values of the parameters, an oscillatory instability having a timescale of 9 months is found. This mode is called the barotropic western boundary current (BWBC) mode. The connection between results from the bifurcation analysis, from the analysis of the observations and from the analysis of the POCM output, is explored in more detail and leads to the conjecture that the 9-month variability is related to a barotropic instability of the wind-driven gyres. In chapter 5 a similar study to the one in chapter 4 is performed, but for the 7-month variability in the Kuroshio region. The conjecture from chapters 4 and 5, i.e. that the nearannual variability of the Kuroshio (Gulf Stream) is caused by a barotropic instability of the mean Kuroshio (Gulf Stream) path near its separation, is supported by: the corresponding features of the near-annual statistical mode from the sea surface height observations, from the POCM output and the BWBC mode in the shallow-water model for each current separately the expected close dynamical correspondence between the Kuroshio and the Gulf Stream the connection between the separate modes (from either the observations, the POCM output or the shallow-water model) of both currents the correspondence in timescales between the statistical modes of both currents despite the difference in size of the North Atlantic and North Pacific basins.

and freshwater at the ocean-atmosphere surface. At midlatitudes, the most prominent features of the mainly wind-driven surface circulation are the large coherent circulation patterns gyres) (chapter 1: Fig. 1.1). In the North Atlantic and North Pacific a relatively strong anticyclonic (subtropical) gyre is seen and a relatively weak cyclonic (subpolar) gyre. Along the western boundaries of the ocean basins strong currents exist. These currents are referred to as western boundary currents and the poleward-flowing currents carry warm water from the tropics to midlatitudes. Hence, they play an important role in the climate system. The major western boundary currents in the North Atlantic and North Pacific are called the Gulf Stream and Kuroshio, respectively. For a long time, observations have been pointing out that the Kuroshio displays bimodal meandering behavior off the southern coast of Japan (chapter 1: Fig. 1.4). For the Gulf Stream, weakly and strongly deflected paths near the coast of South Carolina have been observed (chapter 1: Fig. 1.3). This suggests that bimodal behavior may occur in the Gulf Stream as well, although less pronounced than in the Kuroshio. Apart from this bimodality, both currents show dominant variability on intermonthly timescales. Within a hierarchy of equivalent barotropic models on a ß-plane (chapter 1), it is found that multiple mean flows are dynamically possible for the North Atlantic wind-driven circulation. Besides, the modes of intermonthly variability arising as instabilities on the mean states are closely linked within this hierarchy of models. Therefore, the two main questions addressed in this thesis (chapters 3-5) are: Can the observed (possible) bimodal behavior of the Kuroshio (Gulf Stream) be explained by transitions between multiple steady states? Can the observed variability of the Gulf Stream and the Kuroshio on intermonthly timescales be explained by instabilities of the wind-driven ocean circulation? The approach that is followed in this thesis to answer these questions is to combine data analysis of observations and Ocean General CirculationModels, and dynamical systems analysis of intermediate complexity models. This approach is new, and therefore the connection between both types of analyses is studied in this thesis as well. The associated question is to what extent the statistical techniques are capable to extract dynamically relevant modes from the datasets. This question is addressed in both chapter 2 and chapter 6, where it is shown, in 130 Summary the context of intermediate ocean models with(out) additive noise, that the dominant (and/or statistically significant) statistical modes have dynamical relevance. In chapter 3 evidence is given from a high resolution simulation of the Parallel Ocean Climate Model (POCM) and from intermediate complexity models to support the hypothesis that multiple mean paths of both the Kuroshio and the Gulf Stream are dynamically possible. These paths are found as multiple steady states in a barotropic shallow-water model using techniques of numerical bifurcation theory. In POCM, transitions between similar mean paths are found, with the patterns having similarity to the ones in the observations as well. To study whether atmospheric noise can induce transitions between the multiple steady states, a stochastic component is added to the annual mean wind stress forcing in the barotropic shallow-water model and differences between the transition behavior in the Gulf Stream and Kuroshio are considered. In chapter 4 dominant patterns of variability are determined using multivariate time series analyses of nonseasonal altimeter data and sea surface temperature observations of the North Atlantic, and more specific the Gulf Stream region. A statistically significant propagating mode of variability with a timescale close to 9 months is found, the latter timescale corresponding to dominant variability found in earlier studies. In addition, output from POCM is analyzed, which also displays variability on a timescale of 9 months, although not statistically significant at the 95% confidence level. The vertical structure of this 9-month mode turns out to be approximately equivalent barotropic. Following the idea that this mode is due to internal ocean dynamics, instabilities on the mean states are determined within the same barotropic shallow-water model as used in chapter 3. Within this model, the three different mean flow paths of the Gulf Stream all become unstable to oscillatory modes. For reasonable values of the parameters, an oscillatory instability having a timescale of 9 months is found. This mode is called the barotropic western boundary current (BWBC) mode. The connection between results from the bifurcation analysis, from the analysis of the observations and from the analysis of the POCM output, is explored in more detail and leads to the conjecture that the 9-month variability is related to a barotropic instability of the wind-driven gyres. In chapter 5 a similar study to the one in chapter 4 is performed, but for the 7-month variability in the Kuroshio region. The conjecture from chapters 4 and 5, i.e. that the nearannual variability of the Kuroshio (Gulf Stream) is caused by a barotropic instability of the mean Kuroshio (Gulf Stream) path near its separation, is supported by: the corresponding features of the near-annual statistical mode from the sea surface height observations, from the POCM output and the BWBC mode in the shallow-water model for each current separately the expected close dynamical correspondence between the Kuroshio and the Gulf Stream the connection between the separate modes (from either the observations, the POCM output or the shallow-water model) of both currents the correspondence in timescales between the statistical modes of both currents despite the difference in size of the North Atlantic and North Pacific basins.