North and equatorial Pacific Ocean circulation in the CORE-II hindcast simulations
Introduction
The dynamics of the Pacific Ocean affect not only the regional climate system but also global climate variability, e.g., El Niño Southern Oscillation (ENSO). The Kuroshio, the major western boundary current of the North Pacific, is of particular importance because it transports an enormous amount of mass, geochemical material, and heat from low to mid-latitudes (Imawaki et al., 2013). It also has a significant impact on ocean-atmosphere interactions in the North Pacific Ocean (Kwon et al., 2010). Many previous studies have reported that basin-wide variations of wind stress curl (WSC) have important effects on transport and large scale changes of the Kuroshio and its extension region (e.g., Deser et al., 1999, Sugimoto et al., 2010). These WSC variations are likely related to variations of Aleutian Low activity and the Pacific Decadal Oscillation at the interanual to decadal scales (Soeyanto et al., 2014). However, the mechanisms responsible for the Kuroshio transport and its variability dynamics, remain unclear.
The formation of water masses in the North and tropical Pacific, and their transport pathways, are driven mainly by the WSC in contrast with the thermohaline circulation of the North Atlantic, and as a result the WSC can significantly affect the oceanic uptake of passive tracers such as carbon dioxide. Excluding the deep bottom water (addressed in the Southern Ocean companion paper by Downes et al., 2015), the most important water masses in the North Pacific are the Antarctic Intermediate Water (AAIW), North Pacific Intermediate Water (NPIW) and three mode waters: Central Mode Water (CMW), Eastern Subtropical Mode Water (ESMW) and Subtropical Mode Water (STMW). A list of all acronyms used in this paper is provided in Appendix A and a schematic of the major North and equatorial Pacific surface wind-driven currents and water mass distributions is shown in Fig. 1. Many issues related to water mass formation and transport in the Pacific remain unresolved. For example, the pathway of NPIW from the mixed water region (MWR, Talley et al., 1995), between the Kuroshio and Oyashio, to the western part of the subtropical gyre remains controversial. It is well known that NPIW plays an important role in the uptake of anthropogenic gases in the North Pacific. Talley (1993) assumed NPIW is carried southward into the subtropical gyre by the recirculation of the Kuroshio Extension. Yasuda et al. (1996) suggested that NPIW is ejected from the Kuroshio Extension at the southern edges of the Kuroshio Extension troughs. Some researchers have explored the possibility that NPIW may enter at the westernmost climatological trough due to the existence of cross-frontal flow across the Kursoshio Extension (Joyce et al., 2001). However, a significant amount of NPIW is assumed to be transported eastward past 150°E (Masujima et al., 2003). Based on an inverse model analysis, Nishina (1997) suggested that part of the NPIW turns southwestward around 160°E and reaches the northwestern part of the subtropical gyre south of Japan. This route roughly corresponds to the shortcut pathway proposed in Nakano et al. (2007) and recently verified by Fujii et al. (2013). On the other hand, You, 2003, You, 2010) has suggested that NPIW is generated in the broad area between the subtropical and subarctic gyres and transported eastward to 140°W where the Kuroshio Extension does not reach before entering the subtropical gyre. This coincides with the pathway proposed in Nakano et al. (2007), however, its existence is not completely validated due to the paucity of observations.
The shallow meridional overturning circulation (also known as the subtropical cells, STCs), controls the subsurface transfer of mass, heat and salt between the subtropical and equatorial Pacific (Capotondi et al., 2005). The interior STC transport is compensated by the low-latitude western boundary transport in both hemispheres climatologically. But at interannual scales, the compensation does not exactly hold in the Northern Hemisphere (Chen et al., 2015). The lack of compensation in the Northern Hemisphere is caused by the 7-month phase lag between the interior STC transport and western boundary transport according to Ishida et al. (2008). Recent studies indicate that the STCs have an important effect not only on the redistribution of water properties but also on ocean climate variability. England et al. (2014) have shown that strengthening of the STCs over the last decade is directly related to strengthening of the trade winds, thus resulting in the recent slow-down (up to 2014) in global surface temperatures. In the equatorial Pacific, the STCs involve subtropical water, subducted in eastern areas of the Pacific ocean, that flows westward and equatorward in the upper pycnocline layers. The STCs feed the Equatorial Undercurrent (EUC), and upwelling in eastern equatorial regions, before returning to the subtropics as part of the surface Ekman flow (McCreary and Lu, 1994). Given the STCs are much shallower and narrower than the Atlantic meridional overturning circulation, their time scale is shorter, ranging from interannual to multi-decadal, and cannot be ignored in decadal prediction of Pacific climate (Solomon et al, 2011). Farneti et al. (2014b) recently suggested a potential mechanism linking the STCs with changes in atmospheric circulation in order to explain tropical-extratropical atmosphere-ocean interactions on multi-decadal time scales. Unfortunately our understanding of the STCs, and their variability in relation to the WSC, remains limited due to the lacks of observational data and well-validated model simulations.
In the present study, we shed light on some of the above uncertainties through a detailed examination and intercomparison of a suite of global ocean models integrated from 1948 to 2007 under the Coordinated Ocean-ice Reference Experiments (CORE-II) protocol. The CORE-II was designed to evaluate the performance of ocean and sea-ice models, and to quantify and understand variability on time scales of seasons to decades (Griffies et al., 2009). These hindcast experiments provide a common framework to assess the differences and similarities among models compared with the observation. Several accompanying papers have assessed aspects of sea level variability (Griffies et al., 2014), North Atlantic-mean state (Danabasoglu et al., 2014) and interannual to decadal variability (Danabasoglu et al., 2016), Southern ocean circulation (Downes et al, 2015) and others. In this study, our aim is to assess how well these models can represent the dominant dynamical processes that are unique to the Pacific Ocean if the atmospheric state for recent decades is prescribed? The specific questions are:
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Can we quantify the overall performance of the surface variability which is the first order evaluation of the ocean models?
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How well the western boundary current system is represented in these models, particularly the amount of transport which is the key to move energy to high latitude?
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How are the water masses and their origins represented in such models? Can they help to clarify the unclear pathway of NPIW?
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Can the models adequately simulate the equatorial dynamics, particularly the STCs circulation? What is the role of a better representation of ocean mesoscale eddies?
In this paper, we focus only on the evaluation and discussion of the mean statistical properties and features of the CORE-II simulations over the period of 1963-2007 and leave the study of interannual to decadal variability for future studies. We hope the current evaluation of the mean statistical properties may provide useful guidelines for future variability studies of CORE-II simulations and comparisons of coupled climate simulations in the Pacific.
After a brief description of the 15 participating models and the observational data in 2 Models and observations, 3 Statistical summary of SSH and SST variability provides a statistical summary of the surface variability, which gives us an overall performance of the participating models in the focused region. Section 4 investigates the performance of the models in terms of transport by the Kuroshio Current System (KCS) in the North Pacific. Section 5 and 6 further address the interior pathway and origin of the North Pacific water mass transport by analyzing the mixed layer depth (MLD) and water mass properties. Section 7 addresses the key tropical dynamics linking closely to the interannual and decadal variability. We conclude with a brief summary and suggestions for future research in section 8.
Section snippets
Models
Fifteen model simulations are evaluated in the present study (Table 1). The most extensive descriptions of the models participating in the CORE-II protocol are given in Danabasoglu et al. (2014) except the updated version of FSU model used in this study (which is the FSU2 described in Danabasoglu et al., 2016) and the recent contribution from the Consortium for Climate Change Study (CCLICS) at Academia Sinica, Taiwan (See Appendix B for a brief description of this system). CCLICS uses the
Statistical summary of SSH and SST variability
In order to get the overall performance of the participating ocean models, the statistical properties of two ocean variables of practical and scientific interest, SSH and SST, are first summarized here. Griffies et al. (2014) have already evaluated the mean SSH fields of the CORE-II simulations and so we only discuss them briefly in this paper. We also do not discuss in detail the mean SST because, as will be shown later, the modeled mean SST distribution is very close to the mean of the
Transport of the Kuroshio current system
The transport of the KCS modulates the climate of this region (e.g., Hirose et al., 2009). It is also affected directly by the regional climate variability (Shen et al., 2014). Here, we assess the realism of the mean KCS transport simulated by the CORE-II simulations based on three representative sections although accurate simulation is rather challenging for most of them due to their low resolution.
Mixed layer depth
The following two sections highlight the water masses transport and their origins in the North Pacific by considering the distribution of MLDs and water masses at several sections. MLDs generally reach their seasonal extrema in March and September. Following Danabasoglu et al. (2014), we use a density-based approach to calculate the March MLDs offline, defining the MLD as the depth at which the potential density (referenced to the surface) changes by 0.125 kg m−3 from its surface value. Long
Upper-ocean circulation of the North Pacific
We now further evaluate the water masses formed by the deepening of MLDs in the North Pacific. In order to better understand the three-dimensional structures, we consider two vertical sections first and then three isopycnal surfaces. We first discuss the salinity and potential density distributions along two meridional sections which can give us an overview of the modeled meridional water mass distribution. Then we further investigate the depth, salinity, potential temperature, and potential
Mean tropical dynamics
Finally, we examine the mean tropical dynamics in the Pacific because of its importance in the global climate system (e.g., the impact of ENSO). We first compare the zonal mean current structure along the equator with observations (Johnson et al., 2002) for the 1986-2000 period (Fig. 18). Along the equator, the maximum speed of the EUC ranges from 79 cm s−1 (MRI-FREE) to 114 cm s−1 (KIEL-R050) listed in Table 4 (MRI-ASSIM is excluded). The corresponding core depths (defined as the depth of the
Summary and future work
We have evaluated a suite of global ocean-sea ice models, driven by the CORE-II atmospheric forcing from 1963-2007, focusing on the North and equatorial Pacific. The key results are summarized as follows:
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The overall model performance is clearly quantified by the Taylor diagrams of the first three moments of the surface variability. Our results clearly show that the mean standard deviation and skewness of SSH variability for the altimeter subperiod (1993-2007) deviate from the observations more
Acknowledgments
We thank the constructive and critical comments from Dr. Antonietta Capotondi, Dr. Billy Kessler and four anonymous reviewers, which greatly improved the manuscript. The support and help from the editor Will Perrie are also appreciated. NCAR is sponsored by the U. S. National Science Foundation (NSF). The CESM is supported by the NSF and the U. S. Department of Energy. Y. H. Tseng was supported by the NSF Earth System Model (EaSM) Grant 1419292 (EaSM-3: Collaborative Research: Quantifying
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