Diagnosing the development of seasonal stratification using the potential energy anomaly in the North Pacific Ryohei Yamaguchi, Toshio Suga, Kelvin J. Richards & Bo Qiu Climate Dynamics Observational, Theoretical and Computational Research on the Climate System ISSN 0930-7575 Clim Dyn DOI 10.1007/s00382-019-04816-y 1 23 Your article is protected by copyright and all rights are held exclusively by Springer- Verlag GmbH Germany, part of Springer Nature. This e-offprint is for personal use only and shall not be self-archived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com”. 1 23 Author's personal copy Climate Dynamics https://doi.org/10.1007/s00382-019-04816-y Diagnosing the development of seasonal stratifcation using the potential energy anomaly in the North Pacifc Ryohei Yamaguchi1 · Toshio Suga1,2 · Kelvin J. Richards3 · Bo Qiu4 Received: 8 January 2019 / Accepted: 9 May 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract Upper-ocean seasonal stratifcation (seasonal pycnocline and/or transition layer) is a ubiquitous feature and its vertical structure has large spatial variability. The density stratifcation regulates the stability of the upper ocean and thus can afect the oceanic response to atmospheric forcing and biogeochemical processes by modulating vertical mixing. In this study, we described the development of the seasonal stratifcation in terms of the stability of the water column, using the poten- tial energy anomaly (PEA) as a metric based on Argo profles. PEA budget analysis reveals that over most of the North Pacifc, seasonal stratifcation develops under a vertical one-dimensional energy balance between an increase in PEA (i.e., a strengthening of the stratifcation) driven by atmospheric buoyancy forcing and a decrease in PEA associated with verti- cal mixing within the water column. Horizontal advection of PEA plays a signifcant role in the seasonal development of the stratifcation only in the regions of the western boundary current and equatorial current system south of 10°N. We fnd that, in addition to the total magnitude of the oceanic buoyancy gain, the balance between compositions of the atmospheric forcing (non-penetrating surface buoyancy forcing and penetrating radiative heating) is also important in explaining regional diferences in the development of the seasonal stratifcation. The vertical difusivity in the seasonal stratifcation estimated from the residual of the PEA budget is in the range from 5 × 10−5 m2 s−1 to 5 × 10−4 m2 s−1 and shows spatial and seasonal variability associated with local wind forcing. 1 Introduction across the North Pacifc, its vertical structure difers region- ally (Fig. 1). For example, shallower and sharper seasonal The upper ocean in the warming season consists of a rela- stratifcation tends to develop in the northern part of the tively thin mixed layer (ML) and seasonal stratifcation North Pacifc compared to the southern part. In the Kuroshio (seasonal pycnocline and/or transition layer) below the ML. and its extension regions, a more substantial buoyancy gain Seasonal change in stratifcation is a ubiquitous feature of occurs in the subsurface, leading to stratifcation with more the North Pacifc. In the warming season, the stratifcation linear vertical structures. These features are the result of develops as a result of heating, freshwater supply and wind regional diferences in the dominant processes of forming forcing from the atmosphere, and oceanic lateral processes. the seasonal stratifcation. While the development of a seasonal stratifcation occurs Heat, freshwater, momentum, and chemical tracers exchanged between the atmosphere and ocean are trans- ported into the ocean interior through the ML and the sea- * Ryohei Yamaguchi [email protected] sonal stratifcation. Since the seasonal stratifcation char- acterizes “difculty in mixing (i.e., stability)” of the upper 1 Department of Geophysics, Graduate School of Science, ocean due to its maxima in the density stratifcation, it has Tohoku University, Sendai, Japan great potential of infuences on physical and biogeochemical 2 Research and Development Center for Global Change, processes within the upper ocean. Japan Agency for Marine-Earth Science and Technology, Vertical mixing below the ML during the warming sea- Yokosuka, Japan son is controlled by the strength of stratifcation (Qiu et al. 3 International Pacifc Research Center, University of Hawaii 2004; Cronin et al. 2013). In turn, this can afect the supply at Manoa, Honolulu, HI, USA of nutrients from the subsurface and the vertical transport 4 Department of Oceanography, University of Hawaii of heat, which is an important factor in determining the sea at Manoa, Honolulu, HI, USA Vol.:(0123456789)1 3 Author's personal copy R. Yamaguchi et al. Fig. 1 Seasonal cycle of the upper-ocean potential density stratifca- at labeled location [longitude (°E), latitude (°N)]. Averaged profles tion in the North Pacifc, based on Argo data sampled during 2006– are normalized to the potential density at 200 m in February follow- 2016 (y-axis: depth (m), and x-axis: potential density (kg m−3)]. Pro- ing the procedure of Moisan and Niiler (1998) fles are averaged over a 2° (longitude) × 2° (latitude) region centered surface temperature during the warming season (Hosoda between the surface and subsurface, and the buoyancy et al. 2015). Moreover, not only in the warming season, the frequency (e.g. Tomita et al. 2010; Capotondi et al. 2012; upper ocean stability that develops during the warming sea- Maes and O’Kane 2014). While these metrics are use- son can also afect ocean ventilation via its impact on the ful, being readily obtained from low vertical resolution development of the winter ML (e.g. Qiu and Chen 2006; observational data or climate model output, they do not Kako and Kubota 2007). Despite the importance of seasonal always quantify the “difculty in mixing”, which is the stratifcation for physical and biogeochemical processes, the essence of seasonal stratifcation. To assess the infuences formation and spatial variability of seasonal stratifcation of seasonal stratifcation on biogeochemical processes and have not been widely investigated from an observational air-sea interaction, a metric capable of quantitatively rep- perspective (e.g., Johnston and Rudnick 2009). resenting this “difculty in mixing” of the water column Previous model and observational studies have used is needed. common metrics to quantify upper ocean stratifcation. In this study, for a metric, we used the potential energy These include the density or temperature difference anomaly (PEA; ) advocated by Simpson (1981). PEA 1 3 Author's personal copy Diagnosing the development of seasonal stratifcation using the potential energy anomaly… defined from the surface ( z = 0 ) to the base of a depth are able to discuss quantitatively the development and spatial ( z =−H ) is written as; variability of the seasonal stratifcation in terms of stability. Making use of observational data, the purpose of this 0 0 1 1 study is to describe the development of seasonal stratif- � = ( ̄� − �) gz dz =− ̃� gz dz, (1) H −H H −H cation in terms of “difculty in mixing”, and to discuss the dominant processes forming the stratification and with the vertically averaged potential density being written their balance using a time-dependent equation for PEA. as We also examine the efectiveness of applying the PEA budget analysis, which has generally been used in stud- 1 0 ̄� = � dz, ies of coastal studies, to the seasonal stratifcation in the H −H open ocean. The remainder of this paper is organized as follows. where ̃� (= � − ̄� ) is the deviation from the vertically aver- In Sect. 2, the dataset and the processing methods are aged potential density and g (= 9.80 m s−2) is the accelera- described. The PEA climatological feld and its seasonal tion due to gravity. PEA provides a measure of the amount of cycle are described in Sect. 3. In Sect. 4 we outline the energy per unit volume (J m−3) required to make the density confguration and validation of the PEA budget analysis stratifed water column vertically homogeneous; it, there- and the results of the PEA budget. In Sect. 5 we discuss fore, directly represents the “difculty in mixing”. Examples the diference in the balance of terms in the budget, dem- of calculation of PEA are shown in Fig. 2. Using this PEA onstrating three representative regions as the examples. as a metric, together with its time-dependent equation, we Finally, a summary is provided in Sect. 6. Fig. 2 Examples of the potential energy anomaly (PEA) calculation. 23°N). d–f Corresponding seasonal cycle of PEA with the annual a–c Seasonal cycle of the upper-ocean profle of potential density maxima of ML depth (H) set to d 144 m, e 163 m, f 88 m, respec- stratifcation at (a 153°E, 35°N), (b 177°W, 43°N), and (c 177°W, tively 1 3 Author's personal copy R. Yamaguchi et al. 2 Data and methods gridded QuikSCAT and Advanced Scatterometer (ASCAT) wind stress products made available through the Asian- We mainly used the Advanced automatic Quality Control Pacifc Data-Research Center (http://apdrc .soest .hawai Argo data (AQC Argo data; http://www.jamst ec.go.jp/ i.edu/) were used to obtain the monthly Ekman transport ARGO/argo_web/ancie nt/AQC/index .html) for tempera- feld. ture and salinity profles provided by the Japan Agency We used three daily mean atmospheric datasets to calcu- for Marine-Earth Science and Technology (JAMSTEC).