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The Role of Ocean Eddies East of Taiwan and their Coupling with the Atmosphere in Regulating Sea-Surface Fluxes and Western North Pacific Climate

L.-Y. Oey

Abstract

The ocean surface boundary layer mediates air-sea exchange, and plays an indispensible role in climate studies. Current climate models parameterize surface-layer mixing assuming that it is driven by wind-(and/or buoyancy-flux, e.g. cooling) induced turbulence. Some researchers [e.g. Harcourt & D’Asaro, 2008; Huang et al. 2011] have proposed including also the effects of wind-waves – Langmuir turbulence by the Stokes vortex force, Stokes drift production, and/or the Stokes Coriolis force. We propose a different approach, in part prompted by recent modeling and observational studies which suggest orders-of-magnitude higher levels of turbulence dissipation rate due to the wind acting on the strong vorticity field of oceanic fronts [Thomas & Taylor, 2010; Chang and Oey, 2010, 2011a; D’Asaro et al., 2011]. The high levels of turbulence are caused by strong slantwise convective cells that penetrate to 100’s meters in the ocean’s surface layer. We also hypothesize that a coupled,dynamically active atmospheric boundary layer also exists over these oceanic fronts, as suggested by recent satellite observations over the Gulf Stream front [Minobe et al., 2008]. Here, we propose to apply these ideas of oceanic front-atmosphere boundary-layer coupled dynamics to studying the effects of eddies of the North Pacific Subtropical Counter Current (STCC) east of Taiwan on the overlying atmosphere, and the latter’s feedback on eddy energetics. We will examine the impacts of these coupled wind-eddy dynamics (and thermodynamics) to the interannual and decadal variability of the western North Pacific climate. To achieve these objectives, we propose to develop an advanced air-sea coupled model based on the massively parallel version of the Princeton Ocean Model (mpiPOM) and the sophisticated Weather Research and Forecast (WRF) atmospheric model.

Background

East of Taiwan and northern Luzon Strait, Philippines, at approximately 19oN~23oN latitudes, a weak (a few cm s-1) and shallow (near the surface approximately 100 m) eastward current penetrates into the open Pacific for thousands of kilometers, from 130oE~180oE. This North Pacific Subtropical Counter Current (STCC) was first predicted by Yoshida and Kidokoro [1967a,b] and subsequently confirmed by numerous observations [e.g. White et al. 1978]. The STCC is quite unremarkable were it not for the fact that it exists in a region where one would normally expect sluggish westward flow of the Sverdrup’s anticyclonicgyre, and also were it not for the fact that the region is populated with eddies of both signs, cyclones and anticyclones (fig.1). These eddies have diameters of 300~500 km, and at their centers the SSH is raised (depressed) by approximately 15~20 cm above (below) the background sea-surface. The eddies propagate westward at approximately -8 to -10 km day-1, and their rotational speeds are not particularly strong, about 0.1~0.4 m s-1. However, their sizes and numbers have considerable impacts on the ocean’s circulation of the western Pacific. A recent study [Chang and Oey, 2011b] shows that the Kuroshio transport off the northeastern slope of Taiwan is strongly modulated by the presence of these eddies both at seasonal and interannual time scales.

Fig.1. High-pass filtered (with half-power filter cutoffs of 20° of longitude by 10° of latitude) sea-surface-height (SSH) map on 28 August 1996 constructed from the merged T/P and ERS-1 data from AVISO [ showing a large number of eddies in the STCC (“Study Region”) zone. Red indicates +15cm and purple-blue 15cm. From Chelton et al. [2011].

Fig.2 The Philippines and Taiwan poles are derived in Chang and Oey [2011c] by correlating (as shown here as color & contour interval = 0.2, zero omitted) the 360-day low-pass tide-gauge sea-level difference between Ishigaki and Keelung with 5o×5o-averaged wind stress curl (from ECMWF; leading by 8months); the 95% significance level 0.1. Rectangles show regions used for Taiwan and Philippines poles, and PTO is defined as Philippines minus Taiwan poles.

The STCC eddies are spawned through baroclinic instability of the STCC and the North Equatorial Current (NEC) system. At seasonal time scales, the STCC-NEC system is most unstable in early Spring [March; Qiu, 1999]. At interannual and decadal time scales of particular interest here, Chang and Oey [2011c] show that the STCC system is most unstable, and produces most number of eddies, during years of the so called positive PTO (Philippines-Taiwan Oscillation index) when the wind stress curl east of Philippines (the “Philippines Pole”) is strongly positive and the wind stress curl east of Taiwan (the “Taiwan Pole”) is strongly negative (Fig.2). Chang and Oeythen show that this slow oscillation of the atmospheric wind system drives a correspondingoscillation of the ocean’s thermoclines,such that the positive PTO would correspond to a raised thermocline east of Philippines and a depressed thermocline east of Taiwan. The resulting interannual oscillations of the isotherms correspond to northward (during positive PTO) and southward (negative PTO) shifting of the NEC, giving rise to the STCC-eddy abundance (scarcity) in years of positive (negative) PTO.

Fig.3.June through August climatology of the frequency of high-wind (>20 m s-1) occurrence (%) based on QuikSCAT observations. The 3~4% of strong wind occurrence over the STCC eddy region is almost exclusively due to typhoon visits. The number may seem low except when one realizes that it is the maximum over the entire North Pacific. From SampeXie [2007].

Eddies are simply-connected (i.e. approximately circular) fronts. Recent modeling and observational studies suggest that high levels of turbulence dissipation rate are produced due to the wind acting on the strong vorticity field of oceanic fronts [Thomas & Taylor, 2010; Chang and Oey, 2010, 2011a; D’Asaro et al., 2011]. The high levels of turbulence are caused by strong slantwise convective cells that penetrate to 100’s meters in the ocean’s surface layer. We hypothesize that a coupled, dynamically active atmospheric boundary layer also exists over these oceanic fronts, as suggested by recent satellite observations over the Gulf Stream front [Minobe et al., 2008]. Is the ocean’s surface mixing east of Taiwan modulated by eddies whose abundance varies at interannual and decadal time scales? Does the heat storage in the surface layer also vary? Since eddies carry mass and heat, what is the impact of these variations to the atmospheric system above? It is well-known that the development and intensity of tropical cyclones can be affected by the ocean’s SST and upper-ocean heat content [Emanuel, 2005]. Moreover, the subtropical region east of Taiwan has the highest frequency of typhoon occurrences (fig.3). What impact then, can the variation in STCC-eddy abundance have on the long-term typhoon activities in this region? These are not only interesting questions to ponder, but also practically very important.

Proposed Study

We propose to apply the above ideas of oceanic front-atmosphere boundary-layer coupled dynamics to the eddies of the North Pacific Subtropical Counter Current (STCC) east of Taiwan. We will examine the impacts of these coupled wind-eddy dynamics (and thermodynamics) to the interannual and decadal variability of typhoon intensity in a warming climate. To achieve these objectives, we propose to develop an advanced air-sea coupled model based on the massively parallel version of the Princeton Ocean Model (mpiPOM) and the sophisticated Weather Research and Forecast (WRF) atmospheric model. The ocean component of the proposed modeling study will utilize the Taiwan Ocean Prediction System (TOPS) that we recently proposed to NSC for funding request. This is a comprehensive system that will eventually include also a biogeochemical-ecosystem component. The proposed air-sea coupled modeling can therefore potentially extend to also studying the long-term impact of eddy-typhoon variability on biogeochemical-ecosystem dynamics in East Asian marginal seas. Our plan is to initially conduct a number of somewhat idealized but very high-resolution simulations (grid sizes ~ 1km) of the coupled eddy-atmospheric dynamics, so as to learn the basic dynamical processes. These simulations will yield information on how we may parameterize slantwise convective cells in coarser-resolution climate models. The climate model will then be developed to examine interannual and decadal variability of the typhoon intensity and its dependence on the PTO oscillations of the STCC eddies.

Computing Requirements

5 Million NTD, for 1 Workstation, Cluster CPU & Disk Storage upgrades.

Manpower

1 PI, 1 postdoc, 1 PhD student, 1 MS student, 1 Research Assistant.

References

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