Response of the North Pacific subtropical countercurrent and its variability to global warming

Abstract

Response of the North Pacific subtropical countercurrent (STCC) and its variability to global warming is examined in a state-of-the-art coupled model that is forced by increasing greenhouse gas concentrations. Compared with the present climate, the upper ocean is more stratified, and the mixed layer depth (MLD) shoals in warmer climate. The maximum change of winter MLD appears in the Kuroshio–Oyashio extension (KOE) region, where the mean MLD is the deepest in the North Pacific. This weakens the MLD front and reduces lateral induction. As a result of the reduced subduction rate and a decrease in sea surface density in KOE, mode waters form on lighter isopycnals with reduced thickness. Advected southward, the weakened mode waters decelerate the STCC. On decadal timescales, the dominant mode of sea surface height in the central subtropical gyre represents STCC variability. This STCC mode decays as CO₂ concentrations double in the twenty-first century, owing both to weakened mode waters in the mean state and to reduced variability in mode waters. The reduced mode-water variability can be traced upstream to reduced variations in winter MLD front and subduction in the KOE region where mode water forms.

Appendix: Comparison with observations

Our recent study (Sect. 3a of Xie et al. 2011) compared the control run of GFDL CM2.1, the same model used here, with observations; here we just provide a brief comparison of key variables from 20C3M against previous observational studies.

Using hydrographic data, Suga et al. (2004) showed that the winter MLD exhibits two distinct maximum, one to the south of the KE and one to the north, which are the formation regions for the STMW and the CMW, respectively. Compared to observations, Fig. 1a exhibits a broad, zonally elongated region of deep mixed layer, resulting in too extensive formation of mode waters (Suga et al. 2004; Thompson and Cheng 2008). The zonal elongated region of deep MLD may have caused spatial biases in the maximum subduction rate in Fig. 1a displaced too east compared to observations (e.g., Qiu and Hwang 1995; Qu and Chen 2009). According to Suga et al. (2004), small cross-isopycnal flow in the mixed layer plays a key role in the CMW formation, whereas it is the lateral induction in this model.

In observations, the STMW (CMW) is found in a density range of 25.3–25.7 σ _θ (26.1–26.5 σ _θ ) [Suga et al. 1989, 1997, 2004; Nakamura 1996; Hanawa and Talley 2001]. For 20C3M, the STMW (CMW) is clustered in 25.4–25.7 σ _θ (26.2–26.6 σ _θ ). The model mode waters are slightly denser by 0.1 σ _θ , but the density range of the two mode waters is broadly consistent with observations. However, the typical minimum PV of mode waters is ~1.5 × 10⁻¹⁰ m⁻¹ s⁻¹ in the observed climatology, whereas it is ~0.5 × 10⁻¹⁰ m⁻¹ s⁻¹ in the model (Fig. 7b). Moreover, the low-PV tongue on the isopycnal is much more diffused in observations than in the model, with a broader spatial structure and larger PV values (Fig. 5b of Xie et al. 2011).

In the observed meridional transect (Fig. 3b of Xie et al. 2011), the maximum zonal velocity of STCC is ~3 m s⁻¹ at 26.5°N, anchored by PV minima on 25.5–26.5 σ _θ to the north. The observed zonal velocity of STCC is much weaker than in the model, and these PV minima north of STCC are not as pronounced as in the model. The eastern STCC that we mainly focused on in this study seems to be too strong in 20C3M compared to observations, a bias likely due to too strong mode waters.