Water Masses in the Pacific in CCSM3

Abstract

An examination of model water masses in the North Pacific Ocean is performed in the Community Climate System version 3 (CCSM3) and its ocean-only counterpart. While the surface properties of the ocean are well represented in both simulations, biases in thermocline and intermediate-water masses exist that point to errors in both ocean model physics and the atmospheric component of the coupled model. The lack of North Pacific Intermediate Water (NPIW) in both simulations as well as the overexpression of a too-fresh Antarctic Intermediate Water (AAIW) is indicative of ocean model deficiencies. These properties reflect the difficulty of low-resolution ocean models to represent processes that control deep-water formation both in the Southern Ocean and in the Okhotsk Sea. In addition, as is typical of low-resolution ocean models, errors in the position of the Kuroshio, the North Pacific subtropical gyre western boundary current (WBC), impact the formation of the water masses that form the bulk of the thermocline as well as the properties of the NPIW. Biases that arise only in the coupled simulation include too-salty surface water in the subtropical North Pacific and too deep a thermocline, the source of which is the too-strong westerlies at midlatitudes. Biases in the location of the intertropical convergence zone (ITCZ) and the southern Pacific convergence zone (SPCZ) lead to the opposite hemispheric asymmetry in water mass structure when compared to observations. The atmospheric component of the coupled model acts to compound most ocean model biases.

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

“…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.…”

Response of the Pacific Ocean Circulation to Climate Change

“…1a in Luo et al, 2009b) than in the individual models (e.g., Fig. 7b in Thompson and Cheng, 2008) where the observed two MLD maxima (e.g., Fig. 3a in Suga et al, 2004) are commonly found to coalesce into one large region.…”

An Enhancement of Low-Frequency Variability in the Kuroshio–Oyashio Extension in CCSM3 owing to Ocean Model Biases

“…Biases in mean surface heat and freshwater fluxes for the most part occur because of biases in ocean circulation. They do not originate in the atmospheric component of the model (Thompson and Cheng 2008). However, when one looks in more detail at the western boundary current extension region, larger mean biases become apparent as discussed in the introduction.…”

“…The structure of the wind-driven circulation in the North Pacific in the broadest sense is well represented by climate models, with the location and structure of both the subtropical and subpolar gyres qualitatively correct, once biases in wind stress are taken into account [see Thompson and Cheng (2008) for further discussion of wind stress-forced ocean biases in CCSM3.0]. Biases in mean surface heat and freshwater fluxes for the most part occur because of biases in ocean circulation.…”

“…Large and Danabasoglu (2006) show a large warm SST bias occurs in the KOE in the CCSM3. Thompson and Cheng (2008) compare a fully coupled model against the ocean-only model forced with atmosphere without year-to-year variability and show that biases in SST, circulation, and vertical structure in the ocean component of the coupled model originated in the ocean model for the most part and are not forced by errors in the atmospheric component. In addition, the subarctic North Pacific tends to be too fresh at the surface (Large and Danabasoglu 2006).…”