Seasonal variation of the upper ocean responding to surface heating in the North Pacific

Abstract

Seasonal variations of the upper ocean, such as mixed layer depth (MLD) and sea surface temperature (SST), responding to the atmospheric forcing in the North Pacific (10°N–50°N), are investigated by analyzing the Argo and NCEP/NCAR reanalysis 1 data. The OAFlux data are also used for comparison. During the early heating period in the high‐latitude ocean north of 30°N, where a seasonal thermocline is formed above the deep mixed layer under strong surface heating, the MLD h is found to be scaled as h∝(Lλ)1/2, where L is the Monin‐Obukhov length scale and λ is the Ekman length scale. On the other hand, in the low‐latitude ocean south of 30°N, where the preexisting MLD is shallow and surface heating is weak, h is found to be scaled by λ. It is found that a large amount of heat flux across the MLD occurs, especially in the high‐latitude ocean during the late heating period, in which h is small. It suggests the contribution by turbulent mixing across the MLD in addition to radiation penetration, and the eddy diffusivity in the high‐latitude ocean is estimated as Kv ∼ 10−4−10−3 m2 s−1. The heat budget of the mixed layer reveals that the contribution from the ocean heat transport is much smaller than the surface heat flux in the high‐latitude ocean except in the Kuroshio region, but it is sometimes comparable in the low‐latitude ocean.

Estimating diffusivity from the mixed layer heat and salt balances in the North Pacific

“…All diffusivity estimates were large (>10 24 m 2 /s), in agreement with recent estimates made during the heating season in the North Pacific using the Argo array and reanalysis products [Lee et al, 2015]. Estimates at both locations in this study had a seasonal cycle with larger values during fall and winter, although errors were also larger in winter.…”

“…Turbulence in this transition layer can still be relatively strong so that even weak vertical gradients in the physical or biogeochemical tracers can result in large diffusive fluxes. Consequently, diffusive mixing tends to be a leading-order process governing the surface layer properties [Lee et al, 2015]. Understanding the magnitude and variability of the diffusion coefficient is thus critical for studies of the exchange of heat, freshwater, and biogeochemical tracers between the surface layer of the ocean and the main pycnocline.…”

Scaling wind stirring effects in an oceanic bulk mixed layer model with application to an OGCM of the tropical Pacific

“…In order to fit the logarithmic current profile within the boundary layer, the eddy viscosity coefficient K m = u * L ek and the turbulent Ekman boundary layer is modified as (Rossby and Montgomery 1935). The variation of MLD is found to be proportional to L ek in the low latitude Pacific Ocean (Lee et al 2015). Nevertheless, f vanishes when approaching to the equator and the L ek is too deep to limit the downward propagation of TKE.…”

“…But in the tropical Pacific Ocean, the trade winds are prevailing and the differences are not substantial enough to influence our results. Thus, in this study, the monthly u * fields are calculated from the monthly wind stress fields and the method is commonly employed in the previous studies (e.g., Lee et al 2015;Pookkandy et al 2016). Figure 1a illustrates the annual-mean u * over the tropical Pacific, which is weak in the western equatorial Pacific and the eastern equatorial upwelling regions, but is strong with values exceeding 8 × 10 −3 ms −1 in the areas where the trade winds are prevailing.…”

“…In this formulation, the MLD is primarily determined by the local atmospheric forcing, including surface wind, heat flux, evaporation and precipitation (Lee et al 2015;Yoshikawa 2015;Pookkandy et al 2016). Meanwhile, the stratification at the base of ML also plays a critical role at the deepening phase of the ML (Pollard et al 1972;Kang et al 2010).…”