Structure and effect of ocean biology-induced heating (OBH) in the tropical Pacific, diagnosed from a hybrid coupled model simulation

Abstract

Recent modeling studies have demonstrated that ocean biology plays a significant role in modulating the climate over the tropical Pacific through its effect on the vertical distribution of sunlight in the upper ocean, which can be simply represented by penetration depth (Hp). Previously, remotely sensed ocean color data have been used to derive an empirical model to depict interannual Hp variability (H′p) in the region. The derived H′p model is then incorporated into a hybrid coupled model (HCM) of the tropical Pacific to parameterize ocean biology-induced heating (OBH) effects. In this paper, outputs from the HCM simulations are diagnosed to reveal the structure and variability of OBH terms that are directly influenced by Hp and the depth (Hm) of the mixed layer (ML), including the penetrative solar radiation flux out of the ML (Qpen), the fraction absorbed within the ML (Qabs), and the related time rate of change of the ML temperature (Rsr). Coherent relationships are found among interannual variations in Hp, Hm, Qpen, Qabs and Rsr, with geographical dependence. It is found that Hp tends to have largest interannual variations over the western-central equatorial Pacific where its effects on Qpen are out of phase with those of Hm during ENSO cycles. It is further demonstrated that Qpen is a field whose interannual variability is significantly enhanced by H′p in the western-central equatorial region; the resultant differential heating in the vertical between the ML and subsurface layers acts to modulate the thermal stratification, the stability, vertical mixing and entrainment in the upper ocean. These induced ocean processes further affect sea surface temperatures in the equatorial Pacific.