Observed structural relationships between ocean chlorophyll variability and its heating effects on the ENSO
Ocean chlorophyll (Chl)-induced heating can affect the climate system through the penetration of solar radiation in the upper ocean. Currently, the ocean biology-induced heating (OBH) feedback effects on the climate in the tropical Pacific are still not well understood, and the mechanisms regarding how SST is modulated remain elusive. In this paper, chlorophyll (Chl) data from satellites are combined with physical fields from Argo profiles to estimate OBH-related fields, including the penetration depth (Hp) and the ocean mixed-layer (ML) depth (Hm). In addition, some directly related heating terms with Hm and Hp are diagnosed, including the absorbed solar radiation component within the ML (denoted as Qabs), the rate of ML temperature changes that are directly induced by Qabs (denoted as Rsr = Qabs/(ρ0CpHm)), and the portion of solar radiation that penetrates through the bottom of the ML (denoted as Qpen). The structural relationships between these related fields are examined to illustrate how these heating terms are affected by Hp and Hm. The extent to which Rsr and Qpen are modulated by Hp is strikingly different during ENSO cycles. In the western-central equatorial Pacific, inter-annual variations in Hp tend to be out of phase with those in Hm. A decrease (increase) in Qabs from a positive (negative) Hp anomaly during El Niño (La Niña) tends to be offset by a negative (positive) Hm anomaly. Thus, Rsr is not closely related with Hp, even though Qabs is highly correlated with Hp, indicating that the direct thermal effect through Qabs is not a dominant factor that affects the SST. In contrast, the inter-annual variability of Qpen in the region is significantly enhanced by that of Hp, with their high positive correlation. The Hp-induced differential heating in the ML and subsurface layers from the Qpen and Qabs terms modifies the thermal contrast, stratification and vertical mixing, which represent a dominant indirect ocean dynamical effect on the SST. The revealed relationships between these related fields provide an observational basis for gaining structural insights into the OBH feedback effects and validating model simulations in the tropical Pacific.
KeywordsChlorophyll variability Ocean biology-induced heating ENSO modulations The penetration depth and mixed layer depth Satellite and Argo data
The author would like to thank Drs. Mu Mu, Dunxin Hu, Fan Wang, Dake Chen, Anand Gnanadesikan, Tony Busalacchi, Youmin Tang, and Zhaohua Wu for their comments. The authors wish to thank the anonymous reviewers for their numerous comments that helped to improve the original manuscript. This research was supported by the National Programme on Global Change and Air–Sea Interaction (Grant No. GASI-IPOVAI-06), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA19060102), National Natural Science Foundation of China (Grant Nos. 41690122(41690120), 41490644(41490640), 41421005), the NSFC-Shandong Joint Fund for Marine Science Research Centers (U1406402) and the Taishan Scholarship. Zhi is additionally supported by the Foundation of Key Laboratory of Ocean Circulation and Waves (KLOCW), IOCAS (KLOCW1601), and Kang is additionally supported by the Foundation of KLOCW, IOCAS (KLOCW1809).
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