An ocean‐biology‐induced negative feedback on ENSO as derived from a hybrid coupled model of the tropical Pacific

Abstract

Biological conditions in the tropical Pacific Ocean (e.g., phytoplankton biomass) are strongly regulated by physical changes that are associated with the El Niño‐Southern Oscillation (ENSO). The existence and variation of phytoplankton biomass act to modulate the vertical penetration of the incoming sunlight into the upper ocean, which causes an ocean‐biology‐induced heating (OBH) effect on the climate system. Previously, the penetration depth of solar radiation in the upper ocean (Hp) has been defined to describe the related bioclimate connections. An empirical model for interannual Hp variability that is parameterized in terms of its relationship with the sea surface temperature (SST) in the tropical Pacific was derived from remotely sensed ocean color data and is incorporated into a hybrid coupled model (HCM) to represent the OBH effects. In this paper, several HCM experiments are performed to demonstrate the biofeedback onto the ENSO, including a climatological Hp run (in which Hp is prescribed as only seasonally varying), interannual Hp runs (with different intensities of the interannually varying OBH effects), and a run in which the sign of the OBH effect is reversed. Significant modulating impacts on the interannual variability are found in the HCM and are characterized by a negative feedback between the ocean biology and the climate system in the tropical Pacific; stronger the OBH feedback, weaker the interannual variability. The processes that are involved in the feedback are analyzed. The SST is modulated indirectly by dynamic ocean processes that are induced by OBH. The significance and implication of the OBH effects are discussed in terms of their roles in ENSO variability and the model biases in the tropical Pacific.

Causes of the Regional Variability in Observed Sea Level, Sea Surface Temperature and Ocean Colour Over the Period 1993–2011

“…Zhai et al (2011) demonstrated increased heat loss from the ocean to the air associated with bio-optical heating of the upper ocean caused by the presence of phytoplankton in the Gulf of St. Lawrence. Atmospheric heat gain induced by the presence of phytoplankton has been further examined in coupled ocean-atmosphere models, which show an amplification of the seasonal cycle of temperature in the troposphere, modulating the tropical convection patterns and atmospheric circulation (Shell et al 2003) and affecting large patterns of climate variability such as ENSO (Zhang 2015).…”

A New Hybrid Coupled Model of Atmosphere, Ocean Physics, and Ocean Biogeochemistry to Represent Biogeophysical Feedback Effects in the Tropical Pacific

“…These results have implications for revealing the underlying processes that may be responsible for the biogeophysical effects (Zhang, ). Because interannual H p anomalies are seen to have large modulating effects on the Q pen and Q abs fields, but less so on R sr , this indicates that the feedback effect is not dominantly realized through direct thermal heating effect on SST.…”

“…Large Chl and H p anomalies act as biogeophysical feedbacks on the climate system. As analyzed by previous studies (e.g., Zhang, ), such a biogeochemistry‐climate linkage is realized through the H p ‐induced effects on several heating terms, including the penetrative solar radiation flux through the base of the ML (Q pen ) and the absorbed part within the ML (Q abs ), which are written as where Q sr is the incoming solar radiation flux at the sea surface, H m is the depth of the ML, H p is the penetration depth, and γ is a constant (=0.33) denoting the fraction of the radiation available to penetrate to depths beyond the first few centimeters of the sea surface. The net differences between Q sr and Q pen determine the Q abs part, which leads to the rate of the ML temperature change due to the direct heating effect of solar radiation (R sr ), written as where ρ 0 is the density of sea water and C p is the heat capacity.…”

“…In our previous studies, we adopted a statistical modeling approach to representing the effects of ocean biogeochemistry‐induced heating on the climate system of the tropical Pacific (Zhang, ; Zhang et al, ). More specifically, penetration depth (H p ) is introduced to represent the ocean biogeochemistry‐induced heating effects (Zhang et al, ), serving as a link between the ocean biogeochemistry and physics.…”

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

“…This depth can be significantly affected by marine ecosystem and can be derived from the chlorophyll (Chl) content data which are difficult for in-situ observations at the basin scale. The development of the remote sensing techniques makes satellite-based ocean color data available and the effects of the penetrative radiation on the upper ocean state are reported by the recent studies (Murtugudde et al 2002;Zhang 2015). In this study, merged monthly Chl fields from the GlobColour product (http://globcolour.info), which is developed, validated, and distributed by ACRI-ST, France (Maritorena et al 2010), are used to calculate the fields of h p following the method proposed by Murtugudde et al (2002) …”