ENSO-driven changes in the tropical air–sea system modulating marine isoprene emissions across the Tropical Pacific
image: Key factors and processes driving the difference in marine isoprene emission between El Niño and La Niña years. BIO emission and SML emission exhibit a zonal dipole pattern between the equatorial central Pacific and the equatorial western Pacific. SML emission also shows a meridional tripole pattern in the tropical central Pacific. In the equatorial central Pacific, anomalous enhancement of SST is accompanied by anomalous ascending, increased total cloud cover (TCC), and reduced surface solar radiation downwards (SSRD), collectively suppressing BIO emission and SML emission. In addition, the decline in BIO emission is sensitive to a shallower mixed layer depth (MLD) and reduced sea surface chlorophyll-a concentration (Chl-a), whereas the suppression of SML emission is driven by weakened sea surface wind (SSW) and reduced air-sea exchange rate. Shadings in ocean indicate SST difference between El Niño and La Niña.
Credit: ©Science China Press
The air-sea system plays a key role in global climate system through regulating energy exchange and material cycling. ENSO, as the dominant mode of interannual air-sea variability, modulates sea surface temperature (SST), ocean circulation, and nutrient supply, thereby influencing atmospheric circulation, marine productivity, and the release of marine biogenic substances. Isoprene is the most abundant marine biogenic volatile organic compounds (BVOCs), playing a critical role in secondary organic aerosol formation and climate feedbacks. Marine isoprene emissions mainly comprise the phytoplankton-generated biological isoprene emission flux (BIO emission) and the photochemically generated isoprene emission flux from the sea surface microlayer (SML emission). However, it remains unclear how ENSO drives interannual variations of marine BVOCs emissions due to limited emission dataset and parameterization model.
To address this issue, machine learning algorithms are applied to extend sea surface chlorophyll-a concentration data. Then the extended chlorophyll-a data is integrated into the isoprene emission parameterization model, thereby constructing a long-term marine isoprene emission dataset from 1950 to 2020. By using the long-term isoprene emission data, the interannual variability of marine isoprene emission and the physical processes associated with the ENSO are investigated.
The interannual variability of marine isoprene emissions is largest in the equatorial Pacific region, showing strong correlation with ENSO dynamics. A negative relationship is identified between ONI and isoprene emission in the equatorial Pacific region, which means that under El Niño phase, isoprene emissions decrease, whereas under La Niña phase, emissions increase. During El Niño winters, isoprene emissions in the equatorial Pacific region decrease by 2.34×109 g (16%) compared to La Niña, representing the most pronounced seasonal emission contrast.
ENSO-driven variabilities in SST, solar radiation, surface wind and phytoplankton activity synergistically govern the isoprene air-sea exchange. ENSO-driven positive SST anomaly in the equatorial central Pacific enhances anomalous ascending, increases cloud cover and reduces surface solar radiation, which suppresses BIO and SML emission. In contrast, negative SST anomaly in the equatorial western Pacific favors high BIO and SML emission, creating a zonal dipole pattern across the equatorial Pacific. BIO emission reduction is further linked to a shallower mixed layer and lower chlorophyll-a concentration in the central Pacific. Surface winds induce a meridional tripole pattern in SML emission across the tropical central Pacific, featuring enhanced values in the subtropical regions and reduced intensity near the equator.
By examining marine volatile organic compound emissions from the perspective of interannual variability, this study extends understanding of marine emissions beyond the control of local environmental factors to the regulation by large scale air sea systems. The findings reveal detailed pathways through which air sea systems across different scales and Earth system spheres influence marine organic compound emissions, providing new scientific evidence for understanding the variability of air sea material exchange, evaluating the formation of marine secondary organic aerosols, and assessing their potential climatic effects.
See the article:
Sun M, Song L*, Cui L, Liu H, Xiao Y, Wu L, Hu W, Fu P, Zhu J*. 2026. Interannual variability in marine isoprene emissions modulated by ENSO dynamics. Science China Earth Sciences, 69(5): 1785–1794, https://doi.org/10.1007/s11430-025-1869-6.