Abstract
The MJO modulation of sea surface chlorophyll-a (Chl) examined initially by Waliser et al. in Geophys Res Lett, (2005) is revisited with a significantly longer time-series of observations and a more systematic approach to characterizing the possible mechanisms underlying the MJO-Chl relationships. The MJO composite analysis of Chl and lead-lag correlations between Chl and other physical variables reveal regional variability of Chl and corresponding indicative temporal relationships among variables. Along the path of the MJO convection, wind speed—a proxy for oceanic vertical turbulent mixing and corresponding entrainment—is most strongly correlated with Chl when wind leads Chl by a few days. Composite Chl also displays MJO influences away from the path of the MJO convection. The role of wind speed in those regions is generally the same for Chl variability as that along the path of the MJO convection, although Ekman pumping also plays a role in generating Chl variability in limited regions. However, the wind forcing away from the MJO convection path is less coherent, rendering the temporal link relatively weak. Lastly, the potential for bio-physical feedbacks at the MJO time-scale is examined. The correlation analysis provides tantalizing evidence for local bio-feedbacks to the physical MJO system. Plausible hypothesis for Chl to amplify the MJO phase transition is presented though it cannot be affirmed in this study and will be examined and reported in a future modeling study.
Highlights
Chlorophyll-a (Chl) concentration is an indicator of photosynthetic activity in the ocean
-0.04 means an 8.8% decrease in Madden–Julian Oscillation (MJO) composite compared to the control
It is known that the MJO displays a strong seasonality and MJO composites are separated into boreal winter (November– April) and summer (May–October)
Summary
Chlorophyll-a (Chl) concentration is an indicator of photosynthetic activity in the ocean. Using 5 years of Sea-viewing Wide Field-of-view Sensor (SeaWiFs) data, WET05 averaged satellite-derived ocean color data into pentads around the peak of the MJO in the tropical Indo-Pacific Ocean and identified a number of regions where the Chl anomalies were significant. They further examined the cursory relationships among Chl, wind speed, and surface shortwave flux in those regions. The main finding of their study was that the variability in the entrainment of nutrient-rich waters into the mixed layer could be induced by MJO-related wind variation, and this was likely driving a large part of the spatio-temporal modulation of Chl at intraseasonal time-scales.
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