## 3 Results and Discussion
### CDOM, \(S_{g}\), and DOC Climatology
A feature in the western equatorial Pacific stands out starkly as a crescent stretching from about \(10^{\circ}\) N to \(15^{\circ}\) S and spanning nearly the entire \(100^{\circ}\) longitude range from South America to the Solomon Islands. For brevity, we refer to this as the Western Pacific Crescent (WPC). To test the link between El Nino Southern Oscillation dynamics  
Figure 12: MLR retrievals of \(a_{g}(380)\) by season over the entire MODIS Aqua era (left column), and residuals between Chl and CDOM (right column). Imagery was binned from 4 km resolution monthly composites between August 2002 and January 2014.  
and the CDOM anomaly in the WPC, an average of monthly \(\Delta a_{g}(380)\) within the WPC is compared with the multivariate ENSO index (MEI [72]) and \(\Delta S_{g}(275-295)\) (Figure 14). The MEI provides a convenient index for tracking the dominant characteristics associated with ENSO, namely sea-surface pressure, temperature, wind stress, and cloudiness. Positive MEI represent the warmer EI Nino cycle associated with lower wind stress, a flattening in the trans-Pacific thermocline, and inhibited productivity in the equatorial Pacific, and negative MEI represents La Nina, which is cooler, and more productive. The coupling between MEI and \(\Delta a_{g}(380)\) and MEI and \(\Delta S_{g}(275)\) is remarkably strong and well correlated (Figure 14; r = \(-\)0.77, r = 0.80, respectively, and \(p<<0.01\) in each case). As expected, CDOM and UVB slope are also very well correlated (r = \(-\)0.90, \(p<<0.01\)). The tight correlation between CDOM anomaly, CDOM slope anomaly and MEI may help to predict broad changes in surface CDOM in a future in which warmer sea surface temperatures are expected, particularly in the western Pacific, as the long-term warming trend leads to oceanic conditions favorable to El Nino-like conditions [73]. In fact, sustained deficits in surface CDOM available for photooxidation and microbial remineralization across the WPC, as demonstrated here, is likely to result in a lower partial pressure of CO\({}_{2}\) derived from CDOM, and may therefore increase the flux of CO\({}_{2}\) into the ocean from the atmosphere, although this effect would largely be offset by the decrease in solubility associated with warmer temperatures in the surface ocean. Another consequence of lower CDOM across this region in a warming regime may be decreased surface heating through CDOM absorption, potentially providing some degree of negative feedback to the surface warming trend. Figure 13: CDOM anomaly (left) and slope anomaly (right) from MLR applied to MODIS Aqua during Autumn in El Niño years (2002–2005; top panel) and La Nina years (2007, 2008, 2010, 2011; bottom panel). The Western Pacific Crescent (WPC) feature is defined here as the broad region exhibiting a notable decline in CDOM during El Niño years, and enhancement during La Nina. UV slope shows the opposite pattern, with lower slopes during La Nina, although the percentage change is roughly an order of magnitude lower. The box shows the portion of the WPC subsampled for comparison with MEI (See Figure 14).