Influence of the El Niño - La Niña cycle
on satellite-derived primary production
in the California Current

Mati Kahru, B. Greg Mitchell

Scripps Institution of Oceanography, La Jolla, CA
92093-0218, USA, E-mail: mkahru@ucsd.edu

Introduction

Ecological responses to large-scale changes in physical forcing are difficult to quantify due to time and space variability, aliasing in sampling and uncertainties in temporal coupling of different trophic levels. Satellite remote sensing is a feasible way to sample the high spatial and temporal variability inherent in most oceanic fields over large areas. Several models of phytoplankton net primary production (NPP, g C month^-1) have been developed and successfully implemented [e.g. Behrenfeld et al., 2001]. We applied the Behrenfeld-Falkowski Vertically Generalized Production Model (VGPM) [Behrenfeld and Falkowski, 1997] to satellite-derived fields of surface chlorophyll a concentration (C_sat, mg m^-3), incident photosynthetically active radiation (PAR, Einstein m^-2 Day^-1) and sea-surface temperature (SST, °C), and derived a 5-year time series of NPP for the California Current area.

Results & Discussion

In our previous paper [Kahru and Mitchell, 2000] we showed that in addition to the well-known [Fiedler, 1984; Strub et al., 1990; Thomas et al., 1994] reduction in upwelling and high-chlorophyll surface waters, El Niño events in the California Current are associated with increased surface chlorophyll off Baja California. Here we have quantified the seasonal and inter-annual changes in NPP in relation to the 1997-98 El Niño and the subsequent La Niña.

The effects of the El Niño on NPP were most evident in the 100-300 km band from the coast. Relative to the mean annual cycle NPP dropped by approximately 30% off Southern California but increased by 40% off Southern and Northern Baja during and near the peak of the El Niño (Fig. 2, left panel).

Fig. 2 Left panel, time series of NPP in 100-300 km band in zones CC, SC, NB, SB calculated with Csat from OCTS (‡) and SeaWiFS (®), compared with the mean annual cycle (solid line). The dotted horizontal line is the long-term mean. Minima in the Northern Oscillation Index (¥, relative scale) correspond to peaks in the El Niño (filled black areas). Right panel, anomalies of NPP in 0-1000 km band in zones CC, SC, NB, SB relative to the mean annual cycle.

In the broad 0-1000 km band El Niño resulted in a decrease in NPP off Southern and Central California but in a significant increase off Southern and Northern Baja relative to the 5-year mean (Fig. 2, right panel). During the whole 1997-1998 period NPP decreased in the 0-1000 km band off Southern California by approximately 10-15% (1.5 Tg C month^-1) but increased sharply with the onset of La Niña in late 1998. Increased primary production in the 0-1000 km band off Northern and Southern Baja during the peak of the El Niño caused a positive NPP anomaly of 25-30% or 4.5 Tg C for the two zones combined. This phenomenon shifted the annual NPP maximum from late summer to early spring and caused a negative anomaly in late summer as the maximum in NPP terminated approximately 2 months early. Off Southern California the 10-15% negative anomaly that persisted throughout the 1997-98 El Niño period turned into a 10-20% positive anomaly with the onset of La Niña in late 1998 and increased NPP by approximately 1 Tg C month^-1 relative to the 5-year mean cycle. We detected a trend of increasing NPP and C_sat in both Central and Southern California zones. In Southern California the increase in NPP resembles a step function with lower values in 1997-1998 and higher values in 1999-2001. The shift from -10% anomaly to +15% anomaly occurred with the onset of the La Niña and the associated strong upwelling in late 1998. The total NPP integrated over the four zones peaked in June, 2000 at about 51 Tg C month^-1 for the 0-1000 km band and at 14.9 Tg C month^-1 for the 0-100 km band. The total inter-annual minimum was observed in 1996-97 while the maxima were observed in 1999-2000 and 2000-01 (Table 1).

Table 1. Estimated total annual NPP in Tg C for the
different bands (distance from coast in km)

Zooplankton displacement volume available from CalCOFI cruises showed a downturn during the El Niño period and an increase with the onset of the La Niña.