ENSO and the California Current coastal upwelling response

Abstract

A 31 year sequence of historical analyses of the California Current System (CCS) is used to describe the central CCS (35-43˚N) coastal upwelling response to El Niño-Southern Oscillation (ENSO) variability. The analysis period captures 10 El Niño and 10 La Niña events, including the extreme El Niños of 1982-1983 and 1997-1998. Data-assimilative model runs and backward trajectory calculations of passive tracers are used to elucidate physical conditions and source water characteristics during the upwelling season of each year. In general, El Niño events produce anomalously weak upwelling and source waters that are unusually shallow, warm, and fresh, while La Niña conditions produce the opposite. Maximum vertical transport anomalies in the CCS occur $1 month after El Niño peaks in midwinter, and before the onset of the upwelling season. Source density anomalies peak later than transport anomalies and persist more strongly through the spring and summer, causing the former to impact the upwelling season more directly. As nitrate concentration covaries with density in the central CCS, El Niño may exert more influence over the nitrate concentration of upwelled waters than it does over vertical transport, although both factors are expected to reduce nitrate supply during El Niño events. Interannual comparison of individual diagnostics highlights their relative impacts during different ENSO events, as well as years deviating from the canonical response to ENSO variability. The net impact of ENSO on upwelling is explored through an ''Upwelling Efficacy Index'', which may be a useful indicator of bottom-up control on primary productivity.

On the skill of seasonal sea surface temperature forecasts in the California Current System and its connection to ENSO variability

“…At 6-9 month lead times, a similar pattern continues, with the correlations continuing to decrease in the CCS while correlations between the CCS and the equatorial Pacific decline more slowly. These findings are consistent with our understanding of ENSO's influence on the CCS, with equatorial Pacific SST anomalies leading CCS anomalies by several months (Jacox et al 2015b), and suggest ENSO variability as a likely source of seasonal predictability in the CCS.…”

“…In the case of CanCM4, we attribute the observed skill above persistence primarily to a predictable evolution of the CCS wind (and resultant upwelling) anomalies during moderate to strong ENSO events. Wind-driven upwelling anomalies during ENSO events typically onset in the CCS in ~December (Jacox et al 2015b), consistent with a rapid atmospheric teleconnection from the tropics, and the lag between wind anomalies and their expression in the SST field results in the onset of SST forecast skill in ~January. Upwelling anomalies persist through April/May (Jacox et al 2015b) and continue to drive SST forecast skill, which extends slightly longer (through June or July) due to persistence of the wind-generated anomalies.…”

“…The statistical forecast is constructed using a multiple linear regression, where the observed SST anomaly at a given initialization month and lead time is fit as a function of both the CCS and Niño 3.4 SST anomalies the month prior to forecast initialization. For example, a statistical forecast of June initialized in February (4-month lead time) would fit the observed June SST anomalies as a function of the CCS and Niño 3.4 SST anomalies in January events, with El Niño (La Niña) conditions typically bringing reduced (increased) upwelling intensity and consequently positive (negative) SST anomalies (Jacox et al 2015b;Schwing et al 2002). This atmospheric teleconnection is fast, with the CCS response lagging the tropics by a few weeks to a month, and upwelling anomalies during ENSO events typically persisting through ~April (Alexander et al 2002;Jacox et al 2015b).…”

“…For example, a statistical forecast of June initialized in February (4-month lead time) would fit the observed June SST anomalies as a function of the CCS and Niño 3.4 SST anomalies in January events, with El Niño (La Niña) conditions typically bringing reduced (increased) upwelling intensity and consequently positive (negative) SST anomalies (Jacox et al 2015b;Schwing et al 2002). This atmospheric teleconnection is fast, with the CCS response lagging the tropics by a few weeks to a month, and upwelling anomalies during ENSO events typically persisting through ~April (Alexander et al 2002;Jacox et al 2015b). Local SST anomalies are also impacted by coastally trapped waves that propagate poleward into the CCS domain (Meyers et al 1998), deepening the thermocline and limiting the efficacy of coastal upwelling for cooling the ocean surface.…”

“…These dynamics are sensitive to temporal variability on scales from daily weather to multi-decadal and secular change (Checkley and Barth 2009). In particular, leading modes of basin-scale climate variability including the El Niño-Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the North Pacific Gyre Oscillation (NPGO) exert strong control over CCS upwelling (Di Lorenzo et al 2008;Jacox et al 2014Jacox et al , 2015b, generating pronounced interannual variability in the nearshore environment ( Fig. 1) and its biological communities.…”