ANALYZING FORCED AND INTERNAL VARIABILITY IN LAST MILLENNIUM SUBTROPICAL EASTERN PACIFIC CLIMATE USING AN ALKENONE-DERIVED RECORD OF SST AND PRODUCTIVITY VARIABILITY

Coupled ocean-atmosphere modes of climate variability in the Pacific Ocean play a significant role in the overall interannual to centennial scale variability of the Earth’s climate system and influence global climatic response to radiative forcings. At the forefront of these quasi-periodic climate modes are the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), which is characterized by decadal variations in the strength of the Aleutian Low and subsequent restructuring of regional winds and meridional SST patterns. The dynamical ocean thermostat (DOT) hypothesis suggests that periods of increased (decreased) radiative forcing in the tropical Pacific favors more La Niña-like (El Niño-like) conditions, predicting warm eastern equatorial Pacific (EEP) SST during the cool Little Ice Age (LIA). While some proxy records support the antiphase relationship between radiative forcing and EEP SST, recent studies suggest a more complicated relationship. Furthermore, a dearth of subtropical marine-based climate reconstructions limits our ability to evaluate the extent to which the subtropics may resemble tropical responses to radiative forcings.

We present a high-resolution record of SST and marine paleoproductivty determined using alkenone U_k^’37 unsaturation and C₃₇ concentration proxies spanning the last millennium from a laminated sediment core taken from the San Lazaro Basin (SLB) off Baja California, Mexico (25.18°N, 112.66°W). The SLB is uniquely located beneath the dynamic boundary between cool northerly California Current waters and warm tropical waters, making it strongly sensitive to both the PDO and ENSO. We can thus assess the DOT in the subtropics across multiple climate state transitions and address the influence of tropical phenomena on subtropical responses to radiative forcings. We observe a step-wise transition to a warm LIA with SST warmer or on par with 20^th century temperatures, in agreement with the DOT. Spectral analysis reveals significant power at the 20-30 yr bands indicating strong influence of the PDO. High power at the 190 yr band, particularly during the LIA, suggests a subtropical climatic response to the 200 yr DeVries solar cycle or a multi-centennial period of internal variability.