The West Pacific Gradient tracks ENSO and zonal Pacific sea surface temperature gradient during the last Millennium

Small changes in Pacific temperature gradients connected with the El Niño Southern Oscillation (ENSO) influence the Walker Circulation and are related to global climate anomalies. Therefore, it is of paramount importance to develop robust indices of their past behavior. Here, we reconstruct the difference in sea surface temperature between the west and central Pacific during ENSO, coined the West Pacific Gradient (WPG), based on the Last Millennium Paleo Hydrodynamics Data Assimilation. We show that the WPG tracks ENSO variability and strongly co-varies with the zonal gradient in Pacific sea surface temperature. We demonstrate that the WPG strength is related to significant atmospheric circulation and precipitation anomalies during historical El Niño and La Niña events by magnifying or weakening droughts and pluvials across the Indo-Pacific. We show that an extreme negative WPG coupled to a strong zonal Pacific temperature gradient is associated with enhanced megadroughts in North America between 1400 CE and the late sixteenth century. The twentieth century stands out in showing the most extreme swings between positive and negative WPG conditions over the past Millennium. We conclude that the WPG is a robust index together with ENSO indices to reveal past changes in Pacific zonal sea surface temperature gradient variability.

Seasonal Sensitivity of the Cross-Equatorial Hadley Cell Response to Extratropical Thermal Forcings

This study explores the seasonal sensitivity of tropical circulation responses to an idealized extratropical thermal forcing using Community Atmosphere Model version 5 coupled to a slab ocean. The thermal heating over the Southern Ocean is held constant, and the tropical responses in each month of the year are investigated. An anomalous cross-equatorial cell and a southward tropical rain belt shift occur every month. The anomalous cross-equatorial cell has a strong influence on the strengths of the Hadley cell and the subtropical jet in the winter hemisphere; in contrast, it has nearly no impact on the Hadley cell and the subtropical jet strengths in the summer hemisphere.The seasonal variation of the anomalous cross-equatorial cell is small (<∼ 30% of the annual mean change), and could be understood via the energetic and the sea surface temperature gradient perspectives. Both perspectives point to the seasonality of the anomalous ocean heat uptake within the deep tropics as the key factor explaining the weak seasonality of the anomalous cross-equatorial cell. We propose a hypothesis explaining about 75% of this seasonal variation via the climatological position of the ITCZ relative to the anomalous cross-equatorial cell.The results suggest a modest seasonality in tropical precipitation and circulation responses to extratropical forcing. Also, such seasonality may be partly predicted by the climatological seasonal cycle of the tropical circulations.

Decoupling of temperature and pH gradients along the equatorial Pacific during the Pliocene

Equatorial Pacific dynamics drive tropical climate patterns such as the El Niño-Southern Oscillation and provide nutrients for one of the world’s most productive marine ecosystems. How this region will respond to global warming remains an important area of study with profound implications for both human wellbeing and economic and ecosystem stability. In light of this, numerous studies have investigated equatorial Pacific dynamics during the Pliocene epoch (5.3-2.6 million years ago) as an analogue for future behavior of the region under global warming (1–12). Current paleoceanographic records from the Pliocene tropical Pacific present an apparent paradox, with proxy evidence of a reduced east-west sea surface temperature gradient along the equator(1,5,6,11)– indicative of reduced wind-driven upwelling – conflicting with evidence of enhanced biological productivity in the region (13–15) which is typically driven by upwelling. Here we reconcile these observations by providing new evidence for older, more acidic, and nutrient-rich water reaching the equatorial Pacific by way of a Pacific meridional overturning cell during the Pliocene (16). This provides a mechanism by which enhanced productivity could have existed alongside a reduced east-west sea surface temperature gradient in the Pliocene equatorial Pacific. Furthermore, these results challenge the current paradigm of a decline in biological productivity in warmer worlds due to enhanced thermal stratification (17). Our findings shed a new light on equatorial Pacific dynamics and help constrain potential changes to them in the near-future, given that the Earth is expected to reach Pliocene-like temperatures by the end of the century. The equatorial Pacific is a region of great significance as it hosts one of the most important climate phenomena on the planet, the El Niño Southern Oscillation (18), and supports massively productive fisheries that provide key ecosystem services to numerous communities (19,20), and our results provide novel insight on how it might change as the oceans adjust to a warming world.