El Niño as a mediator of the solar influence on climate (original) (raw)
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ENSO as a mediator of the solar influence on climate
2006
Using a climate model of intermediate complexity, we simulate the response of the El Niño-Southern Oscillation (ENSO) system to solar and orbital forcing over the Holocene. Solar forcing is reconstructed from radiocarbon production rate data, using various scaling factors to account for the conflicting estimates of solar irradiance variability. As estimates of the difference since the Maunder Minimum range from 0.05% to 0.5% of the solar "constant", we consider these two extreme scenarios, along with the intermediate case of 0.2%. We show that for large or moderate forcings, the low-pass filtered east-west sea surface temperature gradient along the equator responds almost linearly to irradiance forcing, with a short phase lag (about a decade). Wavelet analysis shows a statisticallysignificant enhancement of the century-to-millenial scale ENSO variability for even a moderate irradiance forcing. In contrast, the 0.05%-case displays no such enhancement. Orbitally-driven insolation forcing is found to produce a long-term increase of ENSO variability from the early Holocene onwards, in accordance with previous findings. When both forcings are combined, the superposition is approximately linear in the strong scaling case. Overall, the sea surface temperature response is of the magnitude required, and persistent enough, to induce important climatic perturbations worldwide. The results suggest that ENSO may plausibly have acted as a mediator between the Sun and the Earth's climate. A comparison to key Holocene climate records, from the Northern Hemisphere subtropics and midlatitudes, shows support for this hypothesis.
Evolution and forcing mechanisms of El Niño over the past 21,000 years
Nature, 2014
The El Niño Southern Oscillation (ENSO) is Earth's dominant source of interannual climate variability, but its response to global warming remains highly uncertain 1 . To improve our understanding of ENSO's sensitivity to external climate forcing, it is paramount to determine its past behaviour by using palaeoclimate data and model simulations. Palaeoclimate records show that ENSO has varied considerably since the Last Glacial Maximum (21,000 years ago) 2-9 , and some data sets suggest a gradual intensification of ENSO over the past 6,000 years 2,5,7,8 . Previous attempts to simulate the transient evolution of ENSO have relied on simplified models 10 or snapshot 11-13 experiments.
Climate Dynamics, 2012
We use a state-of-the-art 3-dimensional coupled model to investigate the relative impact of long term variations in the Holocene insolation forcing and of a freshwater release in the North Atlantic. We show that insolation has a greater effect on seasonality and La Niña events and is the major driver of sea surface temperature changes. In contrast, the variations in precipitation reflect changes in El Niño events. The impact of ice-sheet melting may have offset the impact of insolation on El Niño Southern Oscillation variability at the beginning of the Holocene. These simulations provide a coherent framework to refine the interpretation of proxy data and show that changes in seasonality may bias the projection of relationships established between proxy indicators and climate variations in the east Pacific from present day records.
Integration of proxy data and model scenarios for the mid-Holocene onset of modern ENSO variability
Quaternary Science Reviews, 2008
Climate model studies have shown a gradual insolation-forced intensification of the El Nin˜o-Southern Oscillation (ENSO) during the Holocene. Proxy records of past climate variability provide important test cases for such model simulations, and are needed to determine the exact mechanisms and dynamics of the ENSO system. We provide an integrated overview of marine and terrestrial paleoclimatic proxy data relevant for detecting ENSO variability. We reconstruct spatial climate patterns during two time-slices, 6-5 and 4.5-3.5 ka cal BP, to examine the mid-Holocene intensification of ENSO. The proxy data consistently indicate that a state change occurred at 5 ka cal BP towards active ENSO cyclicity in the equatorial Pacific. Furthermore, from around 3 ka cal BP the ENSOteleconnected regions are characterized by an increased impact of ENSO, comparable to the present-day high-amplitude fluctuations of ENSO. Model studies have thus far explained the late-Holocene intensification of ENSO by insolation-forced Pacific trade wind reduction during summer. Our review shows that this single mechanism cannot completely explain the observed Holocene changes. An additional mechanism is proposed, involving increased Indo-Pacific Warm Pool (IPWP) heat charging, which is a possible explanation for the late-Holocene increase in ENSO amplitude. r
Orbitally driven insolation forcing on Holocene climate trends reassessed
2004
1] A global spatial pattern of long-term sea surface temperature (SST) trends over the last 7000 years is explored using a comparison of alkenone-derived SST records with transient ensemble climate simulations with a coupled atmosphere-ocean circulation model under orbitally driven insolation forcing. The spatial trend pattern both in paleo-SST data and in model results shows pronounced global heterogeneity. Generally, the extratropics cooled while the tropics experienced a warming during the middle to late Holocene. We attribute these divergent Holocene climate trends to seasonally opposing insolation changes. Furthermore, climate mode changes similar to the Arctic/North Atlantic Oscillation are superimposed on the prevalent pattern. It is concluded that nonlinear changes in the entire seasonal cycle of insolation played a dominant role for the temporal evolution of Holocene surface temperatures. For understanding of marine proxy data, apart from the dominance of summer insolation in high latitudes, a notable shift in the maximum insolation of the year in low latitudes has to be taken into account, which may influence timing of phytoplankton production and thus alters the seasonal origin of temperature signals recorded in the proxies.
Quaternary Science Reviews, 2021
Lack of constraint on spatial and long-term temporal variability of the El Niño southern Oscillation (ENSO) and its sensitivity to external forcing limit our ability to evaluate climate models and ENSO future projections. Current knowledge of Holocene ENSO variability derived from paleoclimate reconstructions does not separate the role of insolation forcing from internal climate variability. Using an updated synthesis of coral and bivalve monthly resolved records, we build composite records of seasonality and interannual variability in four regions of the tropical Pacific: Eastern Pacific (EP), Central Pacific (CP), Western Pacific (WP) and South West Pacific (SWP). An analysis of the uncertainties due to the sampling of chaotic multidecadal to centennial variability by short records allows for an objective comparison with transient simulations (mid-Holocene to present) performed using four different Earth System models. Sea surface temperature and pseudo-d 18 O are used in model-data comparisons to assess the potential influence of hydroclimate change on records. We confirm the significance of the Holocene ENSO minimum (HEM) 3-6ka compared to low frequency unforced modulation of ENSO, with a reduction of ENSO variance of~50 % in EP and~80 % in CP. The approach suggests that the increasing trend of ENSO since 6ka can be attributed to insolation, while models underestimate ENSO sensitivity to orbital forcing by a factor of 4.7 compared to data, even when accounting for the large multidecadal variability. Precessioninduced change in seasonal temperature range is positively linked to ENSO variance in EP and to a lesser extent in other regions, in both models and observations. Our regional approach yields insights into the past spatial expression of ENSO across the tropical Pacific. In the SWP, today under the influence of the South Pacific Convergence Zone (SPCZ), interannual variability was increased by~200 % during the HEM, indicating that SPCZ variability is independent from ENSO on millennial time scales.
We present a high-resolution magnesium/calcium proxy record of Holocene sea surface temperature (SST) from off the west coast of Baja California Sur, Mexico, a region where interannual SST variability is dominated today by the influence of the El Niño–Southern Oscillation (ENSO). Temperatures were lowest during the early to middle Holocene, consistent with documented eastern equatorial Pacific cooling and numerical model simulations of orbital forcing into a La Niña–like state at that time. The early Holocene SSTs were also characterized by millennial-scale fluctuations that correlate with cosmogenic nuclide proxies of solar variability, with inferred solar minima corresponding to El Niño–like (warm) conditions, in apparent agreement with the theoretical “ocean dynamical thermostat” response of ENSO to exogenous radiative forcing.
Climate of the Past, 2006
The coupled global atmosphere-ocean-vegetation model ECBilt-CLIO-VECODE is used to perform transient simulations of the last 9000 years, forced by variations in orbital parameters, atmospheric greenhouse gas concentrations and total solar irradiance (TSI). The objective is to study the impact of decadal-to-centennial scale TSI variations on Holocene climate variability. The simulations show that negative TSI anomalies increase the probability of temporary relocations of the site with deepwater formation in the Nordic Seas, causing an expansion of sea ice that produces additional cooling. The consequence is a characteristic climatic anomaly pattern with cooling over most of the North Atlantic region that is consistent with proxy evidence for Holocene cold phases. Our results thus suggest that the ocean is able to play an important role in amplifying centennial-scale climate variability.
Understanding ENSO dynamics through the exploration of past climates
2010
The palaeoclimate record shows that significant changes in ENSO characteristics took place during the Holocene. Exploring these changes, using both data and models, provides a means of understanding ENSO dynamics. Previous modelling studies have suggested a mechanism whereby changes in the Earth's orbital geometry explain the strengthening of ENSO over the Holocene. Decreasing summer insolation over the Asian landmass resulted in a weakening of the Asian monsoon system. This led to a weakening of the easterly trade winds in the western Pacific, creating conditions more favourable for El Niño development. To explore this hypothesised forcing mechanism, we use a climate system model to conduct a suite of simulations of the climate of the past 8,000 years. In the early Holocene, we find that the Asian summer monsoon system is intensified, resulting in an amplification of the easterly trade winds in the western Pacific. The stronger trade winds represent a barrier to the eastward propagation of westerly wind bursts, therefore inhibiting the onset of El Niño events. The fundamental behaviour of ENSO remains unchanged, with the major change over the Holocene being the influence of the background state of the Pacific on the susceptibility of the ocean to the initiation of El Niño events.