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Understanding El Niño-Southern Oscillation (ENSO) response to past climate forcings is hindered by conflicting paleoclimate evidence. Records from the eastern Pacific show an intensification of ENSO variability from early to late Holocene, while records from the central Pacific show highly variable ENSO throughout the Holocene without an obvious relation to insolation forcing, which is the main climate driver during this interval. Here, we show via climate model simulations that conflicting Holocene records can be reconciled by considering changes in the relative frequency of the three preferred spatial patterns in which El Niño events occur (Eastern Pacific, Central Pacific, and Coastal) and in the strength of their hydroclimatic impacts. The relationship between ENSO diversity and variance is not only crucial for interpreting paleo-ENSO records and understanding ENSO response to external forcings but can also be used across climate model simulations to help evaluate the realism of ENSO projections in a changing climate.

Abstract The large-scale atmospheric circulation of the North Pacific associated with two types of El Niño—the eastern Pacific (EP) and central Pacific (CP)—is studied in relation to Hawaiian winter (December–February) rainfall and temperature. The eastern and central equatorial Pacific undergo active convective heating during EP El Niño winters. The local Hadley circulation is enhanced and an upper-level westerly jet stream of the North Pacific is elongated eastward. Due to the impact of both phenomena, stronger anomalous descending motion, moisture flux divergence anomalies near Hawaii, and reduction of easterly trade winds, which are characteristic of EP winters, are unfavorable for winter rainfall in Hawaii. As a result of this robust signal, dry conditions prevail in Hawaii and the standard deviation of rainfall during EP winters is smaller than the climatology. For CP winters, the maximum equatorial ocean warming is weaker and shifted westward to near the date line. The subtropical jet stream retreats westward relative to EP winters and the anomalously sinking motion near Hawaii is variable and generally weaker. Although the anomalous moisture flux divergence still exists over the subtropical North Pacific, its magnitude is weaker relative to EP winters. Without strong external forcing, rainfall in the Hawaiian Islands duringmore »CP winters is close to the long-term mean. The spread of rainfall from one CP event to another is also larger. The near-surface minimum temperature from three stations in Hawaii reveals cooling during EP winters and slight warming during CP winters.« less

Abstract ENSO and the mean zonal sea surface temperature gradient (dSST) of the tropical Pacific are important drivers of global climate and vary on decadal to centennial time scales. However, the relationship between dSST and ENSO cannot be assessed with the short instrumental record, and is uncertain in proxy data, with intervals of both stronger and weaker ENSO postulated to occur with overall strong dSST in the past. Here we assess the ENSO–dSST relationship during the last millennium using general circulation models (GCMs) participating in phase 3 of the Paleoclimate Modeling Intercomparison Project. Last millennium GCM simulations show diversity in the strength and direction of the ENSO–dSST relationship. Yet, the models that best simulate modern tropical Pacific climate frequently have a more negative ENSO–dSST correlation. Thus, last millennium tropical Pacific climate simulations support the likelihood of enhanced ENSO during decadal to centennial periods of reduced tropical Pacific dSST. However, the alternating directional ENSO–dSST relationship in all model simulations suggests that this relationship is not constant through time and is likely controlled by multiple mechanisms.