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  • Author or Editor: Anthony S. Broccoli x
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Robert P. Harnack
and
Anthony S. Broccoli

Abstract

An attempt was made to verify and further investigate a proposed relationship between the location of the maximum east-west sea surface temperature anomaly gradient (ΔSSTA) and the location of the maximum meridional component of the anomalous 700 mb geostrophic wind (VgA) in the North Pacific on a monthly and seasonal time scale. Previous empirical studies, mostly of a case study type, had suggested collocation of maximum values of these variables in the same time period, particularly during the cold seasons. Using 31 years of monthly sea surface temperature and 700 mb height data for the North Pacific, the two variables wore computed for each month and 3-month periods for each 10° longitude sector from 125°W to 155°E, and for each of three latitude bands (55–40°N, 40–25°N, 55–25°N). From these calculations, the spatial relationships of the two variables wore determined by counting frequencies of the collocation of maximum VgA and ΔSSTA for each month or season and latitude band, and by computing correlation coefficients between VgA and ΔSSTA for each month or season and latitude band. Important seasonal and latitudinal differences were found for the strength of the relationship. It was concluded that the proposed relationship was best for the northernmost latitude band (55–40°N), during winter and summer periods, and for 3-month means when compared to monthly means. Statistically significant relationships were found in several instances, indicating that the proposed relationship is probably a manifestation of real physical coupling between the mean and atmosphere.

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Michael P. Erb
,
Charles S. Jackson
, and
Anthony J. Broccoli

Abstract

The long-term climate variations of the Quaternary were primarily influenced by concurrent changes in Earth’s orbit, greenhouse gases, and ice sheets. However, because climate changes over the coming century will largely be driven by changes in greenhouse gases alone, it is important to better understand the separate contributions of each of these forcings in the past. To investigate this, idealized equilibrium simulations are conducted in which the climate is driven by separate changes in obliquity, precession, CO2, and ice sheets. To test the linearity of past climate change, anomalies from these single-forcing experiments are scaled and summed to compute linear reconstructions of past climate, which are then compared to mid-Holocene and last glacial maximum (LGM) snapshot simulations, where all forcings are applied together, as well as proxy climate records. This comparison shows that much of the climate response may be approximated as a linear response to forcings, while some features, such as modeled changes in sea ice and Atlantic meridional overturning circulation (AMOC), appear to be heavily influenced by nonlinearities. In regions where the linear reconstructions replicate the full-forcing experiments well, this analysis can help identify how each forcing contributes to the climate response. Monsoons at the mid-Holocene respond strongly to precession, while LGM monsoons are heavily influenced by the altered greenhouse gases and ice sheets. Contrary to previous studies, ice sheets produce pronounced tropical cooling at the LGM. Compared to proxy temperature records, the linear reconstructions replicate long-term changes well and also show which climate variations are not easily explained as direct responses to long-term forcings.

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