New paper finds tropical Atlantic temperatures were up to 2.5C warmer 500-1000 years ago

A new paper published in PNAS finds from 2 sediment temperature proxies that the upper 1,500 feet of the tropical Atlantic Ocean experienced abrupt warming of about 3-4C during each of at least two periods over the past 22,000 years due to a natural change in ocean oscillations [i.e. not greenhouse gases]. The paper also shows "the modern mean annual temperature" is about 0.5 to 2.5C colder than the end of the proxy records around 500 years before the present. 

Graph and legend from the paper's supplemental information. The final segments of both temperature proxies in blue and red were added to connect the proxy temperatures to "the modern mean annual temperatures" The Younger Dryas [YD] was a period of natural global warming at a much faster rate than over the 20th century. Horizontal x axis is thousands of years before the present [ky BP].

Related: New Paper finds significant cooling of Atlantic Ocean over past millennium

Impact of abrupt deglacial climate change on tropical Atlantic subsurface temperatures

1. Matthew W. Schmidt^a,¹, 
2. Ping Chang^a,^b, 
3. Jennifer E. Hertzberg^a,
4. Theodore R. Them II^a, 
5. Link J^a, and 
6. Bette L. Otto-Bliesner^c

+Author Affiliations

1. ^aDepartment of Oceanography, Texas A&M University, College Station, TX 77843;
2. ^bPhysical Oceanography Laboratory, Ocean University of China, Qingdao 266003, People’s Republic of China; and
3. ^cNational Center for Atmospheric Research, Boulder, CO 80307

1. Edited by James C. McWilliams, UCLA, Los Angeles, CA, and approved August 1, 2012 (received for review May 8, 2012)

Abstract

Both instrumental data analyses and coupled ocean-atmosphere models indicate that Atlantic meridional overturning circulation (AMOC) variability is tightly linked to abrupt tropical North Atlantic (TNA) climate change through both atmospheric and oceanic processes. Although a slowdown of AMOC results in an atmospheric-induced surface cooling in the entire TNA, the subsurface experiences an even larger warming because of rapid reorganizations of ocean circulation patterns at intermediate water depths. Here, we reconstruct high-resolution temperature records using oxygen isotope values and Mg/Ca ratios in both surface- and subthermocline-dwelling planktonic foraminifera from a sediment core located in the TNA over the last 22 ky. Our results show significant changes in the vertical thermal gradient of the upper water column, with the warmest subsurface temperatures of the last deglacial transition corresponding to the onset of the Younger Dryas. Furthermore, we present new analyses of a climate model simulation forced with freshwater discharge into the North Atlantic under Last Glacial Maximum forcings and boundary conditions that reveal a maximum subsurface warming in the vicinity of the core site and a vertical thermal gradient change at the onset of AMOC weakening, consistent with the reconstructed record. Together, our proxy reconstructions and modeling results provide convincing evidence for a subsurface oceanic teleconnection linking high-latitude North Atlantic climate to the tropical Atlantic during periods of reduced AMOC across the last deglacial transition.

article from Science Daily on this paper:

Past Tropical Climate Change Linked to Ocean Circulation

ScienceDaily (Aug. 23, 2012) — A new record of past temperature change in the tropical Atlantic Ocean's subsurface provides clues as to why Earth's climate is so sensitive to ocean circulation patterns, according to climate scientists at Texas A&M University.

Geological oceanographer Matthew Schmidt and two of his graduate students teamed up with Ping Chang, a physical oceanographer and climate modeler, to help uncover an important climate connection between the tropics and the high latitude North Atlantic. Their new findings are in the current issue ofPNAS (Proceedings of the National Academy of Sciences).

The researchers used geochemical clues in fossils called foraminifera, tiny sea creatures with a hard shell, collected from a sediment core located off the northern coast of Venezuela, to generate a 22,000-year record of past ocean temperature and salinity changes in the upper 1,500 feet of water in the western tropical Atlantic. They also conducted global climate model simulations under the past climate condition to interpret this new observational record in the context of changes in the strength of the global ocean conveyor-belt circulation.

"What we found was that subsurface temperatures in the western tropical Atlantic rapidly warmed during cold periods in Earth's past," Schmidt explains.

"Together with our new modeling experiments, we think this is evidence that when the global conveyor slowed down during cold periods in the past, warm subsurface waters that are normally trapped in the subtropical North Atlantic flowed southward and rapidly warmed the deep tropics. When the tropics warmed, it altered climate patterns around the globe."

He notes that as an example, if ocean temperatures were to warm along the west coast of Africa, the monsoon rainfall in that region would be dramatically reduced, affecting millions of people living in sub-Saharan Africa. The researchers also point out that the southward flow of ocean heat during cold periods in the North Atlantic also causes the band of rainfall in the tropics known as the Intertropical Convergence Zone to migrate southward, resulting in much drier conditions in northern South American countries and a wetter South Atlantic.

"Evidence is mounting that the Earth's climate system has sensitive triggers that can cause abrupt and dramatic shifts in global climate," Schmidt said.

"What we found in our subsurface reconstruction was that the onset of warmer temperatures, thought to reflect the opening of this 'gateway' mechanism, occurred in less than a few centuries. It also tells us that it might be a good idea to monitor subsurface temperatures in the western tropical Atlantic to assess how the strength of the ocean conveyor might be changing over the next few decades as Earth's climate continues to warm."

"One way to prepare for future climate change is to increase our understanding of how it has operated in the recent past."