Tuesday, January 28, 2014

Atlantic sea surface temperatures affect Antarctic climate change

A new source of warming has been found for Antarctica: the North Atlantic and tropical Atlantic Ocean.

Warm waters in the Pacific have been connected to warming in Antarctica, but this is the first time that a connection with the Atlantic Ocean sea surface temperatures has been made.

"Our findings reveal a previously unknown--and surprising--force behind climate change that is occurring deep in our southern hemisphere: the Atlantic Ocean," says Xichen Li, a doctoral student in NYU's Courant Institute of Mathematical Sciences and the study's lead author. "Moreover, the study offers further confirmation that warming in one region can have far-reaching effects in another."

Global warming in recent decades has caused dramatic warming of the Antarctic Peninsula and West Antarctica. Changes in Antarctic climate have been attributed to greenhouse gas emissions and the ozone hole , sea surface temperatures in the central tropical Pacific warming (Qinghua Ding et al 2011) and the impact of El Nino.

Here is how (Frezzotti et al 2013 PDF) paper - A synthesis of the Antarctic surface mass balance during the last 800 yr , described how the tropical Pacific affects the Antarctic.

The central tropical Pacific is a critical region with a significant influence on atmospheric circulation in the Southern Hemisphere through the generation and propagation of a large-scale atmospheric wave train during winter (Lachlan-Cope and Connolley, 2006) that can, in turn, enhance/inhibit blocking high phenomena. Tropical climate signals are primarily communicated to Antarctica via the Pacific-South America pattern and the Southern Annular Mode, driven by Rossby wave trains (Fogt and Bromwich, 2006; Schneider et al., 2012). Indeed, over the past 30 yr, anomalous sea surface temperatures in the central tropical Pacific have generated an atmospheric Rossby wave response influencing the atmospheric circulation over the Amundsen Sea and causing increased advection of warm air to the WAIS and AP (Ding et al., 2011b).

This study of the influence of the Atlantic Ocean sea surface temperatures helps explain longer-term warming along the Antarctic Peninsula or the sea-ice redistribution in the southern hemisphere over winter.

The scientists from New York University studied the North and Tropical Atlantic's Sea Surface Temperature (SST) variability - changes in the ocean's surface temperature, focusing on the last three decades. This metric, the Atlantic Multidecadal Oscillation (AMO), had previously not been considered in seeking explanations for Antarctic climate change.

Using a time-series analysis, in which the scientists matched changes in the North and Tropical Atlantic's SST with subsequent changes in Antarctic climate, the researchers found strong correlations. Specifically, they observed that warming Atlantic waters were followed by changes in sea-level pressure in the Antarctic's Amundsen Sea. In addition, these warming patterns also preceded redistribution of sea ice between the Antarctic's Ross and Amundsen-Bellingshausen-Weddell Seas.

Once the data correlation between the Atlantic and Antarctic had been established, global atmospherical climate models were run to simulate warming in the North atlantic which produced changes in the climate of Antarctica.

The study is important, not only for showing the connnection between the Atlantic sea surface temperatures and Antarctic climate, but also on the way sea ice is redistributed over recent decades.

David Holland, co-author of the study, a professor at NYU's Courant Institute said ""From this study, we are learning just how Antarctic sea-ice redistributes itself, and also finding that the underlying mechanisms controlling Antarctic sea ice are completely distinct from those in the Arctic."

Caption: a, North Atlantic warming (20° S–70° N); b, tropical Atlantic warming (20° S–20° N); and c, mid-latitude north Atlantic warming (20° N–70° N). a and b show a similar SLP response, whereas c exhibits a much weaker low-pressure response over west Antarctica, which implies that the tropical Atlantic plays the key part in the teleconnection. The response of SLP in these three experiments show some linearity, whereas the response of SAT is nonlinear. All three experiments, despite having different amplitudes of SLP response, show similar amplitudes of SAT warming/cooling over west Antarctica and the Amundsen–Bellingshausen–Weddell seas. This nonlinearity in SAT response strongly suggests that the impact of mid-latitude north Atlantic SST warming should not be neglected. Source: Xichen Li et al 2014

The study - Impacts of the north and tropical Atlantic Ocean on the Antarctic Peninsula and sea ice abstract reads:

In recent decades, Antarctica has experienced pronounced climate changes. The Antarctic Peninsula exhibited the strongest warming of any region on the planet, causing rapid changes in land ice. Additionally, in contrast to the sea-ice decline over the Arctic, Antarctic sea ice has not declined, but has instead undergone a perplexing redistribution. Antarctic climate is influenced by, among other factors, changes in radiative forcing and remote Pacific climate variability, but none explains the observed Antarctic Peninsula warming or the sea-ice redistribution in austral winter. However, in the north and tropical Atlantic Ocean, the Atlantic Multidecadal Oscillation (a leading mode of sea surface temperature variability) has been overlooked in this context. Here we show that sea surface warming related to the Atlantic Multidecadal Oscillation reduces the surface pressure in the Amundsen Sea and contributes to the observed dipole-like sea-ice redistribution between the Ross and Amundsen-Bellingshausen-Weddell seas and to the Antarctic Peninsula warming. Support for these findings comes from analysis of observational and reanalysis data, and independently from both comprehensive and idealized atmospheric model simulations. We suggest that the north and tropical Atlantic is important for projections of future climate change in Antarctica, and has the potential to affect the global thermohaline circulation and sea-level change.