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Investigation of Decadal Variability in Southern Hemisphere

PI: Xiaojun Yuan
Institution: Columbia University
Abstract:
The Southern Ocean is a hot spot in climate change. The most striking climate change is the rapid regional warming at the Antarctic Peninsular. At Faraday/Vernadsky station in the western Antarctic Peninsular, surface air temperature exhibits a startling mean annual warming rate of 2.8°C over the last 50 years, contrasting the global mean warming rate of 0.5°C during the same period. It represents the highest rate of warming observed anywhere on earth. In responding to this warming, sea ice in the Southeast Pacific shows a significant decreasing trend, even though the sea ice extent over the entire Southern Ocean has a slightly increasing trend. In the last 25 years, modern instrumental data reveal that the changes in El Niño/Southern Oscillation and Southern Annular Mode influence the sea ice variability in this region but cannot fully explain the trend in the Southeast Pacific sea ice extent. What caused this dramatic regional warming, and associated sea ice reduction, as well as the collapse of ice shelves around the peninsula is still an open question. We suspect that other climate modes in the Southern Ocean and/or combinations of different modes may play roles in this regional warming. Variability and interaction of different climate modes at longer timescales may also be important in this climate change.

We would like to examine the decadal variability in a number of major climate modes in the Southern Hemisphere from 100-150 years of coupled climate model runs. The Southern Annular Mode, marked by zonally-symmetric but out-of-phase pressure anomalies between mid and high latitudes, is a dominant climate mode throughout the Southern Hemisphere. This climate mode impacts the sea ice field, particularly in the South Indian Ocean and South Atlantic Ocean. The Semi-annual oscillation describes another zonally-symmetric mode in the southern extra-tropics, which is characterized by the twice-yearly enhancement in meridional gradients of temperature and pressure fields. The atmospheric convergence line with a strong half-year cycle exerts significant influences on the seasonal asymmetric behavior of ice extent: a slow advance equatorward in winter and a fast retreat in summer. The Pacific South America pattern represents an atmospheric wave train in the pressure field over the South Pacific and South America. This mode is highly related to ENSO variability at the tropics and plays a critical role in the formation of the Antarctic Dipole, the largest interannual variability in sea ice and surface temperature in the Antarctic subpolar regions.

These climate modes collectively dominate the climate variability in the Southern Ocean. We have examined the relationships of these modes with sea ice based on the last 25 years of observations and have a good understanding of the influence from each mode on the sea ice field. The proposed study will extend our current study to longer periods from model runs, which will enable us to examine the climate variability at decadal and longer time scales, and their contributions to sea ice changes. The key objective is to isolate the unique combination of these modes that can cause a significant warming trend in the west Antarctic.

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