Our Current Understanding of the Physical ...

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Thompson and Wallace (1998) identified the. Arctic Oscillation (AO), a pervasive mode of variability in atmospheric circulation, as a major regulator of Arctic.
Our Current Understanding of the Physical Mechanisms Behind Polar Climate Change DAN LUBIN Scripps Institution of Oceanography, UCSD, 9500 Gilman Drive, La Jolla CA 92093-0221, USA ([email protected]); presently at: Goddard Earth Science and Technology Program, NASA Goddard Space Flight Center, Greenbelt MD 20771, USA ([email protected])

Background It has long been expected that a polar amplification of global climate warming should occur, as a result of classical climate "feedback" mechanisms involving surface albedo, solar and terrestrial radiation, and cloud cover. Early global climate model (GCM) simulation experiments predicted the largest surface temperature increases at high latitudes during winter. Observationally, climate warming throughout much of the Arctic, and in Antarctic locations relevant for marine ecology, are well established (e.g., Rigor et al., 2000; Stammerjohn and Smith, 1996). With respect to marine ecology, the alarming aspect of polar climate warming has to do with climatological temperatures being near the triple point of water. Small temperature trends can induce large advances or retreats of sea ice, and hence drastic changes in habitats. The retreat of Arctic sea ice, and smaller but statistically significant advance of Antarctic sea ice, have both been documented by more than twenty years of satellite passive microwave observations. Recent research in atmospheric dynamics has introduced a new perspective on high latitude climate change. Thompson and Wallace (1998) identified the Arctic Oscillation (AO), a pervasive mode of variability in atmospheric circulation, as a major regulator of Arctic surface temperature. Today, the AO and the North Atlantic Oscillation (NAO) index (known for several decades) are considered part of the same dynamical phenomenon known as the Northern Annular Mode (NAM). The NAM is an oscillation in the strength of counterclockwise zonal flow at temperate and high latitudes. In the positive phase of this oscillation, stronger flow isolates colder air to the north, allowing Arctic temperatures at many locations to become warmer. In the negative phase, with weaker zonal flow, colder air is allowed to move farther south at most longitudes. During the past two decades, there has been a shift toward a positive phase in the NAM, and this may be the direct cause of much of the observed Arctic warming (Thompson and Wallace, 1991). Although the NAM appears to lessen the role of direct greenhouse and similar forcings in high latitude warming, these anthropogenic factors may still have a prominent indirect role. Current GCM simulations are beginning to reproduce the NAM (Moritz et al., 2002), and some GCM work suggests that a strengthening of the AO may result from anthropogenic greenhouse forcing. Furthermore, a similar annular mode has been discovered at southern high latitudes, having a similar positive index

bias in recent decades (Thompson and Solomon, 2002) that may be related to the recent anthropogenic springtime ozone decrease.

Aims This paper will review (1) the observed trends in high latitude climate as they relate to the sea ice regime, (2) the classical high latitude ice-albedo and cloudradiation feedbacks, (3) the dynamics of the annular modes and related findings, and (4) the status of GCM studies in these areas.

Conclusion A predictive capability for high latitude climate change requires that GCM simulations contain accurate representations of both fundamental atmospheric radiative properties and the dynamics of the annular modes. Although work remains to be done in both areas (Moritz et al., 2002), current evidence suggests that anthropogenic changes to atmospheric composition may be changing the phase of the annular modes in ways that bring about tropospheric warming throughout much of the Arctic and on the Antarctic Peninsula, and cooling over the Antarctic continent.

References Moritz, R.E.., C.M. Bitz, and E.J. Steig (2002), Dynamics of recent climate change in the Arctic. Science, 297, 1497-1502. Stammerjohn, S.E., and R.C. Smith (1996), Spatial and temporal variability of western Antarctic Peninsula sea ice coverage, AGU Antarct. Res. Ser. 70, 81-104. Thompson, D.W.J., and S. Solomon (2002), Interpretation of recent Southern Hemisphere climate change, Science, 296, 895-899. Thompson, D.W.J., and J.M. Wallace (1998), The Arctic Oscllation signature in the wintertime geopotential height ad temperature fields. Geophys. Res. Lett., 25, 1297-1300. Thompson, D.W.J., and J.M. Wallace (2001), Regional climate impacts of the Northern Hemisphere annular mode. Science, 293, 85-89. Rigor, I.G., R.L. Colony, and S. Martin (2000), Variations in surface air temperature observations in the Arctic, 1979-97. J. Clim. 13, 896-914.