Interannual Atmosphere-Ocean Climate Variability
|Figure 1: The "North Annular Mode" pattern, which describes a large fraction of the monthly to decadal changes in northern hemisphere climate. When there's a deficit of air mass in the red areas, there tends to be a surplus in the blue areas, and vice versa. At the same time, the jet stream winds tend to speed up or slow down in the direction of the green arrows. A similar pattern exists in the southern hemisphere.|
|A cartoon analogy for the random variability of the annular modes. The two tanks of water represent the polar and mid-latitude "reservoirs" of mass in the annular mode (red and blue zones in the figure above). The random mass flows caused by daily weather activity are represented by the pump in the middle. The accumulated mass in the reservoirs varies as a "random walk" process.|
John Marshall (MIT)
Arnaud Czaja (Imperial College, UK)
The variability of mid-latitude marine climate on timescales longer than weather events but shorter than greenhouse gas variations (from weeks to decades) presents a major gap in our understanding, and an important target for improving forecast skill. Variability on these timescales occurs mostly in a few characteristic patterns, most generally named the North and South Annular Modes (Thompson, 2000). These are pole-encircling patterns of atmospheric pressure in either hemisphere. Characteristic sea-surface temperature patterns appear to covary with these atmospheric modes (Sutton and Allen, 1997), but the direction of cause and effect can be confusing (Czaja and Frankignoul, 1999).
Recently, I have taken a step back, and looked at a more basic problem: while
oceanic processes may drive some atmospheric variability on these timescales, much of it is undoubtedly a result of random internal atmospheric fluctuations. If we are to detect and understand fluctuations produced by coupling to the ocean, we must have some understanding of how the atmospheric ?noise? obscuring this signal works. In a recently submitted manuscript, Arnaud Czaja and I (2006) portray the dominant ?annular modes? of atmospheric variability as modulations in the amount of potential vorticity (or atmospheric mass) stored in a polar versus a mid-latitude ?reservoir?. Daily weather activity behaves as a randomly acting pump, moving PV / mass back and forth between these reservoirs as a stochastic process. A simple model of this process suffices to explain the majority of intra- and interseasonal variability of the atmosphere?s annular modes, but importantly, the atmosphere-only stochastic model cannot explain all of the variability: we identify the parts of the data which require a process like ocean-atmosphere coupling to complete the explanation.
Thompson, D. W. J. and J. M. Wallace, 2000. Annular Modes in the Extratropical Circulation. Part I: Month-to-Month Variability. Journal of Climate 13, 1000-1016.
Sutton, R. T. and M. R. Allen, 1997. Decadal predictability of North Atlantic sea surface temperature and climate. Nature, 338, 563-566.
Czaja, A. and C. Frankignoul, 1999. Influence of the North Atlantic SST on the atmospheric circulation. Geophys. Res. Lett. 26(19), 2969-2972.
Goodman, J. and J. Marshall, 1999. A model of decadal middle-latitude atmosphere?ocean coupled modes. Journal of Climate, 12, 621?641.
Goodman, J. C. and J. Marshall, 2002. Using neutral singular vectors to study low- frequency atmospheric variability. Journal of the Atmospheric Sciences, 59(22), 3206?3222.
Goodman, J. C. and J. Marshall, 2003. The role of neutral singular vectors in middle-latitude air?sea coupling. Journal of Climate, 16, 88?102.
Goodman, J. C. and A. Czaja, 2006. Why the West Wind Wobbles: Viewing Annular Modes as Stochastically-Excited Potential Vorticity Reservoirs. Submitted to Quarterly Journal of the Royal Meteorological Society.