Looking at the influence of continentality in atmospheric circulation. (by Diego Fdez-Sevilla)
Comments updated at the bottom. (10/06/2014)
In my research I try to find the state of knowledge about some questions that I find relevant. Through debate I try to find input exploring the perception that others might have about those questions.
In this particular case I explore the state of knowledge justifying the line of research looking at the influence of continentality in atmospheric circulation. Following this idea, the environmental performance derived from land use and cover management would become a relevant factor to look at when considering factors involved in climatic interferences.
The global continent/ocean asymmetry leads to a dominance of water in the Southern Hemisphere and land in the Northern Hemisphere. In contrast, the polar areas of the Southern Hemisphere are land, versus those of the North, which are water. The high elevation (ca. 3,000 m) of the former is distinctive. Both poles are largely covered by ice and snow, so that the albedo is always high but is variable perennially. North / south contrasts are most important at midlatitudes. In the Southern Hemisphere, continents alternate with oceans, whereas in the Northern Hemisphere, the amalgamation of Eurasia and North Africa is not balanced by North America. Climatologically, this imbalance generates powerful fluctuations between zonal (westerly) and meridional (south-north) modes of circulation. “Oceanicity,” or maritime air flow, alternates with “continentality,” or turbulent and meridional air flow. In the Southern Hemisphere, the greater dominance of oceans means that strong continent / ocean contrasts are absent. Global climate systems are therefore dominated by the Northern Hemisphere, given the present continental configuration. Therefore, the Northern Hemisphere will dictate global periodicities during Ice Age fluctuations between glacial and interglacial states.
The differences between the stratosphere in the Southern Hemisphere (SH) and NH are indicative of the important interactions among dynamics, radiation and chemistry. Because the stratosphere is very nearly in geostrophic and hydrostatic balance, the strength of the wintertime westerly vortex that encircles the polar cap region is proportional to the temperature contrast between the polar cap region and lower latitudes. Consistent with the lower temperatures in the polar cap region, the wintertime SH polar vortex is much stronger and longer lasting than its NH counterpart. The SH wintertime stratospheric polar vortex forms about a month earlier in autumn than its NH counterpart, and it persists about 2 months later into the spring .
The wintertime westerly vortex interacts strongly with the flux of planetary wave activity into the stratosphere from below. If the vortex is properly conditioned or the planetary waves are sufficiently strong, planetary waves propagating up from the troposphere can give rise to abrupt midwinter warmings. Planetary wave forcing in the SH is much weaker and vortex variability is much less in winter than in the NH. The weaker wave-driven meridional circulation during the SH winter is reflected in the relative warmth of the tropical tropopause during that season.
The work of Thompson and Wallace (1998) points out that the variability in the Arctic Oscillation (AO) consists of a transfer of mass in and out of the circumpolar polar vortex. A similar mode of variability (the Antarctic Oscillation or AA O) is known to exist in the Southern Hemisphere where the signature in the troposphere is even more symmetric about the pole (Thompson and Wallace, 2000). It seems likely that the bias towards the North Atlantic in the surface and middle troposphere structure of the AO is a consequence of the land-sea contrasts in the Northern Hemisphere (Thompson and Wallace, 1998) and presumably also the mountain ranges (e.g. The Rockies and Greenland) that cut across the path of the tropospheric jet stream.
Jet streams in the upper troposphere usually reach their maximum on the western side of the oceanic basins where the land-sea thermal contrasts are the strongest. East of the same basins, synoptic eddies get larger amplitude and feedback onto the jet streams. This feedback is characterized by the breaking of Rossby waves in the upper troposphere and plays a crucial role in the low-frequency atmospheric variability like the North Atlantic Oscillation (NAO). Two kinds of wave breaking exist; cyclonic wave-breaking pushing the jet southward and closely linked with the negative phase of the NAO and anticyclonic wave-breaking shifting the jet poleward and favoring the positive phase.
The zonal thermal contrast on the western border of the North Atlantic has shown to be responsible for the localization of a dipolar structure in sea-level pressure and height fields, which bears a strong resemblance to the North Atlantic Oscillation (NAO). In this region, the response to thermal land–sea contrast is evident on both annular and planetary- wave patterns, with a NAO-like dipole being the dominant regional feature. In the North Pacific, on the other hand, diabatic forcing is balanced primarily by meridional temperature advection, because here (contrarily to the north- west Atlantic region) the surface temperature gradient is stronger in the meridional than in the zonal direction. Therefore, the modification of the annular mode by zonal temperature advection at the Asia-North Pacific border produces a different pattern from that associated with the thermal balance of a COWL-like planetary wave. This study concludes that ‘thermally-balanced wave mode’ is a dynamically appropriate description for the pattern previously identified as the second EOF of low-frequency 500-hPa height variability, or empirically defined as the Cold-Ocean/Warm-Land pattern. Since this mode accounts for a large proportion of the upper-air inter-decadal variations in the second half of the 20th century, it is being suggested that such variations are dynamically consistent over the hemispheric domain; therefore they should be understood in terms of planetary-scale dynamics, rather than by the casual superposition of regional effects.
Looking at the influence of continentality in atmospheric circulation, how much impact can we expect in the climate from land use and land cover management?
(Free access) Interactions between the atmosphere and terrestrial ecosystems: influence on weather and climate. Global Change Biology (1998) 4, 461–475
Even though the following studies have limited access I believe they are worthy of being acknowledged:
2010 Investigating soil moisture–climate interactions in a changing climate: A review.
2013. Land-atmosphere interactions and climate change: Recent results and new perspectives (Invited). American Geophysical Union, Fall Meeting 2013, abstract #H11L-01. http://adsabs.harvard.edu/abs/2013AGUFM.H11L..01S