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Though in general, they are defined by winter wetness and summer dryness, within the cooler half of the year, I notice that some Mediterranean climates differ in where the peak rainfall is. Some actually have spring or fall as the rainiest rather than the middle of winter, though they still count as Mediterranean climates because it counts as the cool half of the year.
What might cause falls or springs to be rainier? I'm supposing Mediterranean climates get rain from fronts that arrive by the jet stream, which retreats north over summer, thus leading to drought. But what would make more of them arrive more in early spring than late fall or vice versa?
Here's a Med Sea seasonal precipitation distribution map:
Winter is in black, then seasons follow in a counterclockwise order. One can see that, as a rule, the NW Mediterranean has a fall precipitation peak, contrary to the south and NE Med, where the peak is in winter. This study (paragraph 5, pages 8-9-10) suggests that the North African jet stream is to blame for this odd precipitation pattern. In fall, the North African jet stream is weak as the European continent is still warm. High altitude cold air advections are already potent and the warm Med waters are a big source of moisture. The temperature gradient between sea surface and high altitude airmasses is biggest in that season, which enhances instability. Rainfall often comes in the form of "Mediterranean episodes", as we call them in France, 2-3 day periods of heavy rain. Rainfall variability is very high, with some very rainy months and some dry ones.
In winter, SW Europe lies in the NW end of the North African jet stream. In that area, there is a high altitude convergence region caused by the decreasing velocity of cross-jet stream winds. The air moves downwards (subsidence), which hinders cyclogenesis and precipitation over the NW Med. Conversely, in areas located to the northeast of the strongest altitude winds (northeastern Med), the air rises and storms form more easily than in Autumn, when the jet was weak. In spring, the North African jet weakens but still hinders storm activity over the NW Med compared to Autumn. The second rainfall peak in spring over parts of the Iberian peninsula is probably related to the weakening of the jet stream and, for more inland area, to the increasing instability.
As for southern Med climes, due to their more southerly location, the dry season is longer and hinders atypical precip patterns such as fall or spring peaks. Coastal Maghreb is probably south of the winter subsidence area and thus enjoys wetter winters than the Iberian peninsula.
which is rather asymmetrical as early fall is relatively dry; September is no wetter than June. And sunnier than June. Sunshine % drops from 56 to 36 going from September to October.
Maybe Vancouver has a November peak because the ocean is still warmish in that month, the surface - altitude contrast is probably larger than in any other month? California is probably still under the influence of the ridge at that time of year, which hinders rainfall compared to late winter?
Quote:
Originally Posted by nei
it looks like the Atlantic Coast follows the more typical winter precipitation peak.
Probably because the main moisture source is the Atlantic and not the Mediterranean. Atlantic lows are at their strongest in winter.
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