East coast subtropical climates

Mots of the subtropical climates found at eastern coasts of continents are grouped under the humid subtropical Cf, whose rainfall distribution is rather uniform all year round. This is caused by a combination of frontal rain from extratropical cyclons (prevalent in west coast climates Cr, Cs) and convective rain from unstable maritime tropical (mT) air masses coming from the western flank of subtropical anticyclons.

When the latter prevails over the former, we've got a Monsoon or dry winter subtropical (aka Chinese) climate Cw, like the one found in East China and Japan, characterized by a strong rainfall maximum in summer and less rain in winter. Although they're quite different, this climate is often conflated with the humid subtropical Cf, to the point of applying the name "Chinese" to the latter.

Oceanic or Mediterranean? (updated)

 

One of the characteristic climates (8 or more months above 10ºC) of west coasts is the subtropical oceanic Cr¹, whose rainfall pattern reflects the frequency of extratropical cyclons and its westerlies, which is higher in winter than in summer, when the subtropical highs move poleward following the apparent course of the Sun.

Its temperate counterpart is the D_or climate, a subtype of the temperate oceanic D_o (e.g. Brest) of the original Trewartha classification, which for the most part covers the Köppen Cfb climate. The criterion for the r subtype¹ is that at least 65% of the yearly precipitations must be concentrated on the winter half-year (October-March on the northern hemisphere and April-September on the southern hemisphere) and the rainfall of the driest month must be 30 mm or more.

When 1-2 summer months have less than 30 mm of rainfall we've got a Crs climate (e.g. Rome), transitional between the oceanic and the Mediterranean Cs. Its temperate counterpart is the D_ors (e.g. Portland).

These climates where classified as Mediterranean under Köppen, despite the fact they're more rainy (and in the case of D_ors also cooler) than actual Mediterranean climates, as for example San Francisco, whose summers (16.7 ºC in September) are cooler than Portland's because of its coastal location along a cold ocean current, which causes a persistent fog like in the coastal desert climate (Trewartha BM or Köppen BWn) found southward.

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¹ Which stands for rainy and was originally introduced by Trewartha for the tropical rainforest climate Ar (Köppen Af).
² For which Köppen introduced the n subtype, from German Nebel 'fog'.

Trewartha climate classification

In the book An Introduction to Climate (4th edition, 1968), the American geographer Glenn Trewartha introduced his own climate classification, largely based upon Wladimir Köppen's, also known as Köppen-Geiger due to the modifications made by one of his disciples. The most important changes are related to middle latitude climates, which in Köppen were grouped into C (subtropical and warm temperate) and D (cool and cold temperate). Trewartha grouped warm and cool temperate climates into group D, leaving alone subtropical and cold temperate (boreal) ones respectively in C and E groups.

Although Trewartha's goal was to produce a climate classification more adjusted to vegetation zones, it still has some drawbacks, especially regarding C and D climates. This is why it needs some adjustments or "fine tuning" which I'll explain in forthcoming posts.