Ergang, your question 'If the upwelled water in the coastal area is relatively warmer than ambient water, how does it change the local weather?' is a little bit strange. Could you give an example of such situation in real ocean?

Guillermo, Thank your for your kind attention and answers all the time.

Vladimir, I also think my question is a stupid one. My question comes from the phenomenon that upwelling found off the eastern China coast in the southern East China Sea. Warm water, usually called Taiwan Warm Current (Guillermo, you are familiar with) flows northward along the 50 m isobath and primarily  exists in deep layer. West of it, the cold and buoyant eastern china coastal current flows southward. The warm water can upwell when it encounter the buoyant current owing to the intense density gradient front between them. In this situation, the upwelled water might be warmer than ambient cold coastal water. This upwelling zone indeed is smaller than the typical upwelling zone we all known, e.g. Peruvian upwelling. However, I am not familiar with what degree and how likely it would be to influence the local weather.

Hello Ergang. I think that the differenze between air temperature and SST is the reason of fog developement. Differenze in temperature  positive or negative act in the some Way. Cold water with warn air or warm water with cold air produce a condensatIon of vapour and fog.

I do agree with Franco Colombo, and it should have answered your question.

Hello Ergang, as stated in RH Stewart's book, the upwelling regions are characterized by a low cloud deck, a byproduct of the thermal gradient between the upwelled waters and  the sorroundings (as Guillermo said)- take for example the EBUS off Peru, with a nearly permanent cloud deck. Regarding your question, if the upwelled waters were warmer than its sorroundings, in theory, you could expect a relative increase in evaporation and destabilization of the air column. However, this is just speculative.

Upwelling, such as that typical in eastern boundary current regions (California Current, Peru-Chile Current, Benguela Current), brings relatively cooler water to the surface. This creates cool, moist air over the ocean surface. If this air moves over the relatively warmer land mass, the moisture condenses and creates fog.

There is a natural onshore flow that develops in response to the warm thermal low that typically sets up over the land mass adjacent to the eastern boundary, such as in the SW US. This pattern draws cooler marine air over the land, resulting in a persistent foggy condition during the local summer (high upwelling and warm continent). Local vegetation can adapted to this condition, and agriculture interests take advantage of it.

One interesting area of study is how anthropogenic warming will affect this pattern. The hypothesis is that global warming will lead to a warmer continent AND greater upwelling-favorable winds, which may result in greater fog. I am not aware of any substantive study to confirm this. One problem with looking at historical records of fog and SST is that other factors contribute to fog in coastal regions, and the fog is spatially very heterogeneous and localized.