Cold Water Upwelling

[shear_vector]

Question about the proccesses and timescales associated with cold water upwelling after the passage of a tropical cyclone. I was looking at the SSTs around Nadine and noticing the cold anomaly off the coast of Oaxaca where there was TC action producing high quantities of rainfall and the path of Milton is much more pronounced. I under stand that the southward progression of surface fronts may play a role this late in the season.

So I guess my question that in addition to wind driven upwelling how much can runoff of fresh water has a role and in addition to the impact of coastline bathymetry on that process with regards to the upper ocean heat content. Looking also for papers or talks on the subject in addition to what knowledge y'all have. Kinda a few things there but I am curious.

[AJC3]

Question about the proccesses and timescales associated with cold water upwelling after the passage of a tropical cyclone. I was looking at the SSTs around Nadine and noticing the cold anomaly off the coast of Oaxaca where there was TC action producing high quantities of rainfall and the path of Milton is much more pronounced. I under stand that the southward progression of surface fronts may play a role this late in the season.

So I guess my question that in addition to wind driven upwelling how much can runoff of fresh water has a role and in addition to the impact of coastline bathymetry on that process with regards to the upper ocean heat content. Looking also for papers or talks on the subject in addition to what knowledge y'all have. Kinda a few things there but I am curious.

I also looked for papers on this to no avail thus far.

My educated guess is that the rainfall/runoff component wouldn't be as much as one might think, since the majority of rainfall generated by a TC is through warm core processes. In short, the droplet temperature of TC rainfall is by and large higher than that of the higher dbz rainfall generated by the diurnal convective instability that generates Florida thunderstorms - thus it wouldn't help cool SSTs quite as much. And even so, it still wouldn't be nearly as much as the large scale deep water overturning/upwelling that occurs from wave action generated by the TC, especially stronger ones.

Again, just a hypothesis based on what I know.

[USTropics]

Question about the proccesses and timescales associated with cold water upwelling after the passage of a tropical cyclone. I was looking at the SSTs around Nadine and noticing the cold anomaly off the coast of Oaxaca where there was TC action producing high quantities of rainfall and the path of Milton is much more pronounced. I under stand that the southward progression of surface fronts may play a role this late in the season.

So I guess my question that in addition to wind driven upwelling how much can runoff of fresh water has a role and in addition to the impact of coastline bathymetry on that process with regards to the upper ocean heat content. Looking also for papers or talks on the subject in addition to what knowledge y'all have. Kinda a few things there but I am curious.

I also looked for papers on this to no avail thus far.

My educated guess is that the rainfall/runoff component wouldn't be as much as one might think, since the majority of rainfall generated by a TC is through warm core processes. In short, the droplet temperature of TC rainfall is by and large higher than that of the higher dbz rainfall generated by the diurnal convective instability that generates Florida thunderstorms - thus it wouldn't help cool SSTs quite as much. And even so, it still wouldn't be nearly as much as the large scale deep water overturning/upwelling that occurs from wave action generated by the TC, especially stronger ones.

Again, just a hypothesis based on what I know.

I would add to this that wind-driven upwelling is by far the biggest component here (by magnitudes I would imagine), but there could be instances where heavy rainfall (especially runoff) could modulate local sea surface temperature skins. In the Northern Hemisphere, the effect of upwelling is more pronounced on the right-hand side of the storm due to Ekman transport (i.e., pushing water away from the storm's center) and in strong hurricanes, we can actually get a phenomena known as Ekman spirals that can upwell water as deep as 300 feet (statue of liberty is 305 feet as a reference). This goes back to your bathymetry and coastal features question, which we can see depending on the coastline features and how quickly water temperature changes in the vertical, you could see upwelling decreasing SSTs by as much as 6°C in intense cyclones.

As AJC3 stated, tropical moisture is latent heat driven (so warmer) and rainfall nuclei is much smaller over the ocean comparative to land. We have larger initial cloud droplets, but smaller raindrops in tropical cyclones (and typically smaller initial cloud droplets, but larger raindrops from land induced convection) due to the coalescing process. This is because over the ocean, we typically don't have Cloud Condensation Nuclei (e.g., aersols, smoke, dust, etc.) at the same concentration as we do over land.

While that means it likely wouldn't directly modulate SSTs, there are two indirect ways it could. I don't remember which tropical cyclone it was (Isaac or Ida maybe?), but there was such a significant runoff of water into the Mississippi river that it did decrease SSTs near the mouth of the river. Additionally, freshwater has a lower density compared to saline seawater, so it stays near the surface and can reduce vertical mixing (this plays a role in our current transport/downwelling issues of the AMOC as well). This often suppresses the rewarming of the surface layer after the storm has passed, thus contributing to the persistence of cold anomalies.