Color of ocean waters and Ekman pumping - what is the connection?

Question

Is there a connection between the colors of oceanic waters near the coast and Ekman pumping? Along the coastal areas of the Bay of Bengal where I live, occasionally during the monsoon, the colors of the waters become aquamarine, only to become sky blue in color a few hours later as the sea breeze sets in. Can this be explained by coastal upwelling? If this is due to Ekman pumping where does EP take place? Over equatorial waters and is then pushed north due to a coastal current? The phenomenon has been observed during both the Indian monsoons- the south west monsoon as well as the north east monsoon i.e. that the seasonal shift in wind direction does not impact the appearance of the aquamarine color.

The width of the patch is between 200-300 meters from the coast and it is oriented parallel to the coast. Closer to the coast the color is dark greenish brown and as you go further into the open sea it is aquamarine green. There are occasional dark blue "patches"(dark blue bands) which I am presuming is phytoplankton.

Few more personal observations. It appears that the aquamarine color usually manifests itself during the mornings (when there is no breeze whatsoever). The sea presents extremely gentle waves when the aquamarine color is visible. Does that mean that the origin of the aquamarine color is "non - local" ?

Answer 1

The relationship of the observed "aquamarine color" near the coast and the Ekman pumping in the open ocean, while possible is rather unlikely. Ekman pumping is confined to the areas affected by the cyclonic wind stress curl and by definition it is mostly effective in the open ocean, where there is no bottom boundary effect. The color of the coastal waters is likely more affected by the upwelling-favorable wind creating upwelling conditions that provide nutrients to the coastal areas. The fact that the color is affected by the sea-breeze is even more justification for the upwelling mechanism. While transport of nutrient-rich waters to the coastal areas is not uncommon, the time scales associated with primary production especially in coastal areas are quite fast and tend to use the open-ocean nutrients in short time scales.

Phytoplankton changes especially on the order of hours are more likely associated with purely coastal processes. While there might be an input of open-ocean nutrients to the coastal areas, the coastal primary production tends to differ from open-ocean communities and tend to respond rapidly to extra nutrients.

The aquamarine color could be the result of many coastal processes. Without knowing the area, I don't feel qualified to make a strong case about the causes. Reasons for the color could include 1) vertical phytoplankton migrations that settle to the bottom/lower water column when the breeze comes up; 2) the sea breeze resulting on a local near-shore downwelling; 3) a breakdown of the stratification by the breeze that mixes down the surface layers; 4) regional features (e.g., sea level pressure gradient) that become active when the breeze is occurring. Too many unknowns for me. It would be good to know the extent and duration of the "aquamarine color" patch, the width and slope of the continental shelf in the area and the ind and stratification conditions in the near-shore and coastal areas.

Answer 2

This is essentially a part answer and it is hoped people from other basins such as Pacific, Atlantic, etc will corroborate and perhaps extend the findings. Providing an explanation for color of oceanic waters without objective satellite/buoy data may at best be subjective. This answer will attempt to provide an explanation based on work done in two basins - Bay of Bengal as well as Arabian Sea. Vinayachandran et al 1 address the issue by relating phytoplankton blooms to coastal upwelling caused by Ekman transport. While my observations of the aquamarine color were initially noted during the summer monsoon the same authors have established the correlation of phytoplankton blooms to Ekman transport during the autumn monsoon as well as shown in this paper of Vinayachandran et al 2. A more recent paper talks of the same phenomenon for the Arabian Sea 3

With regards to the non locality and the transient nature of the observations as most of the work is qualitative it is hoped that a quantification of the work via observational studies and modeling work will provide some of the answers.

From a layperson's perspective the idea that the color of oceans can actually provide a hint about the occurrence of an important climatological phenomenon such as the Indian Ocean Dipole(IOD) is rewarding by itself.