The temperature of the upper level of the Earth’s oceans can be a major contributing factor to storm development, ocean currents, and even weather patterns as well as key indicator for the best fishing action.
The Seawater Surface Temperatures (SST’s) around Baja creep up every year from late spring to late summer and even early fall some years, largely based on whether it is a “el niño” or “la niña” pattern.
What was noticeably different this year was not only did the SST’s reach chilly temperatures below the norm but how slow they were to reverse that pattern this late spring and early summer.
That pattern appears to be changing here in late July.
With a succession of Tropical Storm systems running from southeast to northwest and originating just off the southern Mexico Pacific coastline, they have maintained a safe distance so far from the southern Baja peninsula. They have, however, begun pushing a series of strong surges bringing high surf accompanied by a wave of warmer water our way over the past few weeks.
And with Hurricane Frank now forecast to follow that similar track a bit closer to the peninsula and maintain tropical storm – hurricane speeds much farther north, expect for these SST’s to continue warming up over the next few weeks.
Ocean: Hurricane’s Energy Source
Development of a tropical depression into a mature hurricane requires energy in the form of heat along with warm vapor evaporating from the ocean’s surface. For this reason, hurricanes do not usually develop over land or outside of the warm tropical oceans where the SST’s are cooler than ~26.5°C (~80°F).
In tropical cyclone development, one of the main sources that aid in the strengthening process is the transfer of heat from the ocean to the atmosphere. The warm sea-surface temperatures acts as a fuel pump by providing the energy needed for further development.
In the Eastern Pacific where Mexico’s Pacific coastline is found with Baja’s long peninsula extending down, parallel to Mexico’s northern Pacific coastline, the mass of warm water grows between May and late summer with September generally having the largest area of the warmest water.
It is not a coincidence that the historical peak of our annual hurricane season also occurs in September.
The warmer the sea surface temperatures are in a specific area, the higher the chances the tropical cyclone will strengthen, assuming the system is moving through a very conducive environment for strengthening to occur; meaning little wind shear and high humidity.
Relationship between SST’s and Hurricanes
The heat transferred to the atmosphere from the ocean is stored in the water vapor as latent heat, or the energy absorbed or released during a change in its physical state (phase), occurring without changing its temperature.
Hurricanes feed off of that energy when passing over warm water below, releasing its latent heat into the upper atmosphere which in turn create spiraling winds.
Absorbing the warm, spiraling moisture in turn can result in helping to maintain the hurricane’s speed or even increase its intensity, both in terms of wind speed as well as overall water mass / size of the storm system.
Conversely, when a hurricane moves over cooler water, the evaporation rates decrease and coupled with the net cooling effect will rapidly reduce a hurricane’s intensity.
As I mentioned previously, ~26.5°C (~80°F) is considered the minimum SST ocean surface temperature for a hurricane to develop or maintain hurricane level speeds while passing overhead. That is commonly referred to as the 26°C isotherm.
When calculating a storm system’s projected strength, scientists will consider not only the SST’s in the system’s projected path in relation to that critical temperature but also the depth of the 26°C isotherm (D26) or in other words, how deep below the surface does the 26°C isotherm reach?
Upwelling Effect Explained
In areas with deeper D26 levels, storm forecasts will tend to predict increased storm intensities as the degrading effects of “upwelling” are normally limited.
Upwelling is a phenomena occurring when the surface winds of a hurricane cause the currents at the sea surface to move cyclonically in rhythm with the hurricane above. The void created below – both from moisture absorption as well as water displaced outward from the storms center – creates conditions for cooler water from below to move up to the sea surface to fill that void.
If the D26 depth is minimum, the tropical cyclone above will quickly lose its main fuel source, however as the D26 depth increases, upwelling will tend to lose its effect on a storm system. .
Upwelling is typically seen as hurricanes come into close contact with coastlines.