What is sea surface temperature rise? How does it affect the formation of tropical cyclones?

What is sea surface temperature rise? How does it affect the formation of tropical cyclones? [10 marks, 2024]

Sea-surface temperature (SST) refers to the temperature of the upper layer of the ocean. SST rise means that this surface water becomes warmer than its long-term average due to factors such as global warming, El Niño or marine heatwaves.

Effect on the formation of tropical cyclones:

1. Warm water provides the initial energy (Tropical cyclones generally form over oceans where the temperature is around 26.5°C or more).
2. Rising moist air creates low pressure → Air from surrounding areas then moves towards this low-pressure region.
3. Condensation supplies additional energy (during condensation, latent heat is released) → further warms the air, makes it rise faster and deepens the low-pressure system.
4. Higher SST can increase the chances of intensification (warmer oceans provide more heat and moisture to an existing disturbance).
5. If ocean waters remain warm for a longer period, conditions favourable for cyclones may persist for a longer season or extend into a wider area.

This chain is self-reinforcing. More evaporation from a warm sea feeds more condensation aloft; each round of condensation releases more latent heat, the warmed air rises faster, and the deepening low draws in still more moist air from the surroundings. The ocean is the storm's fuel tank, which is also why a tropical cyclone begins to weaken soon after it crosses onto land and its supply of warm, moist air is cut off.

However, warm SST alone is not sufficient. Other conditions are also required:

1. an existing low-pressure disturbance;
2. sufficient distance from the Equator for the Coriolis force;
3. abundant atmospheric moisture;
4. low vertical wind shear;
5. favourable upper-air circulation.

Each of these conditions has a clear physical role. The Coriolis force gives the converging winds their spin, and because it is negligible at the Equator, tropical cyclones rarely form very close to it. Low vertical wind shear matters because strong winds aloft would tilt and tear apart the rising column of warm, moist air before an organised storm could develop. Favourable upper-air circulation carries the rising air away from the top of the system, so the surface pressure can keep falling instead of filling up.

Nevertheless, SST rise strengthens the oceanic energy source of tropical cyclones by increasing evaporation, moisture and heat transfer to the atmosphere. A warmer ocean therefore makes formation and intensification more likely and can stretch the cyclone season, but it remains one condition among several: the atmosphere must also cooperate before a cyclone can form.