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

Understanding Sea Surface Temperature Rise

SST is not uniform across the globe and varies with latitude, ocean currents, and seasonal changes. However, a long-term trend of increasing SSTs is evident due to anthropogenic climate change. This warming is primarily caused by the absorption of excess heat trapped by greenhouse gases in the atmosphere.

Tropical Cyclone Formation: A Step-by-Step Process

Tropical cyclones, also known as hurricanes or typhoons depending on the region, are intense low-pressure systems that form over warm tropical or subtropical waters. Their formation requires a confluence of several factors, with SST being a primary driver:

• Warm Ocean Water: SSTs of at least 26.5°C (80°F) to a depth of at least 50 meters are generally considered necessary for tropical cyclone development. This warm water provides the necessary heat and moisture to fuel the storm.
• Atmospheric Instability: A pre-existing disturbance, like a tropical wave, is needed. The warm, moist air rises rapidly, creating an area of low pressure at the surface.
• Low Vertical Wind Shear: Strong changes in wind speed or direction with altitude (vertical wind shear) can disrupt the developing cyclone. Low shear allows the storm to organize and intensify.
• Sufficient Coriolis Force: The Coriolis force, caused by the Earth’s rotation, deflects the moving air, initiating the cyclonic rotation. This force is weak near the equator, which is why cyclones rarely form within 5 degrees latitude of the equator.
• High Humidity: A moist atmosphere provides more water vapor for condensation, releasing latent heat and further fueling the storm.

The Role of SST in Cyclone Intensification

Once a tropical disturbance forms, warmer SSTs provide a continuous supply of energy. The process is as follows:

1. Warm ocean water evaporates, increasing the moisture content in the air.
2. This moist air rises, cools, and condenses, forming clouds and releasing latent heat.
3. The released latent heat warms the surrounding air, causing it to rise further, creating a positive feedback loop.
4. This process lowers the surface pressure, drawing in more air and intensifying the storm.

Impact of Rising SSTs on Tropical Cyclones

Rising SSTs are expected to have several impacts on tropical cyclones:

• Increased Intensity: Warmer waters provide more energy, potentially leading to more intense cyclones with higher wind speeds and heavier rainfall.
• Higher Rainfall Rates: A warmer atmosphere can hold more moisture, resulting in increased rainfall associated with cyclones.
• Rapid Intensification: Cyclones may intensify more rapidly in warmer waters, making forecasting and preparedness more challenging.
• Poleward Migration: Some studies suggest that the regions where tropical cyclones form may shift poleward as ocean temperatures warm.

The IPCC’s Sixth Assessment Report (AR6) confirms that there is high confidence that the proportion of intense tropical cyclones (Category 4-5) has increased globally since the early 1980s. However, the total number of tropical cyclones has not necessarily increased.