Distribution and Frequency of Tropical Storms and Droughts

Tropical Storms: Global Distribution and Frequency

Imagine an engine that only runs on warm ocean water. This is exactly how a tropical storm works, which explains why they only form in very specific parts of the world.

Tropical storms occur in a latitudinal band between 5° and 30° North and South of the Equator. They do not form within 0°–5° of the Equator (an area known as the Coriolis exclusion zone) because the Coriolis effect is too weak there to initiate the necessary rotational spin.

For a storm to form, specific conditions must be met:

• Sea Surface Temperature (SST): Must be at least 27°C.
• Ocean Depth: Warm water must extend to a depth of 50–60 metres to provide sufficient thermodynamic energy.
• Atmospheric Pressure: They require intense low pressure, developing in the Inter-Tropical Convergence Zone (ITCZ) where warm air rises rapidly.

Globally, approximately 80–90 storms form each year, peaking in late summer and autumn when oceans reach their maximum heat capacity. The North-west Pacific is the most active region, averaging over 26 storms annually.

Storms are classified differently depending on their location and wind speed. A Tropical Storm has sustained wind speeds between 63 and 118 km/h. Once it exceeds 119 km/h, it becomes a Tropical Cyclone (known regionally as a Hurricane or Typhoon), measured using the 1–5 Saffir-Simpson Scale.

Have Tropical Storms Changed Over Time?

News headlines often claim we are getting more hurricanes, but the scientific reality is more about power than numbers. When asked to assess changes over time, you must distinguish between frequency and intensity. Crucially, you should evaluate data reliability: long-term frequency comparisons are difficult because pre-satellite historical data (prior to the 1970s) is much less reliable.

The global total frequency of tropical storms has remained relatively steady, though the North Atlantic has seen an increase since the 1970s. However, storm intensity has increased significantly. There has been a 70% increase in the power of storms over the last 30 years, and the number of Category 4 and 5 storms has nearly doubled since 1970.

Evidence linking these changes to climate change includes:

• Thermodynamics: A 0.5°C rise in sea surface temperature correlates to a 5% increase in maximum wind speeds.
• Precipitation: A 4% rise in atmospheric water vapor over the last 25 years causes storms to drop higher rainfall totals.
• Spatial Shifts: As oceans warm, the 27°C isotherm is moving poleward, meaning storms are expanding into higher latitudes where they were previously rare.
• Storm Surges: Global sea levels rose by 0.17m during the 20th century, increasing the destructive baseline height of storm surges.

Droughts: Global Distribution and Frequency

Unlike a storm that arrives in hours, a drought is a "creeping hazard" that quietly builds over months or even years. Droughts are most frequent in bands between the Tropics of Cancer and Capricorn, concentrated at approximately 30° North and South of the Equator.

This spatial distribution perfectly aligns with the global atmospheric circulation system. At the boundary of the Hadley Cell and Ferrel Cell, air descends (sinks). As air sinks, it warms, preventing condensation and cloud formation. This creates permanent high-pressure belts known as an Anticyclone, leading to dry, Arid conditions. Additionally, in sub-tropical regions like the Sahel, droughts occur when the Inter-Tropical Convergence Zone (ITCZ) fails to migrate far enough north or south during its seasonal cycle, depriving regions of their expected wet season.

In terms of temporal frequency, historic data from 1974–2004 shows that the highest frequency of drought disasters occurred in West Africa and Eastern Brazil (recording 7–9 events in 30 years), followed by the East Coast of Australia (5–6 events).

Droughts evolve in a specific sequence, worsening over time:

• Meteorological Drought: A prolonged lack of precipitation compared to the regional average (e.g., 15 days with less than 0.25mm of rain in the UK).
• Agricultural Drought: Insufficient soil moisture to support crops, leading to failure and stress.
• Hydrological Drought: A significant reduction in water stores, such as low river discharge or depleted groundwater aquifers.
• Socioeconomic Drought: When the demand for economic goods (water, food, energy) exceeds the weather-driven supply, leading to severe Water Stress.

Have Droughts Changed Over Time?

Are droughts purely a natural cycle, or is human activity making them worse? To achieve top marks, you must provide a balanced judgement weighing both factors, and recognise that apparent increases in frequency are partially due to better disaster reporting today compared to the early 1900s.

The severity and frequency of droughts have generally increased globally since the 1940s, driven by the Enhanced Greenhouse Effect. Global average temperatures have risen by 0.85°C to 1.1°C since 1880, increasing evaporation rates. According to IPCC predictions, with a 1.5°C increase in global temperatures, a baseline 1-in-10-year drought is predicted to become twice (2x) as likely. In the UK, which has seen its ten warmest years in the last two decades, summer precipitation is projected to decrease by 20–30% by 2050.

However, droughts are also heavily influenced by natural cycles like the El Niño Southern Oscillation (ENSO). During an El Niño event, reversed trade winds push warm water east, causing dry, sinking air and severe droughts in western regions like Australia and Indonesia.

Calculating Frequency Changes

In Geography, you may be asked to calculate how hazards have changed using data. For example, comparing historical drought frequency to modern frequency:

Question: A region historically experienced 2 droughts in 50 years. It now experiences 5 droughts in 50 years. Calculate the percentage increase in drought frequency.

• Step 1: Calculate the historical baseline frequency.
  • $2 / 50 = 0.04$
• Step 2: Calculate the current frequency.
  • $5 / 50 = 0.1$
• Step 3: Use the percentage change formula: $\frac{\text{Difference}}{\text{Original}} \times 100$.
  • $\frac{0.1 - 0.04}{0.04} \times 100 = 150\% \text{ increase}$.