Lecture 12 Midlatitude and Tropical Weather and Climate

Anticyclones (10)

• a large high pressure region

• anticyclonic rotation

• wide isobars = low winds and persistence

• has air spiralling away from it at the surface, creates descent

• descending air gets warmer

• assuming the total water content is conserved, this means the air will get less saturated, so any clouds (liquid water) will evaporate (to water vapour)

• so anticyclones tend to be cloud-free and sunny

• generate hot weather in the summer

• also slow moving, so can persist for several days leading to heatwaves

• well spaced isobars, weak pressure gradients, low winds can accumulate air pollution

Typical Anticyclone Characteristics (5)

• motion: ~0-few 100 km/day

• size: ~4000 km

• lifetime: up to weeks

• pressure gradient: ~<10hPa/1000km

• central pressure: >1020 hPa

Anticyclone Formation (3)

• converging air aloft, descent and surface divergence

• if upper-level convergence is stronger than surface divergence -> surface pressure rises -> surface high deepens

• vertical motion: descends by ~1cm/sec (1km/day)

July 2022 Heatwave (12)

• 16-19th July 2022 ‘unprecedented’

• UK record temperature of 40.3C set in Lincolnshire

• Scotland record temperature of 34.8C

• temperatures above 39C recorded as far north as Yorkshire

• daily minimum temperatures set new record of 25.8C in London

• associated with a longer heatwave in Western Europe and large high pressure systems

• Office of National Statistics attributed 3,271 excess deaths from this heatwave

• led to southerly airflow

• short-lived in the UK due to cold front from low pressure system

• 46 stations exceeded previous UK temperature record

• record high temperatures in last 30 years, especially last 5 years stand out

• exceptional outlier in a 250-year series of central England

ITCZ (4)

• InterTropical Convergence Zone

• easterly trade winds converge near the equator along the ITCZ

• warm moist air rises all the way to the tropopause -> thunderstorms form

• this air then drives the Hadley Cell Circulation with descending air at about 30°

ITCZ Migration (5)

• ITCZ follows the migration of the sun’s overhead position with a delay of 1-2 months

• ocean heats up slower than land

  • moves further north and south over land areas than over water

• in July and August, the ITCZ lies north of the equator over Africa, Asia, and Central America

• in January and February lies further south in South America, central Africa and Australia

• responsible for wet (short and long rains) and dry seasons in the tropics

Monsoons (6)

• important seasonal feature linked to ITCZ

• a regional large-scale sea breeze circulation

• in summer, solar heating over continent leads to low pressure as warm air rises

  • brings moist oceanic air and heavy rainfall

• in winter, land cools faster than water

  • high pressure over ocean

  • wind moves offshore

• wind reversal

  • sea-to-land in summer

  • land-to-sea in winter

• occurs in India, West Africa, East Asia, Northern Australia, Central America

Monsoons: India

• in summer, the ITCZ is known as the monsoon tough

• causes low pressure over North and North West India

• PGF air moves in from sea

• trade winds over India and Indian Ocean blow from southwest

  • lots of rain, moist air

• in winter, ITCZ moves south and the winds reverse (trade winds from northeast)

  • dry, continental air

• rainfall amount and intensity follows ITCZ

East Asia

• in winter, the East Asian monsoon carries cold dry air from the Siberian High Pressure Region offshore

• in summer, the East Asian monsoon carries moist air from the Indian and Pacific oceans to the east Asian continent

Tropical Cyclone Characteristics (3)

• typically ~100-500km, smaller than mid-latitude cyclones

• typical central pressures of 950hPa, but record low of 870hPa, lower than mid-latitude cyclones

• lots of rain

  • Hurricane Irma more than 274 mm per hour

Tropical Cyclone Structure

• eye in centre

• eyewall with outflow cloud shield

• spiralling winds between spiral rainbands

• dry air sinks in centre, winds flow inward and spiral upward

Conditions for Tropical Cyclone Formation (3)

• only form over oceanic regions where SSTs are greater than 26.5C

• do not form within 5° of the equator due to negligible Coriolis force

• form in regions where the vertical wind shear between the surface and upper troposphere is low (less than 23 mph or 10 m/s), no jet stream

Sources of Tropical Cyclones

• form from tropical disturbances

  • a localised area where air is converging

• two mains mechanisms in the tropics for convergence

  • Easterly waves

    • originate over continents as air moves across mountains/deserts e.g. off west coast of Africa

  • ITCZ

    • easterly trade winds converge

Saffir Simpson Scale

• measures tropical storms

• Tropical Depression (TD), Tropical Storm (TS), Hurricane 1, Hurricane 2, Hurricane 3, Hurricane 4, Hurricane 5

2024 Hurricane Season

• June 19 2024 - November 18 2024

• Milton strongest storm

• 18 depressions, 18 storms, 11 hurricanes, 5 major hurricanes

• 401 total fatalities

• $129.5 billion in damage

Hurricane Milton Impacts

• explosive intensification due to warm SSTs in Gulf of Mexico

  • category 5 with winds of 180mph, 897hPa

  • landfall as category 3 in Florida with 120mph winds

  • Mexico

    • evacuations

    • flooding

    • 3 deaths

    • 12,000 people without power

  • Florida

    • 6 million evacuations

    • 32 deaths

    • 46 tornadoes

    • 3 million homes without power

    • misinformation rife