Time runs down the page, longitude across; each row is one month of the subsurface anomaly along the equator from the GODAS reanalysis, at about 95 m or as the depth of the 20 °C isotherm.A Hovmöller diagram is a way of plotting data that changes over both time and space so that patterns of movement become easy to see. Instead of a map at one moment, it shows a slice of the world (for example, along the equator) with time running down the page. Each horizontal line represents conditions at a single time, and the vertical axis stacks these lines in order, so you can watch features like warm water anomalies drift sideways as months pass.
The diagram is named after Ernest Hovmöller, a Swedish meteorologist who introduced it in a 1949 paper to study atmospheric waves.
The horizontal axis is a spatial coordinate (like longitude), and the vertical axis is time, with the earliest time at the top. Each row is a snapshot of a variable—such as temperature anomaly or rainfall—along that spatial line. When a feature (like a warm pool) moves eastward, it appears as a diagonal stripe across the diagram; its slope tells you the speed and direction of movement.
It is a standard tool in meteorology and oceanography for tracking phenomena like El Niño–Southern Oscillation, Kelvin waves, and monsoon patterns in reanalysis and model data.
The same diagram style is used in medical imaging to show how a heartbeat changes over time along a line through the heart.
