Climate variability is often measured by comparing the observed conditions to the long-term mean conditions. The mean state of the tropical Pacific ocean is identified by both its surface and subsurface characteristics, each of which exhibit considerable evolution across the eastern half of the tropical Pacific during the course of the year.Throughout the year the ocean surface is warmest in the west and coldest in the east. The largest difference between the two regions is observed during September and October when temperatures in the eastern Pacific reach their annual minimum. Temperatures across the central and east-central tropical Pacific then normally begin to increase during December and peak in April, while temperatures in the extreme eastern Pacific normally begin to increase during November and peak in March. In contrast, sea surface temperatures across the western tropical Pacific and Indonesia remain warm and nearly constant throughout the year.