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Climate Research Why wind data are crucial for this important endeavor

Our climate is driven by exchanges between two moving systems: the atmosphere and ocean.

Which key process helps steer air-sea exchange...and seagulls? Wind!

Producing decades-long data records for ocean vector winds is critical for ocean and climate communities. That's why NASA began collecting wind data from various satellites in 1973. The ocean wind climate record is built upon decades of NASA investments in product development and improvement, including "homogenization" of measurements from different missions, sensors, and platforms.

Let's take a look at one climate product created by Lucrezia Ricciardulli and the team at Remote Sensing Systems (RSS). This wind climate data record is updated each year in the American Meteorological Society's State of the Climate report. The figure below displays the global wind speed trend over the ocean from satellite radiometers (SSM/I, SSMIS, TMI, GMI, AMSR2, ASMR-E, and WindSat) for the period 1988–2021.

trend – a pattern found in time series datasets; it is used to describe if the data is showing an upward or downward movement for part, or all of, the time series

Where is the trend towards decreasing wind speeds?

Indian Ocean & Western Pacific

Where is the trend towards increasing wind speeds?

Southern Ocean
Coastal regions such as North America
Pacific Ocean trade winds

What is causing these trends?

You may be familiar with east-west atmospheric pressure differences related to El Niña and La Niña phases in the tropical Pacific Ocean. This is sometimes called a "seesaw" effect because, depending on whether it's El Niño or La Niña, sea level is high on one side of the tropical Pacific and low on the other. The shift between these two phases is also called the "Southern Oscillation."

El Niño and the Southern Oscillation (ENSO) is a periodic fluctuation (i.e., every 2–7 years) in sea surface temperature and the air pressure of the overlying atmosphere across the equatorial Pacific Ocean. – Climate.gov

However, this type of oscillation also occurs over much longer time scales. In the Pacific Ocean, this type of oscillation that waxes and waves approximately every 20 or 30 years, thus it is called the Pacific Decadal Oscillation (PDO). As seen with El Niño / La Niña, extremes in the PDO pattern are marked by widespread variations in the Pacific Basin and the North American climate. The extreme phases of the PDO have been classified as being either warm or cool, defined by abnormal ocean temperatures in the northeast and tropical Pacific Ocean.

According to the "State of the Climate in 2021" report, studies indicate that the primary driver of surface wind trends is exerted by decadal ocean-atmosphere oscillations. These, in turn, are modulated by changes in the temperature gradients induced by human-caused greenhouse gases.

What has been happening with winds lately?

For that, we look at data for the year 2021 compared with averages over the previous three decades.

The map below, also produced by RSS, depicts surface wind speed anomalies over the ocean (shaded areas). On land, circles and shaded data are from the ERA5 reanalysis output among others. Anomalies are computed relative to data averaged over 1991 to 2020.

anomaly – something that deviates from what is standard, normal, or expected

Let's start with locations where winds were slower-than-normal...

North Atlantic Ocean
Equatorial Atlantic Ocean
Indian Ocean

Now let's look at faster-than-normal winds...

Bering Sea
Equatorial Pacific Ocean
Southern Ocean & South Pacific

In 2021, the most prominent positive wind anomalies were recorded over the Southern Ocean, focused in the South Pacific. What was the underlying cause?

Antarctic El Niño?

During El Niño, weak trade winds allow warm water to move west, bringing rain to the Americas and drought to New Guinea and Australia. On the flip side, La Niña has stronger-than-normal trade winds that push more warm water and rains toward Asia while colder-than-normal waters off western North America lead to drought in the southern US and heavy rains in the Pacific Northwest and Canada.

This same type of seesaw effect happens in the southern third of our planet, but the atmospheric pressure differences are oriented north-south. Called the Southern Annular Mode or Antarctic Oscillation, it usually defined as the difference in the mean sea level pressure at 40°S (mid-latitudes) and 65°S (Antarctica).

Which winds are affected by the Southern Annual mode? The belt of west-to-east flowing winds that encircle Antarctica. These winds fuel the world’s largest ocean current and the fiercest waves on the planet.

The ocean's tallest waves are getting taller

In a positive phase of the Southern Annular Mode, there is lower-than-normal air pressure over Antarctica, and higher-than-normal air pressure over the mid-latitudes. The westerly wind belt that encircles Antarctica strengthens, as does the oceanic Antarctic Circumpolar Current. Impacts include warming and drying over Patagonia (South America), glacier recession in western Antarctica and the Antarctic Peninsula, and increased summer rainfall in Australia.

In 2021, 73% of the year was in the positive phase, which was the fourth highest percentage since 1979. According to the State of the Climate report, 2021 was Australia’s coolest year since 2012 and wettest since 2016, which helped increase water storage following drought events from 2017 to 2019. Conversely, New Zealand had its warmest year on record. Winds drew down warmth from the tropics and contributed to a marine heatwave event off New Zealand's coast.

Marine Heatwaves

In a negative phase of the Southern Annular Mode, there is higher-than-normal air pressure over Antarctica, and lower-than-normal air pressure over the mid-latitudes. The westerly wind belt around Antarctica weakens and moves towards the equator. Impacts include colder and wetter weather in Patagonia, helping glaciers there to advance. It can also decrease summer rainfall in Australia.

The wind belt and ocean currents around Antarctica are an important coupled climate system that controls climate in the southern third of the world. Furthermore, this system is often tied to El Niño / La Niña events and can reinforce some of their impacts.

Production of the ocean winds climate record has been made possible by decades of research by the NASA's Ocean Vector Winds Science Team.

"Observing and interpreting winds over the ocean is one of the oldest of NASA's traditions. Nearly four decades and several satellite missions later, global and continuous data records of ocean vector winds are the backbone of scientific discovery about our ocean and atmosphere." – Nadya Vinogradova, NASA Program Scientist for Physical Oceanography

References

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