MonitoringMonitoring ElEl NiñoNiño WithWith SatelliteSatellite AltimetryAltimetry

Dr. Don P. Chambers

Center for Space Research The University of Texas at Austin

El Niño Workshop- Online for Education March 16, 1998

Center for Space Research, The University of Texas at Austin Introduction

El Niño has received a lot of attention this year, mostly due to the fact that several different instruments, both in the ocean and in space, detected signals of an impending El Niño nearly a year before the warming peaked. One of these instruments was a radar altimeter on a spacecraft flying nearly 1300 km (780 mi.) over the ocean called TOPEX/Poseidon (T/P). T/P was launched in September 1992 and is a joint mission between the United States and France. It is managed by NASA’s Jet Propulsion Laboratory and the Centre National d’Etudes Spatiales (CNES). In this presentation, I will describe in general the altimeter measurement and discuss how it can be used to monitor El Niño. In particular, I will discuss how the altimetry signal is related to sea surface temperature, and how it is different, and how altimetry can detect El Niño signals months before peak warming occurs in the eastern Pacific. To begin with, I would like to show two recent images of the sea-level measured by TOPEX/Poseidon and point out the El Niño signatures. Then, I will discuss the measurements that went into the image and discuss how we know they are really measuring what we think they are. This is the sea-level anomaly map from the TOPEX/Poseidon altimeter, for January 28 to February 7, 1998. This is the sea-level anomaly map from the TOPEX/Poseidon altimeter, for February 7 to February 17, 1998. Notice the changes occuring in the eastern Pacific. A sea-level anomaly is the difference between the total sea-level and an average sea-level for this time of year. We look at anomalies because the total sea- level measurement made by the altimeter varies from ± 100 meters. Most of this is constant, though, and is due to the Earth’s gravity and the ocean circulation. Sea- level variations caused by El Niño account for less than 1% of the total signal. If the constant part were not removed, the El Niño signal would not be observable. El Niño is evident in the figures on the previous page as higher than normal sea- levels in the east, and lower than normal sea-levels in the west. Sea-level is beginning to drop in the east, though, and indications are that El Niño is beginning to dissipate. The sea-level is actually highest north and south of the equator. This is a sign that El Niño is dissipating, and will be discussed later. Also, note the high sea- levels in the Indian Ocean, which are nearly a mirror-image to El Niño in the Pacific. I will discuss this later in the presentation as well. The altimeter does not directly produce the image shown on the previous slide. Instead, it measures the height of the spacecraft above the ocean surface, which can be converted into sea-level. These sea-level measurements can then be mapped to a uniform grid, color coded, and displayed. However, the conversion to the sea-level measurement is not a simple procedure. Recall that TOPEX/Poseidon orbits the Earth at an altitude of 1300 km. The altimeter sends several thousand radar pulses towards the ocean each second then measures the time for the pulse to return to the spacecraft. Not only does the computer have to match the right transmitted pulse with the right received pulse, but it has to compute the time of transit properly. An error in time of less than a second would mean an error of tens of centimeters. To get the height of the satellite above the ocean, the time for the pulse to return has to be converted to range using the speed of light. However, the speed of light is constant only in a vacuum. The radar pulse travels through the atmosphere twice, where it is refracted by air molecules, water vapor, free electrons, and is partially scattered by surface waves. The size of these errors add up to more than 3 meters. However, all of them can be measured or modeled. Finally, the precise location of the satellite needs to be known, since the sea- level is the difference between the satellite location above the center of the Earth and the height of the satellite above the ocean. If the satellite location were only good to 1 meter, then the sea-level measurement would only be good to 1 meter. But, how accurate is the image on the previous slide? The accuracy is amazing, considering all of these problems listed above: about 2 to 3 cm (1 in). That’s about the diameter of a quarter. We know this because we can compare sea-level measured by T/P with sea-level measured at the ocean surface with tide gauges. Here we have compared altimeter measurements with nearby tide gauges at two sites: Pohnpei in the western Pacific and Baltra in the eastern Pacific. Sea-level in these regions are very large during El Niño events (indicated by arrows). Sea-level is much lower than normal in the west and higher than normal in the east as will be explained later. If the tide gauges are considered to be perfect (which they are not), the accuracy of the altimeter is about 3 cm. However, if a more realistic error is assumed for the tide gauges then the accuracy of the altimeter measurement is around 2 cm. Note that we are using the mean standard deviation as a measure of error; differences at some times can be up to 3 times this number. Also, comparisons at some tide gauges are better or worse. The average is about 2.5 cm.

Pohnpei in Western Pacific Baltra in Eastern Pacific (7°N, 158°E) (0°, 90°W) Standard Deviation of Differences = 3 cm Standard Deviation of Differences = 3.5 cm 20 30 T/P 10 20 Tide Gauge

0 10

-10 0

-20 T/P -10 Tide Gauge Sea-Level Anomaly (cm) Sea-Level Anomaly (cm) -30 -20 1993 1994 1995 1996 1997 1998 1993 1994 1995 1996 1997 1998 Year Year Tide Gauge TAO Moored Buoy T/P Groundtrack 30.0

20.0

10.0

0.0 latitude (°N) -10.0

-20.0

-30.0 50 100 150 200 250 300 longitude (°E) T/P flies in a groundtrack that repeats 30 every 10-days and goes as far north and 20 south as 66° latitude. This means that it samples approximately 400000 points over 10 the ocean every 10-days. Compare that to 0

ship measurements, moored buoys, or tide Latitude (°N) -10 gauges. However, there are still gaps in the data, which means that the image in the -20 earlier slide is based on interpolating real -30 measurements to a constant grid, then 40 50 60 70 80 90 100 110 120 smoothing them and plotting them Longitude (°E) graphically and assigning colors based on Lines are T/P groundtrack and dots the heights. are locations with more than 24 ship measurements in 3 years. Before describing how altimetry can monitor El Niño, I would like to discuss how El Niño begins, and why El Niño warming causes a sea-level variation. During normal conditions, winds blow from east to west, due to differences in the atmospheric pressure. Normally, a high pressure system sits over the eastern Pacific, while a low pressure system sits over the western Pacific. Because of the low pressure system in the west, there is increased upward convection which puts water vapor into the atmosphere and results in more rainfall in the west than in the east. At the same time, the surface currents along the equator generally move east to west. This transports water warmed year round by the sun to the western Pacific, where it tends to pile up before flowing north and south as other currents. This pushes down the thermocline, or the region where the temperature change with depth is the greatest. In the east, cold water upwells, or rises up from great depths to replace the warm water which flowed west, and the thermocline is shallow. All of this will cause a certain sea-level signature, since sea-level is a measure of the integrated water density. Warmer water has a lower density than colder water, and takes up a greater volume. Thus, sea-level is higher where the thermocline is depressed and the upper waters are warm, and sea-level is lower where the thermocline is raised and upper water is cool. Changes in the atmosphere over the western Pacific cause all of this to change. Figure courtesy of NOAA PMEL Several weeks to several months before El Niño begins to manifest in the eastern Pacific, a dramatic change occurs in the atmosphere over the Pacific. The pressure over the western Pacific increases while the pressure in the east decreases. This causes a change in the wind pattern and the convection, as shown in the figure below. This atmospheric change is called the Southern Oscillation. The Trade Winds decrease or even reverse and blow from west to east. The equatorial surface current slows, and a subsurface current that always exists strengthens. This undercurrent has a core at about 100 meters depth and always flows west to east. Most of the time it dies out in the central Pacific. During El Niño, though, the current is strong all the way to the Galapagos Islands off the coast of South America; it sometimes surfaces. Thus, there is a change in the ocean associated with the Southern Oscillation. In the west, the thermocline rises and warm water flows east. In the east, months after the initial wind changes, the thermocline gets deeper. The upper water column warms, and the sea-level rises. This is a simplified model. The ocean takes some time to adjust to the wind changes. One of the primary ways it adjusts during El Niño is through the creation of Kelvin waves. These waves are very different from the surface waves you are probably familiar with and are discussed more in the following slides.

Figure courtesy of NOAA PMEL Cross-Section Looking West; Cross-Section Along Equator, Looking North Winds east to west Although surface currents along the equator typically flow east to west, the net transport of water in the equator occurs deeper and is away from the equator as shown above. This is called the Ekman transport, and is named after the oceanographer who discovered it. The transport is caused by the winds and the Coriolis force, which is caused by the rotation of the Earth. Ekman found that the net water transport was below the surface and that it was perpendicular to the wind direction; to the right of the wind in the north, and to the left of the wind in the south. Thus, since the Trade Winds typically blow east to west in the equatorial region, the net transport is away from the equator. Notice the much deeper thermocline, or boundary between the warm and cold water, in the west than in the east. Most of the sea-level variations measured by altimetry are caused by changes in the thickness of this layer of warm water. Cross-Section Looking West; Cross-Section Along Equator, Looking North Anomalous Winds west to east During the Southern Oscillation, winds are weaker in the west; thus the wind anomaly, or deviation from an average wind for that time of year is from west to east. This causes a change in the Ekman transport in this region. Instead of flowing away from the equator, it flows toward the equator. To balance this anomalous influx of water, the warm water in the upper layer downwells to the lower layer. However, the warm water is lighter than the cooler water, and is naturally more buoyant. The forcing of the warmer water into the cooler water will cause an oscillation, normally at the steepest density gradient, the thermocline. These oscillations will propagate away from the source of the wind anomaly as very long waves. Along the equator, these are called Kelvin waves and they change the thickness of warm water in both the west and the east, which causes large sea-level changes and leads to El Niño. Kelvin waves can cross the Pacific in two months. They can only exist near the equator due to the Earth’s rotation. The amplitude of the Kelvin wave is several tens of meters along the thermocline, and the length of the wave is 1996 thousands of kilometers (1° longitude = 111 km) The figure at the right shows Kelvin waves inside the ocean, computed with temperature data from moored buoys operated by NOAA. It shows the depth of the 20°C temperature level as a series of standard latitude-longitude plots stacked in 1997 time. The latitude width of each time step is 4° (2°S to 2°N) and time is from March 1996 at the top to March 1998 at the bottom. Notice the yellow/orange lines which slope across the Pacific beginning in January 1997 (marked by thick lines). These are Kelvin waves. 1998 Eastward movement is indicated by the slope in time from west to east. These waves set up a change in the warm water thickness in the eastern Pacific beginning in March. Other Kelvin waves are visible after El Niño developed; the first two, however, were early indicators that an El Niño Figure courtesy of NOAA PMEL would probably occur this year. Because a Kelvin wave is associated with density fluctuations inside the ocean, it can be seen in the sea-level measurements made by altimeters, although with a reduced magnitude. The figure to the right shows sea-level anomalies determined from TOPEX/Poseidon. Notice the two Kelvin waves, with amplitudes of 10 and 15 cm in sea-level compared to 30 and 40 meters in thermocline change. Note that a depression in the thermocline (from the previous picture) is associated with an increase in sea- level. The Kelvin waves travel east and set up changes in the eastern Pacific that lead to El Niño by depressing the thermocline there. There are smaller Kelvin waves in 1995 and 1996. These are seasonal, and do not cause El Niño events. However, large Kelvin waves, such as those observed in early 1997 almost always lead to El Niño events.

A larger version of this image is available from http://www.csr.utexas.edu/eqpac An important thing to realize is that the figures on the previous two slide are updated in near-realtime; they are not figures that are produced months to years after the data have been processed. This means that the first Kelvin wave was noticed by scientists connected with the TOPEX/Poseidon project and the TOGA-TAO project in early February 1997. Because the Kelvin wave was large, it was expected that there would be at least a moderate El Niño warming either in the late spring or early summer, although at that time, no one expected that it would surpass the event of 1982-1983. Thus, these scientists were not surprised when an official NOAA El Niño advisory was issued in April.

The plots from the previous slides are still updated on a regular basis and are available over the World Wide Web. The TOGA-TAO data can be accessed from the NOAA Pacific Marine Environmental Laboratory:

http://www.pmel.noaa.gov/toga-tao/realtime.html

The TOPEX/Poseidon data can be accessed from the University of Texas, Center for Space Research:

http://www.csr.utexas.edu/eqpac Sea-Level, Ocean Heating, and El Niño I have mentioned several times the correlation between sea-level and changes in temperature and thickness of the upper layer. Basically, as the temperature and thickness of the upper layer increases, the sea-level will increase. How well does the sea-level follow temperature changes in ocean? In the tropical Pacific, the answer is: very well. One can see this by comparing T/P data to data obtained from the TAO moored buoys in the Pacific. The buoys measure only temperature at various depths, and these can be converted to density and then sea-level due to heating variations only. The agreement is very good, as seen in the figures below. There is more disagreement in the western Pacific than in the east, due to factors other than heating that effect the T/P sea-level measurement in this region. Averaged at TAO Buoys in Western Pacific Averaged at TAO Buoys in Eastern Pacific (5°S to 5°N, 150°E - 175°E) (5°S to 5°N, 260°E - 290°E) Standard Deviation of Differences = 3.8 cm Standard Deviation of Differences = 2.8 cm 30 30

20 T/P 20 TAO 10 10 0 0 -10 T/P -10 -20 TAO Sea-Level Anomaly (cm) Sea-Level Anomaly (cm) -30 -20 1993 1994 1995 1996 1997 1998 1993 1994 1995 1996 1997 1998 Year Year Sea-Level vs. Sea Surface Temperature

Another important type of instrument used to measure El Niño is the Advanced Very-High Resolution Radiometer (AVHRR). These instruments are also based in space, and convert the observed infrared radiation into Sea Surface Temperature (SST). The SST measured by these satellites is really the temperature on the very thin layer at the top of the water column. Since sea-level change during El Niño is caused mainly by heating variations, one would expect that SST and sea-level would be very similar. However, there are significant differences. Ignoring for a moment the sea-level effects of currents, winds, and other forces, recall the previous discussion that much of the early El Niño signal occurs below the ocean surface. It takes some time for heating at 100 m to reach the surface. However, the changes will be seen in sea-level measured by altimetry or the moored buoys which monitor temperatures to a deep level as they are happening. This means that altimetry can actually see El Niño signals slightly before the SST measurements can. The difference in time is only slight, about 2 weeks to a month. However, by combining the two measurements, one can get a more complete picture of how El Niño evolves. The next four slides will demonstrate this by showing complementary sea-level and SST anomalies for each month during 1997. The fifth slide following will summarize important events during the evolution of the 1997 El Niño. cm °C cm °C cm °C cm °C The T/P data show a Kelvin wave in the central Pacific in January, while the SST data indicate cooler than normal surface waters in the region. By February sea- level and SST were both higher than normal in the eastern Pacific after the Kelvin wave reached the coast. In March, a larger Kelvin wave began forming and can be seen in the T/P data; again, it is not observable in the SSTs. The second Kelvin wave moved across the Pacific in April and May, as observed by T/P. By May, there was a tongue of warm water extending westward past 250°E. The T/P data, sensitive to heat changes at depth, shows that the warming approached below the surface from the west. The SST data on the other hand shows the surface warming moved west from the coast after the Kelvin waves reached the eastern Pacific and depressed the thermocline. By July, El Niño had fully developed. Note the similarities in sea-level and SST at this point, since the heating has mixed throughout the water. There are significant differences, though. Peak sea-level was to the west of peak SST, and there were much larger sea-level changes in the western Pacific than SST changes. All of these differences were due mainly to sub-surface heat changes which the SST data cannot detect. Both sea-level and SST continued to rise through the Fall. Both peaked in early December. Note the “lobes” of high sea-level forming north and south of the equator in December (indicated by arrows). These are indicative of another type of internal wave, a Rossby wave. Just like Kelvin waves were created when the ocean was adjusting to the formation of El Niño, Rossby waves are created as it dissipates. These waves move westward, and help re-adjust the thermocline in the west and east to normal. The complete El Niño cycle can be seen in the animation of TOPEX/Poseidon sea level anomalies shown above. Notice the eastwardly traveling Kelvin waves at the beginning of 1997, and the evolution of the anomalous warm pool in the eastern Pacific. Also, notice how in early 1998, sea-level has begun to fall in the east and one can begin to see westward movement north and south of the equator as Rossby waves begin to develop. I pointed out the large sea-level anomalies in the Indian Ocean at the beginning of this presentation, and they can be seen in the animation referenced in the previous slide. Sea-level anomalies in the eastern Pacific began to move westward from the coast of South America in September. Shortly after this, sea-level increased suddenly and moved eastward again, indicating another Kelvin wave. At the same time, large sea-level anomalies developed in the Indian Ocean and began to move westward. By the end of 1997, sea-level in the southwestern Indian Ocean was as anomalously high as it was in the eastern Pacific. What was happening?

cm El Niño in the Indian Ocean In October, there was a large eastwardly wind burst in the central Pacific. At the same time, there was a westwardly wind burst in the central Indian Ocean. Wind bursts in these two oceans are well correlated during El Niño events as shown in the bottom left figure. The wind bursts are associated with the Southern Oscillation, as shown in the bottom right figure. El Niño events are marked with arrows.

Central Pacific Eastwardly Wind Anomalies SOI, 5 month Central Indian Westwardly Wind Anomalies Indian - Pacific Pseudo-Stress Zonal Wind Stress (m

) 50 3.0 40 2 /s

2 2.0 20 1.0

0.0 0

0 SOI -1.0 -20 -2.0 2 /s Zonal Wind-Stress (m -40

-3.0 2 )

-50 -4.0 -60 1970 1975 1980 1985 1990 1995 2000 1970 1975 1980 1985 1990 1995 2000 Year Year Monthly wind stress anomalies, averaged over the central Difference in Indian and Pacific eastwardly Pacific (±5°N, 180°E to 200°E) and the central Indian Ocean wind stress, compared against the Southern (±5°N, 80°E - 100°E). Positive values are eastwardly Oscillation Index (SOI). The SOI is (Pacific) or westwardly (Indian). Data are from Florida State determined from the difference in pressure at Univ., Center for Ocean-Atmospheric Prediction Studies. Darwin, Australia and Tahiti. Recently, several studies have looked at SST warming in the southwestern Indian Ocean and the eastern Pacific, and have found that there is a significant correlation between anomalous warming in the Indian Ocean and El Niño (see figure below), although SST signals in the eastern Pacific are much larger. These studies (Tourre and White, 1995 and Nicholson, 1997) have found correlations between the Indian Ocean and the Pacific for almost every El Niño back to 1946. I have looked at the T/ P altimeter data and see a similar correlation, but the extreme sea-level values are closer to the same magnitude (see below).

E. Pacific SW Indian Ocean

30 Indian SST Anomaly (°C) 4 2 E. Pacific SW Indian Ocean 3 1.5 20

2 1 10

1 0.5 0 0 0 -10 Sea Level Anomaly (cm)

-1 Pacific SST anomaly (°C) -0.5 -20 -2 -1 1993 1994 1995 1996 1997 1998 1982 1984 1986 1988 1990 1992 1994 1996 1998 Year Year SST data averaged over southwestern Indian T/P sea-level anomalies Ocean and eastern equatorial Pacific. The scale averaged over the same region. for Pacific SST anomalies is on the left hand side; the scale for Indian Ocean SST anomalies is on the right. By combining all of these observations with the high resolution maps made from the TOPEX/Poseidon data, one can begin to see how the Indian Ocean warming is related to El Niño. The October wind burst in the Pacific forced a Kelvin wave in the eastern Pacific, which caused sea-level to peak for a second time off the coast of South America. The wind burst in the Indian Ocean created Rossby waves which moved westward in the Indian Ocean. By December, sea-level in the southwestern Indian Ocean was as high as sea-level in the eastern Pacific, and was a near mirror-image of El Niño in the Pacific. Similar variations have been observed during the 1994 El Niño. Based on the altimeter observations, it is beginning to look like El Niño has a very similar mode in the Indian Ocean. Although these results are preliminary, they show that there is still a lot about El Niño that we do not understand. Continued data from satellite altimeters, space- borne AVHRR, and moored buoys will all contribute to improving our knowledge. Numerical models will also play an important part in testing our theories, as I am sure Professor O’Brien will discuss next week. Understanding this apparent El Niño mode in the Indian Ocean may help us explain teleconnections between El Niño and weather patterns in Asia, Africa, Europe, and other sites that are far removed from the Pacific.