SLA和SSHA的区别

翻译过来一个是海平面异常（Sea Level Anomaly），一个是海面高程异常（Sea Surface Height Anomaly）。

起因是 https://www.mdpi.com/1424-8220/21/1/126/htm 中的这么一段话：

Satellite altimeter can provide statistical information of surface eddies by detecting sea surface height anomalies (SSHA) [20], sea level anomalies (SLA) [21], or sea surface temperature (SST) [22]

看了这段话之后开始怀疑, SLA和SSHA不是同一个东西吗?

Sea Level Anomaly(SLA)

根据 http://www.tsgc.utexas.edu/topex/activities/elnino/sld005.html 的解释：

sea-level anomaly is the difference between the total sea-level and the average sea-level for this time of year. We look at anomalies because the total 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 only 1% of the signals. If the constant part were not removed, the El Niño signal would not be observable.

提取重点：SLA是一年中这个时候总海平面与平均海平面之间的差值。

根据 https://coastwatch.noaa.gov/cw/stories/emilys-post/detecting-sea-level-anomalies-with-satellites.html的说法：

The sea surface is a dynamic mixture of bumps and dips resulting from a variety of factors including gravity, ocean currents and the rotation of the Earth. Scientists study variations in sea surface height using radar altimeters on satellites. These altimeters emit radar pulses that bounce off the ocean’s surface and are detected by a sensor on the satellite when they return. Sea surface height derived from satellite altimeters is accurate to within about 3-4 centimeters.
Anomalies in sea level can be identified by calculating the difference between the measured sea surface height and the average sea surface height. By studying sea level anomalies, scientists can improve understanding of ocean circulation patterns and improve forecasts of climatological events such as El Niño and La Niña.

提取重点：SLA定义为实测海面高度与平均海面高度之间的差值。

Sea Surface Height Anomaly(SSHA)

根据 https://sos.noaa.gov/catalog/datasets/sea-surface-height-anomaly-real-time/:

As anyone who has been on a boat knows, the ocean surface is not flat. It ripples and undulates due to wind and waves, rises with the tidal pull of the moon, expands and contracts as it heats and cools, and billows due to circulation patterns. Sea surface height anomaly data is able to characterize these small- and large-scale changes over weeks and years. An anomaly is the difference between the long-term average for different regions of the ocean and what is actually observed by satellites.

提取重点：SSHA是海洋不同区域的长期平均值与卫星实际观测值之间的差异。

根据 https://www.e-education.psu.edu/earth107/node/1493

If you are interested in understanding climate change and you pay attention to in-depth news stories on the topic, you have no doubt frequently heard or read references to sea level and temperature anomalies. Anomaly data are being shared with greater regularity in the media these days, so it is important to understand what we mean by terms like sea surface temperature anomalies and sea surface height anomalies. An anomaly is an inconsistency or deviation from the norm, so images are created to show where change is taking place in the ocean in either a positive or negative trend when comparing to previous data. Sometimes this is referred to as Sea Surface Height Deviation data or SSHD. Sea surface height anomalies are calculated using data from satellite altimeters. Many years’ worth of thousands of measurements provide a historical mean sea surface height and the difference between the historical mean and the sea surface measurement for a particular date is called the sea surface height deviation. This can be calculated for points over the ocean surface, providing the data for the incredible maps we are seeing that show color-coded variations in sea surface across the globe and the changes in these measurements over time and space.

提取重点：历史平均值与特定日期的海面测量值之间的差异称为SSHA。

总结

http://www.altimetry.info/radar-altimetry-tutorial/data-flow/data-processing/reference-surfaces/ 提到了：

Sea Level Anomalies (SLA, also called SSHA, Sea Surface Height Anomalies) is the sea surface height with respect to a Mean Sea Surface or a Mean profile
So, SSH = (Altitude – Range – corrections) = geoid + ADT = MSS + SLA/SSHA = geoid + MDT + SLA/SSHA

海平面异常（SLA，也称为 SSHA，海面高度异常）是相对于平均海面或平均剖面的海面高度。
因此这两者是等同的。

计算方式

SSH：卫星高度计直接测出的是海面高程 , 由于轨道是根据参考椭球面确定的, 因此SSH是相对于参考椭球面的。
SSH=轨道高度−高度计测量SSH = 轨道高度 - 高度计测量SSH=轨道高度−高度计测量

MSS：根据一段时间(N)的SSHSSHSSH均值求出平均海面MSSNMSS_NMSSN。
MSSN=∑k=1N(SSHk)NMSS_N = \frac{\sum_{k=1}^N(SSH_k)}{N}MSSN=N∑k=1N(SSHk)

以上两个值都是基于参考椭球面(Ellipsoid of Reference)确定的，实际应用中使用相对大地水准面的计算更加合理。

MDT：平均动态地形(MDT)与平均海面(MSS)类似, 只不过是以大地水准面计算的。
MDTN=MSSN−大地水准面MDT_N = MSS_N - 大地水准面MDTN=MSSN−大地水准面

ADT：绝对动态地形(ADT)与海面高程(SSH)类似, 只不过是以大地水准面计算的。
ADT=SLAN+MDTN=SSH−MSSN+MDTN=SSH−大地水准面ADT = SLA_N + MDT_N = SSH - MSS_N + MDT_N = SSH - 大地水准面ADT=SLAN+MDTN=SSH−MSSN+MDTN=SSH−大地水准面

SLA：海平面异常(SLA)或海面高程异常(SSHA)是SSH或ADT与其均值的差值，与基准面无关。
SLA=SSHA=SSH−MSSN=ADT−MDTNSLA = SSHA = SSH - MSS_N = ADT - MDT_NSLA=SSHA=SSH−MSSN=ADT−MDTN

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Sea Level Anomaly 和 Sea Surface Height Anomaly 的区别

SLA和SSHA的区别

Sea Level Anomaly(SLA)

Sea Surface Height Anomaly(SSHA)

总结

计算方式

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