OceanCurrent- DM03_vel

OceanCurrent - Current Meters - Do altimeters and tide gauges measure coastal currents?

Introduction

The OceanCurrent team has been combining satellite altimeter measurements of ocean sea level with tide gauge data to produce our Gridded Sea Level Anomaly product for longer than IMOS has existed. Indeed, the inclusion of tide gauges has long been a distinguishing feature of our product. It enables us to estimate the average across the shelf of the geostrophic component of the surface current velocity with more confidence than if we relied solely on the satellite measurements. How much confidence? In summary, it depends where you look, as shown below..

Summary maps of skill, for three 6-year periods
	These maps (of which there are three, click the image to expand then click through to see the others) give a nation-wide summary of the skill of the geostrophic velocity at all the locations of the ANMN shelf array of ADCPs. We define skill as 1-MAE/(MM+MO), where MAE is the mean (over a 6-year period) of the absolute value of the (de-meaned) daily error (the vector difference of the two velocity estimates), MM is the mean absolute (de-meaned) model (i.e. the geostrophic velocity at the closest grid point to the ADCP) and MO is the mean absolute (de-meaned) observation (i.e. the ADCP data, averaged between 30m and 80m and over a 50h period). FAQs: 1) Why demean everything? A: Because errors of the mean velocity are determined by the mean absolute topography, not the altimeter or tide gauge observations. 2) why average the ADCP between 30 and 80m? A: because the ADCP data has more error and non-geostrophic velocity signal (surface reflections, Ekman) closer to the surface. We show skill in three categories. For 2009 to 2014, it is only at 22% (12/54) of locations that the skill is more than 0.5. For 2015-2020, however, this rises to 37% (14/38) while in 2021-2025 it is 27% (8/29). The intermediate category of skill (0.4 to 0.5) occurs at about the same number of locations. The next series of maps look at these statistics more closely.

Summary maps of skill, for three 6-year periods

These maps (of which there are three, click the image to expand then click through to see the others) give a nation-wide summary of the skill of the geostrophic velocity at all the locations of the ANMN shelf array of ADCPs. We define skill as 1-MAE/(MM+MO), where MAE is the mean (over a 6-year period) of the absolute value of the (de-meaned) daily error (the vector difference of the two velocity estimates), MM is the mean absolute (de-meaned) model (i.e. the geostrophic velocity at the closest grid point to the ADCP) and MO is the mean absolute (de-meaned) observation (i.e. the ADCP data, averaged between 30m and 80m and over a 50h period). FAQs: 1) Why demean everything? A: Because errors of the mean velocity are determined by the mean absolute topography, not the altimeter or tide gauge observations. 2) why average the ADCP between 30 and 80m? A: because the ADCP data has more error and non-geostrophic velocity signal (surface reflections, Ekman) closer to the surface.

We show skill in three categories. For 2009 to 2014, it is only at 22% (12/54) of locations that the skill is more than 0.5. For 2015-2020, however, this rises to 37% (14/38) while in 2021-2025 it is 27% (8/29). The intermediate category of skill (0.4 to 0.5) occurs at about the same number of locations. The next series of maps look at these statistics more closely.

3-month averages of the whole FishSOOP dataset compared with a historical climatology reveals how the ocean has changed. Click for more depth layers. (The plots also show the data coverage history.) Click image below then step through regions and depth layers to explore the spatial-, temporal- and depth-dependence of the anomalies

Background about altimetry and tide gauges