Two-Dimensional Mapping and Tracking of a Coastal Upwelling Front by an Autonomous Underwater Vehicle Yanwu Zhang, James G
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Two-Dimensional Mapping and Tracking of a Coastal Upwelling Front by an Autonomous Underwater Vehicle Yanwu Zhang, James G. Bellingham, John P. Ryan, Brian Kieft, and Michael J. Stanway Abstract—Coastal upwelling is a wind-driven ocean process. on ocean ecosystems. Detection and tracking of an upwelling It brings cooler, saltier, and usually nutrient-rich deep water front is important for investigating the formation and evolution upward to the surface. The boundary between the upwelling of the upwelling process, and enables targeted sampling of the water and the normally stratified water is called the “up- welling front”. Upwelling fronts support enriched phytoplankton different water types across the front. and zooplankton populations, thus having great influences on In our prior work, we developed a method of using an ocean ecosystems. In our prior work, we developed and field autonomous underwater vehicle (AUV) to autonomously iden- demonstrated a method of using an autonomous underwater tify an upwelling front, map the vertical structure across the vehicle (AUV) to autonomously identify an upwelling front, map front, and track the front’s movement [3], [4]. In upwelling the vertical structure across the front, and track the front’s movement on a fixed latitude (i.e., one-dimensional tracking). In water, the vertical temperature difference between shallow and this paper we present an extension of the method for mapping and deep depths is small due to the upwelling process; but in tracking an upwelling front on both latitudinal and longitudinal stratified water, the vertical temperature difference is large dimensions (i.e., two-dimensional) using an AUV. Each time the (warm at surface and cold at depth). Figure 1 illustrates AUV crosses and detects the meandering front, the vehicle makes the distinct vertical temperature structures in stratified and a turn at an oblique angle to recross the front, thus zigzagging through the front to map it. The AUV’s zigzag tracks alternate in upwelling water columns, as well as in the narrow front northward and southward sweeps, so as to track the front as it between them. To characterize the difference between these moves over time. From 29 May to 4 June 2013, the Tethys long- temperature structures, and to differentiate between upwelling range AUV ran the algorithm to autonomously detect and track and stratified water columns, we set up a key classification an upwelling front in Monterey Bay, CA. The AUV repeatedly 2 metric — the vertical temperature difference between shallow mapped the frontal zone over an area of about 200 km in more than five days. and deep depths [5]. Based on this, we subsequently developed an improved metric — the vertical temperature homogeneity Index Terms—Autonomous underwater vehicle (AUV), up- index, defined as follows [4]: welling front, mapping, tracking. 1 1 I. INTRODUCTION ΔTemp = ΣN |Temp − ΣN Temp | vert N i=1 depth i N i=1 depth i Coastal upwelling is a wind-driven ocean process that brings (1) cooler, saltier, and usually nutrient-rich deep water upward, where i is the depth index, and N is the total number replacing warmer, fresher, and nutrient-depleted surface water. of depths included for calculating ΔTempvert. Tempdepth i 1 N The nutrients carried up by upwelling have significant impact is the temperature at the ith depth. N Σi=1Tempdepth i is on primary production and fisheries. When a northwesterly the average temperature of those depths. |Tempdepth i − 1 N wind persists in spring and summer along the California coast- N Σi=1Tempdepth i| measures the difference (absolute value) line, intense upwelling takes place at Point A˜no Nuevo (to the between the temperature at each individual depth and northwest of Monterey Bay, CA), and the upwelling filaments the depth-averaged temperature. The average difference spread southward to the mouth of the bay. The upwelling ΔTempvert (averaged over all participating depths) is a mea- process breaks down stratification and makes water properties sure of the vertical homogeneity of temperature in the water more homogeneous over depth. In northern Monterey Bay, column. ΔTempvert is significantly smaller in upwelling however, the water column typically remains stratified because water than in stratified water. the region is sheltered from upwelling-inducing wind by Suppose an AUV flies from stratified water to upwelling the Santa Cruz mountains, and sheltered from the upwelling water on a sawtooth (i.e., yo-yo) trajectory (in the vertical filaments by the coastal recess. Hence an “upwelling shadow” dimension). Figure 2 illustrates the algorithm for the AUV forms in the northern bay. The boundary between the up- to determine that it has departed from the stratified water welling water and the stratified water is called the “upwelling column and entered the upwelling water column. On each yo- front”. Upwelling fronts support enriched phytoplankton and yo profile (descent or ascent), the AUV records temperature zooplankton populations [1], [2], thus having great influences at the participating depths to calculate ΔTempvert.When ΔTempvert falls below a threshold threshΔTemp for a num- All authors are with the Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039. Email of the corresponding author ber of consecutive yo-yo profiles, the AUV determines that it Yanwu Zhang: [email protected] has entered the upwelling water column. Conversely, suppose 978-0-933957-40-4 ©2013 MTS This is a DRAFT. As such it may not be cited in other works. The citable Proceedings of the Conference will be published in IEEE Xplore shortly after the conclusion of the conference. tracking method from one-dimensional (on a fixed latitude) to two-dimensional (on both latitude and longitude). Upwelling Stratified water Uppgwelling water front II. TWO-DIMENSIONAL FRONT TRACKING ALGORITHM Building upon our previous work, we have developed a method for mapping and tracking an upwelling front on both latitudinal and longitudinal dimensions (i.e., two-dimensional) using an AUV. A key change is that the AUV makes turns at an oblique angle (rather than reversing course) after detecting Depth the front. This way, the vehicle zigzags through the front to map it. The AUV’s zigzag tracks alternate in northward and southward sweeps, so as to track the front as it moves over time. The following steps describe the new algorithm, which Fig. 1. Illustration of different vertical temperature structures in stratified and is illustrated in Figure 3. upwelling water columns, and in the narrow front between them. Temperature from high to low is represented by color ranging from orange to blue (i.e., • The AUV starts the mission, say, from stratified water the temperature decreases with depth). (where ΔTempvert is high), flying westward towards upwelling water (where ΔTempvert is low), on a yo-yo trajectory (in the vertical dimension). When ΔTempvert an AUV flies from upwelling water to stratified water on a yo- falls below threshΔTemp for a number of consecutive yo trajectory. When ΔTempvert rises above threshΔTemp for yo-yo profiles, the AUV determines that it has passed the a number of consecutive yo-yo profiles, the AUV determines front and entered the upwelling water column. that it has entered the stratified water column. To avoid false • The AUV continues flight in the upwelling water for some detection due to measurement noise or existence of isolated distance to sufficiently cover the frontal zone, and then water patches, the algorithm only sets the detection flag when turns 135◦ to fly back to the stratified water. ΔTemp vert meets the threshold for a number of consecutive • On the way back to the stratified water, when ΔTempvert yo-yo profiles. rises above threshΔTemp for a number of consecutive yo-yo profiles, the AUV determines that it has passed the front and entered the stratified water column. ∆Tempvert = ∑ | ∑ | on each yo-yo profile • where 5 , 10 , 20 , 30 The AUV continues flight in the stratified water for If ∆Tempvert(n) ≤ thresh∆Temp ◦ for 5 consecutive yo-yo profiles, some distance, and then turns −135 to fly back to the classify as upwelling water. upwelling water. Stratified water column Upwelling water column • The AUV repeats the above cycle, thus zigzagging through the front. Condition met: • classify as upwelling water. The AUV’s zigzag track is confined to a pre-set “oper- … ational zone” (based on bathymetry and logistical con- thresh ∆Temp siderations). On each single leg of the zigzag track, if the AUV reaches the western or eastern bound without … detecting the front, the vehicle will turn at the bound and begin the next zigzag leg. On a northward-sweeping n-9 n-8 n-7 n-6 n-5 n-4 n-3 n-2 n-1 n Index of yo-yo profile zigzag, when the AUV reaches the northern bound, 5 m the vehicle turns around to start a southward-sweeping 30 m zigzag. Conversely, on a southward-sweeping zigzag, when the AUV reaches the southern bound, the vehicle turns around to start a northward-sweeping zigzag. The Fig. 2. Illustration of the algorithm for an AUV to determine that it has departed from a stratified water column and entered an upwelling water entire mission terminates once the prescribed mission column. duration has elapsed. • As the front moves over time (due to variations of wind and ocean circulation), the AUV tracks it in two In our one-dimensional front tracking algorithm, on each dimensions, thus producing a complete depiction of the east-west AUV transect, the vehicle detects the front, continues dynamic front. flight for some distance (to sufficiently cover the frontal zone), and then reverses course. The AUV repeats this behavior to effectively track the front on a fixed latitude. In multiple III. FIELD PERFORMANCE experiments in Monterey Bay in 2011 and 2012, MBARI’s The Tethys long-range AUV [7], shown in Figure 4, is 2.3 m Tethys AUV and Dorado AUV ran this algorithm to track long and 0.3 m (i.e., 12 inches) in diameter at the midsection.