OceTHE OFFICIALa MAGAZINEn ogOF THE OCEANOGRAPHYra SOCIETYphy CITATION Rudnick, D.L., S. Jan, and C.M. Lee. 2015. A new look at circulation in the western North Pacific. Oceanography 28(4):16–23, http://dx.doi.org/10.5670/oceanog.2015.77. DOI http://dx.doi.org/10.5670/oceanog.2015.77 COPYRIGHT This article has been published in Oceanography, Volume 28, Number 4, a quarterly journal of The Oceanography Society. Copyright 2015 by The Oceanography Society. All rights reserved. USAGE Permission is granted to copy this article for use in teaching and research. Republication, systematic reproduction, or collective redistribution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The Oceanography Society. Send all correspondence to: [email protected] or The Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. DOWNLOADED FROM HTTP://TOS.ORG/OCEANOGRAPHY FROM THE GUEST EDITORS A New Look at Circulation in the Western North Pacific INTRODUCTION TO THE SPECIAL ISSUE By Daniel L. Rudnick, Sen Jan, and Craig M. Lee ABSTRACT. The currents in the low-latitude western North Pacific are critical to the general circulation. The North Equatorial Current flows westward, bifurcating off the coast of the Philippines to form the northward-flowing Kuroshio and the southward- flowing Mindanao Current. Below the thermocline, undercurrents flow opposite to the currents above and include the southward-flowing Luzon Undercurrent, the northward-flowing Mindanao Undercurrent, and an eastward-flowing series of North Equatorial Undercurrents. Two complementary programs, Origins of the Kuroshio and Mindanao Current (OKMC) and Observations of Kuroshio Transport and Variability (OKTV), focused on observations and modeling of these currents. Results address the region’s circulation, water masses, and eddies. The connectivity between the currents as determined by water masses is of special interest. 16 Oceanography | Vol.28, No.4 INTRODUCTION Most of the work reported here is not conceivable for subthermocline cir- The currents off the east coasts of Taiwan derived from two linked programs: culation in the region at the beginning of and the Philippines are of critical impor- Origins of the Kuroshio and Mindanao these studies, but we believe such a frame- tance to the general circulation of the Current (OKMC), sponsored by the work has now been achieved. Establishing Pacific Ocean (Figure 1). The North US Office of Naval Research, and the connectivity of these flows has tra- Equatorial Current (NEC) heads west- Observations of Kuroshio Transport ditionally been achieved through track- ward centered roughly at 13°N, hits the and Variability (OKMC), sponsored by ing water masses defined by extrema in coast of the Philippines, and bifurcates into the Ministry of Science and Technology, water properties. In this region, the dom- the Kuroshio heading northward and the Taiwan (ROC). OKMC focused on inant salinity extrema are the maximum Mindanao Current (MC) heading south- the NEC, the bifurcation region, the near 200 m depth known as North Pacific ward. The Kuroshio starts as a weak and Mindanao Current, and the Kuroshio in Tropical Water (NPTW), and the salinity variable current off the Philippine coast, the environs of Luzon Strait. OKTV con- minimum near 400 m that is the signal of and it strengthens as it passes Luzon Strait centrated efforts on the Kuroshio off the North Pacific Intermediate Water (NPIW). (Lien et al., 2014), sometimes looping coast of Taiwan. Together, the observa- These water masses and their modifica- into the strait. By the time the Kuroshio tion and modeling results of OKMC/ tion through mixing are the second theme progresses northward off the east coast OKTV provide a new view of the region running through many of the articles in of Taiwan, it is a strong and well-defined that is reported in the following pages. this issue. The oldest theories of gyre cir- current, but with marked temporal and Three themes have arisen through the culation, found in the work of Norwegian spatial variations (Jan et al., 2015). In con- course of our investigations in the western oceanographer Harald Sverdrup (1888– trast, the Mindanao Current is strong very Pacific. The first, and perhaps overarch- 1957), suggest that meridional flow driven close to the bifurcation region, apparently ing, theme is circulation. The general out- by the Ekman pumping of wind stress being fed by the relatively steady southern line that the NEC feeds the Kuroshio and curl across the basin must be balanced portion of the NEC. This special issue of MC must be correct, but to flesh out this by the flow in intense western bound- Oceanography is devoted to a new descrip- outline requires knowledge of how flow ary currents. Thus, the control of west- tion of these important currents and their is partitioned in the bifurcation region, ern boundary currents is set by condi- associated water masses. how each boundary current strengthens tions to the east, and this control arrives at Undercurrents in the region are less as it proceeds away from this region, and the western boundary through westward- well observed and understood theoret- whether these processes are predictable in propagating waves and eddies. Mesoscale ically than the currents that extend to models. A broad framework like this was eddies are a dominant feature of the flow the surface. Fundamentally new find- ings concerning the undercurrents have been reported during the past few years 26°N and are described in the articles in this Kuroshio issue. While the existence of the Luzon 24°N Undercurrent beneath the Kuroshio, the Mindanao Undercurrent, and an under- 22°N STCC current beneath the NEC have been sug- gested by observations for some time, 20°N the sustained observations reported here LUC 18°N provide convincing evidence for the per- NEUC sistence of these undercurrents. 16°N 14°N BR NEUC FIGURE 1. Map of the western Pacific, including major currents and fea- tures above (red) and below (blue) the thermocline. The North Equatorial Current (NEC) is a broad westward flow that hits the Philippine coast in 12°N NEC the Bifurcation Region (BR). From the BR, the Mindanao Current (MC) NEUC heads southward. The Kuroshio is difficult to identify close to the BR, but 10°N it strengthens as it proceeds northward. The Subtropical Countercurrent (STCC) is a weak flow in a region dominated by eddies. The Mindanao MUC Eddy is a semi-permanent cyclonic feature off the coast of Mindanao, 8°N Philippines. Subthermocline boundary currents include the Mindanao ME Undercurrent (MUC) heading northward and the Luzon Undercurrent 6°N MC (LUC) heading southward. These boundary currents feed the eastward North Equatorial Undercurrent (NEUC) jets below the NEC. 120°E 125°E 130°E 135°E Oceanography | December 2015 17 field east of the Kuroshio in the region of METHODS 15 m depth, were deployed between 2010 the Subtropical Countercurrent (STCC). A variety of observational platforms were and 2013 on a roughly regular sched- Thus, a third theme has to do with the used in our studies, but there was spe- ule of eight drifters per month. A vari- eddy processes and their effects on the cial emphasis on autonomous platforms ety of moorings were deployed in stra- large-scale circulation. The observant that could collect observations with- tegic locations, including current meters reader may identify the three themes out direct support from a research vessel in Lamon Bay in the bifurcation region, of (1) circulation, (2) water masses, (Figure 2). This autonomous approach to six 75 kHz acoustic Doppler current pro- and (3) eddies repeated throughout regional oceanography distinguishes our filers (ADCPs) and five HPIES (a combi- this special issue. effort from past studies in the region. For nation of horizontal electric field sensors In the following, we summarize high- example, underwater gliders (Rudnick and pressure inverted echo sounders) lights of recent work, including mate- et al., 2004) were used extensively to in the southern Luzon Strait, and three rial covered in the papers to follow. The measure the major currents in the region, ADCPs, five PIESs, one CPIES (a PIES discussion is organized by the dominant including the NEC, the Kuroshio, and with a current sensor), and more than current in the region, starting with the the MC. In total, gliders completed over 18 ship survey cruises in the Kuroshio NEC, followed by the weak currents in 40 missions, with 13,000 dives to as off eastern Taiwan. the bifurcation region. A great deal of our deep as 1,000 m, in 4,700 days. A total The programs had a goal of contribut- effort was spent in the Kuroshio, rang- of 27 profiling floats were deployed, pro- ing to establishing predictability through ing from its passage across Luzon Strait viding profiles to 2,000 m on a five-day state-of-the-art numerical models. A to its powerful presence off Taiwan. We repeat cycle, significantly augmenting global eddying ocean model, the Parallel close with the Mindanao Current, which Argo coverage in a region where floats Ocean Program (POP), was used to pro- is perhaps the least studied of the cur- deployed elsewhere rarely tend to go. A vide context on the longest times and rents in this region. total of 260 surface drifters, drogued at largest length scales. The MITgcm assim- ilating model was used to provide hind- cast state estimates using the array of observations. The intention was to embed these modeling activities throughout the observational program, so there is no separate modeling article in this issue. However, the reader will find modeling results in several of the articles to follow. This combined observational/modeling approach to regional oceanography has proven effective, and we hope it to be an example for the future.