Barrier layers in the subtropical gyres of the worlds oceans

Kanako Sato,1 Toshio Suga,1,2 and Kimio Hanawa2

Received 28 December 2005; revised 8 March 2006; accepted 22 March 2006; published 21 April 2006.

[1] Barrier layers (BLs) in the subtropical gyres of theworlds oceans are detected using all temperature andsalinity profiles obtained by Argo floats from Jan. 2000 toJun. 2005. The synoptic BLs are distributed patchily in thearea where climatological BLs are continuously distributed.They are thicker and occur more frequently during Jan.Apr. (Jul.Nov.) in the Northern (Southern) Hemispherewith some differences in thickness and occurrence ratesamong the oceans. We suggest that the same formationmechanism of BLs as that in the North Pacific, i.e., thesubduction of high salinity water at sharp salinity fronts on asmall scale of approximately 100 km is at work in the othersubtropical gyres. Seasonal change of the mixed layer depthcontributes to the seasonality of BL thickness and frequencyin all subtropical gyres. Citation: Sato, K., T. Suga, andK. Hanawa (2006), Barrier layers in the subtropical gyres of the

worlds oceans, Geophys. Res. Lett., 33, L08603, doi:10.1029/

2005GL025631.

1. Introduction

[2] Salinity fields in the tropics often have a significantinfluence on the density stratification of the water columnby producing a barrier layer (BL [e.g., Lukas and Lindstrom,1991]). A BL is a layer between the bottoms of the mixedlayer (ML) and of the isothermal layer (IL); this layer iscaused by the strong stratification of salinity, withouttemperature stratification, under the bottom of the ML.Numerous previous studies have examined the BL and itsformation mechanism in the tropics of all over the world[e.g., Sprintall and Tomczak, 1992]. In addition, the relationbetween the BL and air-sea interaction events, i.e., IndianOcean dipole events and El Nino events has been recentlyindicated [e.g., Masson et al., 2004; Vialard and Delecluse,1998]; for example, in the equatorial Pacific, BL plays animportant role in the development of El Nino [Vialard andDelecluse, 1998].[3] While less attention has been paid to BLs in the

subtropical gyres compared to those in equatorial regions,Sprintall and Tomczak [1992] reported that a wide, contin-uous BL region as thick as 25 m appeared in the subtropicalgyres of the winter hemisphere, and argued that subductionof high salinity water must be responsible for the BLformation, based on climatology data. Recently, using theArgo profiling float data, Sato et al. [2004] showed that theBLs in the North Pacific subtropical gyre are patchy andmuch thicker in the synoptic field compared with those

determined by climatology, and that BL formation occurson a timescale as short as 10 days. They further suggestedthat subduction of high salinity water on a small spatialscale (O(100 km)) is responsible for the rapid formation ofpatchy BLs. Their results complement the argument ofSprintall and Tomczak [1992] by depicting the process onsmaller space and time scales, which the climatology didnot resolve. These findings raise the question as to whethersimilar synoptic features and formation mechanism arepresent in the subtropical BLs in other oceans. One ofthe purposes of this study is to analyze the Argo data overthe worlds oceans to address this question.[4] While the BLs in the North Pacific subtropical gyre

are detected year-round based on the Argo data, BL occur-rence rate and thickness are higher and greater, respectively,from Jan. to May [Sato et al., 2004]. Causes of thisseasonality have not been explored yet. Another purposeof this study is to elucidate the factors controlling theseasonal changes of the thickness and frequency of BLsin the North Pacific subtropical gyre and also in othersubtropical gyres.

2. Data and Methods

[5] The real-time quality controlled and the delayedmode data of Argo profiling floats deployed in the subtrop-ical and tropical (30S30N) oceans worldwide from Jan.2000 to Jun. 2005 are obtained from the United StatesGlobal Ocean Data Assimilation Experiment (USGODAE)Argo Global Data Assembly Center (GDAC). We use43,402 profiles containing both temperature and salinitydata of which quality flags in all the measured levels arecoded as 1 (i.e., good data), and satisfying the con-ditions that the measured depth nearest the sea surface isshallower than 10 dbar and that the vertical samplinginterval from the sea surface to 100 dbar is less than 15 dbar.Each profile of temperature and salinity is interpolatedvertically on 10-m grids using Akimas spline [Akima,1970]. The vertical resolution of most of original profilesis 10 m in the top 200 m, which is the most relevant depthrange for the BL.[6] The BL features captured by the Argo data are

compared to those from the seasonal climatology of WorldOcean Atlas 2001 (WOA01 [Boyer et al., 2002; Stephens etal., 2002]). Climatological profiles of temperature andsalinity at each 1 1 grid are also interpolated verticallyon 10-m grids using Akimas spline, although the verticalgrid spacing in WOA01 is 2050 m in the top 200 m.[7] The BL thickness is defined as the difference between

the IL and ML depths. The definitions of ML and IL depths,and the BL thickness used here are the same as those givenby Sprintall and Tomczak [1992]. The IL depth is defined asthe depth where the temperature is equal to the sea surfacetemperature (SST) minus 0.5C. The ML depth is calculatedas the depth where the density is equal to the sea surface

GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L08603, doi:10.1029/2005GL025631, 2006

1Institute of Observational Research for Global Change, Japan Agencyfor Marine-Earth Science and Technology, Yokosuka, Japan.

2Department of Geophysics, Graduate School of Science, TohokuUniversity, Sendai, Japan.

Copyright 2006 by the American Geophysical Union.0094-8276/06/2005GL025631$05.00

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density (SSD) plus the increment in density equivalent to atemperature decrease of 0.5C with salinity fixed at the seasurface salinity (SSS). In this study, the temperature, salinityand density at 10-m depth are regarded as the SST, SSS andSSD, respectively, in order to avoid possible effects of thediurnal heating cycle and short-duration rainfall events.

3. Results

[8] Following Sato et al. [2004], BLs thicker than 10 mare considered as real features in accord with the verticalresolution of profiles observed by the Argo floats. BLs thusdefined appear all over the tropical and subtropical oceans(Figure 1). Of all the profiles used in this study, 20.75 %have BLs. BLs with over 40-m thickness are detected by theArgo data in regions where the climatological BL is alsorelatively thick. These thick BLs appear both during Jan.Mar. and during Jul.Sep. in the western equatorial Pacificand Atlantic, and north of 5S in the Indian Ocean. On theother hand, the thick BLs in the subtropical gyres occur

almost exclusively in the winter hemisphere, i.e., in theNorth Pacific (1030N, 120E120W) and North At-lantic (1030N, 9020W) during Jan.Mar., and in theSouth Pacific (205S, 14090W), South Atlantic(155S, 40W0), and South Indian Ocean (3010S, 50120E) during Jul.Sep. It is obviously recog-nized that the synoptic features of the BLs observed in theNorth Pacific subtropical gyre [Sato et al., 2004] arecommon to those of the BLs in all subtropical gyres; theyare thicker than the climatological BL, occur all the yearround, and show patchy distribution (Figures 1 and 2).[9] While the BLs in all the subtropical gyres have

common synoptic features qualitatively, they show quanti-tative differences among oceans (Figure 2). BLs in theNorth and South Pacific had the greatest maximum thick-ness, exceeding 100 m. The relative frequency of the BLs inthe South Indian Ocean reaches 42 % in July, which is thehighest monthly rate among all the subtropical gyres. Thetotal occurrence rate for the entire analysis period is highestin the North Atlantic, with the smallest seasonal change in

Figure 1. Seasonal distribution of profile data obtained by Argo floats from Jan. 2000 to Jun. 2005: (a) Jan.Mar. and(b) Jul.Sep. The BL thickness at each profile is denoted by color symbols. Background shadings denote the BL thicknesscalculated from the corresponding seasonal WOA01.

Figure 2. Seasonal changes of the BL thickness in the subtropical gyres of the worlds oceans in the Argo float data:(a) North Pacific, (b) North Atlantic, (c) South Pacific, (d) South Atlantic, and (e) South Indian Ocean. Background bargraphs denote the relative frequency of profiles with the BL exceeding 10 m to the total profiles of Argo float data in eachregion and each month. The numbers at the right-hand edge denote the rate of BLs in all profiles from Jan. 2000 to Jun.2005 in each region.

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the occurrence rate. Typical thickness and relative frequencyof the BLs in the South Atlantic are lowest among all thesubtropical gyres. In spite of these differences, the generalseasonality of greater thickness and higher occurrence ratein winter common to all the subtropical gyres is still clearlydiscernible.[10] Now, we examine the BL formation mechanism in

the subtropical gyres other than the North Pacific. To beginwith, we got a result that the local freshening of the surfaceassociated with precipitation is not a main cause of the thicksubtropical BL in winter, because evaporation dominatesprecipitation in all areas of the BL distribution in winter (notshown), by analyzing the freshwater flux calculated fromthe latent heat flux and precipitation of NCEP/NCARreanalysis data [Kalnay et al., 1996].[11] According to the WOA01 climatology, the subtrop-

ical BL in each ocean meets the same three conditions asthose in the North Pacific: (1) the BL is located beneath theSSS front between regions with low SSS near the equatorand with high SSS in the central part of each subtropicalgyre (Figure 3), (2) stable stratification of salinity tends tobe induced by the subducted saline water which is referredas, for example, the South Pacific tropical water [e.g.,Tsuchiya, 1968], when it crosses the SSS front as indicatedby the near-surface flow (dynamic height at 50 dbar) and theSSS contours (Figure 3), and in contrast, (3) the relativelylow horizontal gradient of SST is unlikely to yield substan-tial temperature stratification associated with the subduc-tion. These facts are qualitatively consistent with the ideasuggested by Sprintall and Tomczak [1992]. However, thisis an unsatisfactory scenario because the contribution ofsalinity variation along the flow to density is equivalent tothat of temperature variation and not large enough toproduce a BL in the climatology. On the other hand, wefound in several hydrographic sections in winter that theSSS fronts can be much stronger in the synoptic field thanin the climatology: salinity change across the SSS fronts inthe North Atlantic and South Pacific has about twice orthree times as large effect on density change as temperaturechange (not shown) in the same way as Sato et al. [2004]demonstrated in the North Pacific. Besides, these SSS frontshave a spatial scale of about 100 km. Therefore, we inferthat the subduction of high salinity water associated with asharp SSS front is responsible for the BL formation in thesubtropical gyres of the worlds oceans as in the NorthPacific. The patchy distribution (Figure 1) and the rapid

occurrence of the BLs captured by quasi-Lagrangian timeseries by the Argo floats (not shown) as in the North Pacificsupport this inference.[12] Note that an extremely thick BL is found near 10

15N, 6050W in the North Atlantic in the climatology(Figure 3). The cause of this BL is presumably very lowSSS yielded by a large amount of river discharge and theadvection of freshwater associated with such dischargesalong the northwestward flow [e.g., Masson and Delecluse,2001]. The formation mechanism inferred above is notapplicable to this exceptional BL.[13] In summer, precipitation dominates evaporation in

the part of the region where the subtropical BLs appear (notshown) and thus may be one of the causes of the BLs. Evenunder this more favorable condition for BL formation insummer, BLs in the subtropical gyres show smaller thick-ness and lower occurrence rate in summer than in winter, incontrast to the BLs without seasonality in the equatorialPacific and Atlantic. The cause of this seasonality is soughtbased on the relationship between BLs and ILs as follows.[14] In the nature of BLs, BLs can get thicker when ILs

are deeper, as demonstrated for the North Pacific as anexample (Figures 4a and 4b). Note that all IL depth-BLthickness pairs are plotted even when the BLs are thinnerthan 10 m and that ILs without significant BLs simply mean

Figure 3. Wintertime climatological maps of sea surface salinity (solid lines) and dynamic height at 50 dbar (dashed lines)based on WOA01. All figures are superimposed on the climatological BL thickness (shading) for reference. Contourintervals of salinity and dynamic height are 0.2 psu and 1 m2/s2, respectively.

Figure 4. (top panels) Scattering diagrams of the IL depthversus BL thickness in the subtropical North Pacific during(a) Jan.Apr. and (b) Jun.Sep. (bottom panels) Relativefrequency (bar) of profiles with the BLs exceeding 10 m tothe profiles in each IL depth class during (c) Jan.Apr. and(d) Jun.Sep., superimposed over the relative frequency(line) of profiles in each IL depth class to the total profiles.

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MLs. Due to larger negative buoyancy forcing in winter(Jan.Apr.), MLs or ILs are generally deeper in winter (seeline with dots in Figures 4c and 4d) and allow thicker BLsto develop, which accounts for the thicker BLs in winter.[15] The relative frequency of BLs exceeding 10 m for

each class of IL depth (see bars in Figures 4c and 4d)indicates that this occurrence rate of BLs is relatively lowand similar for winter and summer in classes with IL depthsless than 60 m, where most of the summer IL depth ispopulated (Figure 4d). On the other hand, the occurrencerate of BLs is relatively high in the IL depth classes greaterthan 60 m, where the majority of winter IL depth ispopulated (Figure 4c). Assuming that the IL depth afterBL formation is not very different from the ML depthbefore BL formation, this statistics implies that MLs deeperthan 60 m are more preferable for BL formation thanshallower MLs, and that the higher occurrence rate ofBLs in winter is associated with the higher occurrence ofdeeper MLs. The same relationships between BL thicknessand IL depth are found in the other subtropical gyres.Therefore, the seasonal change of the ML depth presumablycontributes to that of the thickness and relative frequency ofBLs in all subtropical gyres.

4. Concluding Remarks

[16] Analysis of the Argo data from Jan. 2000 to Jun.2005 demonstrated that the synoptic BLs in the subtropicalgyres of the worlds oceans are thicker than the climatolog-ical BL, occur all the year round, and show patchy distri-bution; these features are similar to those found in the NorthPacific by Sato et al. [2004]. Since the synoptic thick BLsappear in the areas where the continuous climatological BLis thick, the climatological BL is to be interpreted as aspatially and temporally smoothed picture of the synopticBLs. It is worth to note that BL in the recent less smoothedclimatology [de Boyer Montegut et al., 2004] shows mod-erately patchy distribution, supporting the present result.Analysis of freshwater flux, WOA01 data, and synopticsections indicates that large-scale subduction process ofhigh salinity water alone is hardly responsible for the BLformation, and suggests importance of the smaller scalesubduction process (O(100 km)) for BL formation in allsubtropical gyres as in the North Pacific. It is also suggestedthat the seasonality of the thickness and relative frequencyof the BLs common to the subtropical gyres of the worldsoceans are attributable to seasonal change in the ML depth.[17] According to this study and Sato et al. [2004], a SSS

front much sharper than the climatological front and thesubduction across this front are essential for BL formationin the subtropical gyres. Since the regions of synoptic BLdistribution include regions with large eddy kinetic energy,especially in the North Pacific and the Indian Ocean [Ducet

et al., 2000], mesoscale variations such as mesoscale eddiespossibly relate to the BL formation process. The Argo floatsmeasure mesoscale features but do not resolve them. Inorder to quantitatively clarify the BL formation process ineach subtropical gyre of the worlds oceans, analyses ofoutput from eddy-resolving ocean general circulation mod-els would be effective, which is underway as a separatestudy. It will be also interesting to examine how BLs in eachsubtropical gyre affect the mixed layer processes and theair-sea interactions.

[18] Acknowledgments. We thank the members of the Japan Agencyfor Marine-Earth Science and Technology (JAMSTEC) Argo group and thePhysical Oceanography Group, Tohoku University for their comments anddiscussion. The Argo float data used in this study were collected and madefreely available by the International Argo Project and the national programsthat contribute to it (http://www.argo.net).

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