Variability of Oceanic Barrier Layer and Compensated Layer

6 Observed subseasonal variability of oceanic barrier and compensated

7 layers

10 Hailong Liu, Semyon A. Grodsky, and James A. Carton 11

13 Submitted to Journal of Climate 14 December 10, 2008 15 16 17 18 19 20 21 22 23 24 25 26 27Department of Atmospheric and Oceanic Science 28University of Maryland, College Park, MD 20742 29 30 31Corresponding author: Semyon Grodsky ([email protected])

1 32Abstract 33 34A monthly gridded analysis of barrier layer/compensated layer thickness covering the period

351960-2007 is explored for evidence of subseasonal variability and the causes of these

36components of mixed layer variability. The strongest variability from monthly climatology is

37found in the subpolar latitudes of the North Atlantic in winter, where it reaches 100m. A

38temperature-salinity compensated layer in the eastern North Atlantic varies interannually in

39conjunction with a North Atlantic Oscillation-like pattern of anomalous sea level pressure. Thus

40thickening/thinning of this layer occurs in conjunction with the meteorological changes

41associated with strengthening/weakening of the Azores sea level high pressure center.

43In the winter subpolar North Pacific a salinity stratified barrier layer exists. While further south

44along the Kuroshio extension a compensated layer exists. The thickness of both of these layers

45varies from year to year by up to 60m. A significant component of this variability is a long-term

46trend of shrinkage of the barrier layer in the subpolar gyre and growth of the compensated layer

47along the Kuroshio Extension. This trend is shown to result from trends in meteorological

48variables. In the subpolar gyre the barrier layer has shrunk in response to a strengthening of the

49Aleutian low pressure system, the resulting strengthening of dry northerly winds, and a decrease

50of precipitation. In the Kuroshio Extension the compensated layer thickness has increased in

51response to this pressure system strengthening and related amplification of the midlatitude

52westerly winds, the resulting increase of net surface heat loss, and its effect on the temperature

53and salinity of the upper ocean water masses.

55The tropical Pacific and Atlantic both have barrier layers with variability that is less than 20m

2 2 56but is still important in influencing mixed layer properties. In the tropical Pacific west of 160oE,

57the barrier layer thickness changes by approximately 5m in response to 10 unit change in the

58South Oscillation Index. It thickens during La Ninas as a result of the abundant rainfall and thins

59during dry El Ninos. But east of 160oE variations in zonal advection of salt becomes as important

60as local rainfall in influencing the thickness of the barrier layer.

3 3 611. Introduction

62The ocean mixed layer is a nearsurface layer of fluid with quasi-uniform properties such as

63temperature, salinity, and density. The thickness of this mixed layer and its time rate of change

64both strongly influence the ocean’s role in air-sea interaction. However, the thickness of the

65nearsurface layer of quasi-uniform temperature, MLT, may differ from the thickness of the

66nearsurface layer of quasi-uniform density, MLD. MLT may be deeper than MLD when positive

67salinity stratification forms a barrier layer (BL=MLT-MLD) isolating the shallower and deeper

68levels of the mixed layer as was originally found in the western equatorial Pacific (Lukas and

69Lindstrom, 1991). Elsewhere MLT may be shallower than MLD when negative salinity

70stratification compensates for positive temperature stratification (or the reverse situation) to form

71a Compensated Layer (CL=MLD-MLT) (Stommel and Fedorov, 1967; Weller and Plueddemann,

721996). Changes in the seasonal thicknesses of BLs and CLs from one year to the next may cause

73corresponding changes in the role of the mixed layer in air-sea interaction by altering the

74effective depth of the mixed layer or the temperature of water at the mixed layer base (e.g., Ando

75and McPhaden, 1997). Here we examine the global historical profile data set covering the period

761960-2007 for corresponding year-to-year changes in the BL/CL thickness distribution.

78Four studies; Sprintall and Tomczak (1992), Tomczak and Godfrey (1994), de Boyer Montegut et

79al. (2007), and Mignot et al. (2007); have provided an observational description of the seasonal

80cycle of BL/CL distribution over much of the global ocean. BLs are a persistent feature of the

81tropics as well as high latitudes during winter. In high latitudes BLs occur where freshening in

82the near-surface is produced by excess precipitation over evaporation, river discharge, or ice

83melting (de Boyer Montegut et al., 2007) and may be most evident in regions where upward

4 4 84Ekman pumping ( wEk ) acts against the effects of vertical mixing such as occurs in the subpolar

85gyre (Kara et al., 2000). BLs are also observed equatorward of the salty subtropical gyres where

86subduction leads to a shallow subsurface salinity maximum (Sato et al., 2006). The North Pacific

87provides an example of this, the result of southward subduction of salty North Pacific

88Subtropical Mode Water below fresher tropical surface water (as originally suggested by

89Sprintall and Tomczak, 1992). BLs also occur in the Southern Ocean south of the Polar Front as a

90result of near surface freshening and weak thermal stratification (e.g. de Boyer Montegut et al.,

912004).

93BLs also occur seasonally in the tropics in regions of high rainfall and river discharge such as the

94Arabian Sea and Bay of Bengal, where layers as thick as 20-60m have been observed (Thadathil

95et al., 2008). Similarly, BLs occur in the western Equatorial Pacific under the high precipitation

96regions of the Intertropical Convergence Zone and South Pacific Convergence Zone (Lukas and

97Lindstrom, 1991) and in the western tropical Atlantic (Pailler et al., 1999; Ffield, 2007).

99Much less is known about subseasonal variations in BL/CL. In their examination of mooring

100time series Ando and McPhaden (1997) show that BLs do have interannual variability in the

101central and eastern Pacific, and conclude that the major driver is precipitation variability

102associated with El Nino. At 0oN 140oW, for example, the BL thickness increased from 10m to

10340m in response to the enhanced rains of the 1982-3 El Nino. In contrast, Mignot et al. (2007)

104suggest that changes in zonal advection of high salinity water in response to El Nino winds are

105important in regulating BLs in midbasin. In the west an observational study by Cronin and

106McPhaden (2002) is able to document the response of the mixed layer to intense westerly wind

5 5 107bursts and accompanying precipitation and show how these lead to both the formation and

108erosion of BLs.

110CLs in contrast may result from excess evaporation over precipitation, such as occurs in the

111subtropical gyres, or by differential advection where it leads to cooler fresher surface water

112overlying warmer saltier subsurface water (Yeager and Large, 2007). de Boyer Montegut et al.

113(2004) summarize several additional possible mechanisms of CL formation, such as subduction-

114induced advection, Ekman transport, slantwise convection and density adjustment. CLs are most

115prominent in the eastern subpolar North Atlantic and in the Southern Ocean (de Boyer Montegut

116et al., 2007). In the eastern North Atlantic a CL is formed by transport of the warm and salty

117North Atlantic Current above fresher colder subpolar water. Further east the North Atlantic

118Current splits into a northern branch comprising the Norwegian and Irminger Currents, and the

119southward Canary Current, all of which also develop CLs.

121In this study we build on previous observational examinations of the seasonal cycle of BL/CL

122development, to explore year-to-year variability. This study is made possible by the extensive 7.9

123million hydrographic profile data set contained in the World Ocean Database 2005 (Boyer et al.,

1242006) supplemented by an additional 0.4 million profiles collected as part of the Argo observing

125program. We focus our attention primarily on the Northern Hemisphere because of its higher

126concentration of historical observations.

1282. Data and methods

129This study is based on the combined set of temperature and salinity vertical profiles archived in

6 6 130the World Ocean Database 2005 for the period 1960-2004 and Argo floats from 1997 to 2007.

131The first dataset was adopted from the study of variability of the oceanic mixed layer by Carton

132et al. (2008) where issues of data quality control and processing are detailed.

134Mixed layer depth is defined here following Carton et al. (2008) which in turn combines the

135approaches of Kara et al. (2000) and de Boyer Montegut et al. (2004). The mixed layer depth is

136the depth at which the change in temperature or density from its value at the reference depth of

13710m exceeds a specified value (for temperature: T | 0.20 C |). This reference depth is

138sufficiently deep to avoid aliasing by the diurnal signal, but shallow enough to give a reasonable

139approximation of monthly SST. The value of T | 0.20 C | is chosen following de Boyer

140Montegut et al. (2004) as a compromise between the need to account for the accuracy of mixed

141layer depth retrievals and the need to avoid sensitivity of the results to measurement error. The

142absolute temperature difference instead of the negative temperature difference is used following

143Kara et al. (2000) in order to accommodate for temperature inversions that are widespread at

144high latitudes. The specified change in density used to define the density-based mixed layer

145depth follows the variable density criterion (e.g. Monterey and Levitus, 1997) to be locally

146compatible with the specified temperature value, (i.e.   ( / T )  0.20 C ). In this study the

147thickness (or width) of either a barrier layer or compensated layer is defined as a difference of

148isothermal mixed layer depth and isopycnal mixed layer depth, MLT-MLD. As a result of these

149definitions a positive MLT-MLD difference indicates the presence of a BL while a negative

150difference indicates the presence of a CL. We compute BL/CL thicknesses for each profile which

151are then passed through a subjective quality control to eliminate outliers and binned into 2o×2o×1

152month bins without attempt to fill in empty bins.

7 7 153

154In order to quantify the relative impact of temperature and salinity stratification within BLs and

155CLs we introduce a bulk Turner Angle, defined following Yeager and Large (2007) as:

1 1 1 156Tub  tan [(T  S) /(T  S)], where     / T and     / S are the

157expansion coefficients for temperature, T , and salinity, S . In this study the changes in

158temperature and salinity T and S are defined between the top, zt  min(MLT, MLD) , and

159the bottom, zb  max(MLT, MLD) , of either a BL or CL based on analysis of individual vertical

160profiles. The bulk Turner angle is then evaluated from spatially binned values of T and S .

161Fig. 1 shows the correspondence between values of T , S , Tub , and the presence of BLs and

o 162CLs. Angles | Tub |<45 correspond to BLs stabilized by both, temperature and salinity ( T  0 ,

o 163 S <0). A perfect BL stabilized by salinity and homogeneous in T corresponds to Tub =-45 ,

o o 164while Tub =45 corresponds to a pure thermal stratification. Angles greater than 45 correspond to

165the most frequently occurring CLs where positive temperature stratification compensates for

166negative salinity stratification (mixed layer is saltier than the thermocline). Less frequently

o 167occurring CLs are observed at high latitudes below cool and fresh mixed layers (-90

o 168-72 ). The transition point is associated with the density ratio R  T / S  0.5 or

1 0 169Tub  tan (3)  72 . Thus bulk Turner angle provides an alternate way of displaying BL/CL

170distribution.

8 8 172We explore the role that surface forcing plays in regulating mixed layer properties through

173comparison of the BL/CL distribution to fluxes from the NCEP-NCAR reanalysis of Kalnay et

174al. (1996). Satellite QuikSCAT scatterometer winds (see Liu, 2002), which begin in mid-1999,

175are used to characterize the finer scale spatial patterns of wEk . To better characterize precipitation

176in the tropics, we also examine the Climate Prediction Center Merged Analysis of Precipitation

177(CMAP) of Xie and Arkin (1997), which covers the period 1979 -present.

179Limitations in the historical observational coverage limit our ability to explore the mechanisms

180giving rise to subseasonal BL/CL variability. In order to address these mechanisms we repeat our

181analysis on output from a 140-year long control simulation by the NOAA/Geophysical Fluid

182Dynamics Laboratory CM2.1 global coupled climate model forced with natural and

183anthroprogenic forcings beginning in year 1860 (after a 220 year spin-up period, Delworth et al.,

1842006). This coupled simulation maintains full freshwater and heat flux balances at the air-sea

185interfaces. The ocean model component is based on Modular Ocean Model V4 numerics with

18610m vertical resolution in the upper 220 m and including vertical mixing with mixing rates

187determined based on the K-profile parameterization (KPP) mixed layer scheme of Large et al.

188(1994). Comparison of the simulated and observed mixed layer shows encouraging

189correspondence in both the geographic position and seasonal timing of BL/CL variability.

1913. Results

192We begin with a brief review of features of the mean and seasonal distribution.

1943.1. Time mean and seasonal patterns Global seasonal patterns of BL/CL display many features

9 9 195revealed by previous analyses (de Boyer Montegut et al., 2007). Throughout the year there are

196persistent BLs in the tropics in areas of high precipitation (Figs. 2a, 2b). BLs are thick under the

197Intertropical Convergence Zone and the South Pacific Convergence Zone. BLs are generally

198thickest in the western side of the tropical Pacific and Atlantic Oceans reflecting higher levels of

199rain as well as (in the case of the Atlantic) Amazon river discharge. BLs are thickest on the

200eastern side of the tropical Indian Ocean due to the presence of the Java and Sumatra high

201precipitation area (Qu and Meyers, 2005). Rainfall in the southern Intertropical Convergence

202Zone (Grodsky and Carton, 2003) may contribute to freshening of the mixed layer along 10oS in

203the South Atlantic during austral winter. In midlatitude BL/CLs occur mainly in winter and early

204spring. In boreal winter BLs exceeding 60 m are observed in the North Pacific subpolar gyre

205(Fig. 2a). Similarly thick BLs occur in the Atlantic Ocean north of the Gulf Stream. In both

206locations the BLs appear coincident with a seasonal increase of precipitation.

208Sea surface salinity (SSS) increases drastically across the Gulf Stream front leading to a switch

209from the BL regime north of the front to a CL regime south of the front (Fig. 2a). Thick CLs

210(thicker than 30m) are also observed along the Gulf Stream due to cross-frontal transport of low

211salinity water. And even thicker CLs (thicker than 60m) are observed further northeast along the

212path of the North Atlantic Current where its warm, salty water overlies cooler, fresher water.

213Interestingly, despite the presence of warm and salt western boundary currents in both the

214Atlantic and Pacific Oceans, the winter CLs are much less pronounced in the North Pacific than

215the North Atlantic. Explanation for the basin-to-basin difference likely lies in the higher surface

216salinity of the Atlantic (Fig. 2a) and consequently larger values of S (Fig. 3a).

10 10 218CLs are also evident in the southern subtropical gyres of the Pacific and Atlantic Oceans as well

219as the South Indian and Southwest Pacific Oceans (Fig. 2b) south of the 30oS SSS. The presence

220of CLs in this region reflects the northward advection of cold and fresh water which subducts

221under the water of the SSS maximum (Sprintall and Tomczak, 1993; note correspondence

222between CL in Fig. 2b and S in Fig. 3b).

224A similar mechanism is used to explain BL formation in subtropical gyres (e.g. Sato et al., 2006).

225In the southern Indian Ocean BLs north of 30oS form as salty water from the region of the SSS

226maximum subducts northward under relatively fresh surface water (Fig. 2b). Similar meridional

227dipole-like patterns with CLs to the south and BLs to the north of local subtropical SSS maxima

228are seen during austral winter in the South Pacific and the South Atlantic in the regions of

229downward wEk (Fig. 2b). In the Northern Hemisphere the areas of CLs in the North Pacific and

230Atlantic Oceans extend well north of the downward wEk regions (Fig. 2a) suggesting that

231horizontal transport of warm salt waters by the western boundary currents plays a role.

233Spatial patterns of BL/CL (Fig. 2) are in close correspondence with the spatial patterns of the

234vertical changes of salinity, S , (Figs. 3a, 3b). As expected, the barrier layers are distinguished

235by a stable salinity stratification, S  0 , where salinity increases downward below the mixed

236layer. In contrast, compensated layers have unstable salinity stratification, S  0 . As discussed

237above, regions of fresh mixed layer trace major areas of precipitation (like the Intertropical

238Convergence Zone) and river runoff (the Bay of Bengal). Different type of BL is observed on the

239equatorward flanks of subtropical salinity maxima. In these areas the ocean accumulates salt due

240to an excess of evaporation over precipitation. Here the equatorward propagation of subducted

11 11 241water produces meridional dipole-like BL/CL and S structures that are most pronounced

242during local winter in the Southern Hemisphere (Figs. 2b, 3b). Here barrier layers are observed

243to the north of SSS maxima where mixed layer tops saltier water below while compensated

244layers are observed to the south where mixed layer is saltier than thermocline.

246The spatial patterns of the bulk Turner angle (Figs. 3e,f) indicate that majority of CL cases are

o 247associated with warm, salty water overlaying colder, fresher water (thus Tub >45 ). CLs widen

248during winter leading to higher values of Tub in winter than summer. When cold, fresh water

o 249overlays warmer, saltier water a CL can also be formed where Tub <-72 . This latter type of

250density compensation is observed only in limited regions of the Labrador Sea and near Antarctica

251during winter.

2533.2 Subseasonal variability Subseasonal variability of BL/CL thickness is similar in amplitude to

254seasonal variability (compare Fig. 4 and Fig. 2). Subseasonal variability is stronger at higher

255latitudes reflecting weaker temperature stratification. The highest variability of BL/CL thickness

256of up to 100m occurs in the North Atlantic. It is also high in the western Atlantic and Pacific due

257to the impact of subseasonal variations in precipitation and river discharge.

259We next consider BL/CL thickness separated by season and roughly 15-year averaging periods

260(Fig. 5). During the first period 1960-1975 thick BLs are evident during local winter in the North

261Pacific, tropical western Pacific and western Atlantic, northern Indian Ocean, and Southern

262Ocean (the latter being evident even in austral summer). CLs during this early period appear

263primarily in the eastern North Atlantic. Little can be said about the existence of BLs in the

12 12 264Southern Ocean in austral winter due to the lack of data during this period. By the last period,

2651991-2007 several changes are evident. CLs have appeared in the subtropical North Pacific

266during winter and have strengthened in the eastern North Atlantic. Strong CLs are also evident

267on the northern side of the Circumpolar Current during austral winter (in fact these may have

268existed earlier but simply not been observed). By this last period the extensive winter BLs noted

269in the North Pacific in earlier decades had declined. We next examine monthly time series

270describing four regions: (1) BLs in the subpolar North Pacific, NP/BL box (2) CLs in the

271subtropical North Pacific, NP/CL box; (3) CLs in the eastern North Atlantic; and (4) BLs in the

272western equatorial Pacific (the regions outlined in Fig. 5c).

274The monthly time series of the northern subpolar North Pacific BL region and the subtropical CL

275region both show a long-term trend towards thinner BLs and thicker CLs interrupted by

276occasional reversals (Figs. 6a, 6b). Indeed, the subtropical CL region actually supported a 10-

27720m thick BL prior to 1980s. One direct cause of this change from BL to CL seems to be the

278gradual deepening of the late winter-spring mixed layer in the central North Pacific noted by

279Polovina et al. (1995) and Carton et al. (2008). This observed 20 m deepening into the cooler,

280fresher sub-mixed layer water has the effect of strengthening density compensation (the ‘spice

281injection’ mechanism is discussed by Yeager and Large, 2007). Carton et al. (2008) attribute the

282cause of mixed layer deepening to changes in the atmospheric forcing associated with the

283deepening of the Aleutian sea level pressure low after 1976. These changes led to strengthening

284of the midlatitude westerlies and the ocean surface heat loss in the North Pacific, hence the

285deepening of the mixed layer. The deepening of the mixed layer has opposite impacts on the

286width of CLs and BLs. Deepening widens CLs by injecting saltier water from the mixed layer

13 13 287into fresher thermocline. In contrast, stronger atmospheric forcing normally destroys near-

288surface BLs by enhancing mixing. These mechanisms likely explain the narrowing of BLs and

289the widening of CLs in the North Pacific during recent decades (Figs. 6a, 6b).

291The BL and CL thicknesses in the NP/BL and NP/CL boxes both seem to be associated with

292wind changes resulting from changes in the Aleutian pressure low (Fig. 7). Widening of CLs

293(that corresponds to the inverse of regression patterns shown in Fig. 7a) is linked to

294strengthening of westerly winds and latent heat loss (hence, deeper mixed layers) in the NP/CL

295box. Winds also strengthen in the NP/BL box to the north. This wind strengthening may reduce

296the BL due to enhanced mixing. But, in addition the strengthening dry northerly winds to the east

297of the low that decreases moisture transport from the south and thus reduces precipitation that is

298vital to maintain the BL (Fig. 7b). Limitations in the historical data record limit the extent to

299which these relationships can be identified based on data analysis. To overcome this data

300limitation we examine the processes regulating the mixed layer of the CM2.1 coupled model.

302We begin by comparing the simulated and observed winter/spring BL/CL distributions. In the

303simulation as in the observations a BL forms in the North Pacific north of approximately 35oN

304and a CL south of the BL region (compare Fig. 2a, Fig. 8a). The southern CL region can be

305divided into two zones. The first CL zone lies in the Kuroshio-Oyashio confluence at

306approximately 40oN in the northwest. The second CL zone extends across the North Pacific

307along approximately 30oN. This latter zone corresponds to the NP/CL region previously defined

308and is collocated with the subtropical SSS maximum and is bounded to the north by the northern

309edge of the downward wEk area (Fig. 8b). Subduction to the north of the subtropical SSS

14 14 310maximum produces negative salinity stratification in the region of SSS maximum while

311subduction at the SSS maximum produces positive salinity stratification to the south (see

312examples in Fig. 9).

314We next explore year to year changes in the NP/CL region by contrasting March 1953 when the

315simulated CL is the thickest of the 140 year record (25m) with March 1956 when the CL has

316been replaced by a 10m thick BL (Fig. 8c). The striking difference between these two years

317seems to be the different surface forcing. In 1953 downward wEk extends throughout the

318subtropics leading to a deep mixed layer (Fig. 9a). During this year the isohaline contours along

319180oE indicate an exaggeration of the normal situation in which there is southward advection of

320high salinity water below the fresh nearsurface layer. These conditions lead to a CL layer north

o o 321of 25 N and BL layer to the south. In contrast, in 1956 wEk was upward north of 28 N trapping

322the freshwater introduced through precipitation in the surface layer and thus causing a shallow

323BL to form. The lag correlations in Fig. 8d confirm that CLs in the NP/CL region thicken in

324years when the mixed layer is anomalously deep, in line with the spice injection mechanism of

325Yeager and Large (2007), and BLs in the NP/BL thin as a result of the enhanced mixing

326associated with deep mixed layers. Thus deepening of the subtropical mixed layer may explain

327the trends observed in Figs. 6a, 6b.

329Correspondence between temporal changes of BL/CL width and atmospheric pressure patterns

330seen in both, observations and simulations in the north Pacific suggests that similar

331correspondence with local atmospheric patterns may take place in the north Atlantic. But the

332time series of variations in CL region of the North Atlantic is striking in showing primarily year-

15 15 333to-year variations (Fig. 6c) even though the winter-spring meteorology of this region does

334exhibit decadal variations frequently referred to as the North Atlantic Oscillation (Hurrell, 1995).

335We suspect that some of this variability is the result of the smaller spatial scales of variability

336relative to the North Pacific which has been insufficiently sampled by the historical observing

337systems. During the past ten years when the data coverage has been increasingly extensive (due

338to implementation of the Argo floats) CL thickness in the North Atlantic CL region has

339undergone 15m variations from year-to-year with thick layers in 1999, 2002-3, and 2006-7 and

340thinner layers during the other years (Fig. 10a). This pattern of CL variation is related to the

341strength of the subtropical high in sea level pressure (Fig. 10a). Indeed, a regression of sea level

342pressure, latent heat flux, and winds on CL thickness in the North Atlantic CL box shows basin-

343scale relationships (Fig. 10b). Vertical expansion of CLs (more negative CL width) corresponds

344to NAO-like pattern of the atmospheric pressure with stronger Azores high and deeper Iceland

345low. This pressure pattern strengthens winds and latent heat losses in the east while weakens

346them in the central and western subtropics (Fig. 10b). Anomalously strong winds and net surface

347heat losses suggest deepening of the mixed layer that, in turn, expands width of CLs by the spice

348injection mechanism. This is in line with 2002-3 and 2006-7 data when anomalously thick CLs

349in the NA/CL box occur in phase with anomalously deep mixed layers. But in 1999 this

350relationship is opposite (Fig. 10a). This may suggest that alternative mechanisms including salt

351advection play a role. Indeed, in the north-east it is clear that advection plays a central role in

352variability of the mixed layers (Nilsen and Falck, 2006 show this process at work in the

353Norwegian Sea).

355Finally, we consider year-to-year variability of the BL region of the tropical Pacific. BL thickness

16 16 356in the western Pacific west of 160oE, which is generally 10-20m, varies in thickness coherently

357with the value of the Southern Oscillation Index (Fig. 11c). The eastward shift of precipitation

358during El Nino decreases precipitation west of 160oE by 20% or 1 mm/dy (in response to a 20

359unit decrease of the SOI) and as a result BLs shrink by 20% or 2-5m (Figs. 11a, 11b). Thus, in

360this region variations in BL thickness respond primarily to surface freshwater flux. East of 160oE

361BLs are thinner and occur at much shallower depths so that they are affected more strongly by

362solar radiation, advection and entrainment.

3644. Conclusions

365This study examines subseasonal changes of barrier and compensated layers (BLs and CLs)

366based on analysis of profiles of temperature and salinity covering the years 1960-2007. Because

367of data limitations we focus mainly on the Northern Hemisphere and tropics. The processes that

368regulate subseasonal variability of BL/CL thickness are similar to those which regulate their

369seasonal appearance: fluctuations in surface freshwater flux, Ekman pumping, and horizontal

370advection. Thus, the spatial distribution of subseasonal variability reflects aspects of the

371subseasonal variability of these forcing terms.

373The most striking interannual variability of BL width occurs in the rainy tropical oceans with

374deep thermoclines such as the Arabian Sea and Bay of Bengal in the Indian Ocean and the

375western tropical Pacific under the Intertropical Convergence Zone and the South Pacific

376Convergence Zone. Precipitation in these regions varies strongly interannually. During high

377precipitation years the mixed layers in these regions show capping fresh layers and thick BLs. In

378contrast, during low precipitation years mixed layer salinities increase and BL thickness

17 17 379decreases. Thus, in the western Equatorial Pacific between 120oE and 160oE, the BL thickens by

3802m to 5m during La Nina while during the El Nino the BL thins by a similar amount. In the

381central and eastern equatorial Pacific variations of BL thickness are not nearly as well correlated

382with the phase of ENSO. We think this happens because in the central basin salt advection plays

383an important role in regulating BL thickness as well, while further east the mixed layer shallows

384and mixed layer entrainment variability becomes increasingly important.

386In the subtropics and midlatitudes during late winter-spring we find alternating regions of CLs

387and BLs in the seasonal climatology. The northern tropics of both the Pacific and Atlantic (the

388southern edge of the subtropical gyres) show broad regions of BLs where salty subtropical

389surface water formed further north has subducted, advected equatorward, and affected the water

390properties of the winter mixed layer. Within the evaporative subtropical North Pacific and eastern

391North Atlantic we find CLs resulting from mixed layers with positive temperature stratification

392but negative salinity stratification. In the eastern subpolar North Atlantic this CL merges with a

393subpolar CL resulting from negative temperature stratification but positive salinity stratification.

394In the North Pacific, strengthening of the Aleutian pressure low during successive winters, thus

395strengthening the midlatitude westerly winds has led to deeper mixed layers, cooler SSTs, and a

396long-term increase in the thickness of this CL. Further north in the North Pacific a supolar BL

397region is evident, which does not have a counterpart in the North Atlantic. The same changes in

398meteorology which include strengthening of the Aleutian pressure low have led to an increase in

399dry northerly winds which in turn cause a thinning of this BL from ~40m before 1980s to ~ 20m

400afterwards.

18 18 402In the North Atlantic the broad area of CL also undergoes fluctuations in thickness associated

403with surface meteorology. In this basin the fluctuations in winter thickness also seem to vary in

404response to meteorological changes associated with changes of the Azores sea level high

405pressure. Estimation of the timescale of variability is more difficult in this basin as the analysis is

406more noisy. Here our examination focuses on the changes that are apparent in the shorter 1997-

4072007 period when the data coverage is better. During this period the thickness of the winter-

408spring CL in the northeastern Atlantic undergoes 4-5 year timescale fluctuations from 45m to

40960m in phase with the strengthening/weakening of the Azores high.

411Determining the basin-scale BL/CL structure tests the limits of the historical observing system.

412Further progress in understanding BL/CL variability and its role in air-sea interactions will likely

413require further exploration of models that provide reasonable simulations of observed variability.

415Acknowledgements We gratefully acknowledge the Ocean Climate Laboratory of the National

416Oceanographic Data Center/NOAA and the Argo Program data upon which this work is based.

417Support for this research has been provided by the National Science Foundation (OCE0351319)

418and the NASA Ocean Programs.

19 19 419References

420Ando, K., and M.J. McPhaden, 1997: Variability of surface layer hydrography in the tropical

421 Pacific Ocean. J. Geophys. Res., 102, 23063-23078.

422Boyer, T.P., J.I. Antonov, H.E. Garcia, D.R. Johnson, R.A. Locarnini, A.V. Mishonov, M.T.

423 Pitcher, O.K. Baranova, and I.V. Smolyar, 2006: World Ocean Database 2005. S. Levitus,

424 Ed., NOAA Atlas NESDIS 60, U.S. Government Printing Office, Washington, D.C., 190 pp.,

425 DVDs.

426Carton, J.A., S.A. Grodsky, and H. Liu, 2008: Variability of the Oceanic Mixed Layer, 1960–

427 2004. J. Climate, 21, 1029–1047.

428Cronin, M. F., and M. J. McPhaden, 2002: Barrier layer formation during westerly wind bursts.

429 J. Geophys. Res., 107(C12), 8020, doi:10.1029/2001JC001171.

430de Boyer Montégut, C., G. Madec, A. S. Fischer, A. Lazar, and D. Iudicone, 2004: Mixed layer

431 depth over the global ocean: An examination of profile data and a profile-based climatology.

432 J. Geophys. Res., 109, C12003, doi:10.1029/2004JC002378.

433de Boyer Montégut, C., J. Mignot, A. Lazar, and S. Cravatte, 2007: Control of salinity on the

434 mixed layer depth in the world ocean: 1. General description. J. Geophys. Res., 112, C06011,

435 doi:10.1029/2006JC003953.

436Delworth, T.L., and Coauthors, 2006: GFDL's CM2 Global Coupled Climate Models. Part I:

437 Formulation and Simulation Characteristics. J. Climate, 19, 643–674.

438Ffield, A., 2007: Amazon and Orinoco River Plumes and NBC Rings: Bystanders or Participants

439 in Hurricane Events? J. Climate, 20, 316–333.

440Grodsky, S. A., and J. A. Carton, 2003: Intertropical convergence zone in the South Atlantic and

441 the equatorial cold tongue. J. Climate, 16(4), 723-733,

20 20 442Hurrell, J. W., 1995: Decadal trends in the North-Atlantic oscillation - regional temperatures and

443 precipitation. Science, 269, 676-679.

444Kalnay, E., & co-authors, 1996: The NCEP/NACR 40-year reanalysis project. Bull. Amer.

445 Meteor. Soc., 77, 437-471.

446Kara, A. B., P. A. Rochford, and H. E. Hurlburt, 2000: Mixed layer depth variability and barrier

447 layer formation over the North Pacific Ocean. J. Geophys. Res., 105(C7), 16,783–16,801.

448Large, W., J. McWilliams, and S. Doney, 1994: Oceanic Vertical Mixing: A Review and a Model

449 with a Nonlocal Boundary Layer Parameterization. Rev. Geophys., 32(4), 363-403.

450Liu, W.T., 2002: Progress in scatterometer application. J. Oceanogr., 58(1), 121-136.

451Lukas, R., and E. Lindstrom, 1991: The mixed layer of the western equatorial Pacific Ocean. J.

452 Geophys. Res., 96 (Supplement), 3343 –3357.

453Mignot, J., C. de Boyer Montégut, A. Lazar, and S. Cravatte, 2007: Control of salinity on the

454 mixed layer depth in the world ocean: 2. Tropical areas. J. Geophys. Res., 112, C10010,

455 doi:10.1029/2006JC003954

456Monterey, G., and S. Levitus, 1997: Seasonal Variability of Mixed Layer Depth for the World

457 Ocean, NOAA Atlas NESDIS 14, 100 pp., Natl. Oceanic and Atmos. Admin., Silver Spring,

458 MD.

459Nilsen, J. E. Ø., and E. Falck, 2006: Variations of mixed layer properties in the Norwegian Sea

460 for the period 1948–1999. Prog. Oceanogr., 70, 58–90.

461Pailler, K., B. Bourlès, and Y. Gouriou, 1999: The Barrier Layer in the Western Tropical Atlantic

462 Ocean. Geophys. Res. Lett., 26(14), 2069-2072.

21 21 463Polovina, J.J., G.T. Mitchum, and G.T. Evans, 1995: Decadal and basin-scale variation in MLD

464 and the impact on biological production in the central and North Pacific, 1960-88. Deep-Sea

465 Res., 42, 1701-1716.

466Qu, T., and G. Meyers, 2005: Seasonal variation of barrier layer in the southeastern tropical

467 Indian Ocean. J. Geophys. Res., 110, C11003, doi:10.1029/2004JC002816.

468Sato, K., T. Suga, and K. Hanawa, 2006: Barrier layer in the North Pacific subtropical gyre.

469 Geophys. Res Lett, 31, L05301, doi:10.1029/2003GL018590.

470Sprintall, J., and M. Tomczak, 1992: Evidence of the Barrier Layer in the Surface Layer of the

471 Tropics. J. Geophys. Res., 97(C5), 7305–7316.

472Sprintall, J., and M. Tomczak, 1993: On the formation of central water and thermocline

473 ventilation in the Southern Hemisphere. Deep Sea Res., Part I, 40, 827–848.

474Stommel, H., and K. N. Fedorov, 1967: Small-scale structure in temperature and salinity near

475 Timor and Mindanao. Tellus, 19, 306-325.

476Thadathil, P., P. Thoppil, R.R. Rao, P.M. Muraleedharan, Y.K. Somayajulu, V.V. Gopalakrishna,

477 R. Murthugudde, G.V. Reddy, and C. Revichandran, 2008: Seasonal Variability of the

478 Observed Barrier Layer in the Arabian Sea. J. Phys. Oceanogr., 38, 624–638.

479Tomczak, M., and J. S. Godfrey, 1994: Regional Oceanography: An Introduction, Pergamon

480 Press, New York, 422 pp. (pdf edition is available at

481 http://gyre.umeoce.maine.edu/physicalocean/Tomczak/regoc/pdfversion.html)

482Weller, R. A., and A. J. Plueddemann, 1996: Observations of the vertical structure of the oceanic

483 boundary layer. J. Geophys. Res., 101, 8789–8806.

22 22 484Xie, P., and P.A. Arkin, 1997: Global Precipitation: A 17-Year Monthly Analysis Based on Gauge

485 Observations, Satellite Estimates, and Numerical Model Outputs. Bull. Amer. Meteor. Soc.,

486 78, 2539–2558.

487Yeager, S.G., and W.G. Large, 2007: Observational Evidence of Winter Spice Injection. J. Phys.

488 Oceanogr., 37, 2895–2919.

23 23 489Figure captions.

490 Figure 1. Climatological cold season barrier layer/compensated layer thickness versus the

491bulk Turner angle evaluated using temperature ( T ) and salinity ( S ) difference between the

492top and bottom of a barrier or compensated layer. Vertical bars show the mean and the standard

493deviation for consecutive 22.50 interval of the Turner angle. Grey dots are the cold season data

494that aggregate January-March data from the Northern Hemisphere and July-September data from

495the Southern Hemisphere. The Turner angle range -720 to 450 corresponds to BL. CL occurs

496outside this interval.

497 Figure 2. Climatological (a) January-March and (b) July-September barrier layer width

498(positive) and compensated layer width (negative). Climatological sea surface salinity (  35 psu

499solid contours, below 35 psu dashed contours) and downward wind-driven vertical velocity

500(hatching) are overlain. Salinity data are from Boyer et al. (2006).

501 Figure 3. (a,b) Salinity ( S ) and (c,d) temperature ( T ) difference between the top zt =

502min[MLT, MLD] and the bottom zb =max[MLT, MLD] of barrier/compensated layer, (e,f) bulk

503Turner angle calculated from S and T between the same two depths. (left) January - March

504(JFM) values, (right) July - September (JAS) values. Turner angles in the range from -72o to 45o

505corresponds to barrier layers, while compensated layers occur outside this range

507 Figure 4. Standard deviation (STD) of monthly anomalous BL/CL thickness.

508 Figure 5. Quasi-decadal mean barrier layer (positive) and compensated layer (negative)

509thickness in (left) northern winter and (right) austral winter. Rectangles in (c) show locations of

510the North Pacific barrier layer box (NP/BL 140oE-140oW, 50o-60oN), North Pacific compensated

511layer box (NP/CL 140oE-160oW, 25o-35oN), equatorial Pacific barrier layer box (EP/BL 120oE-

24 24 512160oE, 5oS-5oN), and North Atlantic compensated layer box (NA/CL 30oW-10oW, 40o-60oN)

513 Figure 6. Box averaged BL/CL width in (a) North Pacific barrier layer region, (b) North

514Pacific compensated layer region, and (c) North Atlantic compensated layer region. Thin lines

515are monthly average values, bold lines are January-March average values. JFM values are shown

516if at least 10 data points are available. Box locations are shown in Fig. 5c.

517 Figure 7. Time regression of JFM barrier layer thickness in (a) North Pacific compensated

518layer box on latent heat flux (Wm-2/m, shading), winds (ms-1/m, arrows), and sea level pressure

519(mbar/m); (b) North Pacific barrier layer box on precipitation (mm h-1/m). See also Fig.5c for

520box locations. Atmospheric parameters are from the NCEP/NCAR Reanalysis.

521 Figure 8. CM2.1 model simulations in the north Pacific. (a) March climatological BL/CL

522width with climatological sea surface height (SSH, CI=0.2m) and sea surface salinity (SSS, bold

523contours) overlain. CL and BL boxes are drawn in red and blue, respectively. (b) Zonally

524averaged March climatological Ekman pumping (positive – downward is shaded gray). (c) Time

525series of the box-averaged CL width. Two contrasting model years with thick (1953) and thin

526(1956) CL are marked by ‘o’. (d) Lagged correlation of anomalous (blue) box-averaged CL

527width and MLD, (red) box-averaged BL width and MLD. (e) Time regression of March box-

528averaged CL width on wind stress and Ekman pumping (mdy-1/m).

529 Figure 9. Latitude-depth section of salinity along 180oE for March of the two contrasting

530model years with thick (1953) and thin (1956) CL. Ekman pumping (positive – downward is

531shaded gray) is shown against the right-hand vertical axes.

532 Figure 10. (a) JFM compensated layer width (solid) and monthly mixed layer depth

533(compensated layer depth range between MLT and MLD is shaded) averaged in the North

534Atlantic compensated layer box, JFM anomalous mean sea level pressure (dashed) averaged

25 25 53540oW-30oW, 30oN-40oN. (b) Time regression of the 1997-2007 JFM compensated layer width on

536latent heat flux (Wm-2/m, shaded), anomalous mean sea level pressure (mbar/m, contours), and

537winds (ms-1/m, arrows).

538 Figure 11. Lag regression of SOI index on 5oS-5oN averaged (a) anomalous barrier layer

539width, (b) precipitation. Time mean BL width (solid), salinity (dashed), and precipitation are

540shown against right hand vertical axes. Precipitation is Xie and Arkin (1997). (c) Time series of

541annual running mean SOI (shaded) and anomalous BL width averaged in the western equatorial

542Pacific box (120oE-160oE, 5oS-5oN). See Fig. 5c for the box location.