lavin etal 2006 onthesummerpoleward

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On the summer poleward coastal current off SW Mexico

M. F. Lavn,1 Emilio Beier,2 Jose Gomez-Valdes,1 Vctor M. Godnez,1 and J. Garca1

Received 18 September 2005; revised 29 November 2005; accepted 6 December 2005; published 17 January 2006.

[1] For the first time on the basis of direct observations(June 2003 and June 2005), the characteristics of thesummer-time coastal poleward current off SW Mexico arereported. Only the surface evidence of this coastal currenthas previously been described, from ship drift compilations.In June 2003 the current was 90180 km wide, 400 m deep,with speed 0.3 ms1 and transport 4 S v ( 1 S v =106 m3 s1). In June 2005, its width was 90 km, it was250 300 m deep, with mean speed 0.15 m s1 andtransport 2.5 Sv. California Current water (CCW) andequatorward flow were found further offshore. Mesoscaleeddies significantly affected the coastal current, and

transported CCW into the coastal zone. Citation: Lavn,M. F., E. Beier, J. Gomez-Valdes, V. M. Godnez, and J. Garca

(2006), On the summer poleward coastal current off SW Mexico,

Geophys. Res. Lett., 33, L02601, doi:10.1029/2005GL024686.

1. Introduction

[2] Knowledge of the oceanographic province of theEastern Tropical Pacific that spans from the Gulf of Cal-ifornia entrance to Panama has long been based on thecompilations byWyrtki[1965, 1967]. An important piece ofthe oceanic current pattern in the zone is the polewardcoastal current off SW Mexico, between the Gulf of

Tehuantepec and Cabo Corrientes, which was describedby Wyrtki [1965] from monthly ship drift charts. Heindicated that in June and July the flow reached as far asCabo Corrientes, that it was the northernmost extension ofthe Costa Rica Coastal Current (CRCC), and therefore ofthe North Equatorial Countercurrent (NECC) before itjoined the North Equatorial Current (NEC). The importanceof this poleward current was reviewed by Badan-Dangon[1998], who proposed calling it The Mexican Currentand remarked that it remained one of the least explored ofthe major coastal currents of the world.

[3] Recent analyses of hydrographic data banks [Kessler,2002, 2006] and satellite altimetry data [Strub and James,2002] mention this early-summer poleward coastal flow.

From satellite altimetry,Strub and James[2002] find that inMayJune coastal high sea-level anomalies start forming inSW Mexico and Central America and by JulyAugust theyoccur as far north as the tip of the Baja California peninsula.However, in order to calculate geostrophic current speeds,the Levitus mean hydrography was used, which contain toofew data points in the area, and is too coarse to resolve the

coastal current. The geostrophic calculations by Kessler[2006] show a weak subsurface current (0.03 m s1) witha transport 1 2 Sv. These calculations, however, arebased on XBT data averaged into a 1 latitude 1longitude mesh, which is too coarse for resolving the coastalcurrent, and do not include the potentially important effectof the salinity on the estimates of the current speed.

[4] The lack of direct observations at regional scale ishampering progress in the understanding of this importantpoleward coastal current. Therefore, we report here itscharacteristics, based on direct observations carried out inJune 2003 and June 2005. The findings are put in the

context of recent evidence questioning the accepted originof the current.

2. Data

[5] The data come from two surveys made in the R/VFrancisco de Ulloa, on 626 June 2003, and 319 June2005. The sampled area is centered on Cabo Corrientes andthe station grids are shown in the corresponding figuresbelow. A factory-calibrated SeaBird CTD model 911 pluswas lowered to a maximum of 4000 m in 2003 and to1000 m in 2005.

[6] The surface geopotential anomaly was obtained (after

smoothing the temperature and salinity cross-sections byobjective mapping) relative to 1000 m, and the distributionof geostrophic velocity normal to the transects was calcu-lated by assuming no flow at 1000 m, or at the bottom forstations in the continental shelf or slope. The geopotentialmaps are presented only for the area where bottom depthreached or exceeded 1000 m.

[7] Velocity profiles along the ship track were obtainedwith an RDI hull-mounted 153.6 kHz broad-band ADCP,and processed according to Firing et al. [1995]. In deepwater, the 5-minute mean current velocity has an uncertainty0.05 m s1. With bottom tracking, overall error was0.018 m s1. Reliable data were obtained to 120 m.

3. Results

3.1. June 2003

[8] The 20 40 m depth-mean ADCP currents in June2003 (Figure 1a) show mostly poleward flow, with a verystrong coastal jet (0.5 m s1) from Manzanillo to CaboCorrientes. Equatorward flow is present in the offshore endof the longest lines (Figure 1a, transects T2, T3 and T8).The pattern of ADCP currents remains the same down to atleast 100 m, although decreasing in intensity (not shown).There is good agreement between the circulation suggestedby the surface geopotential field (Figure 1b) and the ADCPdata; both show the poleward coastal current and the

equatorward flow in the offshore ends of the longest lines.In addition, they suggest that the equatorward flow in

GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L02601, doi:10.1029/2005GL024686, 2006

1Departamento de Oceanografa F sica, Centro de InvestigacionCientifica y de Educacion Superior de Ensenada, Ensenada, BajaCalifornia, Mexico.

2Centro de Investigacion Cientifica y de Educacion Superior deEnsenada, Estacion La Paz, La Paz, Baja California Sur, Mexico.

Copyright 2006 by the American Geophysical Union.0094-8276/06/2005GL024686$05.00

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transects T2 and T3 turns toward the coast and strengthensthe poleward coastal flow. In transects T4 and T5, shore-ward flow also seems to add to the coastal current. Figure 1suggests that these shoreward flows are branches of meso-scale features (eddies or meanders).

[9] Surface salinity in most of the area (Figure 1c) wasabove 34.9, which indicates that it is Gulf of CaliforniaWater. In the two extremes of T8 low salinity (34.4) wasobserved (Figure 1c). The low salinity water in the offshoreend of T8 is probably California Current Water (CCW),although its temperature is somewhat high. The coastal lowsalinity water could be Tropical Surface Water since thecurrent is poleward there; however, it could be CCWbrought there from offshore by the mesoscale eddy sugestedby Figure 1b (T7 T8), as seems to have occurred furthernorth. Identification of surface water masses in this zone isdifficult because their characteristics change smoothly,rather than sharply across fronts [Wyrtki, 1967].

[10] The vertical distribution of geostrophic velocitynormal to the transects (Figure 2) shows the characteristicsof the coastal poleward current. It was 90180 km wide,with maximum speed of0.35 m s1 (transects T5 and T7)at the surface and close to the coast. In transect T2 the

maximum speed (0.3 m s1

) occurred inside San Blas Bay.In transect T6 the maximum speed (0.25 m s1) is in a

subsurface core around 75 m. The coastal current was 400m deep (except in T2, where it was 300 m), as measured bythe depth of the 0.05 m s1 isotach. The mean polewardtransport (due to speeds over 0.05 m s1) was 3.6 Sv(1 Sv = 106 m3s1) in transects T4 to T8, and 5.5 Sv intransects T2 and T3; this northward increase in transport isprobably due to the injection of offshore water mentioned

above.[11] The equatorward current in the western extreme of

the sampled area is best defined in T3 (Figure 2), where themaximum speed (0.15 m s1) was found in a subsurfacecore centered at a depth of 75 m.. The core of theequatorward current in transects T2 and T3 carried low-salinity water (not shown), whose characteristics (tempera-ture 1520C, salinity 34.434.6, and surrounding salinity34.7) indicate modified CCW. Surface salinity at the off-shore end of T2 and T3 (Figure 1c) is also lower than that ofthe surrounding water (34.8 vs. 35.0).

3.2. June 2005

[12] The ADCP current data for the 2040 m layer(Figure 3a) again show the coastal poleward flow, but thestrongest currents are found offshore, south of the tip of theBaja California Peninsula; the coastal current is weaker thanin 2003. The surface geopotential anomaly relative to1000 m (Figure 3b) shows a strong current entering theregion from the NW, and an anticyclonic meander directlyoffshore of Cabo Corrientes. A cyclonic eddy in thesouthern part of the sampled area extends nearshore toreinforce the coastal current. The surface salinity (Figure 3c)

Figure 1. Surface distributions in June 2003. (a) ADCPcurrent (m s1), averaged from 20 to 40 m depth.

(b) Geopotential anomaly at the surface (m2

s2

) relativeto 1000 m. (c) Surface salinity.

Figure 2. Vertical distribution of geostrophic velocity (cms1), relative to 1000 m or to the bottom, in June 2003,across transects T1T8. QV is the volume transported by the

poleward current (speeds over 0.05 m s1

), in Sv (1 Sv =106 m3 s1).

L02601 LAVIN ET AL.: POLEWARD CURRENT OFF SW MEXICO L02601

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is very low (34.0) in the NW corner, which indicates that the

intruding current is a part of the California Current system.In the rest of the sampled area, surface salinity is lower thanin June 2003, with a mean value of 34.7 in sections T4T8.

[13] The vertical distribution of geostrophic velocitynormal to the transects is shown in Figure 4. In all thesections the coastal current is well defined; it was narrowerthan in 2005, at 90 km, and the speeds are slower,with a mean value 0.15 m s1. The coastal current was300400 m deep in the four southern sections and notice-ably (200250 m) shallower in T1 and T3. The current wasalso stronger and wider in the southern sections (T4T7)than in the two northern sections (T1 and T3). The maxi-mum speed was found in T7, at 0.35 m s1. In mosttransects the maximum speed was at the surface, except inT5 where the maximum speed (0.15 m s1) wa s i n asubsurface core centered around 100 m. The mean polewardtransport (considering only speeds 0.05 m s1) was 3 Svin transects T4 to T7, and 1.4 Sv in transects T1 and T3.

[14] Like in 2003, equatorward flow was encounteredbeyond the coastal current, often in the most offshore part.In section T7 (Figure 4), this flow was the offshore side ofthe eddy suggested in the geopotential (Figure 3b). Insection T3 the equatorward flow is due to the cyclonicmeandering of the current intruding from the NW.

4. Discussion and Conclusions

[15] We report here the first direct observations of thesummer poleward coastal current off SW Mexico. The

observed surface speed (0.15 to 0.35 m s1) encompassesthe estimated 0.24 m s1 ofWyrtki[1965, Figure 15] but ismuch faster than the 0.03 m s1 estimate ofKessler[2006].In June 2003 the coastal current was 90 180 km wide,400 m deep, with mean speed 0.3 m s1 and transport3.15.4 Sv. In June 2005 the coastal current was narrower(90 km), shallower (200400 m), slower (0.15 m s1)

and transported less volume (1.53.2 Sv). This transportin both occasions was lower than the 10 Sv estimated byWyrtki [1967] for the CRCC.

[16] The data show that at least a part of the coastalcurrent continues along the coast into the Gulf of California,but we have no data to support a possible connection acrossthe Gulfs entrance to join with the California Countercur-rent [Badan-Dangon, 1998] nor a westward deviation tofeed the NEC [Wyrtki, 1967].

[17] Having established the characteristics of the current,it is worth noting that recent work is opening the question ofits origin. Numerical models [Beier et al., 2003] andclimatological geostrophic calculations [Kessler, 2006]

show that in the temporal mean it is disconnected fromthe CRCC. Rather, a connection seems to occur with theCalifornia Current, a branch of which veers toward the SWcoast of Mexico, both in the average and in spring-summer.Our data also shows that CCW is found offshore, and thatsome of it can be injected into the coastal current bymesoscale eddies or meanders. The most obvious localforcing mechanisms are the elevated surface height in theregion during summer [Strub and James, 2002; Kessler,2006], and the wind stress, which is to the SE most of theyear but weakens (and occasionally reverses) from June toSeptember. The difference between the two cruises may bedue to the interannual variability of the forcings, whichwould, for instance, change the timing of the current onset.

The origin of this current and its connections with the global

Figure 3. Surface distributions in June 2005. (a) ADCPcurrent (m s1), averaged from 20 to 40 m depth. (b)Geopotential anomaly at the surface (m2 s2) relative to1000 m. (c) Surface salinity.

Figure 4. Vertical distribution of geostrophic velocity (cms1), relative to 1000 m or to the bottom, in June 2005,

across transects T1T7. QV is volume transported by thepoleward current (speeds over 0.05 m s1), in Sv.


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current system needs to be addressed by observationalists,theoreticians and numerical modelers.

[18] Acknowledgments. This work was supported by CONACYT(Mexico) through contracts G34601-S, D41881-F and SEP-2003-C02-42941/A-1, and through regular CICESE budget. This is a contribution tothe scientific agenda of EPCOR (IAI). M.F.L. is currently at SIO-UCSD asrecipient of a UCMEXUS-CONACYT sabbatical scholarship.

ReferencesBadan-Dangon, A. (1998), Coastal Circulation from the Galapagos to the

Gulf of California,The Sea, vol. 11,The Global Coastal Ocean,RegionalStudies and. Syntheses, edited by A. R. Robinson and K. H. Brink, pp.315343, John Wiley, Hoboken, N. J.

Beier, E., M. F. Lavn, J. Gomez, V. Godnez, and J. Garca (2003), LaCorriente Costera Mexicana, in GEOS, Reunion Anual 2003 de la UnionGeofsica Mexicana, vol. 23, p. 152, Port Vallarta, Mexico.

Firing, E., J. Ranada, and P. Caldwell (1995), Processing ADCP data withCODAS software system version 3. 1, Users Manual, 226 pp., Univ. ofHawaii, Honolulu.

Kessler, W. S. (2002), Mean three-dimensional circulation in the northeast-ern tropical Pacific, J. Phys. Oceanogr., 32, 24572471.

Kessler, W. S. (2006), The circulation of the eastern Tropical Pacific: Areview, Progr. Oceanogr., in press.

Strub, P. T., and C. James (2002), Altimeter-derived surface circulation inthe large-scale NE Pacific gyres: Part 1. Seasonal variability, Progr.Oceanogr., 53, 163183.

Wyrtki, K. (1965), Surface currents of the eastern equatorial Pacific Ocean,Inter Am. Trop. Tuna Comm. Bull., IX, 269304.

Wyrtki, K. (1967), Circulation and water masses in the eastern equatorialPacific Ocean, Int. J. Oceanol. Limnol., 1, 117 147.

E. Beier, CICESE, Estacion La Paz, Unidad La Paz, Miraflores 334,Fracc. Bellavista, La Paz, Baja California Sur, Mexico 23050.

J. Garca, V. M. Godnez, J. Gomez-Valdes, and M. F. Lavn,Departamento de Oceanografa Fsica, CICESE, P.O. Box 43844, SanDiego, CA 92143-4844, USA. ([email protected])


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