Seasonal and Interannual Modulation of Mixed Layer Variability at 0°, 110°W

Deep-Sea Research I 49 (2002) 1–17

Seasonal and interannual modulation of mixed layer variability at 01, 1101W

Meghan F. Cronin*, William S. Kessler National Oceanographic and Atmospheric Administration (NOAA), Paciﬁc Marine Environmental Laboratory (PMEL), 7600 Sand Point Way NE, Seattle, WA 98115, USA

Received 22 August 2000; received in revised form 9 April 2001; accepted 29 June 2001

Abstract

Long, high resolution time series from the 01, 1101W tropical atmosphere ocean mooring in the eastern equatorial Pacific are used to analyze how warm and cold phases of El Nino-Southern* Oscillation (ENSO) and the annual cycle modulate the near-surface stratification and sea-surface temperature (SST) diurnal cycle. During the annual warm season (February–April), when solar warming is large and wind mixing weak, the isothermal-mixed layer depth (MLDT ) is shallow (typically 10 m deep) and the 1 m SST diurnal cycle amplitude is large (typically up to 0.41C). Likewise during the remainder of the year when SST is generally cool, typically the diurnal cycle amplitude is less than 0.21C and the isothermal-mixed layer is deeper than 20 m. Thus, annual variations in wind and insolation, which lead to an annual cycle in SST, also cause annual modulation of the SST diurnal cycle and near-surface stratification, consistent with one-dimensional mixed layer physics. However, on interannual time scales, mixed layer physics are more complicated. In particular, the diurnal cycle amplitude and MLDT anomalies are out of phase with the SST anomalies. MLDT is anomalouslydeep and the SST diurnal cycleamplitude is anomalouslylow during the warm phase of ENSO. On these longer timescales, MLDT tends to be stronglyinfluenced bythermocline-depth variability.In addition, salinity stratified barrier layers large enough to support temperature inversions were often observed at 01, 1101W during the final stage of El Ninos.* As SST rose above 28.51C during the final stage of the 1997–1998 El Nino,* a regime shift was observed, with large temperature inversions, a relative increase in SST diurnal cycle amplitude, and large variability in the mixed-layer depth. It is likely that barrier layers (inferred from temperature inversions) allowed warm conditions to remain, even as the thermocline and mixed-layer depths shoaled. r 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Diurnal variations; Diurnal thermocline; Surface mixed layer; Seasonal variations; El Nino;* Temperature inversions

1. Introduction time cooling; a 20–40 dayband associated with tropical instabilitywaves; an annual cycle,which is Sea-surface temperature variabilityin the east- dominant despite the fact that the sun crosses the ern equatorial Paciﬁc occurs primarilywithin four equator twice per year; and a 2–7 year band frequencybands (Kessler et al., 1996): a diurnal associated with the El Nino/Southern* Oscillation cycle associated with daytime warming and night- (ENSO) cycle. There has been considerable work showing the cross-scale interactions at the lower *Corresponding author. Fax: +1-206-526-6744. end of the spectrum, such as phase locking E-mail address: [email protected] (M.F. Cronin). between ENSO and the annual cycle (Rasmusson

0967-0637/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0967-0637(01)00043-7 2 M.F. Cronin, W.S. Kessler / Deep-Sea Research I 49 (2002) 1–17 and Carpenter, 1982; Deser and Wallace, 1987; indistinguishable from the surface in the OLR Xie, 1995; Harrison and Larkin, 1998) and annual records. Weak winds and deep convection asso- and interannual modulation of tropical instability ciated with the ITCZ are far north of the equator waves (Halpern et al., 1988; Qiao and Weisberg, during the cold season (August–October), when 1995, 1998). In this paper we investigate the cross- the equatorial cold tongue is fullyformed, and are scale interaction with the high end of the spectrum. nearest to the equator during the warm season In particular, we investigate how the warm and (February–April), when the equatorial waters are cold phases of ENSO and the seasonal cycle warmest (Fig. 1). On interannual time scales, modulate and in turn are affected bythe diurnal equatorial trade winds varyin relation to the cycle in SST and short lived temperature inver- zonal location of the warm pool’s deep atmo- sions. spheric convection (Figs. 2 and 3). During El Nino* Solar warming occurs onlyduring the daytime, events, the warm pool, deep convection and trade and mixing occurs primarilyat nighttime in winds feeding into the convection, all shift east- regions of light winds (Moum et al., 1989; Peters ward. During La Nina* events, the system shifts et al., 1994; Bond and McPhaden, 1995). Thus, westward. On both seasonal and interannual time seasonal and interannual variations in wind scales, the tropical Pacific ocean and atmosphere mixing and surface heating can be expected to are coupled, so that variations in insolation and cause variations in the SST diurnal cycle, as well as trade wind forcing are both caused byvariations in seasonal and interannual variations in SST. For the SST pattern and also generate changes in the example, increased wind mixing will tend to both SST through surface-heat fluxes, mixing, and cool the SST and reduce the SST diurnal cycle, and horizontal and vertical advection. solar warming will tend to both warm the SST and Adding to the complicated dynamics in the increase the SST diurnal cycle. Thus, based on eastern equatorial Pacific, since the thermocline is simple one-dimensional mixed layer physics, one veryshallow in this region, mixing and vertical might expect an increased SST diurnal cycle advection can bring verycool water to the surface during warm phases of the low-frequencySST (Figs. 2 and 3). Indeed, because the mean thermo- variability. Further, since increased solar warming cline is so shallow, thermocline variabilityis often and reduced wind mixing (which give rise to a used as a proxyfor mixed layerdepth variability large SST diurnal cycle) also tend to cause surface in the eastern equatorial Pacific (Xie, 1995). Thus, thermal restratification, one might also expect a we pose the following questions: (1) If the SST shallow isothermal-mixed layer, separated from diurnal cycle is affected primarily by surface one- the deeper thermocline bya fossil layer, during the dimensional mixed layer processes (e.g. wind warm phases. mixing, solar warming), processes that also affect However, in general, eastern equatorial Pacific the lower frequencySST variability,what is the SST is controlled bycomplicated physicsthat are relationship between the SST diurnal cycle and not purelyone-dimensional. For example, at the lower frequencySST anomalies? Is the SST equator, the thermocline can respond both to diurnal cycle larger during the warm phases? (2) Kelvin waves generated byremote wind-forcing Since the thermocline is so shallow in the eastern and to Ekman upwelling and downwelling caused tropical Pacific, is it reasonable to use thermocline- bylocal variations in zonal wind. On seasonal time depth variabilityas a proxyfor mixed-layerdepth scales, the equatorial trade winds varyin relation variability? to the meridional location of the intertropical convergence zone (ITCZ) (Fig. 1). In Figs. 1–3 low outgoing longwave radiation (OLR) indicates the 2. Data presence of cold cloud tops (e.g. tall cumulus towers) associated with deep tropical convection. Data used in this analysis are primarily from the Low-level, hence relativelywarm, stratus clouds 01, 1101W tropical atmosphere ocean (TAO) that often form over seasonallycool SST are mooring. Originallysupported byNOAA’s Equa- M.F. Cronin, W.S. Kessler / Deep-Sea Research I 49 (2002) 1–17 3

Fig. 1. Time-latitude plot of 1101W monthlyaveraged sea-surface temperature (SST), outgoing longwave radiation (OLR), and wind vectors from January1986 through February1998.

torial Pacific Ocean Climate Studies (EPOCS) several of the sites, including 01, 1101W, ocean program (Halpern, 1987) and later incorporated temperature was measured byminiature tempera- into the TAO array(McPhaden et al., 1998), the ture recorders (MTRs) with a sample rate of mooring has one of the longest time series of the typically 10–20 min. MTR depths were typically 1, array, extending back to 1980. With approxi- 10, 25, 45, 60, 80, 100, 120, 140, 200, 300, and mately70 moorings in the tropical Pacific, the 500 m. At other eastern Pacific TAO moorings, TAO arraywas designed to improve monitoring, vertical resolution of temperature was typically understanding, and forecasting of ENSO. Stan- 20 m above 140 m, not sufficient to resolve mixed- dard measurements on all TAO moorings include layer depth. hourlysurface wind speed and direction, air During the August 1997 deployment, the moor- temperature, relative humidity, 1 m depth SST, ing was also enhanced with an optical rain gauge and dailyaveraged subsurface temperatures. At (ORG), a Seabird Electronics conductivityand 4 M.F. Cronin, W.S. Kessler / Deep-Sea Research I 49 (2002) 1–17

Fig. 2. Time-longitude plot of equatorial SST, OLR, zonal winds, and depth of the 201C isotherm (Z20) from June 1986 through June 1998. Zonal wind, Z20 and SST are measured by TAO buoys. At each longitude, monthly means from buoys at 21S, 01,21N are averaged to produce the equatorial value then ﬁltered in time with a 1-2-1 ﬁlter. The black line along 1101W longitude from September 1997 through February1998 indicates the time and location of the high resolution monitoring.

temperature sensor at 1 m to monitor sea surface For all TAO moorings, dailyaveraged surface salinity1 (SSS), and a set of additional MTRs so data (including SST) are telemetered via satellite in that hourlyaveraged subsurface temperature had near real-time; hourlysurface data and all MTR 5 m vertical resolution within the top 50 m. The subsurface temperature data are available only period from August 1997 through February1998 after recoveryof the mooring. Thus, mooring spans the peak stage of the historic 1997–1998 El failure and vandalism can cause data gaps for up Nino* and will be referred to as the enhanced to full deployment periods in the hourly and monitoring period (EMP). subsurface MTR data, and for generallyshorter periods in the dailyaveraged telemetered data. 1 Salinityis reported on the PSS-78 scale (Lewis and Because of an extensive gap after February1998 in Fofonoﬀ, 1979), which is denoted in this paper byPSU the subsurface data, our analysis will extend only (practical salinityunit). until February1998, the end of the EMP. Hourly M.F. Cronin, W.S. Kessler / Deep-Sea Research I 49 (2002) 1–17 5

Fig. 3. Time-longitude plot of interannual anomalous equatorial band SST, OLR, zonal winds, and Z20 from June 1986 through June 1998. The anomalies are computed bysubtracting the 1984–1998 seasonal climatologies from the fields shown in Fig. 2. As in Fig. 2, the black line indicates high-resolution monitoring. data at 01, 1101W began in late 1985 and ‘‘mixed-layer depth’’ (MLD), which is defined in encompasses at least part of five El Ninos* (1986– terms of density. In regions with a near-surface 1987, 1991–1992, 1993, 1994–1995, and 1997– freshwater stratification, the mixed-layer depth 1998), and two La Ninas* (1988 and 1996). Daily MLD can be shallower than the isothermal-mixed averaged data have more complete records and layer depth MLDT : The layer between the shallow also include the 1982–1983 El Nino.* MLD and deeper MLDT is often referred to as a In our analysis, we define the near-isothermal- ‘‘barrier layer’’ since the salinity gradient iso- mixed layer depth (hereinafter referred to as lates the surface from the cool thermocline water MLDT ) as the depth at which the subsurface and acts as a barrier to turbulent mixing of temperature is 0.51C cooler than the SST, i.e. heat (Lukas and Lindstrom, 1991). However, without subsurface salinitydata, MLD cannot be MLD ¼ zðT ¼ SST À 0:51CÞ: ð1Þ T directlyevaluated and barrier layerscan onlybe MLDT is distinct from the near-isopycnal inferred from temperature inversions, which must 6 M.F. Cronin, W.S. Kessler / Deep-Sea Research I 49 (2002) 1–17 be supported bya salinitystratification to remain between 1 and 5 m was essentiallyzero and MLDT stable. showed no substantial diurnal variability. The Analyses of the upper ocean (near isothermal) upper ocean in fact was nearlyisothermal down to temperature stratification require veryaccurate B50 m, and the depth of the 201C isotherm (Z20) temperature measurements with high vertical was B125 m. resolution. Errors in MLDT due to vertical reso- As the SST rose above 28.51C in earlyDecember lution are estimated as a fraction m of the depth 1997, a regime shift was observed, involving an difference (Dz) between the spanning sensors: abrupt increase in rainfall and change of chara- cter of mixed-layer depth and surface tempe- erðMLDT Þ¼mDz: ð2Þ rature and salinityvariability(Fig. 4). Indeed, For linear stratification, m will be near zero; but with verywarm surface temperatures, a deep for step-like stratification, m can approach one. thermocline, low winds, and increased convective Since the stratification is unknown between clouds and rainfall, conditions at 01, 1101W were temperature sensors, we use a value of 0.5 for m: more typical of the western equatorial Pacific Thus, a 5 m vertical resolution (as for example warm pool. As in the warm pool, individual rain during the EMP) corresponds to a MLDT error of events showed transient SSS fresh anomalies 2.5 m. On the basis of laboratorypredeployment (typically 1 PSU) lasting from a few hours to a and postdeployment calibrations, MTR tempera- day(Cronin and McPhaden, 1998). Consistent ture accuracyis 0.03 1C (Freitag et al., 1994). with freshwater stratification supporting tempera- Because the MTR casing is reflective, small, and ture inversions (cold water above warm water), has high thermal conductivity, it is expected that inversions often occurred during low SSS (less there is negligible thermal warming of the sensor than B33 PSU), and during times of heavyrain due to solar radiation when deployed in the eu- and low (less than 4 msÀ1) wind speeds. The photic zone. Thus, with the vertical and temporal vertical lines in Fig. 4 indicate periods in which resolution available here, MTR data are suitable subsurface temperature at anydepth was more for mixed layer studies. than 0.21C warmer than SST for more than 24 h. For example, on 12 February1998, the daily averaged 20 m temperature was more than 0.61C 3. Results greater than the SST. Likewise, on 21 February 1998, the dailyaveraged 10 m temperature was 3.1. Enhanced monitoring at 01, 1101Wdurin g about 0.31C greater than the SST. During the the 1997–1998 El Nino* 3 month period from 5 December 1997 through 28 February1998, ten inversions greater than 0.2 1C The enhanced monitoring period from August and lasting longer than 24 h were observed, 1997 through February1998 straddles the peak representing 8% of the record. Because a tem- interannual SST anomalies associated with the perature inversion must be supported bysalinity 1997–1998 El Nino* (see line indicating enhanced stratification to remain stable, during periods with monitoring period in Figs. 2 and 3). For a temperature inversions, the mixed-layer depth complete discussion of the large-scale progression must be shallower than MLDT : Thus, temperature of the 1997–1998 El Nino,* see McPhaden (1999), inversions can be used to infer the presence of a Johnson et al. (2000), Slingo (1998). The EMP was salinitystratified barrier layer. However, this marked bytwo distinct regimes (Fig. 4). Prior to represents onlya subset of the times a barrier December 1997, SST at 01, 1101W was always less layer existed at this site, since periods when the than 291C, rainfall was infrequent, and SST and barrier layer was isothermal cannot be identified sea-surface salinity(SSS) had several 20–25 day without subsurface salinitydata. oscillations (apparentlydue to tropical instability Although temperature inversions tended to waves) and minimal high frequency(diurnal) produce a relativelydeep MLDT (and byinference variability. Likewise, the temperature difference a shallow MLD), on clear afternoons with weak M.F. Cronin, W.S. Kessler / Deep-Sea Research I 49 (2002) 1–17 7

Fig. 4. Hourlytime series at 0 1, 1101W during the enhanced monitoring period, 20 August 1997 through 28 February1998. (a) SST at 1 m depth (1C), (b) SST minus 5 m temperature (1C), (c) dailyaveraged wind speed (m s À1), (d) rain rate (mm hÀ1), (e) sea-surface salinity(practical salinityunit), and (f) Z20 and isothermal-mixed layerdepth ( MLDT ) (m). The vertical graybars indicate periods when a temperature inversion greater than 0.21C occurred for more than 24 h. winds there was up to a 11C temperature an isothermal-mixed layer to form and extend stratification between 1 and 5 m (Fig. 4b) so that down to near the top of the thermocline. When the there was essentiallyno mixed layer(Fig. 4f ). thermocline was deep, as it was in December 1997, However, even during these periods of solar- daytime warming and nighttime mixing produced induced surface restratification, nighttime surface an MLDT diurnal cycle ranging from near the cooling and consequent convective mixing caused surface to well below 50 m. However, as the 8 M.F. Cronin, W.S. Kessler / Deep-Sea Research I 49 (2002) 1–17 thermocline shoaled from December 1997 through equatorial Pacific onlyduring the final stages of February1998, the maximum depth of the MLDT El Ninos?* Although the mooring had best resolu- shoaled to B25 m. tion during the enhanced monitoring period Note that the coincidence of the SST and described above, long time series of surface and MLDT diurnal cycles depends to a certain extent subsurface data (albeit with less vertical resolu- upon the choice of DT in the MLDT definition (1). tion) are available at this site. In this section we Although afternoon near-surface temperature analyze the low-frequency variability of the back- stratification was at times as large as 11C in the ground state, which sets the context for our top 5 m, more typically, the peak-to-peak SST analysis of the seasonal and interannual modula- diurnal cycle amplitude and near-surface stratifi- tion of SST diurnal cycle and typically short-lived cation were less than 0.51C. Thus, MLDT as temperature inversions. defined by (1) did not always have a diurnal cycle Because of extensive data gaps in the long associated with the SST diurnal cycle. Likewise, hourlyrecords, for this purpose, MLDT and Z20 because the depth of nighttime mixing is limited by are computed from dailyaveraged temperature, the thermocline’s depth, the MLDT diurnal cycle rather than from hourlydata. Since the Z20 was larger when the thermocline was deeper. calculation is linear, dailyaveraged Z20 is In summary, during the final stages of the 1997– equivalent to Z20 computed from dailyaveraged 98 El Nino,* as the ‘‘western equatorial Pacific’’ temperature. However, this is not necessarilytrue warm pool extended across the entire Pacific and for mixed-layer depth. For example, a very large surface waters at 01, 1101W warmed above 28.51C, daytime thermal surface stratification can appear the site experienced a regime shift with increased as a weak stratification in the dailyaveraged convective clouds and rainfall, temperature in- temperature. Consequently, if DT in (1) is too versions, and increased SST diurnal cycle and small, MLDT computed from dailyaveraged mixed-layer depth variability. The regime shift also temperature can be shallower than the depth of marked the deepest extent of the thermocline and nighttime mixing. We posit that DT should be near-isothermal layer (MLDT ). Prior to mid- larger than the amplitude of the SST diurnal cycle November, both the thermocline and MLDT so that the dailyaveraged SST minus DT is cooler tended to deepen (although not necessarilyat the than the nighttime SST. If so, then MLDT same time); after earlyDecember, both tended to computed from dailyaveraged temperature will shoal. Thus, in terms of the questions posed in be a rough measure of the depth of nighttime Section 1, we find that the SST diurnal cycle does mixing. Fig. 5 shows a comparison of MLDT appear to have increased as SST warmed above computed from hourlyand dailyaveraged tem- 28.51C, and that the nighttime mixed-layer depth peratures. Note that while there are occasional tended to be deep when the thermocline was deep periods when the hourlymixed layeris deeper than and shallow when the thermocline was shallow. MLDT from dailyaveraged temperature (e.g. 2nd week of December, 2nd week of January), most of 3.2. Annual and interannual variability at 01 1101W these periods appear to be associated with temperature inversions. During inversions, MLD Clearly several types of mixed layer regimes based on densityis shallower than MLDT ; and it is occur at 01, 1101W. However, with onlya 7-month unlikelythat mixing extends throughout MLDT : record it is not possible to determine how the However, in most cases, MLDT defined by(1) and December 1997 regime shift relates to the seasonal computed with dailyaveraged temperature ap- and ENSO cycles. Typically how large is the SST pears to adequatelycapture the depth of nighttime diurnal cycle amplitude in September versus mixing represented bythe deep nighttime values of January? Was the SST diurnal cycle amplitude hourly MLDT : during this historic El Nino* warm event larger or In order to analyze the seasonal and interannual smaller than during La Nina* cool events? Are variability, all variables in Fig. 6 (OLR, wind temperature inversions present in the eastern speed, SST, SSTÀ10 m temperature, MLDT ; and M.F. Cronin, W.S. Kessler / Deep-Sea Research I 49 (2002) 1–17 9

Fig. 5. MLDT computed from hourlytemperature data (thin black line) and from dailyaveraged temperature data (thick greyline) for the 01, 1101W enhanced monitoring period from September 1997 through February1998.

Z20) were monthlyaveraged and subsampled to when the ITCZ is at its northernmost latitude. once per month, then averaged to produce a Verylow OLR values (signifyingdeep atmospheric seasonal climatology. The seasonal climatology convection) occurred at this location onlyduring was subtracted from the monthlyaveraged time El Ninos* (Figs. 3 and 7). Consequently, the OLR series and smoothed with a 3-month boxcar filter annual cycle at 01, 1101W was not significant at the to produce the interannual anomalies shown in 95% confidence level. Fig. 7. Because the vertical resolution of the The thermocline depth (Z20) seasonal cycle subsurface temperature was typically 10–20 m, (Fig. 6f ) was also not significant at the 95% the daily MLDT measurement error was typically confidence level. MLDT ; however, did have a 5–10 m according to (2). With an integral time significant seasonal cycle and was shallowest scale of B10 days and assuming these to be during March, the warm season, when on average random errors, the measurement error of the there was nearlya 1 1C stratification in the top monthlyaveragedpffiffiffi MLDT is reduced byapproxi- 10 m (Fig. 6de). As shown in the next section, the mately1/ 3 to be B3–6 m. The measurementpffiffiffiffiffi SST diurnal cycle amplitude is also larger during error is further reduced bya factor of 1/ 13 to be the warm season, suggesting that the shallow 1–2 when climatologies based on 13 years of data mixed layer during the warm season is due to are computed. Thus, onlyconfidence limits for solar-induced thermal restratification caused by estimating climatological means for each month seasonal weak winds and high insolation. are shown in Fig. 6. El Ninos* (1986–1987, 1991–1992, 1993, 1994– Although the sun crosses the equator twice per 1995, and 1997–1998) and La Ninas* (1988 and year, in March and again in September, SST has 1996) are clearlyseen in the interannual anomaly predominantly a one-cycle per year oscillation at time series shown in Fig. 7. In particular, the 01, 1101W, with a warm season from February commonlyused southern oscillation index (surface through May, and a cold season from June pressure difference between Darwin and Tahiti) is through November (Fig. 6c). Consistent with the highlyanti-correlated at zero lag with the interannual migration of the ITCZ, wind speed is low annual SST anomalies at 01, 1101W. Typically, El during the warm season when the ITCZ is closest Nino* events (negative SOI) are characterized at 01, to the equator, and high during the cold season 1101W bywarm SSTs, increased convection, 10 M.F. Cronin, W.S. Kessler / Deep-Sea Research I 49 (2002) 1–17

Fig. 6. Monthlyaveraged time series (left panels) and seasonal averages (right panels) at 0 1, 1101W. (a) OLR (W mÀ2), (b) wind speed À1 (m s ), (c) SST (1C), (d) SST minus temperature at 10 m (1C), (e) MLDT (m), and (f) Z20 (m). The 95% conﬁdence level is shown for the seasonal cycles (dotted line). slightlyhigher wind speeds, and deep Z20 and the thermocline. Therefore, when the thermocline MLDT : Likewise, at this location, La Nina* events is veryshallow, as it is during La Ni nas,* MLDT (positive SOI) are characterized bycooler SSTs, will correspondinglybe limited in depth. For slightlylower wind speeds, reduced convection, example, during the 1988 La Nina,* the 201C iso- and shallower thermocline and MLDT (Fig. 7). therm outcropped and there was a 2.51C tempera- Note that MLDT cannot be deeper than the top of ture diﬀerence between 1 and 10 m, indicating that M.F. Cronin, W.S. Kessler / Deep-Sea Research I 49 (2002) 1–17 11

Fig. 7. Same as Fig. 6, but for interannual anomalies, smoothed with a 90-dayboxcar ﬁlter. The Southern Oscillation Index (SOI) is shown as a dotted line in (c).

there was essentiallyno mixed layerat this time. 3.3. Annual and interannual modulation of high Thus, both surface process (e.g. wind mixing and frequency variability at 01, 1101W surface heating) and thermocline variabilityact to produce a shallow mixed layer during La Nina* 3.3.1. Temperature inversions cold events and a deep mixed layer during El Nino* As illustrated during the 1997–1998 El Nino* en- warm events. Consequently, interannual Z20 and hanced monitoring period (Fig. 4), large (B0.21C) MLDT anomalies are in phase and have a cross- temperature inversions are typically short-lived. correlation of 0.94. Only11 events during the EMP had inversions 12 M.F. Cronin, W.S. Kessler / Deep-Sea Research I 49 (2002) 1–17 larger than 0.21C and lasting longer than 24 h. deep atmospheric convection resulted in strong Since a 24-h event mayspan parts of two different rainfall. Because the temperature inversions must days, the inversions may appear to be weaker and be supported bya freshwater stratified barrier longer when estimated with dailyaveraged tem- layer, we can conclude that barrier layers were perature data centered at 12:00 GMT. However, present during these periods. although large inversion events maybe identifiable The coincidence of temperature inversions, rain in the dailyaveraged data, these short-lived events events and weak winds (Figs. 4 and 8) suggests are unlikelyto register in monthlyaveraged data. barrier layers were formed by local rain events. It is therefore not surprising that based on Near the dateline, saltywarm water can be monthlyaveraged data, 1 m SST was always subducted under the western Pacific warm fresh warmer than the temperature at 10 m (Fig. 6d). water to form thick barrier layers (Lukas and To determine the prevalence of inversions, we Lindstrom, 1991; Vialard and Delecluse, 1998). use the long time series of dailyaveraged Further studyis needed to determine whether temperatures to compute temperature differences subduction also plays a role in forming barrier relative to 1 m SST. Fig. 8 shows for each daythe layers in the eastern equatorial Pacific. maximum positive differences (i.e. inversions) that are greater than 0.031C (the accuracyof the 3.3.2. SST diurnal cycle sensor), monthlyaveraged precipitation from Xie The enhanced monitoring period during the and Arkin (1995), and the SOI. Since changing 1997–1998 El Nino* also showed variations in the vertical resolution can cause apparent changes in SST diurnal cycle amplitude, with minimal diurnal the magnitude of the temperature inversion, cycle amplitudes before SSTs rose above 28.51Cin magnitudes in Fig. 8 should be viewed with December 1997. In order to determine the seasonal caution. With these caveats, Fig. 8 shows that and interannual modulation of SST diurnal cycle, temperature inversions at 01, 1101W occurred we performed a complex demodulation (Bloom- almost exclusivelyduring the final stages of strong field, 1976; Kessler et al., 1995) on the long time El Nino* warm events, when winds were weak and series of hourlySST. The procedure expresses SST

Fig. 8. Temperature inversion time series at 01, 1101W computed from dailyaveraged data (solid bars), compared to the SOI (grey line) and monthlyrain rates (dotted line). A negative SOI (plotted upward here) is commonlyused as an indicator of an El Ni no* warm event. The stripe across the top of the plot shows periods when temperature inversion could be estimated. Onlyinversions greater than the accuracyof the sensor (0.03 1C) are shown. Rain rates are from Xie and Arkin (1995). M.F. Cronin, W.S. Kessler / Deep-Sea Research I 49 (2002) 1–17 13 as a diurnal cycle with slowly varying amplitude During the 8-yr record, the monthly averaged AðtÞ and phase FðtÞ superimposed upon the rest of SST diurnal cycle amplitude ranged from below the variability ZðtÞ 0.11C during the earlypart of the 1997–1998 El Nino* to nearly0.5 1C during the warm seasons of SSTðtÞ¼AðtÞ cosðot À FðtÞÞ þ ZðtÞ; 1986 and 1988 (a La Nina* year) (Fig. 9). The SST where o ¼ 2p/day. Peak-to-peak diurnal varia- diurnal cycle was modulated both by the seasonal tions are equivalent to twice the amplitude AðtÞ: As cycle and by ENSO. During the warm season in Figs. 6 and 7, the monthlyaveraged AðtÞ time (February–April), the diurnal cycle typically had series is decomposed into annual and residual an amplitude of up to 0.41C, but at other times of interannual components (Fig. 9). The diurnal cycle the year the amplitude was typically less than nearlyalwayshas maximum SST at 16:00 local; 0.21C. The ENSO cycle caused approximately therefore, the phase modulation FðtÞ is not shown. 70.11C variations in the SST diurnal amplitude,

Fig. 9. SST diurnal cycle amplitude (solid line; 1C) and wind speed (dotted line; m sÀ1). Top panel: amplitude time series. Middle panel: interannual anomalies. Bottom panel: seasonal cycles. 14 M.F. Cronin, W.S. Kessler / Deep-Sea Research I 49 (2002) 1–17 with higher amplitudes during La Ninas* and lower cycle modulation. Local wind forcing and surface amplitudes during El Ninos.* On both the seasonal heating processes that produce seasonal warming and interannual time scales, the SST diurnal cycle and cooling also increase and decrease the SST amplitude varied in relation to local winds and diurnal cycle amplitude. For example, weaker deep atmospheric convection, with higher ampli- wind speeds and stronger solar radiation in March tudes during periods of weak winds and clear contribute to both seasonal warming and an skies, and lower amplitudes during periods of increased SST diurnal cycle. For this reason, the strong winds and deep atmospheric convection seasonal modulation of the SST diurnal cycle is in (Figs. 6 and 9). Although the SST diurnal cycle phase with the SST seasonal cycle. increased following the December 1997 regime In contrast, interannual variations in east Pacific shift (Fig. 4), it was still weaker than typically SST are controlled primarilybylarge-scale three- found during warm seasons because of the reduced dimensional processes. Shifts in the western and insolation associated with this historic El Nino.* central Pacific trade winds generate Kelvin waves, which result in a basin-wide adjustment of the thermocline depth. During La Ninas,* maximum 4. Discussion and summary trade winds and deep atmospheric convection shift westward, resulting in clear skies and relatively In answer to the first question stated in Sec- weak winds and consequentlyanomalouslyhigh tion 1, ‘‘is the SST diurnal cycle amplitude largest diurnal cycle amplitude (11C) in the eastern equa- during warm phases of the seasonal and ENSO torial Pacific. During El Ninos,* the reverse occurs cycles?’’, we find that on seasonal time scales, SST (Rasmusson and Carpenter, 1982; Harrison and diurnal cycle amplitude is largest during the warm Larkin, 1998). In particular, during the earlystage season; but on interannual timescales, the SST of the 1997–1998 El Nino* warm event, maximum diurnal cycle is largest during the cold phase of trade winds shifted eastward so that wind speeds ENSO. These results highlight the different physics were anomalouslyhigh at 0 1, 1101W through at playon these two timescales. Surface heating October 1997. Then as the waters warmed above and local wind-forced upwelling and mixing are B28.51C in December 1997, deep convective dominant processes controlling seasonal SST clouds shifted eastward and the winds weakened variability, but are secondary processes on ENSO (Figs. 2 and 3). Thus, first higher winds and then time scales. to a lesser extent deep tropical convection at 01, Although the sun crosses the equator twice per 1101W caused the SST diurnal cycle amplitude to year, in March and again in September, SST is be anomalouslylow ( À11CtoÀ0.51C) during the warmest during Februarythrough April and 1997–98 El Nino* warm event. Consistent with coolest during August through October. Heat empirical interannual heat budgets (Wang and balance analyses of this region (Enfield, 1986; McPhaden, 2000), on ENSO timescales, local Hayes et al., 1991; Kessler et al., 1998; Swenson surface heating acts as a damping term, tending and Hansen, 1999; Wang and McPhaden, 1999) to warm the SSTs during La Nina* cold events and attribute the seasonal variations to a varietyof cool the surface during El Nino* warm events. processes, including the equinoctial increase in The second question posed in Section 1 was solar radiation, changes in upwelling and turbu- whether the thermocline displacements could be lent mixing associated with the local trade wind used as a proxyfor mixed-layerdepth variations. forcing, meridional heat fluxes associated with The isothermal-mixed layer depth (MLDT )is tropical instabilitywaves, and the reduction in always shallower than the thermocline depth as solar radiation due to the extensive stratocumulus represented bythe depth of the 20 1C isotherm decks that cover the eastern Pacific during August (Z20); mean depths of these two surfaces are 22 to November. Of these seasonal heating and cool- and 58 m (Table 1). Likewise, interannual MLDT ing processes, onlythe tropical instabilitywaves anomalies are smaller than Z20 anomalies. As have no corresponding effect on the SST diurnal shown in Table 1, interannual anomalies have M.F. Cronin, W.S. Kessler / Deep-Sea Research I 49 (2002) 1–17 15

Table 1 Standard deviations and means of variables shown in Figs. 6 and 7 (outgoing longwave radiation (OLR), wind speed, sea-surface temperature at 1 m (SST), temperature difference between 1 and 10 m, isothermal-mixed layer depth (MLDT ) defined by(1), and depth of the 201C isotherm (Z20)). Standard deviations of the annual cycle amplitude, interannual anomalies, and monthly averaged time series are listed. Annual cycles that are significantly different than zero at the 95% confidence limit are indicated in bold

Standard deviations

Annual cycle Interannual anomalies Monthly averages Means

OLR (WmÀ2) 2 9 9 269 Wind speed (msÀ1) 0.88 0.86 1.21 4.80 SST (1C) 1.31 1.61 2.02 24.03 SST - T(10 m) (1C) 0.22 0.28 0.36 0.37 MLDT (m) 6 12 14 22 Z20 (m) 6 24 25 58

standard deviations of 12 m for MLDT and 24 m dominates over precipitation. Furthermore, using for Z20. However, with these caveats, on inter- Levitus (1982) climatological data, Sprintall and annual time scales, Z20 can be used as a proxyfor Tomczak (1992) showed that there was essentially MLDT variabilitysince these two surfaces are well no mean barrier layer in the eastern equatorial correlated (0.94), both being deep during El Nino* Pacific. However, based on CTD data collected warm events and anomalouslyshallow during La from 1976 to 1994, Ando and McPhaden (1997) Nina* events. However, on seasonal timescales, showed that the composite El Nino* had a 10–20 m MLDT and thermocline depth tend to be unrelated thick barrier layer across the entire equatorial (Fig. 6). The isothermal-mixed layer depth is Pacific. shallowest during the warm season, March, and Although no subsurface salinitydata is available deepest in November, with a standard deviation at the 01, 1101W TAO mooring, temperature of 6 m (Table 1). In contrast, the thermocline inversions, implying a shallow halocline and the depth seasonal cycle is not significant at the 95% presence of a barrier layer, were observed during confidence level. Thus, on seasonal timescales, periods of high rainfall and weak winds. Following thermocline depth cannot be used as a proxyfor the regime shift in earlyDecember 1997, tempera- mixed-layer depth. ture inversions greater than 0.21C and lasting In general, mixed-layer depth should be defined longer than a daymade up 8% of the record. Over in terms of densityrather than temperature. In the full 18 year time series of daily averaged sub- particular, if the near surface is stratified in salinity surface temperature, inversions were observed but not in temperature, the isothermal layer almost exclusivelyduring El Ni no* warm events, between the shallow halocline and the top of the and primarilyduring the two ‘‘El Ni nos* of the thermocline can act as a barrier to turbulent century’’ (1982–1983 and 1997–1998). During entrainment of cool thermocline water into the these warm events, the inversions were frequent surface mixed layer (Lukas and Lindstrom, 1991). and were often greater than 0.11C when averaged Cooling processes at the surface and solar radia- over a day. Since it is possible to have a barrier tion penetrating into the salinitystratified ‘barrier layer without having a temperature inversion, layer’ can cause temperature inversions to develop barrier layers must have occurred even more without causing the densityprofile to become frequentlythan this. Since barrier layers shield convectivelyunstable (Anderson et al., 1996). In the surface from turbulent mixing of cold thermo- this case, turbulent entrainment of warm barrier cline water, it is likelythat the barrier layer layer water produces surface warming. In the supporting these temperature inversions helped eastern Pacific, salinityeffects are usuallyignored maintain warm SSTs even as the thermocline since the thermocline is shallow and evaporation shoaled during the termination of the El Nino.* 16 M.F. Cronin, W.S. Kessler / Deep-Sea Research I 49 (2002) 1–17

Although the ocean is forced bythe atmosphere, Freitag, and the TAO Project Office for their effort the atmosphere also responds to ocean surface in providing this data set. Paul Freitag played a conditions. In particular, based on the coupled critical role in collecting the extra thermistor data ocean atmosphere response experiment (COARE) on the August 1997 deployment. Funding for the bulk flux algorithm (Fairall et al., 1996a, b; Cronin raingauge and salinitysensor was provided in part and McPhaden, 1997), a 70.51C SST variation bythe NASA TRMM program. Funding for this from mean conditions at 01, 1101W can cause a analysis was provided by NOAA’s Office of 712 WmÀ2 latent heat flux variation and a Global Programs. This is NOAA Pacific Marine 74WmÀ2 sensible heat flux variation. Thus, the Environmental Laboratorycontribution No. 2246. SST diurnal cycle modulation may also be associated with a modulation of the latent and sensible heat fluxes. Similarly, SST variations also affect carbon References dioxide (CO2) flux between the ocean and atmosphere. For most of the world’s oceans, the ther- Anderson, S.P., Weller, R.A., Lukas, R., 1996. Surface buoy- ancyforcing and the mixed layerof the western equatorial modynamic effect of SST variations on the partial Pacific warm pool: observations and 1-D model results. pressure of carbon dioxide is approximately4.23% Journal of Climate 9, 3056–3085. per 11C (Takahashi et al., 1993). Often, ‘‘surface’’ Ando, K., McPhaden, M.J., 1997. Variabilityof surface layer hydrographic bottle measurements are at 10 m hydrography in the tropical Pacific ocean. Journal of depth and underwayship intake surface measure- Geophysical Research 102, 23063–23078. Bloomfield, P., 1976. Fourier Decomposition of Time Series: ments are at 5–8 m depth. As discussed byMcNeil An Introduction. Wiley, New York, 258pp. and Merlivat (1996), daytime temperature stratifi- Bond, N.A., McPhaden, M.J., 1995. An indirect estimate of the cation above 10 m depth can cause a 2% increase diurnal cycle in upper ocean turbulent heat fluxes at the equator, 1401W. Journal of Geophysical Research 100, in the dailysea-to-air CO 2 flux in the eastern equatorial Pacific and a 20% increase in the western 18369–18378. Cronin, M.F., McPhaden, M.J., 1997. The upper ocean heat equatorial Pacific. In addition, trapping within the balance in the western equatorial Pacific warm pool during shallow warm layer may also affect the surface September–December, 1992. Journal of Geophysical Re- dissolved gas concentration and thus the air–sea search 102, 8533–8553. flux. Our results suggest that this diurnal pumping Cronin, M.F., McPhaden, M.J., 1998. Upper ocean salinity will have seasonal and interannual modulations. balance in the western equatorial Pacific. Journal of Geophysical Research 103, 27567–27587. Surface conditions should be measured well above Deser, C., Wallace, J.M., 1987. El Nino* events and their 10 m during the warm season (February–April) relation to the southern oscillation: 1925–1986. Journal of and during La Nina* cold events (Figs. 6 and 7). Geophysical Research 92, 14189–14196. At the equator, zonal trade winds can cause Enfield, D.B., 1986. Zonal and seasonal variations in the near- substantial upwelling, mixing, and surface cooling. surface heat balance of the equatorial ocean. Journal of Physical Oceanography 16, 1038–1054. The resulting SST gradients in turn affect the winds Fairall, C., Bradley, E.F., Rogers, D.P., Edson, J.B., Young, and cloud structure, causing the ocean-atmosphere G.S., 1996a. Bulk parameterization of air-sea fluxes for system to be highly coupled. Understanding the tropical ocean-global atmosphere coupled-ocean atmo- processes bywhich the mixed layerproperties vary sphere response experiment algorithm. Journal of Geophy- is essential for quantitative diagnostics of the cou- sical Research 101, 3747–3764. Fairall, C., Bradley, E.F., Godfrey, J.S., Wick, G.A., Edson, pled ocean and atmosphere system and its effect on J.B., Young, G.S., 1996b. Cool skin and warm layer effects the biogeochemical ecosystem. on sea surface temperature. Journal of Geophysical Research 101, 1295–1308. Freitag, H.P., Feng, Y., Mangum, L.J., McPhaden, M.J., Acknowledgements Neaner, J., Stratton, L.D., 1994. Calibration procedures and instrumental accuracyestimates of TAO temperature, relative humidityand radiation measurements. NOAA The authors wish to thank Chris Sabine for Technical Memo. ERL PMEL-104, 32 pp. Pacific Marine helpful discussions, and Michael McPhaden, Paul Environmental Laboratory, NOAA, Seattle, WA. M.F. Cronin, W.S. Kessler / Deep-Sea Research I 49 (2002) 1–17 17

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