Second peak in the far eastern Pacific sea surface temperature anomaly following strong El Niño events

GEOPHYSICAL RESEARCH LETTERS, VOL. 40, 4751 4755, doi:10.1002/grl.50697, 2013 Second peak in the far eastern Pacific sea surface temperature anomaly following strong El Niño events WonMoo Kim 1 and Wenju Cai 1 Received 18 May 2013; revised 19 June 2013; accepted 24 June 2013; published 9 September 2013. [1] The eastern Pacific sea surface temperature anomaly (SSTa) associated with El Niño usually peaks in December, January, and February, and decays in the ensuing months; sometimes, however, after a strong El Niño, a second peak in SSTa is observed in the far eastern equatorial Pacific. It is found that an enhanced westerly wind stress anomaly over the equatorial Pacific basin, usually associated with a meridional swing of South Pacific Convergence Zone (SPCZ), precedes the occurrence of the second peak. As the equatorial thermocline slope balances with the zonally integrated zonal-wind stress, the westerly wind stress anomaly induces deepened thermocline and suppresses upwelling in the eastern Pacific for the double-peak cases. On the other hand, enhanced latent heat release and reduced shortwave radiation cause a pause in warming between the two peaks. As a result, a strong El Niño, usually accompanied by a zonal SPCZ, exhibits distinct double-peak characteristics. Citation: Kim, W. M., and W. Cai (2013), Second peak in the far eastern Pacific sea surface temperature anomaly following strong El Niño events, Geophys. Res. Lett., 40, 4751 4755, doi:10.1002/grl.50697. 1. Introduction [2] El Niño-Southern Oscillation (ENSO) is a dominant mode of the coupled atmosphere-ocean variability in the equatorial Pacific. The positive phase of ENSO, El Niño, generally disappears by the following boreal spring season [An and Wang, 2001; An and Choi, 2009; Galanti and Tziperman, 2000; Tziperman et al., 1998]; however, in some cases, after an extremely strong El Niño event [Chiodi and Harrison, 2010; Vecchi, 2006; Vecchi and Harrison, 2006], a second sea surface temperature (SST) anomaly peak is observed at the far eastern Pacific [cf. Lengaigne and Vecchi, 2010]. It is surprising that, as shown in Lengaigne and Vecchi [2010], some of the climate models also reproduce this distinct double-peak characteristics of the strong El Niño. This double-peak feature can have profound impact on global circulation and regional climate, especially on the Western North Pacific and Asian Monsoon system [Nigam, 1994; Wang et al., 2001], as well as Atlantic and American [Chiang et al., 2000; Kane, 1999] climates. Although it seems that the double peaks of SST anomalies are a robust feature of strong El Niño s, only few previous studies pay attention to the second peak; most previous studies treat them as a prolonged or delayed termination of ENSO [cf. 1 CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia. Corresponding author: WonMoo Kim, CSIRO Marine and Atmospheric Research, Aspendale, VIC 3195, Australia. (WonMoo.Kim@csiro.au) 2013. American Geophysical Union. All Rights Reserved. 0094-8276/13/10.1002/grl.50697 Fedorov, 2002]. A very relevant study by Vecchi and Harrison [2006] analyzed the unusual termination of 1997 1998 El Niño and pointed out that the disappearance and the return of easterlies due to a southward movement of the intertropical convergence zone (ITCZ; c.f. [McPhaden, 1999]) and decoupling of surface and subsurface are responsible for prolonged warm SST and rapid termination under shallow thermocline condition. However, the double-peak feature in the far eastern Pacific has not been fully discussed. [3] In this study, the underlying dynamics and physics of the second peak are examined. The data used are summarized in section 2. In section 3, the double-peak features of strong El Niño events are analyzed through the heat budgets of the upper layer of the far eastern Pacific where the second peak is observed. The dynamics of the occurrence of the second peak is investigated, and an explanation of the pause between the two peaks of the SST anomalies is given at the section 4. 2. Data and Definition [4] The Extended Reconstructed SST (ERSSTv3b) [Smith et al., 2008] is analyzed and compared to NOAA Optimum Interpolation Sea Surface Temperature version 2 (OISSTv2; available at http://www.esrl.noaa.gov/psd/) [Reynolds et al., 2002] for verification of the result. For a calculation of the dynamic heating of the ocean, Simple Ocean Data Assimilation (SODA 2.2.4) [Carton and Giese, 2008] is used. Some atmospheric variables including radiative, sensible, and latent heat fluxes, and cloud covers are derived from the 20 th Century Reanalysis version 2 (20CRv2) [Compo et al., 2011]. Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP, provided by the NOAA/ OAR/ESRL PSD, Boulder, Colorado, USA) [Xie and Arkin, 1997] is analyzed for calculating equatorial mean precipitation rate. All data are analyzed during the period of 1960 2010, except for CMAP (1979 2010). [5] In this study, El Niño events are defined when the December February (DJF) average Niño3 index (area-averaged SST anomaly over 150 W 90 W and 5 S 5 N) is greater than 1.0 K, which is about one standard deviation value. The second peak is definedwhentheapril May June (AMJ) average sea surface temperature anomaly (SSTa) over the far eastern equatorial Pacific (Niño2, 80 W 90 W, 5 S EQ)ishigherthan1.0KafteranEl Niño event. El Niño (not) followed by a second SST peak is classified as a (single-) double-peak event. All indices are detrended by subtracting a 31-year-window moving average. Heat budgets are calculated for the dynamic heating terms ( Ũ T, where Ũ is the three-dimensional oceanic current vector and T is temperature) by averaging top 35 m of the far eastern Pacific region. 4751

KIM AND CAI: DOUBLE PEAK EL NIÑO Figure 1. Hovmöller diagrams of (a,b) SST (shadings, K) and precipitation rate (contours, mm day 1) anomalies, and (c,d) sea surface height (shadings, cm) and zonal-wind stress (contours, 10 2 N m 2) for single-peak (left) and double-peak (right) events. 3. Results [6] In many of the historical El Niño events, the SSTa peaks around boreal winter and decays during the following spring season; however, Figure 1 shows that after strong El Niño events [Chiodi and Harrison, 2010; McPhaden, 1999], the SSTa exhibits marked double peaks. The second peak occurs during boreal spring to early summer (March July) in the far eastern Pacific. A composite of SST anomalies for these double-peak events (i.e., 1969, 1983, 1987, 1992, and 1998) shows that the second peak is distinguished from a westward-propagating El Niño (first peak) SSTa which peaks in boreal winter, and the westward anomaly propagation is more obvious in single-peak events compared to the double-peak events (cf. Figures 1a and 1b). In the single-peak cases (i.e., 1964, 1966, 1973, 1995, and 2003; the 2010 event [Kim et al., 2011] is excluded from either cases as its AMJ Niño2 index remains in a moderate, positive range while others have either strong positive (> +1.0 K) or negative values), the precipitation anomaly resides around the dateline during the ENSO-peak period (DJF) and decays during boreal spring without showing any significant sign of propagation. In the doublepeak cases, on the other hand, the equatorial precipitation anomaly is centered around 160 W during the El Niño mature phase and continues to expand to the equatorial eastern Pacific during the following seasons until the second maximum in the precipitation anomaly occurs in concurrence with the second peak of the eastern Pacific SST. [7] Although the SSTa shows a distinct second peak, the subsurface structure reveals a close continuous connection between the two peaks (Figure 1d). For a single-peak El Niño, the sea surface height (and the thermocline depth) anomalies quickly recover and the equatorial Pacific discharges by the end of boreal winter; after spring, the equatorial Pacific basin including the far eastern Pacific shows a shallower-thannormal thermocline depth and a significant cooling is observed. In the double-peak cases, on the other hand, significant positive thermocline depth anomalies persist after their mature phase and Figure 2. Composite of FMA mean 10 m zonal wind (contours, m s 1) and precipitation rate (shadings, mm day 1) for (a) single-peak, (b) double-peak cases, and (c) their differences, respectively. Significant differences of zonal wind (t test) and precipitation (bootstraping, nb = 10,000) rate are indicated by diagonal strokes and dots for wind and rainfall, respectively. 4752

Figure 3. Relationship between the AMJ Niño2 far eastern Pacific SST anomaly and (a) precipitation rate (FMA) over 160 W 90 W, 5 S 2.5 N, and (b) preceding zonal wind stress (FMA) forcing over 130 E 90 W, 5 S EQ. El Niño years are marked as black circle and year for single-peak and double-peak cases, respectively. slowly propagate eastward before their disappearance in the far eastern Pacific during the following summer. [8] Figure 2 shows composite of total precipitation rate (mm day 1 ; shadings) and total zonal wind (m s 1 ; contours) for (a) single-peak, (b) double-peak cases, and (c) their differences. Statistically significant differences at the 95% confidence level (Student s t-test for zonal wind and bootstrapping (n B = 10,000) for precipitation) between the two El Niño categories (Figure 2c) are indicated by diagonal strokes for wind stress and colored dots for precipitation. As the difference in SST tendency between the two categories is the largest after (before) the first (second) peak, we choose February March April (FMA) average for a comparison. During this period, in the single-peak cases, the precipitation and the zonal wind stress are recovering from the anomalous El Niño state, although enhanced ITCZ rainfall and westerlies are still found over the eastern Pacific (Figure 2a); in the central Pacific, however, there are no significant anomalies, and there exist distinct ITCZ-South Pacific Convergence Zone (SPCZ) branches. In contrast, the rainfall is significantly increased over the central equatorial Pacific in the double-peak cases, and the ITCZ and the SPCZ are merged onto the equator to form the zonally elongated SPCZ [Cai et al., 2012; Vincent et al., 2011]. Concurrent with the increased precipitation in the central equatorial Pacific, stronger westerly zonal wind is observed over the equatorial Pacific. [9] Scatter diagrams in Figure 3 show a strong nonlinear relationship between the AMJ mean far eastern Pacific SST and the leading indices of FMA-mean (a) precipitation over 160 W 90 W, 5 S 2.5 N and (b) zonal wind stress forcing over 130 E 90 W, 5 S EQ. The FMA mean precipitation is strongly positively skewed, i.e., there exist strong positive values greater than 4.0 mm day 1 and up to 11 mm day 1, while most of the cases lie within 1.9 ± 0.4 mm day 1. These strong-el Niño years include 1983, 1992, and 1998, which are also classified as zonal SPCZ events in the previous studies [Cai et al., 2012; Vincent et al., 2011], in which the DJF SPCZ swings toward the equator such that it is parallel to the equator. Associated with this extreme swing of the SPCZ, an enhanced westerly anomaly in the equatorial Pacific is observed. The basin-wide zonal wind anomaly in FMA precedes the following AMJ SSTa second peak over the eastern Pacific. Vecchi and Harrison [2006] and Lengaigne and Vecchi [2010] postulated that the southward movement of the ITCZ plays a key role, as it effectively decouples/couples the subsurface and surface, which results in a prolonged warming/abrupt cooling in the Niño3-nearby Figure 4. Temperature tendency in FMA contributed by dynamic advection (K mon 1 )andsurfaceflux (W m 2 )for single- and double-peak cases. Blue, red, and green bars indicate advection of anomalous temperature by mean currents, mean temperature advection by anomalous currents, and nonlinear advection, respectively. Also shown on the right are anomalous latent heat (LHF), sensible heat (SHF), longwave radiation (LW), and shortwave radiation (SW) fluxes at the surface. The bars for the fluxes are scaled so that they are equvalent to the temperature tendency in K mon 1 if they were equally distributed to approximately top 30 m of the ocean. 4753

region; however, in the far eastern Pacific, because the local coupling and movement of the ITCZ plays an indirect role (see budget analysis later), the focus will be on the westerly anomaly, usually accompanied by a zonal SPCZ. The westerly wind stress forcing across the equatorial Pacific is balanced by the zonal gradient of sea surface height or thermocline depth, as typically observed for the double-peak cases (Figure 1d). Also, with the existence of the dissipation and friction in a semiclosed basin, there is anomalous downwelling (suppressed upwelling) over the eastern Pacific in response to the anomalous wind stress forcing, conducive to warm SSTa in this region. [10] The anomalous warming in the far eastern Pacific is accomplished by local air-sea interactions and/or by ocean dynamics. Processes such as evaporation-wind-sst feedback and ocean dynamics are known to play a dominant role in the regulation of oceanic temperature over the eastern Pacific [Li et al., 2000]. Therefore, we examine possible differences in the role of dynamic heating and local air-sea interactions between the single-peak and the double-peak events. Figure 4 shows the top 35 m average dynamical advective heating rates, surface heat fluxes, and radiative fluxes for the far eastern Pacific Niño2 region. After the mature phase of single-peak El Niño events, the far eastern Pacific starts to cool from FMA (total anomalous cooling rate for the three months, T a / t= 0.78 K mon 1 ). This cooling is mostly accomplished by an anomalous zonal circulation, i.e., an anomalous upward motion at the far eastern Pacific in association with the termination of ENSO, which upwells colder mean subsurface water ( w a T m / z= 0.44 K mon 1 ; anomalies indicated by X a, and mean indicated by X m ), and the anomalous westward zonal current advects cooler surface water to the west ( u a T m / x= 0.25 K mon 1 ). For double-peak El Niño events, however, the top 35 m layer of the far eastern Pacific is still warming in FMA at the rate of T a / t = +0.40 K mon 1. As the zonally integrated wind stress, which is anomalously westerly, acts on the equatorial Pacific, a significant deepening of the thermocline in the eastern Pacific with suppressed upwelling occurs. The deeper-than-normal thermocline forms anomalously warm water at the subsurface level, and the mean upwelling adiabatically delivers heat to the surface ( w m T a / z=+0.25k mon 1 ). Suppressed upward vertical motion or anomalous downwelling also warms the far eastern Pacific at the rate of w a T m / z=+0.42k mon 1,in contrast to the enhanced cooling in the single-peak cases ( w a T m / z= 0.44 K mon 1 ). However, the surface fluxes, especially the latent heat flux and the shortwave radiative flux, contribute to a strong cooling in the double-peak events. With higher SST, cloud cover significantly increases (by 30% and 24% in the middle- and high-cloud cover, respectively, compared to the single-peak cases) over the far eastern Pacific, which obstructs shortwave radiation (double-peak SW a = 11.5 W m 2 ; single-peak SW a =+0.6Wm 2 ). At the same time, the latent heat flux (double-peak LHF a = 23.4 W m 2 ; single-peak LHF a =+9.5Wm 2 ) strongly damps the SSTa. Due to this strong surface cooling, there exists a pause between the first and the second SST peaks. 4. Summary and Discussion [11] Our analysis shows that during strong El Niño events, usually accompanying a meridional swing of the SPCZ, anomalous oceanic circulation supported by concurrent wind anomalies is primarily responsible for the occurrence of the second peak of the far eastern Pacific SSTa, while the cloud (i.e., radiative) feedback and evaporation (i.e., latent heat) feedback lower the SST. Most of the anomalous second-peak warming comes from the anomalous zonal circulation and deepened thermocline depth. The oceanic vertical structure or thermocline slope balances with the zonally integrated zonal wind stress at the equator, so the strong westerly anomaly associated with zonally elongated SPCZ results in a warmer subsurface temperature (i.e., deeper thermocline) in the eastern Pacific, as well as the enhanced anomalous downward motion (i.e., suppressed upwelling). Although oceanic waves continuously transmit the warming signal to the east, the SSTs show a pause between the first and the second peaks. A sharp climatological evolution and radiative-convective constraint of SST over the far eastern Pacific during FMA appear to separate the otherwise continuous anomalies, and the pause is observed at the subsurface. For example, the ocean dynamics alone warms the top 35 m layer of the far eastern Pacific at the rate of +0.30 K mon 1 and cools at the rate of 0.11 K mon 1 in DJF for doubleand single-peak events, respectively, but the SSTa tendencies are both negative ( 0.20 and 0.43 K mon 1 for doubleand single-peak, respectively). The significant damping of SST by latent heat flux ( 16.8 W m 2 and +0.3 W m 2 in DJF, respectively) and the reduced shortwave radiation ( 10.3 W m 2 and 9.0 W m 2 in DJF, respectively) due to the increased cloud cover anomalously cool the far eastern Pacific surface, resulting in a pause between the two peaks. After the pause, the dynamic heating dominates over the surface cooling, and the second peak of the SST appears over the far eastern Pacific, which is ultimately terminated by the end of the season. 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