Anomalous Zonal Advection Research Articles

The Physical Processes Dominating the Impact of the Summer North Atlantic Oscillation on the Eastern Tibetan Plateau Summer Rainfall

Abstract The summer North Atlantic Oscillation (SNAO), an important climate signal in regulating the interannual variability of Tibetan Plateau (TP) summer rainfall, is closely related to a meridional precipitation dipole pattern between the southeastern and northeastern TP. In this study, based on diagnoses of observations and multiple realizations of the CESM2 historical simulation, we find that there are fundamental differences between the formation processes dominating the SNAO-related summer rainfall anomaly in the southeastern and northeastern TP. An atmospheric moisture budget analysis reveals that the anomalous vertical (horizontal) moisture advection makes the largest contribution to the southeastern (northeastern) TP summer rainfall anomaly. During the negative phase of SNAO, the increased precipitation in the southeastern TP is related to the anomalous ascending flows, which are driven by two processes according to the moist static energy budget. The first is the southward shift of the subtropical westerly jet stream, which produces positive anomalous zonal advection of the climatological moist enthalpy in the upper-middle troposphere over the southeastern TP. The second is related to the enhanced transport of anomalous warm moist air to the south of TP, which produces positive anomalous meridional advection of anomalous moist enthalpy into the lower troposphere over the southeastern TP under the control of climatological monsoonal meridional circulations. This positive moist enthalpy advection enhances the atmospheric moist static energy and facilitate enhanced local convection. For the northeastern TP, the decreased precipitation is dominated by negative anomalous horizontal moisture advections due to the SNAO-induced equivalent-barotropic anomalous cyclone near the eastern edge of the TP.

Dynamic Causes of ENSO Decay and Its Asymmetry

AbstractEl Niño and La Niña exhibit asymmetric evolution characteristics during their decay phases. The decay speed of El Niño is significantly greater than that of La Niña. This study systematically and quantitatively investigates the relative contributions of the equatorial western Pacific (WP) and central-eastern Pacific (CEP) wind stress anomalies to ENSO decay and its asymmetry through data analysis, numerical experiments, and dynamic and thermodynamic diagnoses. It is demonstrated that the sea surface temperature anomalies (SSTAs) forced by the wind stress anomalies in the equatorial CEP play a dominant role in ENSO decay and contribute to ENSO decay asymmetry, while the forcing by the equatorial WP wind stress anomalies has a small contribution. Diagnoses of the oceanic mixed layer heat budget indicate that anomalous zonal advection term and vertical advection term forced by the wind stress anomalies in the equatorial CEP are the most important dynamic terms contributed to ENSO decay. Both terms in El Niño decay phase are much larger than in La Niña decay phase, resulting in a larger decay speed in El Niño than in La Niña. The contributions of these two terms do not depend on the equatorial WP wind field, confirming that the equatorial WP wind stress anomalies do not act as a pivotal part in ENSO asymmetric decay. Moreover, it is demonstrated that within the equatorial CEP, dominant contribution comes from the wind stress anomalies in the equatorial central Pacific, in which those in the equatorial southern central Pacific play a major role.Significance StatementPrevious studies proposed why wind fields in the equatorial western Pacific (WP) or central-eastern Pacific (CEP) are asymmetric and how the asymmetric wind fields affect ENSO decay and decay asymmetry. By using an oceanic general circulation model, we quantitatively estimate the relative contributions of the wind stress anomalies over the equatorial WP and CEP. It is demonstrated that the wind stress anomalies over the equatorial CEP and the associated ocean response play a dominant role in the asymmetric decay. Additionally, it is further illustrated the predominant role comes from the wind stress anomalies in the equatorial southern central Pacific within the equatorial CEP. Our study provides a physical explanation on the ENSO decay and its asymmetry.

Influence of El Niño events on sea surface salinity over the central equatorial Indian Ocean

The El Niño event is a major large-scale air-sea interaction phenomenon over the tropical Pacific region. Previous studies classified El Niño into three types — canonical El Niño, El Niño Modoki I, and El Niño Modoki II. This research reveals that different types of El Niño present dramatic effects on the interannual variability of sea surface salinity over the central equatorial Indian Ocean in the boreal autumn. The decreasing (increasing) sea surface salinity during the canonical El Niño and the EI Niño Modoki I (the EI Niño Modoki II) events is identified. The salinity budget analysis is performed to identify the dominant factors responsible for the variability of sea surface salinity over the central Indian Ocean. The results indicate that the wind-driven anomalous zonal advection plays an important role in sea surface salinity variability during the El Niño events associated with the forcing from the anomalous Walker circulation over the equatorial Indian Ocean.

Why 1986 El Ni\xf1o and 2005 La Ni\xf1a evolved different from a typical El Ni\xf1o and La Ni\xf1a

1986 El Niño (EN) and 2005 La Niña (LN) experienced distinctive evolution features compared to the typical EN and LN events. The 1986 EN persisted for more than 2 years, whereas the 2005 LN transitioned into a warm episode in the following winter. The physical mechanisms that caused the distinctive evolution features are investigated through an oceanic mixed-layer heat budget (MLHB) analysis. For both cases, major differences with typical EN and LN lie in wind induced anomalous zonal advection and thermocline feedback terms. An anomalous cyclone appeared over the western North Pacific (WNPC) in late 1986, in contract to an anomalous anticyclone (WNPAC) during the mature winter of the typical EN. The eastward propagation of anomalous convective heating from tropical Indian Ocean in late 1986 holds a key for generating the anomalous WNPC in the mature winter of 1986/87 EN. The equatorial westerly anomaly south of the WNPC triggered downwelling Kelvin waves, prolonging the positive SSTA throughout 1987. The negative SSTA center of the 2005 LN shifted eastward by at least 20° in longitude compared to the typical LN. As a result, an anomalous WNPC, with pronounced westerly anomalies at the equator, developed in the 2005 LN mature phase. This led to a rapid decay of the 2005 LN, and by the following boreal summer a positive SSTA formed. The season-dependent air-sea feedback further strengthens the warming in northern fall, leading to a transition from LN to EN in the succeed winter.

Role of the meridional dipole of SSTA and associated cross-equatorial flow in the tropical eastern Pacific in terminating the 2014 El Niño development

In the boreal spring of 2014, the oceanic and atmospheric conditions were favorable for an El Nino's development. It was predicted that in 2014, a super El Nino or at least a regular El Nino with normal magnitude, would initiate. However, the growth rate of the sea surface temperature anomaly (SSTA) in the equatorial eastern Pacific suddenly declined in the boreal summer. The physical processes responsible for the termination of the 2014 El Nino were addressed in this study. We hypothesized that a meridional dipole of SSTA, characterized by a pronounced warm SSTA over the eastern North Pacific (ENP) and cold SSTA over the eastern South Pacific (ESP), played a crucial role in blocking the 2014 El Nino’s development. The observational analysis revealed that the meridional dipole of SSTA and the relevant anomalous cross-equatorial flow in the tropical eastern Pacific, induced anomalous westward (\({u^\prime } 0\)) currents in the equatorial eastern Pacific, leading to negative anomalous zonal advection term (\(- {u^\prime }\partial \overline T /\partial x<0\)) and anomalous upwelling advection term (\(- {w^\prime }\partial \overline T /\partial z<0\)). Additionally, the anomalous cross-equatorial flow also induced northward meridional current anomalies that transported subtropical cold water to the equator. All the changes of the oceanic dynamic terms collectively caused negative SSTA tendency in the boreal summer, and thus killed off the budding 2014 El Nino. The idealized numerical experiments further confirmed that the 2014 El Nino’s development could be suppressed by the meridional dipole of SSTA, and both the ENP pole and ESP pole make a contribution.

A new dipole index of the salinity anomalies of the tropical Indian Ocean.

With the increased interest in studying the sea surface salinity anomaly (SSSA) of the tropical Indian Ocean during the Indian Ocean Dipole (IOD), an index describing the dipole variability of the SSSA has been pursued recently. In this study, we first use a regional ocean model with a high spatial resolution to produce a high-quality salinity simulation during the period from 1982 to 2014, from which the SSSA dipole structure is identified for boreal autumn. On this basis, by further analysing the observed data, we define a dipole index of the SSSA between the central equatorial Indian Ocean (CEIO: 70°E-90°E, 5°S-5°N) and the region off the Sumatra-Java coast (SJC: 100°E-110°E, 13°S-3°S). Compared with previous SSSA dipole indices, this index has advantages in detecting the dipole signals and in characterizing their relationship to the sea surface temperature anomaly (SSTA) dipole variability. Finally, the mechanism of the SSSA dipole is investigated by dynamical diagnosis. It is found that anomalous zonal advection dominates the SSSA in the CEIO region, whereas the SSSA in the SJC region are mainly influenced by the anomalous surface freshwater flux. This SSSA dipole provides a positive feedback to the formation of the IOD events.

Migration of atmospheric convection coupled with ocean currents pushes El Niño to extremes

AbstractThe warm phase of El Niño–Southern Oscillation can grow much stronger than the cold phase, but the associated dynamics are not well understood. Here we show that the anomalous zonal advection of warm water is the major process that pushes El Niño to extremes and that this anomalous advection results from the coupling of oceanic currents with eastward migration of the atmospheric convection; a greater zonal advection is associated with a greater extent of the eastward migration. By contrast, there is a limited extent for westward migration during La Niña. Climate models that successfully simulate the amplitude asymmetry display a systematic linkage of a greater longitudinal movement of the convection center with a stronger zonal advection and greater El Niño amplitude. In a warming world, the longitudinal migration of convection response increases, as does the role of zonal advection, increasing the frequency of future extremes of El Niño.

Analyzing the 2010–2011 La Niña signature in the tropical Pacific sea surface salinity using in situ data, SMOS observations, and a numerical simulation

AbstractThe tropical Pacific Ocean remained in a La Niña phase from mid‐2010 to mid‐2012. In this study, the 2010–2011 near‐surface salinity signature of ENSO (El Niño‐Southern Oscillation) is described and analyzed using a combination of numerical model output, in situ data, and SMOS satellite salinity products. Comparisons of all salinity products show a good agreement between them, with a RMS error of 0.2–0.3 between the thermosalinograph (TSG) and SMOS data and between the TSG and model data. The last 6 months of 2010 are characterized by an unusually strong tripolar anomaly captured by the three salinity products in the western half of the tropical Pacific. A positive SSS anomaly sits north of 10°S (&gt;0.5), a negative tilted anomaly lies between 10°S and 20°S and a positive one south of 20°S. In 2011, anomalies shift south and amplify up to 0.8, except for the one south of 20°S. Equatorial SSS changes are mainly the result of anomalous zonal advection, resulting in negative anomalies during El Niño (early 2010), and positive ones thereafter during La Niña. The mean seasonal and interannual poleward drift exports those anomalies toward the south in the southern hemisphere, resulting in the aforementioned tripolar anomaly. The vertical salinity flux at the bottom of the mixed layer tends to resist the surface salinity changes. The observed basin‐scale La Niña SSS signal is then compared with the historical 1998–1999 La Niña event using both observations and modeling.

Structures and mechanisms of the first-branch northward-propagating intraseasonal oscillation over the tropical Indian Ocean

The first-branch northward-propagating intra- seasonal oscillation (FNISO) over the tropical Indian Ocean (IO) often triggers the onset of the Asian summer monsoon. In this study we investigate the structures and mechanisms associated with FNISO through the diagnosis of ERA-Interim reanalysis data for the period of 1990-2009. A composite analysis is conducted to reveal the structure and evolution characteristics of the FNISO and associated background circulation changes. It is found that the FNISO convection originates from the southwest- ern IO and propagates eastward. After reaching the eastern IO, the major convective branch moves northward toward the northern Bay of Bengal (BoB). Two possible mecha- nisms may contribute to the northward propagation of the FNISO. One is the meridional asymmetry of the back- ground convective instability. A greater background con- vective instability over the northern BoB may destabilize Rossby waves and cause convection to shift northward. The other is the meridional phase leading of perturbation humidity in the planetary boundary layer (PBL). Maximum PBL moisture appears to the north of the convection center, which promotes a convectively unstable stratification ahead of the convection and leads to the northward prop- agation of the FNISO. A PBL moisture budget analysis reveals that anomalous zonal advection is a dominant process in contributing to the moisture asymmetry.

Ocean temperature and salinity components of the Madden–Julian oscillation observed by Argo floats

New diagnostics of the Madden–Julian oscillation (MJO) cycle in ocean temperature and, for the first time, salinity are presented. The MJO composites are based on 4 years of gridded Argo float data from 2003 to 2006, and extend from the surface to 1,400 m depth in the tropical Indian and Pacific Oceans. The MJO surface salinity anomalies are consistent with precipitation minus evaporation fluxes in the Indian Ocean, and with anomalous zonal advection in the Pacific. The Argo sea surface temperature and thermocline depth anomalies are consistent with previous studies using other data sets. The near-surface density changes due to salinity are comparable to, and partially offset, those due to temperature, emphasising the importance of including salinity as well as temperature changes in mixed-layer modelling of tropical intraseasonal processes. The MJO-forced equatorial Kelvin wave that propagates along the thermocline in the Pacific extends down into the deep ocean, to at least 1,400 m. Coherent, statistically significant, MJO temperature and salinity anomalies are also present in the deep Indian Ocean.

Correlative Evolutions of ENSO and the Seasonal Cycle in the Tropical Pacific Ocean

Abstract This study examines whether shifts between the correlative evolutions of ENSO and the seasonal cycle in the tropical Pacific Ocean can produce effects that are large enough to alter the evolution of the coupled atmosphere–ocean system. The approach is based on experiments with an ocean general circulation model (OGCM) of the Pacific basin, in which the seasonal and nonseasonal (interannually varying) components of the surface forcing are prescribed with different shifts in time. The shift would make no difference in terms of ENSO variability if the system were linear. The surface fluxes of heat and momentum used to force the ocean are taken from 1) simulations in which the OGCM coupled to an atmospheric GCM produces realistic ENSO variability and 2) NCEP reanalysis data corrected by Comprehensive Ocean–Atmosphere Data Set climatology for the 20-yr period 1980–99. It is found that the response to the shifts in terms of eastern basin heat content can be 20%–40% of the maximum interannual anomaly in the first experiment, whereas it is 10%–20% in the second experiment. In addition, the response to the shift is event dependent. A response of this magnitude can potentially generate coupled atmosphere–ocean interactions that alter subsequent event evolution. Analysis of a selected event shows that the major contribution to the response is provided by the anomalous zonal advection of seasonal mean temperature in the equatorial band. Additional OGCM experiments suggest that both directly forced and delayed signals provide comparable contributions to the response. An interpretation of the results based on the “delayed oscillator” paradigm and on equatorial wave–mean flow interaction is given. It is argued that the same oceanic ENSO anomalies in different times of the oceanic seasonal cycle can result in different ENSO evolutions because of nonlinear interactions between equatorially trapped waves at work during ENSO and the seasonally varying upper-ocean currents and thermocline structure.

ENSO amplitude modulation related to Pacific decadal variability

Decadal modulation of El Niño/Southern Oscillation (ENSO) amplitude is reproduced by a long‐term simulation with an atmosphere‐ocean general circulation model. A frequency of the modulation is in phase with Pacific decadal variability (PDV). A detailed analysis of the budget of sea surface temperature (SST) shows that effect of a change in anomalous zonal advection of mean temperature correlated to ENSO development has a dominant role in low‐frequency modulation of ENSO. That is because an equatorial zonal SST gradient changes periodically in the PDV cycle as a change in the background mean state for ENSO. Such characteristics are consistent with an observed climate shift and changes in ENSO characteristics that occurred in mid‐1970s.

Near-Annual SST Variability in the Equatorial Pacific in a Coupled General Circulation Model

Abstract Equatorial Pacific sea surface temperature (SST) anomalies in the Center for Ocean–Land–Atmosphere Studies (COLA) interactive ensemble coupled general circulation model show near-annual variability as well as biennial El Niño–Southern Oscillation (ENSO) variability. There are two types of near-annual modes: a westward propagating mode and a stationary mode. For the westward propagating near-annual mode, warm SST anomalies are generated in the eastern equatorial Pacific in boreal spring and propagate westward in boreal summer. Consistent westward propagation is seen in precipitation, surface wind, and ocean current. For the stationary near-annual mode, warm SST anomalies develop near the date line in boreal winter and decay locally in boreal spring. Westward propagation of warm SST anomalies also appears in the developing year of the biennial ENSO mode. However, warm SST anomalies for the westward propagating near-annual mode occur about two months earlier than those for the biennial ENSO mode and are quickly replaced by cold SST anomalies, whereas warm SST anomalies for the biennial ENSO mode only experience moderate weakening. Anomalous zonal advection contributes to the generation and westward propagation of warm SST anomalies for both the westward propagating near-annual mode and the biennial ENSO mode. However, the role of mean upwelling is markedly different. The mean upwelling term contributes to the generation of warm SST anomalies for the biennial ENSO mode, but is mainly a damping term for the westward propagating near-annual mode. The development of warm SST anomalies for the stationary near-annual mode is partially due to anomalous zonal advection and upwelling, similar to the amplification of warm SST anomalies in the equatorial central Pacific for the biennial ENSO mode. The mean upwelling term is negative in the eastern equatorial Pacific for the stationary near-annual mode, which is opposite to the ENSO mode. The development of cold SST anomalies in the aftermath of warm SST anomalies for the westward propagating near-annual mode is coupled to large easterly wind anomalies, which occur between the warm and cold SST anomalies. The easterly anomalies contribute to the cold SST anomalies through anomalous zonal, meridional, and vertical advection and surface evaporation. The cold SST anomalies, in turn, enhance the easterly anomalies through a Rossby-wave-type response. The above processes are most effective during boreal spring when the mean near-surface-layer ocean temperature gradient is the largest. It is suggested that the westward propagating near-annual mode is related to air–sea interaction processes that are limited to the near-surface layers.

A Near-Annual Pacific Ocean Basin Mode

Some fairly regular and nearly annual variability in the equatorial Pacific after the major 1997/98 El Nino event is studied. Sea level, sea surface temperature (SST), and surface wind anomalies of this variability are tied together in a way similar to the slow cycles of El Nino-Southern Oscillations (ENSO). Despite a slightly longer-than-annual time scale, similar variability was also found in the datasets prior to 1997/98. This fast- coupled mode is superimposed on the slow 3-5-yr ENSO cycles in the tropical Pacific. It contributed to the occurrences of some minor El Nino and La Nina events. Zonal currents associated with the equatorial waves play a dominating role in generating SST anomalies of this fast mode through anomalous zonal advection. It is suggested that this fast mode may be best understood as a coupled Pacific Ocean basin (POB) mode. The existence of this fast mode, which appears independent of the slow ENSO mode, has important implications in understanding and predicting the tropical Pacific SST anomaly.

A near‐annual coupled ocean‐atmosphere mode in the equatorial Pacific ocean

A near‐annual coupled ocean‐atmosphere mode in the equatorial Pacific is studied using the NCEP ocean assimilation data set. This fast mode of tropical Pacific climate variability is superimposed on the slow 3–5 year El Niño‐Southern Oscillation (ENSO) phenomenon. Anomalous zonal advection plays a crucial role in generating this fast mode. It is suggested that this fast mode can be understood in terms of a coupled Pacific basin mode. It is responsible for the occurrence of some minor El Niño and La Niña events and has implications for the prediction of ENSO events.

Interannual variability of surface currents in the tropical Pacific during 1987–1993

Buoy drifts and current meter records between January 1987 and December 1993 are used to investigate the interannual variability of the equatorial Pacific currents at a depth of 15 m. The sampling is coarse until mid‐1988 but more complete afterward, so that the large‐scale features of the anomaly currents can be documented on the seasonal to yearly timescale. Using objective analysis, bimonthly current anomalies are mapped between 20°N and 20°S on a 1° × 5° grid, and the error covariance matrix of the analyzed fields are estimated. The current anomalies are primarily zonal, with largest amplitudes within about 8° from the equator, and they are largely linked to the El Niño‐Southern Oscillation phenomenon. In particular, broad, basin‐wide westward anomaly currents were encountered during the 1988 La Niña, and strong eastward currents persisted from July–August 1991 to January–February 1992, followed by westward currents from May–June to July–August 1992. An empirical orthogonal function (EOF) analysis shows that the first EOF of zonal current anomaly is largely uniform in the equatorial band, while the next two EOFs describe large‐scale currents of opposite sign across the equator and across 160°W, respectively. The EOFs are rather smooth and the errors on the principal component time series relatively small, which indicates that the sampling is adequate to describe the large‐scale, low‐frequency zonal current fluctuations. As the dominant EOFs of meridional current are noisy and the relative errors on the principal components larger, the meridional current fluctuations are not as well captured by the data set. Correlation analysis and a singular value decomposition are used to investigate the influence of advection by the large‐scale, low‐frequency currents on sea surface temperature (SST) anomalies during 1987–1993. Although the data set is noisy and other terms play an important role in the SST anomaly equation, the effect of zonal, and, to a lesser extent, meridional advection is seen in much of the central and eastern equatorial Pacific. The dominant terms are the anomalous zonal advection of mean SST, the mean zonal, and, intermittently, meridional advection of SST anomalies.

Equatorial wave sequence associated with warm pool displacements during the 1986–1989 El Niño‐La Niña

In the Pacific equatorial band the zonal displacement of the eastern edge of the warm pool, subject to insignificant seasonal variations, is dominated by strong interannual variations almost in phase with the Southern Oscillation Index. Over the 1971–1973 El Niño‐La Niña period, Gill (1983) suggested that such displacement was caused solely by horizontal advection by zonal current anomaly. Basin‐wide Geosat‐derived zonal surface current anomalies are available during the 1986–1989 period, and they agree quite well with observed equatorial currents in the western and central Pacific. Within the 4°N–4°S equatorial band the cumulative contribution of first‐baroclinic Kelvin and first‐symmetric (m = 1) Rossby modes to surface zonal equatorial current anomalies account for most of the Geosat‐derived current variability. From these Geosat‐derived currents it is demonstrated that the eastward (westward) displacement of the eastern edge of the warm pool was primarily due to horizontal advection by zonal current anomalies during the 1986–1987 El Niño (1988–1989 La Niña). As a corollary, the El Niño (La Niña) warm (cold) sea surface temperature (SST) anomaly in the central western Pacific was the result of anomalous zonal advection. The 1986–1989 displacement of the warm pool appeared as a low‐frequency movement resulting from high‐frequency forcing, i.e., a succession of local wind forcing and its remote Kelvin and m = 1 Rossby wave responses in the equatorial waveguide. In the central western Pacific a m = 1 downwelling Rossby wave, issued from the eastern Pacific, shifted the displacement from eastward to westward and therefore seems to have been the cause for El Niño to turn to La Niña in boreal fall 1987. In the eastern Pacific the simultaneous occurrence of free‐propagating downwelling (upwelling) Kelvin waves with the seasonal SST warming (cooling) appeared necessary for the development of local El Niño (La Niña) SST anomaly. Over most of the equatorial Pacific, slow migration of SST anomalies clearly showed up, distinct during El Niño and La Niña. These zonal migrations appeared to result from the delayed association of two phenomena: the zonal advection in the central western Pacific and the arrival of downwelling (upwelling) Kelvin waves into the eastern Pacific, synchronous with the seasonal SST warming (cooling). Albeit different in terms of equatorial wave action, as compared to the delayed action oscillator theory, our results suggest that zonal advection and wave dynamics were both important for the generation and migration of El Niño‐ Southern Oscillation SST anomalies in 1986–1989. Finally, our observational results about warm pool displacements, equatorial wave sequence, the shift from El Niño to La Niña, and the slow SST migrations are presented in a schematic form involving sequential ocean‐atmosphere coupling which appears continuously over the November 1986 to February 1989 period.