Interannual Modes Research Articles

The relationship between the simulated climatic variability modes of the tropical Atlantic

Two main modes of climatic variability occur in the tropical Atlantic Ocean at inter-annual time-scales: the equatorial mode, similar to the El Nino phenomenon in the Pacific Ocean, and the meridional mode, or dipole-like mode, with no Pacific counterpart. The Atlantic equatorial mode is characterized by the occurrence of alternating warm and cold episodes at the equator, on the eastern side of the basin. These events are associated with abnormal variations in the zonal equatorial slope of the thermocline. The meridional mode is characterized by an inter-hemispheric gradient in the sea-surface temperature (SST). The mean position of the Inter-tropical Convergence Zone (ITCZ) separates positive and negative SST signals. It was recently shown with observational indices that there is significant correlation between these two climatic modes of variability. This study goes one step further, by using a multi-year numerical simulation, where an oceanic general circulation model is forced by the 1979-1993 ECMWF reanalysis. Model computed indices representing the two main modes of variability compare well with observations. The two inter-annual modes of variability are shown to have the same physics as the annual variability does, which is related to the latitudinal displacement of the ITCZ. Furthermore, it is suggested that the ocean dynamics (as opposed to thermodynamic processes) is the principal cause of climate variability in the region.

Quantifying Persistence in ENSO

The seasonal dependence of predictability in ENSO manifests itself in the so-called spring barrier found in the cyclostationary lag autocorrelations, or persistence. This work examines the statistics of persistence, with particular focus on the phase-of-year-dependent pattern found in ENSO data, the barrier. Simple time series of one sine wave produce a barrier if the frequency is a biennial cycle or one of its harmonics. Time series of two sine waves produce a barrier if one frequency is a biennial cycle or a harmonic thereof. They additionally produce a barrier if their frequencies sum to unity. Time series with continuous but narrow spectral peaks at barrier-producing frequencies produce barriers only if the phase angles vary slowly or coherently across the peaks. The shape of the barrier seen in these simple time series is used to construct a model persistence map, which is a combination of an idealized barrier and the persistence of a red-noise process. A nonlinear least squares fit of the persistence of a time series to the model persistence provides a quantitative measure of the properties of the persistence barrier in any time series. Application of the measure to the Southern Oscillation index and sea surface temperature in the NINO3 region of the equatorial Pacific indicates that the ENSO persistence barrier is a feature that is statistically distinguishable from the theoretical persistence of a red-noise process. The ENSO barrier results from phase coherency of the continuum of interannual modes near the biennial frequency. Measuring the barrier on windowed data shows that there was a weak persistence barrier from approximately 1915 to 1945, a strong barrier during the 1960s and early 1970s, and a weakening barrier in the late 1970s.

A mechanism for generating ENSO decadal variability

A coupled ocean‐atmosphere model of the Pacific basin is used to illustrate a mechanism by which El Niño and the Southern Oscillation (ENSO) may be modulated on decadal time scales. For reasonable choices of model parameters, solutions exhibit two types of oscillation, an ENSO‐like interannual mode and a decadal one. The decadal mode affects the equatorial zone by means of an oceanic teleconnection that involves transport variations of the North Pacific Subtropical Cell. Since almost half of the cool, thermocline water that upwells in the eastern equatorial Pacific participates in this cell, these variations significantly alter the extent of the cold tongue, and hence provide an efficient mechanism for modulating ENSO.

Seasonal to interannual modes of sea level variability in the western Pacific and eastern Indian oceans

Almost six years of TOPEX/Poseidon altimeter data were analyzed to discern low frequency (monthly to interannual) variability in the region from the eastern Indian Ocean east to the dateline. An empirical orthogonal function (EOF) analysis shows that from September 1992 through April 1998 the dominant mode of sea level variability in the western Pacific/eastern Indian Ocean is on the ENSO time scale, and annual variability is secondary. The amplitude of the annual cycle becomes even less in the latter part of the time period (in the years prior to the 1997/98 El Niño event). The pattern that emerges on interannual time scales is one of western Pacific warm pool buildup (during non‐El Niño years) and discharge (during El Niño years). Mixed with these signals is a large amplitude signal in the eastern Indian Ocean, south of the equator (off the south coast of Sumatra), which has both annual and interannual variations associated with it. Harmonic analysis shows the annual cycle is dominated by Rossby waves in both the equatorial Pacific and at higher latitudes in the eastern Indian Ocean.

Monsoonal oscillation revealed by the upper-troposphere water vapor band brightness temperature

The measurements of brightness temperature (BT) from the upper-troposphere water vapor channel 12 of the National Oceanic and Atmospheric Administration polar satellites from 1979 through 1995 are used to analyze the interannual variations of the global monsoon strength. Results show that in the interannual time-sclae the BT variability in the equatorial eastern Pacific (EEP) is out of phase with the BT variabilities in other four regions, i. e. South Asia, tropical south American, two subtropical areas in the South and North Pacific. The BT interannual variation mode may be called monsoonal oscillation (MO). The MO is the result of the atmospheric circulation anomaly in the troposphere.

Interannual variability in the troposphere and stratosphere of the southern hemisphere winter

Interannual variability in the tropospheric and stratospheric circulation of the southern‐hemisphere winter is studied over the 16‐year period from 1979 to 1994. In particular, the interannual mode of variability in the seasonal march from autumn to spring is investigated by means of a multiple empirical orthogonal function analysis. The extracted dominant mode of variability is characterized by the poleward and downward propagation of anomalous zonal‐mean zonal winds, accompanied by changes in wave activity. The overall features of the variability are similar to those found in the northern hemisphere except that the entire sequence of evolution is closely locked to the annual cycle and requires a longer period of 6 months. The time coefficients of this mode exhibit a very clear biennial oscillation and increasing trend. A similar spatial pattern of variability is also found in the simulated data of an atmospheric general circulation model with climalogical sea surface temperatures as the lower boundary conditions. The similarities in the time evolution of the spatial pattern suggest that this mode is essentially an internal mode of variability in the atmosphere, which is modulated by external conditions.

Modulation of the seasonal signal of the Kuroshio Extension during 1994 from satellite data

Interannual modulation of the seasonal signal in the Kuroshio Extension region is studied using TOPEX altimeter and advanced very high resolution radiometer (AVHRR) data. Meridional profiles of the surface eddy kinetic energy indicate a significant drop in eddy energy levels beginning early in 1994 and persisting until the summer of 1995. The low eddy energy levels are most apparent during the summer and fall 1994. The contribution of the convergence of Reynolds stresses to the zonal momentum budget, , is much weaker in 1994 than in either 1993 or 1995, especially during summer. The low values of Reynolds stresses lead the low eddy kinetic energy values by about a season, consistent with the characteristic timescale of the eddy energy, which is about 90–110 days in this region. AVHRR‐derived estimates of sea surface temperature indicate that the subpolar gyre was cooler during summer 1994 relative to the summers of 1993 and 1995. Calculations of the baroclinic zonally symmetric circulation indicate that the Kuroshio Extension was frontolytic during summer 1994, which could account for the observed drop in eddy kinetic energy. In particular, convergence of heat due to eddy flow appears to play a crucial role in the interannual modulation of the seasonal signal of the Kuroshio Extension.

Seasonal and interannual variations of atmospheric CO2 and climate

Interannual variations of atmospheric CO2 concentrations at Mauna Loa are almost masked by the seasonal cycle and a strong trend; at the South Pole, the seasonal cycle is small and is almost lost in the trend and interannual variations. Singular-spectrum analysis (SSA) is used here to isolate and reconstruct interannual signals at both sites and to visualize recent decadal changes in the amplitude and phase of the seasonal cycle. Analysis of the Mauna Loa CO2 series illustrates a hastening of the CO2 seasonal cycle, a close temporal relation between Northern Hemisphere (NH) mean temperature trends and the amplitude of the seasonal CO2 cycle, and tentative ties between the latter and seasonality changes in temperature over the NH continents. Variations of the seasonal CO2 cycle at the South Pole differ from those at Mauna Loa: it is phase changes of the seasonal cycle at the South Pole, rather than amplitude changes, that parallel hemispheric and global temperature trends. The seasonal CO2 cycles exhibit earlier occurrences of the seasons by 7 days at Mauna Loa and 18 days at the South Pole. Interannual CO2 variations are shared at the two locations, appear to respond to tropical processes, and can be decomposed mostly into two periodicities, around (3 years)-1 and (4 years)-1, respectively. Joint SSA analyses of CO2concentrations and tropical climate indices isolate a shared mode with a quasi-triennial (QT) period in which the CO2 and sea-surface temperature (SST) participation are in phase opposition. The other shared mode has a quasi-quadrennial (QQ) period and CO2 variations are in phase with the corresponding tropical SST variations throughout the tropics. Together these interannual modes exhibit a mean lag between tropical SSTs and CO2 variations of about 6–8 months, with SST leading. Analysis of the QT and QQ signals in global gridded SSTs, joint SSA of CO2 and δ13C isotopic ratios, and SSA of CO2 and NH-land temperatures indicate that the QT variations in CO2 mostly reflect upwelling variations in the eastern tropical Pacific. QQ variations are dominated by the CO2 signature of terrestrial-ecosystem response to global QQ climate variations. Climate variations associated with these two interannual components of tropical variability have very different effects on global climate and, especially, on terrestrial ecosystems and the carbon cycle.

Greenhouse Warming, Decadal Variability, or El Niño? An Attempt to Understand the Anomalous 1990s

The dominant variability modes in the Tropics are investigated and contrasted with the anomalous situation observed during the last few years. The prime quantity analyzed is anomalous sea surface temperature (SST) in the region 308S-608N. Additionally, observed tropical surface wind stress fields were investigated. Further tropical atmospheric information was derived from a multidecadal run with an atmospheric general circulation model that was forced by the same SSTs. The tropical SST variability can be characterized by three modes: an interannual mode (the El Nino-Southern Oscillation (ENSO)), a decadal mode, and a trend or unresolved ultra- low-frequency variability. The dominant mode of SST variability is the ENSO mode. It is strongest in the eastern equatorial Pacific, but influences also the SSTs in other regions through atmospheric teleconnections, such as the Indian and North Pacific Oceans. The ENSO mode was strong during the 1980s, but it existed with very weak amplitude and short period after 1991. The second most energetic mode is characterized by considerable decadal variability. This decadal mode is connected with SST anomalies of the same sign in all three tropical oceans. The tropical Pacific signature of the decadal mode resembles closely that observed during the last few years and can be characterized by a horseshoe pattern, with strongest SST anomalies in the western equatorial Pacific, extending to the northeast and southeast into the subtropics. It is distinct from the ENSO mode, since it is not connected with any significant SST anomalies in the eastern equatorial Pacific, which is the ENSO key region. However, the impact of the decadal mode on the tropical climate resembles in many respects that of ENSO. In particular, the decadal mode is strongly linked to decadal rainfall fluctuations over northeastern Australia in the observations. It is shown that the anomalous 1990s were dominated by the decadal mode. Considerable SST variability can be attributed also to a linear trend or unresolved ultra-low-frequency vari- ability. This trend that might be related to greenhouse warming is rather strong and positive in the Indian Ocean and western equatorial Pacific where it accounts for up to 30% of the total SST variability. Consistent with the increase of SST in the warm pool region, the trends over the tropical Pacific derived from both the observations and the model indicate a strengthening of the trade winds. This is inconsistent with the conditions observed during the 1990s. If the wind trends reflect greenhouse warming, it must be concluded that the anomalous 1990s are not caused by greenhouse warming. Finally, hybrid coupled ocean-atmosphere model experiments were conducted in order to investigate the sensistivity of ENSO to the low-frequency changes induced by the decadal mode and the trend. The results indicate that ENSO is rather sensitive to these changes in the background conditions.

Evidence for a relationship between spring snow cover in North America and summer rainfall in New Mexico

Two possible mechanisms for interannual modulation of precipitation anomalies in the Southwest U.S. are examined. The association between El Niño/Southern Oscillation (ENSO) fluctuations and New Mexico precipitation anomalies is found to vary with season: positive Pacific Ocean surface temperature anomalies are associated with positive winter‐spring precipitation anomalies in New Mexico but negative summer anomalies. We propose that the spring snow extent across the west‐central U.S. acts to enhance or suppress the subsequent summer monsoon circulation affecting the American Southwest, such that excessive snow leads to deficient summer rain, and deficient snow leads to abundant rain. This mechanism is consistent with the observed seasonal variation in the ENSO—precipitation relationship.

Interannual modulation of extratropical high frequency variability

A simple explanation is presented for the observed interannual changes in the dominant space and time scales of Northem Hemisphere winter extratropical high frequency variability. It is found that such changes can suc- cessfully be predicted by linearizing a 2-level quasi-geostrophic mode] in spherical geometry around the ob- served zona] mean states. The mechanisms responsible for the selection of the most unstable normal mode are investigated.

Unstable interannual oscillation modes in a linearized coupled ocean-atmosphere model and their associations with ENSO

By using a coupled ocean-atmosphere model with an oceanic surface boundary layer, including linear atmospheric and oceanic dynamics and linearized SST prognostic equation with respect to spatially varying climatological background states, we have investigated the eigenvalue problem of the linearized coupled system in the tropical Pacific, including the characteristic periods, horizontal structures, temporal-spatial evolution and instability of the unstable interannual oscillation characteristic modes and their associations with ENSO. The main results show that the quasi-biennial (QB) oscillation was found to act as the most unstable mode in the tropical Pacific coupled air-sea system. Only the most unstable QB mode displays the ENSO-like structure and temporalspatial evolution, and its existence seems likely to have no essential dependence on the climatological annual cycle (AC). Unfortunately, from the linearized coupled system we have not derived a most unstable mode relevant to the observed principle mode with the preferred 3–4 year lower-frequency (LF) oscillation period in the real world ENSO variability. Therefore, we infer that the LF mode would likely result from certain nonlinear interaction, in which the QB mode that acts as the shortest ENSO cycle could be fundamentally important. Also, we believe that the results in present work could be helpful to fully understand the multiple time scales and the associated mechanism responsible for the real world ENSO variability.

Inter-annual climate modes over southern Africa from satellite cloud OLR 1975?1994

Principal components analysis of satellite outgoing longwave radiation (OLR) anomalies over southern Africa forms the basis of this work. OLR anomalies, which are closely correlated with rainfall anomalies and vegetation cover, are formulated for early and late summer, the season when convective clouds develop over southern Africa. The first four principal component modes account for 82 to 85% of the total inter-annual convective variability in the period 1975–1994. The dominant terrestrial modes take the form of truncated dipoles (36%). NW-SE oriented loading patterns contributed by cloud bands and poleward tropical outflows, form the second most important climate mode (25%). Lower order modes are dominated by monsoon effects (12%) and isolated cells (10%). The dominant modes of convective variability appear related to the phase of the Southern Oscillation and its regional signals. The principal component results indicate that attention should be given to understanding mechanisms underlying dipole circulation systems.

Interannual Modes of Variability in Atmospheric Angular Momentum

Abstract The interannual variability of atmospheric angular momentum over a 26-yr period is studied regionally using monthly analyses of zonal winds derived from the global rawinsonde network. Variations in zonal-mean momentum, filtered to emphasize interannual timescales, exhibit a coherent propagating signal emanating from low latitudes, as identified in other studies using shorter records. Applying extended empirical orthogonal function (EEOF) analyses to zonally varying data, the authors isolate a dominant pair of eigenvectors whose principal component time series and spatial patterns are in quadrature with one another, indicating oscillatory behavior. The oscillation described by the two EEOFs has a period of about 36 months and is linked a posteriori to the time evolution of the El Nino-Southern Oscillation phenomenon. Beginning as an anomaly over the Tropics that extends from the Indian Ocean into the Pacific, the signal is observed to progress eastward and poleward into both hemispheres, leading t...

Empirical orthogonal function analysis of global TOPEX/POSEIDON altimeter data and implications for detection of global sea level rise

Two years of TOPEX/POSEIDON altimeter data are examined to determine the dominant spatial features and timescales of sea surface height variability in the global oceans and to estimate the rate of global sea level rise. Empirical orthogonal function (EOF) decomposition of 69 cycles of TOPEX altimeter data into the significant modes of variability reveals dominant annual and interannual timescales. The annual modes include the hemispheric‐scale changes in steric height due to seasonal heating variations, changes in the strength of the major current systems in the equatorial Pacific, and the reversing monsoonal circulation in the Indian Ocean. The interannual modes capture oscillations in the tropical Pacific characteristic of recent El Niño events. A 2‐year history of the change in mean sea level derived from TOPEX altimeter data reveals a rise of 5.2 mm/yr. By analyzing the contribution of each EOF mode to global mean sea level variations, we find that 82% of the rise in mean sea level is caused by a single interannual mode of variability. Altimeter data spanning only 2 years, however, are insufficient to resolve a complete El Niño‐Southern Oscillation (ENSO) cycle which dominates the interannual EOF modes. Thus most of the rise in mean sea level derived from TOPEX altimetry is an artifact of incomplete temporal sampling of interannual variability. When a longer time series of TOPEX altimeter data is obtained and a complete ENSO cycle is observed, a significant reduction in the rate of global mean sea level rise estimated from TOPEX altimetry is expected. Most of the remaining rise in global mean sea level is explained by the annual EOF modes, suggesting a possible connection between sea level rise and changes in the steric component of sea surface height.

Seasonal variations in the equatorial Indian Ocean and their impact on the Lombok throughflow

In order to identify the origin of the nonequilibrated pressure field which introduces semiannual variations to the Lombok throughflow, one of the major components of the Indonesian throughflow, seasonal sea level records along the eastern boundary of the Indian Ocean and subsurface temperature records in the equatorial Indian Ocean are compared with the results from ocean general circulation models forced by climatological winds. It is shown, as discussed by Clarke and Liu [1993] using a simple analytical model, that the annual component of the sea level is excited locally by monsoonal winds, whereas the semiannual component is excited remotely by zonal winds in the central equatorial Indian Ocean during the monsoon transition periods. The eastward semiannual propagation observed both in the model and in the 20°C isotherm depth anomaly along the equatorial Indian Ocean is interpreted in terms of the second mode equatorial Kelvin wave excited by the zonal winds during the monsoon transition periods. The propagation direction is westward in the off‐equatorial region along 4°N, suggesting that the equatorial Kelvin wave reflects as an equatorial Rossby wave twice a year at the coast of Sumatra. Since the seasonal winds responsible for those semiannual oceanic variations suffer short‐term climate variabilities such as El Niño‐Southern Oscillation events, they may introduce interannual modulations to the seasonal cycle of the Lombok throughflow. As a corollary, we suggest an ageostrophic limitation due to a nonequilibrated pressure field on the use of Godfrey's [1989] island rule in estimating the throughflow variations.

Mechanistic analysis of interhemispheric transport

After a strong interannual modulation in methyl chloroform was observed at Samoa, it was hypothesized that a mechanism other than convection, namely the propagation of Rossby waves through the tropical westerly ducts, may also be responsible for interhemispheric transport. We assess the role of these mechanisms in this exchange using the National Center for Atmospheric Research (NCAR)'s Community Climate Model 2 (CCM2). In our analysis of Northern Hemisphere wintertime transport we find that convective outflow is the dominant mechanism for cross‐equatorial transport. Rossby wave propagation through westerly ducts contributes significantly less. Furthermore, we find that the observed modulation of methyl chloroform mixing ratio at Samoa is due to a shift in the low level convergence pattern during El Nino, resulting in a more southeasterly flow at the surface over Samoa.

Long‐term variability in the solar diurnal tide observed by HRDI and simulated by the GSWM

Observations of the mesosphere and lower thermosphere winds obtained by the High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) during 1991 to 1995 reveal a semiannual variation in the amplitude of the (1, 1) diurnal tide. The global‐scale wave model (GSWM) represents the first numerical modeling attempt at simulating this seasonal variability, and a preliminary comparison of the GSWM tidal results with HRDI measurements is presented. The results of the comparison and of numerical tests point to some vital and unresolved questions regarding tidal dissipation and tropospheric forcing. In addition to the seasonal variability, HRDI has revealed a strong interannual modulation of the diurnal tide with amplitudes observed to change by nearly a factor of 2 from 1992 to 1994.

Outgoing longwave radiation biases and their impacts on empirical orthogonal function modes of interannual variability in the tropics

The effects of different equator‐crossing times on the outgoing longwave radiation (OLR) data record are examined. A simple procedure proposed by Kousky and Kayano (1994) is used to reduce these effects. Large‐scale interannual modes of OLR within the tropics are determined by performing empirical orthogonal function analyses on OLR anomalies and on bias‐corrected OLR anomalies. Both analyses result in two physically meaningful leading modes, which describe different large‐scale anomalous features of the OLR related to the southern oscillation. A mode related to changes in the satellite observing system is found only in the analysis for the uncorrected OLR anomalies. Evidence is presented indicating that estimates of the mean daily OLR are affected by changes in the equator‐crossing times due either to the replacement of satellites or to long‐term drifts from the orbital plane during the lifetime of individual satellites.

Principal Modes of Outgoing Longwave Radiation and 250-mb Circulation for the South American Sector

Abstract Principal modes of anomalous upper-tropospheric circulation and outgoing longwave radiation (OLR) are determined for the South American sector. A combined EOF analysis is performed using anomalous 250-mb zonal and meridional wind components plus anomalous OLR. The data are filtered so that modes on both intraseasonal and interannual timescales can be studied separately. The resulting patterns show a consistent relationship between anomalous OLR and anomalous upper-tropospheric circulation features. The first and second interannual modes contain many atmospheric features related to extremes in the Southern Oscillation. These include 1) variations in the pattern of tropical convection, and 2) changes in the intensity of upper-tropospheric zonal flow in the equatorial band and in the subtropics. The first intraseasonal mode has its greatest loadings in the Tropics and is most active during the southern summer season. For positive (negative) amplitudes of this mode, anomalous upper-tropospheric weste...