ENSO Timescales Research Articles

Clustering tropical cyclone genesis on ENSO timescales in the Southwest Pacific

Tropical cyclones (TCs) as a natural hazard pose a major threat and risk to the human population globally. This threat is expected to increase in a warming climate as the frequency of severe TCs is expected to increase. In this study, the influence of different monthly sea surface temperature (SST) patterns on the locations and frequency of tropical cyclone genesis (TCG) in the Southwest Pacific (SWP) region is investigated. Using principal component analysis and k-means clustering of monthly SST between 1970 and 2019, nine statistically different SST patterns are identified. Our findings show that the more prominent ENSO patterns such as the Modoki El Niño (i.e., Modoki I and Modoki II) and Eastern Pacific (EP) El Niño impact the frequency and location of TCG significantly. Our results enhance the overall understanding of the TCG variability and the relationship between TCG and SST configurations in the SWP region. The results of this study may support early warning system in SWP by improving seasonal outlooks and quantification of the level of TC-related risks for the vulnerable Pacific Island communities.

Variability of lightning activity over India on ENSO time scales

ENSO, the reliable indicator of inter-annual climate variation of the ocean-atmosphere system in the tropical Pacific region, can affect the overall lightning activity which is another atmospheric phenomenon. In the present study, the impact of the ENSO on the total lightning activity over India has been studied for the period 2004–2014. During the El-Nino period (July 2004–April 2005 and July 2009–April 2010), total number of lightning flashes increased by 10% and 18% respectively and during La-Nina period (July 2010–April 2011 and August 2011 to March 2012), the total number of lightning flashes decreased approximately by 19% and 28% respectively as compared to the mean of corresponding period (2004–14) of the Non-ENSO. Seasonal variation of flash density is also examined for the El-Nino and La-Nina period. The result shows that in the El-Nino period of the pre-monsoon and monsoon seasons, there is an increment in the flash density approximately by 48% and 9% respectively than the Non-ENSO and the spatial variation also having high flash density along the foot of Himalayas region. In the post-monsoon season, there is a marginal change in the flash density between El-Nino and the Non-ENSO. In the winter season, there is an increment in flash density in the El-Nino period approximately by 45% than the Non-ENSO. In the La-Nina period of the pre-monsoon and monsoon seasons, there is the decrement in the flash density approximately by the 44% and 24% respectively than the Non-ENSO. In the Post-monsoon season and winter season of La-Nina, the flash density is increased by about 24% and 33% over India. These findings can be applied to do proper planning of lightning induced hazard mitigation as lightning is of one of the major natural disasters of India.

Vertical energy flux at ENSO time scales in the subthermocline of the Southeastern Pacific

AbstractThe question of how energy is redistributed in the ocean has renewed the interest for the processes leading to midlatitude subthermocline variability at low frequency. Here we investigate a process that has been disregarded although potentially relevant for climatic studies dealing with the planetary energy budget. The focus is on the Southeastern Pacific where an efficient oceanic teleconnection takes place, linking the remote surface equatorial momentum forcing with the subthermocline through the vertical propagation of low‐frequency long‐wavelength extratropical Rossby waves (ETRW). A high‐resolution model is used to document the vertical energy flux associated with ETRW at interannual to decadal time scales. The analysis of a long‐term (1958–2008) simulation reveals that the vertical energy flux can be interpreted to a large extent as resulting from the coastally forced ETRW as far south as 35°S, so that heat content variability can be predicted along theoretical trajectories originating from the coast below the thermocline. It is shown that the vertical energy flux associated with the El Niño Southern Oscillation forms beams below the thermocline that account for a large fraction of the total vertical energy flux at interannual time scales. Extreme El Niño events are the dominant contributor to this flux, which is hardly impacted by mesoscale activity. The energy beams experience a dissipation processes in the ocean below 1000 m that is interpreted as resulting from vertical turbulent diffusion. Our results suggest that the ETRW at ENSO time scales are strongly dissipated at the surface but still can modulate the heat content in the deep ocean of the Southeastern Pacific.

Variability of lightning flash and thunderstorm over East/Southeast Asia on the ENSO time scales

The variability of lightning flash and thunderstorm on the ENSO time scales over East/Southeast Asia was investigated by using 17-year (1995–2011) lightning data from the Optical Transient Detector (OTD) and Lightning Imaging Sensor (LIS), and 14-year (1998–2011) Tropical Rainfall Measuring Mission satellite (TRMM) precipitation feature data. In addition, ERA-Interim reanalysis data of the European Centre for Medium-Range Weather Forecasts (ECMWF) were used to present related environmental characteristics. It was found that the response of lightning flash to ENSO events shows remarkable seasonal and regional variations. The regions of positive (negative) lightning anomaly are mainly located at both sides of 5°–20°N (5°–15°N) in El Niño (La Niña) spring and winter, and located north of the equator in summer and autumn. There is a significantly positive correlation between lightning anomaly and the Oceanic Niño Index (ONI) over both East China and Indonesia during El Niño episodes, but no obvious correlation during La Niña episodes. The positive thunderstorm anomalies during El Niño periods are dispersed. The distribution of thunderstorm anomalies in La Niña summer and autumn is almost opposite to that in spring and winter. The correlation between thunderstorm anomaly and ONI is better over East China than that over Indonesia. In general, lightning variation follows thunderstorm intensity (number) variation over East China during El Niño (La Niña) episodes, and follows a combination of thunderstorm intensity and number variations over Indonesia on ENSO time scales. During ENSO time scales, variations of surface wind can be considered as one of the key factors to LAs. More lightning flashes present in the regions where warm moist flows intersection, and less in the regions where surface wind changes slightly or diverges. Dramatic lightning increases also occur with higher values of convective available potential energy (CAPE). In addition, higher (lower) 850hPa relative humidity generally follows with more (less) lightning flashes, which is more obvious during El Niño episodes.

Isolation of sea surface salinity maps on various timescales in the tropical Pacific Ocean

To isolate sea surface salinity (SSS) maps on seasonal, ENSO, decadal, and long-term trend timescales in the tropical Pacific Ocean, the ensemble empirical mode decomposition (EEMD) is applied on an SSS data set covering 1950–2009, concerning three key regions including the western equatorial Pacific Warm Pool (WP), South Pacific Convergence Zone (SPCZ), and Inter Tropical Convergence Zone (ITCZ); then a self-organizing map is performed on the intrinsic mode function maps decomposed by the EEMD, concerning the whole basin. The ENSO and decadal signals concern mainly the western Pacific, in contrast to the seasonal signal mostly notable in the east. (1) The modulated annual cycle has smaller (larger) amplitudes during El Nino (La Nina) years and later appearance of minima during El Nino years; one unique annual cycle is observed at the northwestern edge of the ITCZ lagging the well-known ITCZ cycle by ~3 months. (2) The pronounced 1999–2001 SSS-related La Nina event in the SPCZ was reinforced twice by the decadal shift in the 1990s; the eastern Pacific and central Pacific ENSO-related SSS features are compared. (3) The contrasted anomalies between the western equatorial and non-equatorial regions were pronounced during 1977–1996, whereas they were less pronounced during 1971–1976, 2005–2008, and a roughly opposite pattern appeared with strong and abrupt decrease shift prevailing in large areas over the southwestern basin during 1997–2004. (4) The freshening at the western equator and the saltening located east of the SPCZ SSS front together amplify the geographical SSS contrasts exhibited by the WP and SPCZ SSS fronts.

Sea surface height variations in the Mindanao Dome region in response to the northern tropical Pacific winds

Sea surface height (SSH) variability in the Mindanao Dome (MD) region is found to be one of the strong variations in the northern Pacific. It is only weaker than that in the Kuroshio Extension area, and is comparable to that in the North Pacific Subtropical Countercurrent region. Based on a 1.5-layer reduced gravity model, we analyzed SSH variations in this region and their responses to northern tropical Pacific winds. The average SSH anomaly in the region varies mainly on a seasonal scale, with significant periods of 0.5 and 1 year, ENSO time scale 2-7 years, and time scale in excess of 8 years. Annual and long-term variabilities are comparably stronger. These variations are essentially a response to the northern tropical Pacific winds. On seasonal and ENSO time scales, they are mainly caused by wind anomalies east of the region, which generate westward-propagating, long Rossby waves. On time scales longer than 8 years, they are mostly induced by local Ekman pumping. Long-term SSH variations in the MD region and their responses to local winds are examined and discussed for the first time.

Drivers of the projected changes to the Pacific Ocean equatorial circulation

Climate models participating in the third Coupled Model Inter Comparison Project (CMIP3) suggest a significant increase in the transport of the New Guinea Coastal Undercurrent (NGCU) and the Equatorial Undercurrent (EUC, in the central and western Pacific) and a decrease in the Mindanao current and the Indonesian Throughflow. Most models also project a reduction in the strength of the equatorial Trade winds. Typically, on ENSO time scales, a weakening of the equatorial easterlies would lead to a reduction in EUC strength in the central Pacific. The strengthening of the EUC projected for longer timescales, can be explained by a robust projected intensification of the south‐easterly trade winds and an associated off‐equatorial wind‐stress curl change in the Southern Hemisphere. This drives the intensification of the NGCU and greater water input to the EUC in the west. A 1½‐layer shallow water model, driven by projected wind stress trends from the CMIP3 models demonstrates that the projected circulation changes are consistent with a purely wind driven response. While the equatorial winds weaken for both El Niño events and in the projections, the ocean response and the mechanisms driving the projected wind changes are distinct from those operating on interannual timescales.

Predictability of the Atlantic Overturning Circulation and Associated Surface Patterns in Two CCSM3 Climate Change Ensemble Experiments

Abstract Predictability of the Atlantic meridional overturning circulation (AMOC) and associated oceanic and atmospheric fields on decadal time scales in the Community Climate System Model, version 3 (CCSM3) at T42 resolution is quantified with a 700-yr control run and two 40-member “perfect model” climate change experiments. After taking into account both the mean and spread about the mean of the forecast distributions and allowing for the possibility of time-evolving modes, the natural variability of the AMOC is found to be predictable for about a decade; beyond that range the forced predictability resulting from greenhouse gas forcing becomes dominant. The upper 500-m temperature in the North Atlantic is even more predictable than the AMOC by several years. This predictability is associated with subsurface and sea surface temperature (SST) anomalies that propagate in an anticlockwise direction along the subpolar gyre and tend to be prominent during the 10 yr following peaks in the amplitude of AMOC anomalies. Predictability in the North Atlantic SST mainly resides in the ensemble mean signals after three to four forecast years. Analysis suggests that in the CCSM3 the subpolar gyre SST anomalies associated with the AMOC variability can influence the atmosphere and produce surface climate predictability that goes beyond the ENSO time scale. However, the resulting initial-value predictability in the atmosphere is very weak.

ENSO frequency change in coupled climate models as response to the increasing CO2 concentration

The response of ENSO frequency to the increasing CO2 concentration and associated mechanism are examined with outputs of four coupled climate models (GFDL/CM2.0, CNRM/CM3, IPSL/CM4 and INM/CM3.0) submitted to the IPCC Fourth Assessment Report (IPCC AR4). Results reveal a significant change of ENSO frequency as response to the increasing CO2 concentration. However, such a change exhibits an evident model dependence. The ENSO frequency tends to increase in GFDL/CM2.0 and CNRM/CM3 models and decreases in IPSL/CM4 and INM/CM3.0 models. The model dependence is found to be determined by how the model climatological background state of the tropical Pacific responds to the increasing CO2 concentration. It is demonstrated that the change of zonally- and vertically-averaged climatological background upper ocean temperature gradient between the equator and off-equator is crucially responsible for the ENSO frequency change. As response to the increasing CO2 concentration, the climatological background temperature gradient is increased in GFDL/CM2.0 and CNRM/CM3 models and decreased in IPSL/CM4 and INM/CM3.0 models. In terms of the recharge-discharge oscillator theory for ENSO, the increased (decreased) climatological background temperature gradient between the equator and off-equator induces a faster (slower) exchange of oceanic heat content between the equator and off-equator, thus giving rise to a shorter (longer) ENSO timescale and a higher (lower) ENSO frequency.

Seasonal-to-Interannual Variability of Ethiopia/Horn of Africa Monsoon. Part I: Associations of Wavelet-Filtered Large-Scale Atmospheric Circulation and Global Sea Surface Temperature

Abstract Horn of Africa rainfall varies on multiple time scales, but the underlying climate system controls on this variability have not been examined comprehensively. This study therefore investigates the linkages between June–September Horn of Africa (especially Ethiopian) rainfall and regional atmospheric circulation and global sea surface temperature (SST) variations on several key time scales. Wavelet analysis of 5-day average or monthly total rainfall for 1970–99 identifies the dominant coherent modes of rainfall variability. Several regional atmospheric variables and global SST are then identically wavelet filtered, based on the rainfall frequency bands. Regression, correlation, and composite analyses are subsequently used to identify the most important rainfall–climate system time-scale relationships. The results show that Ethiopian monsoon rainfall variation is largely linked with annual time-scale atmospheric circulation patterns involving variability in the major components of the monsoon system. Although variability on the seasonal (75–210 days), quasi-biennial (QB; 1.42–3.04 yr), and El Niño–Southern Oscillation (ENSO; 3.04–4.60 yr) time scales accounts for much less variance than the annual mode (210 days–1.42 yr), they significantly affect Ethiopian rainfall by preferentially modulating the major regional monsoon components and remote teleconnection linkages. The seasonal time scale largely acts in phase with the annual mode, by enhancing or reducing the lower-tropospheric southwesterlies from the equatorial Atlantic during wet or dry periods. The wet QB phase strengthens the Azores and Saharan high and the tropical easterly jet (TEJ) over the Arabian Sea, while the wet ENSO phase enhances the Mascarene high, the TEJ, and the monsoon trough more locally. The effects of tropical SST on Ethiopian rainfall also are prominent on the QB and ENSO time scales. While rainfall–SST correlations for both the QB and ENSO modes are strongly positive (negative) over the equatorial western (eastern) Pacific, only ENSO exhibits widespread strong negative correlations over the Indian Ocean. Opposite QB and ENSO associations tend to characterize dry Ethiopian conditions. The relationships identified on individual time scales now are being used to develop and validate statistical prediction models for Ethiopia.

Interdecadal variability of the relationship between the East Asian winter monsoon and ENSO

The authors examine relationships between the East Asian winter monsoon and the ENSO, particularly on the interdecadal timescales. Based on the analyses of SLP data from 1899 to 1997, the East-Asian winter monsoon index (WMI) is defined as the zonal difference of SLP between ∼120° E and ∼160° E. It is found that 18 out of 28 strong winter monsoon years are either before the development of an El Nino or during the decaying La Nina event, 12 out of 28 weak winter monsoon are before the development of a La Nina or during the decaying El Nino event. There is a significant positive correlation coefficient value of about 0.49 between the normalized 11-yr running mean of WMI and ENSO index, however, the WMI-ENSO relationship is not consistently highly correlated. The temporal evolution of correlation between WMI and ENSO indices in both 11-yr and 21-yr moving window shows that the WMI-ENSO relationship clearly undergo low-frequency oscillation. Obviously, both observational and IPSL air-sea coupled modeling WMI index has a near-decadal peak with PDO timescales and internal peaks with ENSO timescales by applying the Multitaper method. Moreover, the cross wavelet and wavelet coherence analysis of WMI/ENSO indicate that there is a larger significant sections with an in phase behavior between WMI and ENSO at period of 20–30 yrs, suggesting that the interdecadal variation of the WMI-ENSO relationship might exist.

Hydrological prediction in a non-stationary world

Abstract. The paper discusses evidence that common assumptions in the analysis of hydrological time series (homogeneous variability in random fluctuations about a constant mean value) may not be appropriate for some South American drainage basins. Relatively rapid changes have occurred, and are occurring, as a consequence of replacing mature forest by short crops and urban development. Some research claims to have detected non-linear trends in streamflow in rivers draining the south-eastern part of the sub-continent, together with decadal fluctuations and interannual peaks at ENSO timescales. The paper discusses the implications of such changes for hydrological practices now in widespread and largely unquestioned use.

Multi-year predictability in a coupled general circulation model

Multi-year to decadal variability in a 100-year integration of a BMRC coupled atmosphere-ocean general circulation model (CGCM) is examined. The fractional contribution made by the decadal component generally increases with depth and latitude away from surface waters in the equatorial Indo-Pacific Ocean. The relative importance of decadal variability is enhanced in off-equatorial “wings” in the subtropical eastern Pacific. The model and observations exhibit “ENSO-like” decadal patterns. Analytic results are derived, which show that the patterns can, in theory, occur in the absence of any predictability beyond ENSO time-scales. In practice, however, modification to this stochastic view is needed to account for robust differences between ENSO-like decadal patterns and their interannual counterparts. An analysis of variability in the CGCM, a wind-forced shallow water model, and a simple mixed layer model together with existing and new theoretical results are used to improve upon this stochastic paradigm and to provide a new theory for the origin of decadal ENSO-like patterns like the Interdecadal Pacific Oscillation and Pacific Decadal Oscillation. In this theory, ENSO-driven wind-stress variability forces internal equatorially-trapped Kelvin waves that propagate towards the eastern boundary. Kelvin waves can excite reflected internal westward propagating equatorially-trapped Rossby waves (RWs) and coastally-trapped waves (CTWs). CTWs have no impact on the off-equatorial sub-surface ocean outside the coastal wave guide, whereas the RWs do. If the frequency of the incident wave is too high, then only CTWs are excited. At lower frequencies, both CTWs and RWs can be excited. The lower the frequency, the greater the fraction of energy transmitted to RWs. This lowers the characteristic frequency (reddens the spectrum) of variability off the equator relative to its equatorial counterpart. At low frequencies, dissipation acts as an additional low pass filter that becomes more effective, as latitude increases. At the same time, ENSO-driven off-equatorial surface heating anomalies drive mixed layer temperature responses in both hemispheres. Both the eastern boundary interactions and the accumulation of surface heat fluxes by the surface mixed layer act to low pass filter the ENSO-forcing. The resulting off-equatorial variability is therefore more coherent with low pass filtered (decadal) ENSO indices [e.g. NINO3 sea-surface temperature (SST)] than with unfiltered ENSO indices. Consequently large correlations between variability and NINO3 extend further poleward on decadal time-scales than they do on interannual time-scales. This explains why decadal ENSO-like patterns have a broader meridional structure than their interannual counterparts. This difference in appearance can occur even if ENSO indices do not have any predictability beyond interannual time-scales. The wings around 15–20°S, and sub-surface variability at many other locations are predictable on interannual and multi-year time-scales. This includes westward propagating internal RWs within about 25° of the equator. The slowest of these take up to 4 years to reach the western boundary. This sub-surface predictability has significant oceanographic interest. However, it is linked to only low levels of SST variability. Consequently, extrapolation of delayed action oscillator theory to decadal time-scales might not be justified.

Meteorology and oceanography of the Northern Gulf of Alaska

The Gulf of Alaska shelf is dominated by the Alaska Coastal current (ACC), which is forced by along-shore winds and large freshwater runoff. Strong cyclonic winds dominate from fall through spring, and substantial runoff occurs from late spring through fall with annual distributed freshwater discharge greater than that of the Mississippi River. We examine the ACC from Icy Bay to Unimak Pass, a distance of over 1500 km. Over this distance, the ACC is a nearly continuous feature with a marked freshwater core. The annual mean transport, as measured from current meters, is approximately 1.0×10 6 m 3 s −1 along the Kenai Peninsula, with transport decreasing as the ACC travels westward. Even though the coastal GOA is a predominately downwelling system, it supports a productive ecosystem. Macro nutrients from the basin are provided to the coastal system through a number of processes including topographic steering, eddies, upwelling in response to horizontal shear in the barrier jets, and during winter the on-shelf flux in the surface Ekman layer. Micronutrients (e.g., iron) are supplied from mechanisms such as resuspension of shelf sediments and river discharge. While strong seasonal cycles and interannual variability are dominant scales in atmospheric forcing and the oceanic response, there is also forcing on ENSO and decadal time scales.

Is interannual fluctuation of atmospheric CO2 dominated by combined effects of ENSO and volcanic aerosols?

The influence of ENSO, volcanic eruptions, and the North Atlantic Oscillation (NAO) on fluctuations of the atmospheric CO2 concentration were globally investigated on the ENSO timescale for the period 1958 to 1994. Two deseasonalized and detrended monthly time series of anomalous CO2 fluxes were generated: (1) modeled terrestrial biospheric CO2 flux anomalies calculated by the High Resolution Biosphere Model (HRBM) driven by realistic temperature and precipitation data but without considering radiation effects and (2) anomalous CO2 fluxes deduced from atmospheric measurements. While comparing the two time series, three types of periods could be distinguished: “a” periods with a phase shift close to zero between the two series, “b” periods with a phase shift of up to 11 months, and “c” periods with the two curves completely out of phase. During the “c” periods the modeled data show carbon release by the biosphere while the observed data show carbon uptake. The periods of type “c” are accompanied by major volcanic eruptions with considerable aerosol production. Enhanced atmospheric aerosol loading can increase the diffuse fraction of the solar radiation. Diffuse radiation in many cases penetrates better into plant canopies thus potentially enhancing photosynthesis. The resulting additional uptake of carbon may have overcompensated the carbon release caused by ENSO effects thus possibly bringing about a net uptake of CO2. The length variations of the time lags (“b” periods) may be attributed to impacts of vegetation fires. The influence of the NAO remained unclear.

Downslope convection north of Elephant Island, Antarctica: Influence on deep waters and dependence on ENSO

We present a long time series (1992–2001) of bottom temperature from 1040m on the continental slope north of Elephant Island, Antarctic Peninsula. A strong annual signal is apparent, with minimum temperatures during or following the austral winter, and a range similar to that found in much shallower waters on the adjacent shelf. Individual convective events are observable as large‐amplitude cold spikes that are confined to the period of seasonally coldest temperatures. These plumes are dense enough to seasonally displace the Circumpolar Deep Water, and directly inject cold water into the deep open ocean. The coldest water occurred when anomalous sea ice conditions were prevalent, caused by the strong 1997/98 ENSO event. We thus expect repeats of such strong convective events on ENSO timescales. The observed long‐term warming and reduction in sea ice along the Peninsula may, however, act to suppress this mode of ventilating the deep Southern Ocean.

Earth rotation and ENSO events: combined excitation of interannual LOD variations by multiscale atmospheric oscillations

Time series of the length of day characterizing the rate of Earth rotation, the atmospheric angular momentum and the Southern Oscillation Index from 1962 to 2000 are used to reexamine the relationships between the ENSO events and the changes in the length of day, as well as the global atmospheric angular momentum. Particular attention is given to the different effects of the 1982–1983 and 1997–1998 ENSO events on the variations of Earth rotation. The combined effects of multiscale atmospheric oscillations (seasonal, quasi-biennial and ENSO time scales) on the anomalous variations of the interannual rates of Earth rotation are revealed in this paper by studying the wavelet spectra of the data series.

Links between intraseasonal (extended MJO) and ENSO timescales: Insights via geodetic and atmospheric analysis

The relationship of intraseasonal variability to ENSO is investigated using daily length‐of‐day (LOD), atmospheric angular momentum (AAM), west equatorial Pacific (WPAC) wind stress and monthly Nino3 SST data for the years 1976–1999. Strong coherence is found for LOD, AAM, and their residual (representative of oceanic angular momentum and / or unmodeled/underrepresented atmospheric variability) with the regional wind stress at periods near 75 days, a preferred time scale for WPAC Kelvin wave generation. Amplitude envelopes of LOD and AAM filtered at similar periods (60–100 days) are significantly correlated with Nino3 anomalies at an SST lag of 8 months; the LOD‐AAM residual correlates at a shorter lag (5 months), with the difference representing an oceanic spin‐up time. No significant correlations are found using a band (36–60 days) typifying the global Madden‐Julian Oscillation, accounting for the lack of previous MJO‐ENSO correlations in the literature.

On the relationship between Indian Ocean sea surface temperature and Asian Summer Monsoon

Indian Ocean SST has been thought to play a weaker role in Indian summer monsoon rainfall than does the equatorial eastern Pacific SST. In this study we show that on the tropical biennial oscillation (TBO, 2–3 year) time scale the Indian monsoon rainfall has significant positive correlations with the Indian Ocean SST and moisture flux transport in the preceding winter and spring. The effect of this SST influence is quite different from the remote forcing of the Indian monsoon rainfall by the eastern Pacific SST, which is more dominant on the El Nino‐Southern Oscillation (ENSO, 3–7 year) time scale. We conclude that while the eastern Pacific SST and the Eurasian land temperature both may affect the monsoon on the ENSO time scale, they are not important on the TBO time scale. Our results support the tropical and local feedback theories of TBO that this most important component of the monsoon variation is largely influenced by the Indian Ocean SST and interactions within the tropical atmosphere‐ocean system (Chang and Li, Nichols).

A conceptual framework for time and space scale interactions in the climate system

Interactions involving various time and space scales, both within the tropics and between the tropics and midlatitudes, are ubiquitous in the climate system. We propose a conceptual framework for understanding such interactions whereby longer time scales and larger space scales set the base state for processes on shorter time scales and smaller space scales, which in turn have an influence back on the longer time scales and larger space scales in a continuum of process-related interactions. Though not intended to be comprehensive, we do cite examples from the literature to provide evidence for the validity of this framework. Decadal time scale base states of the coupled climate system set the context for the manifestation of interannual time scales (El Nino/Southern Oscillation, ENSO and tropospheric biennial oscillation, TBO) which are influenced by and interact with the annual cycle and seasonal time scales. Those base states in turn influence the large-scale coupled processes involved with intraseasonal and submonthly time scales, tied to interactions within the tropics and extratropics, and tropical–midlatitude teleconnections. All of these set the base state for processes on the synoptic and mesoscale and regional/local space scales. Events at those relatively short time scales and small space scales may then affect the longer time scale and larger space scale processes in turn, reaching back out to submonthly, intraseasonal, seasonal, annual, TBO, ENSO and decadal. Global coupled models can capture some elements of the decadal, ENSO, TBO, annual and seasonal time scales with the associated global space scales. However, coupled models are less successful at simulating phenomena at subseasonal and shorter time scales with hemispheric and smaller space scales. In the context of the proposed conceptual framework, the synergistic interactions of the time and space scales suggest that a high priority must be placed on improved simulations of all of the time and space scales in the climate system. This is particularly important for the subseasonal time scales and hemispheric and smaller space scales, which are not well simulated at present, to improve the prospects of successfully forecasting phenomena beyond the synoptic scales.