Interannual variability of the simulated hydrology in a climatic model-implications for drought

A 10-year simulation of the atmosphere has been performed with a two-level global general circulation model. The model takes account of realistic topography and the hydrological cycle, and computes its own cloud cover, snowfall and sea ice distributions. The seasonally varying sea surface temperatures are specified from climatology. The results presented in this paper are restricted to hydrological aspects of the integration, particularly the soil moisture variations. The model exhibits considerable temporal and regional variability in its simulation of the hydrological cycle, generally reproducing quite fair agreement with observations. Some geographical regions were found to have very noticeable interannual variability, with the occurrence of annual or multi-annual drought a feature at a number of locations. The implications of such naturally occurring drought are discussed.     

Decadal climate variability simulated in a coupled general circulation model

A 1000 year integration of the CSIRO coupled ocean-atmosphere general circulation model is used to study low frequency (decadal to centennial) climate variability in precipitation and temperature. The model is shown to exhibit sizeable decadal variability for these fields, generally accounting for approximately 20 to 40% of the variability (greater than one year) in precipitation and up to 80% for temperature. An empirical orthogonal function (EOF) analysis is applied to the model output to show some of the major statistical modes of low frequency variability. The first EOF spatial pattern looks very much like that of the interannual ENSO pattern. It bears considerable resemblance to observational estimates and is centred in the Pacific extending into both hemispheres. It modulates both precipitation and temperature globally. The EOF has a time evolution that appears to be more than just red noise. Finally, the link between SST in the Pacific with Australian rainfall variability seen in observations is also evident in the model. Received: 29 August 1998 / Accepted: 31 July 1999     

Interannual variability of Mediterranean evaporation and its relation to regional climate

Gridded monthly evaporation data for 1958–2006 from the Woods Hole Oceanographic Institution data set are used to investigate interannual variability of Mediterranean evaporation during cold and hot seasons and its relation to regional atmospheric dynamics, sea surface temperature and atmospheric elements of the hydrological cycle. The first EOF mode of Mediterranean evaporation, explaining more than 50% of its total variance, is characterized by the monopole pattern both in winter and summer. However, despite structural similarity, the EOF-1 of Mediterranean evaporation is affected by different climate signals in cold and hot seasons. During winter the EOF-1 is associated with the East Atlantic teleconnection pattern. In summer, there is indication of tropical influence on the EOF-1 of Mediterranean evaporation (presumably from Asian monsoon). Both in winter and summer, principal components of EOF-1 demonstrate clear interdecadal signals (with a stronger signature in summer) associated with large sea surface temperature anomalies. The results of a sensitivity analysis suggest that in winter both the meridional wind and the vertical gradient of saturation specific humidity (GSSH) near the sea surface contribute to the interdecadal evaporation signal. In summer, however, it is likely that the signal is more related to GSSH. Our analysis did not reveal significant links between the Mediterranean evaporation and the North Atlantic Oscillation in any season. The EOF-2 of evaporation accounts for 20% (11%) of its total variance in winter (in summer). Both in winter and summer the EOF-2 is characterized by a zonal dipole with opposite variations of evaporation in western and eastern parts of the Mediterranean Sea. This mode is associated presumably with smaller scale (i.e., local) effects of atmospheric dynamics. Seasonality of the leading modes of the Mediterranean evaporation is also clearly seen in the character of their links to atmospheric elements of the regional hydrological cycle. In particular, significant links to precipitation in some regions have been found in winter, but not in summer.     

Climatology and interannual variability simulated by the ARPEGE-OPA coupled model

A 10-year simulation with a coupled ocean-atmosphere general circulation model (CGCM) is presented. The model consists of the climate version of the Météo-France global forecasting model, ARPEGE, coupled to the LODYC oceanic model, OPA, by the CERFACS coupling package OASIS. The oceanic component is dynamically active over the tropical Pacific, while climatological time-dependent sea surface temperatures (SSTs) are prescribed outside of the Pacific domain. The coupled model shows little drift and exhibits a very regular seasonal cycle. The climatological mean state and seasonal cycle are well simulated by the coupled model. In particular, the oceanic surface current pattern is accurately depicted and the location and intensity of the Equatorial Undercurrent (EUC) are in good agreement with available data. The seasonal cycle of equatorial SSTs captures quite realistically the annual harmonic. Some deficiencies remain including a weak zonal equatorial SST gradient, underestimated wind stress over the Pacific equatorial band and an additional inter-tropical convergence zone (ITCZ) south of the equator in northern winter and spring. Weak interannual variability is present in the equatorial SST signal with a maximum amplitude of 0.5°C.     

Ocean–atmosphere coupled Pacific Decadal variability simulated by a climate model

Climate Dynamics - Currently, the mechanisms for Pacific Decadal Oscillation (PDO) are still disputed, and in particular the atmosphere response to the ocean in the mid-latitude remains a key...     

Interannual variability of the upper tropospheric circulation

Summary Seventeen years of sea level pressure (SLP), 200-hPa zonal wind and 500-hPa geopotential height data were used to investigate the boreal winter and summer interannual (IA) circulation patterns. The IA patterns for these variables and for their zonally asymmetric (ZA) part were determined by performing empirical orthogonal function (EOF) analyses on the SLP and on ZA SLP. The corresponding patterns for the other variables were obtained by correlating their time series with the amplitude time series of these EOF analyses. For both seasons, the SLP and ZA SLP show a zonal wavenumber one pattern extending from the tropics into the winter hemisphere extratropics, which is consistent with the circulation anomalies related to the El Niño/Southern Oscillation (ENSO) cycles. The zonal wavenumber one pattern observed for the boreal winter describes the SLP and ZA SLP variations related to the mature state of the El Niño and La Niña episodes, and that for the summer, the SLP and ZA SLP variations associated with the initial or decay stages of these phenomena. The 200-hPa zonal wind and 500-hPa geopotential height patterns exhibit strong seasonal dependence, and the ZA parts of these two variables show even more pronounced seasonal differences. These results indicate that the seasonal cycle of the atmospheric circulation, in particular at the upper tropospheric levels, might play an important role in extending the IA wavetrain-like structure into the subtropics as noted for the 200-hPa zonal wind and its ZA part in the Pacific/Americas sector. This wavetrain-like structure shows its Southern Hemisphere (SH) and Northern Hemisphere (NH) branches for the boreal winter, and only its SH branch, for the boreal summer. Thus, the effects of the seasonal cycle of the atmospheric circulation on the IA patterns seem to be stronger for the NH.With 9 Figures     

Decadal and centennial variability of the southern semiannual oscillation simulated in the GFDL coupled GCM

 We investigate the behavior of the semiannual oscillation (SAO) in surface pressure and 500 hPa baroclinicity at high southern latitudes in a 1000-year GFDL coupled ocean-atmosphere GCM run. The model represents this feature but is shown to underestimate its amplitude and percentage of variance explained in the midlatitudes. South of 60 °S the simulation of the pressure oscillation, although somewhat too weak, is considerably better. Our analysis reveals significant interannual, decadal and centennial variability in the modeled SAO. While there is only a short historical record of observational data in the middle and high southern latitudes, existing studies suggest that the strength of the SAO does show significant variability on at least the first two of these time scales. Strong relationships between the semiannual cycles of surface pressure and baroclinicity are apparent in the model output, reinforcing the findings in earlier studies that the differing annual march of temperature between the midlatitudes and the Antarctic coast leads to a semiannual component in the baroclinicity and thence the surface pressure. We draw attention to extended periods when the model SAO is weak and strong, and have investigated the nature of the circulation during each period. The GFDL model results suggest that a significant proportion of the SAO variation was associated more with variations in the strength of the winter pressure maximum rather than the springtime minimum. The extent to which this and other aspects of the modeled longterm variability are related to actual atmospheric structure must await the availability of longer data records. Received: 11 November 1996 / Accepted: 28 July 1997     

Interannual relationships between Indian Summer Monsoon and Indo-Pacific coupled modes of variability during recent decades

Various SST indices in the Indo-Pacific region have been proposed in the literature in light of a long-range seasonal forecasting of the Indian Summer Monsoon (ISM). However, the dynamics associated with these different indices have never been compared in detail. To this end, the present work re-examines the variabilities of ISM rainfall, onset and withdrawal dates at interannual timescales and explores their relationships with El Ni?o-Southern Oscillation (ENSO) and various modes of coupled variability in the Indian Ocean. Based on recent findings in the literature, five SST indices are considered here: Ni?o3.4 SST index in December?CJanuary both preceding [Nino(?1)] and following the ISM [Nino(0)], South East Indian Ocean (SEIO) SST in February?CMarch, the Indian Ocean Basin (IOB) mode in April?CMay and, finally, the Indian Ocean Dipole (IOD) averaged from September to November, also, both preceding [IOD(?1)] and following the ISM [IOD(0)]. The respective merits and associated dynamics of the selected indices are compared through various correlation and regression analyses. Our first result is a deceptive one: the statistical relationships with the ISM rainfall at the continental and seasonal scales are modest and only barely significant, particularly for the IOD, IOB and Nino(?1) indices. However, a detailed analysis shows that statistical relationships with the ISM rainfall time series are statistically biased as the ISM rainfall seems to be shaped by much intraseasonal variability, linked in particular to the timing of the onset and withdrawal of the ISM. Surprisingly, analysis within the ISM season shows that Nino(?1), IOB and SEIO indices give rise to prospects of comparatively higher ISM previsibility for both the ISM onset and the amount of rainfall during the second half of the ISM season. The IOD seems to play only a secondary role. Moreover, our work shows that these indices are associated with distinct processes occurring within the Indian Ocean from late boreal winter or early spring onwards. The regression analyses also illustrate that these (local) mechanisms are dynamically and remotely linked to different phases of ENSO in the equatorial Pacific, a result which may have useful implications in terms of forecasting strategies since the choice of the better indices then hinges on the concurrent phasing of the ENSO cycle.     

Interannual variability of the South Pacific Ocean in observations and simulated by the NCEP Climate Forecast System,version 2

The mechanism of the South Pacific Ocean Dipole (SPOD) mode is examined, using a 50-year simulation of the Climate Forecast System, version 2 (CFSv2) and 50-year observation-based ocean–atmosphere analyses (1961–2010). It is shown that the SPOD, a sea surface temperatures (SST) seesaw between the subtropics and extratropics, is the dominant mode of the interannual variability in the South Pacific in both observations and CFSv2 simulation. CFSv2 also reproduces the seasonal phase-locking of the observed SPOD, with the anomaly pattern developing in austral spring, peaking in summer, and decaying in autumn. Composite analyses based on both observational and model data suggest that in the warm phase of SPOD, positive SST anomaly (SSTA) is initiated by weakened westerly winds over the central South Pacific in austral spring, which suppress the surface evaporative heat loss and reduce the oceanic mixed layer depth, both contributing to the SST warming. The wind-SST-mixed layer anomalies then evolve coherently over the next two seasons while the cold SSTA develops to the north. The wind perturbations are in turn a response to El Niño-Southern Oscillation (ENSO), which forces an atmospheric planetary wave train, the Pacific-South American pattern, emanating from an anomalous heat source in the tropical western Pacific. Moreover, SPOD is significantly correlated with the southern annular mode (SAM) while the latter is also significantly correlated with the ENSO index. This suggests that ENSO’s influence on the SPOD may be partially conveyed through SAM.     

Interannual variability and predictability of African easterly waves in a GCM

The interannual variability of African Easterly Waves (AEWs) is assessed with the help of spatio-temporal spectral analysis (STSA) and complex empirical orthogonal functions methods applied to the results of ten-member multiyear ensemble simulations. Two sets of experiments were conducted with the Météo-France ARPEGE-Climat GCM, one with interactive soil moisture (control), and the other with soil moisture relaxed towards climatological monthly means calculated from the control. Composites of Soudano–Sahelian AEWs were constructed and associated physical processes and dynamics were studied in the frame of the waves. It is shown that the model is able to simulate realistically some interannual variability in the AEWs, and that this dynamical aspect of the West African climate is potentially predictable (i.e. signal can be extracted from boundary conditions relatively to internal error of the GCM), especially along the moist Guinean coast. Compared with ECMWF 15-year reanalysis (ERA15), the maximum activity of AEWs is located too far to the South and is somewhat too zonal, but the main characteristics of the waves are well represented. The major impact of soil moisture relaxation in the GCM experiments is to reduce the seasonal potential predictability of AEWs over land by enhancing their internal variability.     

Interannual variation of the diurnal convection cycle in the western North Pacific

Summary As revealed from the interannual variation of outgoing longwave radiation in the western Pacific, deep cumulus convection along the Meiyü-Baiu front and ITCZ is modulated by the anomalous summer circulation in the following manner: when the sea surface temperatures on the eastern tropical Pacific are anomalously warm (cold), cumulus convection is enhanced (suppressed) along the equator east of 150° E and along the Meiyü-Baiu front, but is suppressed (enhanced) along the equator west of 150° E and along a longitudinal zone (10° N–30° N) extending from the northern section of the South China Sea to the International Dateline. Since tropical deep cumulus convection exhibits a pronounced diurnal variation, the diurnal convection cycle in the western Pacific may undergo an interannual variation coherent with that of deep tropical cumulus convection. This inference is substantiated by our analysis of the diurnal convection cycle for 1980–1993 with 3-hour equivalent black-body temperature observed by the Japanese Geostationary Meteorological Satellite (GMS). As expected, the diurnal convection cycle in the western Pacific is subjected to an interannual variation in accordance with deep cumulus convection along the Meiyü-Baiu front and ITCZ. Except along the equator east of 150° E, the diurnal convection cycle does not exhibit a drastic interannual change in phase.     

Interannual temperature variability in the United States since 1896

The variability of mean monthly temperatures in the United States since 1896 is examined. The results show that the interannual variability reached a peak in the decade centered on 1930 and decreased fairly steadily to a minimum in the decade centered on 1970. This temporal trend is almost completely explained by changes in the variability of winter (December, January, February) mean monthly temperatures. The greatest percent decrease in variability occurred in the Midwest.     

Interannual variability of the convective activities associated with the east asian summer monsoon obtained from tbb variability

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Winter synoptic-scale variability over the Mediterranean Basin under future climate conditions as simulated by the ECHAM5

Changes of the winter climate in the Mediterranean Basin (MB) for future A2 conditions are investigated for the period 2071–2100 and compared with the control period 1961–1990. The analysis is based on time-slice simulations of the latest version of the ECHAM model. First, the control simulation is evaluated with reanalysis data. The emphasis is given to synoptic and large-scale features and their variability in the MB. The model is found to be capable of reproducing the main features of the MB and southern Europe in the winter season. Second, the A2 simulation is compared with the control simulation, revealing considerable changes of the synoptic variability. Focusing on the synoptic spatio-temporal scale aims to unfold the dynamic background of the climatic changes. The Mediterranean cyclones, which are individually detected and tracked, decrease by 10% in the Western Mediterranean (WM) whereas no significant change is found in the Eastern Mediterranean. The cyclone intensity is slightly reduced in the entire region. To understand these changes, the underlying dynamical background is analyzed. It is found that changes in baroclinicity, static stability, transformation from eddy kinetic energy to kinetic energy of the mean flow and stationary wave activity are significant in particular in the WM and the coastline of North Africa. The reduction of cyclonic activity severely impacts the precipitation mainly in the southern part of the WM.