João Corte-Real

Regional climate change in Portugal: precipitation variability associated with large-scale atmospheric circulation

Four major circulation patterns, associated with daily precipitation in Portugal, are classified from daily sea level pressure fields over the northeastern Atlantic and western Europe, based on the K-means clustering algorithm coupled with principal component analysis. A rainy pattern is clearly identified with a probability of rain of 74.6%, as well as two distinct dry patterns, one prevailing in summer and the other occurring frequently in winter; a blocking-like pattern with a probability of rain of 36.8% has also been identified. These patterns are quasi-stationary, normally persisting for 1 week and sometimes even for 1 month, especially the dry ones; they represent the principal weather regimes associated with precipitation in Portugal. Interannual variations in monthly precipitation associated with the circulation patterns are also investigated; results show that these variations match fluctuations in the frequencies of occurrence of both the rainy and the dry patterns. The decreasing trend of March monthly rainfall in southern Portugal is closely related to corresponding trends in the frequencies of both the rainy pattern and the summer dry pattern. Long term trends are not significant either in other monthly rainfall sequences or in the frequencies of different circulation patterns. Interannual variations seem, in most months, to be quasi-periodic. Singular spectrum analysis (SSA) is performed on these sequences to detect quasi-periodic oscillations. Relationships between oscillations in rainfall and in frequencies of occurrence of circulation patterns are studied. Results show that four weather circulation patterns or weather regimes are important for investigating regional climate change in Portugal and its relationship with variability of large-scale atmospheric circulation.

Dependence of winter precipitation over Portugal on NAO and baroclinic wave activity

The relationship between winter (DJF) rainfall over Portugal and the variable large scale circulation is addressed. It is shown that the poles of the sea level pressure (SLP) field variability associated with rainfall variability are shifted about 15° northward with respect to those used in standard definitions of the North Atlantic Oscillation (NAO). It is suggested that the influence of NAO on rainfall dominantly arises from the associated advection of humidity from the Atlantic Ocean. Rainfall is also related to different aspects of baroclinic wave activity, the variability of the latter quantity in turn being largely dependent on the NAO. A negative NAO index (leading to increased westerly surface geostrophic winds into Portugal) is associated with an increased number of deep (ps<980 hPa) surface lows over the central North Atlantic and of intermediate (980<ps<1000 hPa) surface lows over North-western Europe. It is suggested that these distant surface lows have no direct influence on local Portuguese precipitation, but rather contribute to advection at their southern flanks. The other aspect of baroclinic wave activity varying with the NAO is the mid-tropospheric storm track (defined by the 500 hPa bandpass-filtered geopotential height variance). A possible local influence of the storm track due to vertical motions ahead of the upper air troughs cannot be unambiguously separated from the effect of advection. A separate influence of local surface cyclones over the Iberian peninsula which may, for instance, arise from the large scale ascent of air, is revealed by the statistics: for a given advection, rainfall amounts for months with local cyclone cores over the considered region tend to exceed those without. Copyright

Is the North Atlantic Oscillation the most important atmospheric pattern for precipitation in Europe

Signatures of the North Atlantic Oscillation (NAO) are widely detected in climatic variability, particularly in the extratropical latitudes surrounding the North Atlantic Ocean. However, it is still controversial whether the NAO is the most important pattern of nonseasonal variability of atmospheric circulation related to precipitation over Europe. This paper is an attempt to contribute to this issue. The spatial modes of nonseasonal variability of monthly fields of mean sea level pressure (MSLP) over the northeastern Atlantic and western Europe and precipitation over Europe are investigated mainly by using principal component analysis. The relationships between the two fields are studied via canonical correlation analysis (CCA). The data sets used refer to the period from 1911 to 1990. The most important spatial mode of MSLP is the NAO pattern, its corresponding principal component being closely related to the NAO index. Interestingly however, the NAO pattern seems to be responsible only for the second empirical orthogonal function in precipitation, while the most important spatial mode of precipitation corresponds to the third EOF of MSLP (North Sea pattern). Furthermore, the second EOF of MSLP (Scandinavian pattern) is highly associated with the third EOF of precipitation. Significant pairs of canonical patterns between the MSLP and precipitation fields obtained from canonical correlation analysis are coherent with the conclusions above. These results can potentially be used to assess possible changes of precipitation over Europe due to increasing greenhouse gases based on the variability of MSLP simulated by general circulation models.

Downscaling GCM information to regional scales: a non-parametric multivariate regression approach

The multivariate adaptive regression splines (MARS) model was used to capture the observed relationships between sea level pressure (SLP) anomalies over the Euro-Atlantic sector and the winter time (December–February) monthly rainfall at eight sites in Portugal; possible anthropogenic changes of the rainfall in a perturbed future climate were then estimated by using both the observed SLP-rainfall relationships, described by MARS models, and the GCM simulated SLP, taken from the output of the Hadley Centre Transient Climate Change Experiment (UKTR). Also, principal component analysis was carried out to reduce the dimensionality of the SLP data, and to assess the ability of the GCM in simulating the large-scale circulation; the first six principal components were retained as predictors in the MARS model. The MARS model were built up by using the data for 1946–1991 as the training set and that for 1901–1943 as the testing set, showing satisfactory prediction skills. It is concluded that the UKTR control simulation successfully reproduced main features of the large-scale circulation, but the observed relationship between SLP and the regional rainfall was not well preserved. With respect to the 54-year experiment of perturbed run, the MARS estimation of rainfall and the relevant direct GCM rainfall output possess similar multi-annual variations; however, there are substantial differences regarding details; the change in the area mean of winter time mean monthly rainfall in Portugal estimated by MARS (indirect GCM output) is about −12.7 mm per 54-year, and the relevant direct GCM output is −16.9 mm/54-year. This reduction tendency is consistent with previously reported findings respecting rainfall in the Iberian Peninsula, which were based on the MPI (Max-Planck Institute for Meteorology) transient simulations.

On the relationships between daily circulation patterns and precipitation in Portugal

Daily circulation (sea level pressure) maps over east Atlantic-Europe were classified into six circulation patterns (CPs) by using k-means clustering coupled with principal component analysis. The relationships between winter monthly rainfall in Portugal and the occurrence frequencies of the CPs were also examined. The CP classification scheme is effective not only in discriminating regional wet/dry weather conditions, but also in differentiating between various conditions associated with different precipitation intensities. For southern Portugal, CP6 and CP3 are the wettest patterns: under their presence the rainfall occurrence probability and the rainy day rainfall intensity are twice as high as those associated with the dry CPs; with only about 26.6% occurrence probability the two wet CPs together contribute about 66.8% of the total rainfall. For northern Portugal, CP6 and CP4 turned out to be the wettest patterns. They produce over 60% of the total rainfall with only 31% chance of occurrence. In terms of long-term variation the precipitation is also intimately related to the occurrence frequencies of the CPs. The occurrence frequencies of CPs were found to be good predictors for the regional precipitation. In addition, it is evident that the decrease of March rainfall throughout the country in the last 3 decades has been a consequence of a reduction in the occurrence frequency of the wettest circulation pattern, CP6, which coincides with the upward trend of the North Atlantic Oscillation index and could be related to the northeastward shifting and strengthening of the Azores High.

Impacts of climate and land use changes on the hydrological and erosion processes of two contrasting Mediterranean catchments.

The impacts of climate and land use changes on streamflow and sediment export were evaluated for a humid (São Lourenço) and a dry (Guadalupe) Mediterranean catchment, using the SWAT model. SWAT was able to produce viable streamflow and sediment export simulations for both catchments, which provided a baseline for investigating climate and land use changes under the A1B and B1 emission scenarios for 2071-2100. Compared to the baseline scenario (1971-2000), climate change scenarios showed a decrease in annual rainfall for both catchments (humid: -12%; dry: -8%), together with strong increases in rainfall during winter. Land use changes were derived from a socio-economic storyline in which traditional agriculture is replaced by more profitable land uses (i.e. corn and commercial forestry at the humid site; sunflower at the dry site). Climate change projections showed a decrease in streamflow for both catchments, whereas sediment export decreased only for the São Lourenço catchment. Land use changes resulted in an increase in streamflow, but the erosive response differed between catchments. The combination of climate and land use change scenarios led to a reduction in streamflow for both catchments, suggesting a domain of the climatic response. As for sediments, contrasting results were observed for the humid (A1B: -29%; B1: -22%) and dry catchment (A1B: +222%; B1: +5%), which is mainly due to differences in the present-day and forecasted vegetation types. The results highlight the importance of climate-induced land-use change impacts, which could be similar to or more severe than the direct impacts of climate change alone.

SPATIAL-TEMPORAL STRUCTURES OF QUASI-PERIODIC OSCILLATIONS IN PRECIPITATION OVER EUROPE

Based on a 5°5° gridded monthly precipitation data set from 1911 to 1990, quasi-periodic oscillations are identified by performing a multichannel singular spectrum analysis (MSSA) on the first ten principal components (PCs; explaining 53.1% of total variance) of normalized monthly precipitation fields over Europe. Two robust quasiperiodic oscillations are isolated with periods of 23.8 months (2.0 years) and 43.5 months (3.6 years), respectively. Applying a reconstruction technique and composite analysis, spatial structures and temporal propagation of the quasi-periodic oscillations are revealed. The two oscillations behave differently: the 43.5-month oscillation clearly propagates northward; however, the 23.8-month oscillation seems quasi-stationary, although it propagates slowly southward. This may imply that the causes of these two oscillations are different. Oscillatory behaviour in the time series of a North Atlantic Oscillation (NAO) index has also been analysed by performing singular spectrum analysis (SSA) for the same period used for the analysis of precipitation. However, in the time series of the NAO index, the dominant oscillation is a 27.7-month (2.3-year) period, with no robust oscillation with a period of 3.6 years being detected. Furthermore, the two oscillations cannot be identified by MSSA from the precipitation fields regressed upon the NAO index. Therefore, it appears that the two quasi-periodic oscillations are not well related to the NAO, which may not be the most important atmospheric pattern associated with non-seasonal precipitation variability over Europe. Copyright

Modes of variability in the Northern Hemisphere's mid-tropospheric large-scale circulation

SummaryIn this paper the preferential modes of non-seasonal variation of the 500 hPa height fields and the temporal variations thereof are explored at pentad time steps using S-mode Principal Component Analysis, maximum entropy spectrum analysis, single-channel singular spectrum analysis and the Kendall test of randomness.In a pentad analysis the modes similar to the well-known circulation regimes identified by applying different methods on monthly mean or 1-month mean series (Wallace and Gutzler, 1981; Barnston and Livezey, 1987) are identified and also a number of new modes. Unlike in the previous studies, we obtain some information about the amplitudes of the preferential circulation modes by using composite analysis; significant trends were detected and extracted by performing singular spectrum analysis, in combination with the Kendall test of randomness, and low-frequency fluctuations of about 30–50 days are revealed as being intimately associated with blocking activities. Moreover, a quasi-cycle of about 3.86-year is detected, in association with the so-called PNA teleconnection, which can be related to the low-frequency component of the ENSO phenomenon.

Intraseasonal oscillations and associated spatial‐temporal structures of precipitation over China

On the basis of daily rainfall series for 122 stations well covering eastern China (east of 97°E), intraseasonal oscillations of the monsoonal precipitation and the associated spatial-temporal structures were explored by performing multichannel singular spectrum analysis. Three oscillations with quasi-periods of 43, 21, and 12 days were found to be robust, being confined to the region south of 40°N latitude. The spatial-temporal structures, i.e., the propagation of rainfall anomalies, associated with these quasi-periodicities are presented. Also, some information about the amplitudes of these oscillations is provided by using composite analysis.

COUPLED BISTABLE MAPS: A TOOL TO STUDY CONVECTION PARAMETERIZATION IN OCEAN MODELS

We present a study of ocean convection parameterization based on a novel approach which includes both eddy diffusion and advection and consists of a two-dimensional lattice of bistable maps. This approach retains important features of usual grid models and allows to assess the relative roles of diffusion and advection in the spreading of convective cells. For large diffusion our model exhibits a phase transition from convective patterns to a homogeneous state over the entire lattice. In hysteresis experiments we find staircase behavior depending on stability thresholds of local convection patterns. This nonphysical behavior is suspected to induce spurious abrupt changes in the spreading of convection in ocean models. The final steady state of convective cells depends not only on the magnitude of the advective velocity but also on its direction, implying a possible bias in the development of convective patterns. Such bias points to the need for an appropriate choice of grid geometry in ocean modeling.