Miocene shift of European atmospheric circulation from trade wind to westerlies.

The modern European climatic regime is peculiar, due to its unitary winter but diverse summer climates and a pronounced Mediterranean climate in the south. However, little is known on its evolution in the deep time. Here we reconstruct the European summer climate conditions in the Tortonian (11.62-7.246 Ma) using plant fossil assemblages from 75 well-dated sites across Europe. Our results clearly show that the Tortonian Europe mainly had humid to subhumid summers and no arid climate has been conclusively detected, indicating that the summer-dry Mediterranean-type climate has not yet been established along most of the Mediterranean coast at least by the Tortonian. More importantly, the reconstructed distribution pattern of summer precipitation reveals that the Tortonian European must have largely been controlled by westerlies, resulting in higher precipitation in the west and the lower in the east. The Tortonian westerly wind field appears to differ principally from the trade wind pattern of the preceding Serravallian (13.82-11.62 Ma), recently deduced from herpetofaunal fossils. Such a shift in atmospheric circulation, if ever occurred, might result from the development of ice caps and glaciers in the polar region during the Late Miocene global cooling, the then reorganization of oceanic circulation, and/or the Himalayan-Tibetan uplift.

The Late Miocene, a critical time period in Cenozoic climatic evolution, saw profound environmental changes and geological events, which together regulated the then climate distribution pattern12 and gradually propagated to modern climate configuration34. In North America a similar-to-modern climate was established as early as in the late Middle Miocene, marked by the onset of Great Plain landscape56, while significantly intensified aridification in the vast Asian interior did not occur until the Late Miocene to Early Pliocene367. In Western Eurasia, however, although the Miocene climate has been extensively investigated891011, little is known about the early evolution of the regime, especially in regard to the hydrological dynamics and atmospheric circulation61112.

The modern Europe experiences a prevalence of westerlies in winter, while the summer climates are highly diverse, mainly consisting of a maritime climate along the western coast with evenly dispersed precipitation throughout the year, a Mediterranean climate in the south with dry summers, and a subhumid–semiarid continental climate to the east1314. Such a pattern is jointly controlled by the westerlies and the seasonal shift of Azores subtropical high between ~30° N in winter and ~35° N in summer414. By contrast, during the Serravallian (late Middle Miocene; 13.82–11.62 Ma), the distribution of summer precipitation suggests that Europe was likely dominated by a trade wind system flowing from the northeast to southwest15. Therefore a dramatic shift of the wind field must have occurred between the Serravallian and present. However, neither proxy data nor modelling results have ever evidenced this upheaval before. Here we reconstruct the Tortonian (early Late Miocene; 11.62–7.246 Ma) hydrological dynamics of Europe using palaeobotanical proxy data supplemented by modelling experiments, with a specific focus on the summer condition, to infer the wind direction and consequently the prevailing atmospheric circulation patterns following the Serravallian. Our results reveal a dramatic transition in atmospheric circulation that the so called Serravallian trade winds15 gave way to the Tortonian westerlies that predominate over Western Eurasia.

Results and Discussion

Tortonian summer hydrological distribution

The estimated means of warmest month precipitations (WMP) of the Tortonian Europe range between ~67 mm and ~164 mm, with corresponding warmest month temperatures (WMT) between 21.2°C and 27.8°C (Fig. 1A, 1B; and Supplementary Information Table 1 for complete data ranges). Locations with high WMPs are found in West Europe, except for the southerly coastal regions of the Mediterranean, while relatively lower WMPs are mainly in the coastal areas of Lake Pannon and northern coast of the Black Sea to the east (Fig. 1A). In spite of the spatial differentiation of WMP and WMT, the overall summer climate of the Tortonian Europe was humid to subhumid, but with semi-arid to subhumid in the Andalucian coast of Spain, which is in agreement with the vegetation data, showing a latitudinal gradient1617 (Fig. 1C). No arid type is clearly detected according to the climatic classification of the Köppen Aridity Index (AI)7 (Supplementary Information Table 1).

Figure 1

Reconstruction of the Tortonian climate of Europe.

(A) Warmest month precipitation. Arrows show direction of prevalent wind direction of particular age. The pink arrow denotes the Serravallian trade wind direction favored by European herpetofaunal data15, while the blue arrow indicates the westerlies system inferred from the palaeobotanical data (this study). (B) Warmest month temperature. (C) Summer climatic condition described by Köppen Aridity Index. Palaeoclimatic data are available in Supplementary Information Table 1. The palaeoclimate maps were generated by ArcGIS, Esri.

Humid to subhumid climates during the Tortonian throughout the major parts of Europe are also suggested by mammal hypsodonty data618, small-mammal community structure12, and herpetofaunal composition19. Although possibly punctuated by dry conditions in particular periods and/or at some locations, especially for that of the middle Tortonian northeastern Iberian Peninsula2021, both faunal and floral data indicate an overall (sub)humid climate in the Tortonian346121822. Moreover, it can be assumed that the Andalucian coastal area and the northern Black Sea coast experienced drier summers.

Notably our results show that humid to subhumid Tortonian summers existed in most of Europe, probably except for its southernmost parts (Fig. 1A). The spatial pattern of summer precipitation strongly indicates that the modern Mediterranean climate, with summer drought, had not yet been established by the Tortonian (Fig. 1C; Supplementary Information Table 1). This is in close agreement with fossil mammal community and the majority of palaeovegetation types from Spain, Italy, Greece, and Turkey, supporting the presence of evergreen broadleaved and mixed forests122324. Although sclerophyll woodland and shrubland vegetation did occasionally occur in northeast Spain in the Late Miocene3416, it was likely resulted from a rain shadow effect caused by the uplift of metamorphic complexes at the basin margins25. Therefore, it appears that the northward migration of the subtropical high, which causes the modern Mediterranean dry summer must have had minimal impact on the Mediterranean summer during the Tortonian and affected only the southernmost parts.

Summer wind direction

Terrestrial rainfall in the extra-tropics is primarily controlled by wind transport of moisture from the oceans, with the total amount of precipitation decreasing towards continental interiors1114, while topography can cause local rain shadow effects526. Therefore, large-scale seasonal precipitation distribution can reflect well the associated wind direction given the moisture transport is the dominant factor in relation to precipitation1215. The overall topography of Europe with huge mountains such as Alps and Pyrenees had largely been formed prior to the Late Miocene with exception of the final retreat of the Paratethys in the east27282930.

Our quantitative palaeoclimate reconstruction suggests a zonal eastward decrease of summer precipitation in Tortonian Europe (Fig. 1A), as also implied by palaeovegetation types and mammal data1216. This pattern clearly shows the dominant impact of westerlies on summer moisture transport and the existence of a high pressure system over the mid-latitudes of the Atlantic in summer that drove the moist air mass eastward from the ocean to the continent. However, the high pressure system must have been farther away from the European mainland than the modern Azores High, because the continental climate was mainly (sub)humid in the summer.

A rain shadow effect defined by local palaeotopography also indicates an overall westerly or south westerly wind in the Tortonian. For instance, WMPs experienced in the west of the Spanish Inner Plateau are much higher than those to the east (Fig. 1A). In addition, the WMPs to the south of the Alps are higher than those to the north, which indicates that the northward wind might also be important component of summer air flow in this region. In eastern Europe, the dominance of westerly wind can also be inferred by the relatively high WMPs in the coastal areas of Lake Pannon compared to that in the northern coast of Black Sea. Although the Tortonian WMP differentiations between two sides of plateaus or mountains are much less than those in the modern climate, the topographical effects in blocking water transport are evident (Fig. 1A).

Our inferences of the Tortonian summer wind direction based on WMPs are supported by the Tortonian climate model simulation. As illustrated in Fig. 2, there is relatively high pressure over north Atlantic and low pressure over central Eurasia in the warmest month in our Tortonian run (Fig. 2A), which leads to a strong westerly wind over Europe. Since the high pressure over the north Atlantic is more zonally oriented in the Tortonian run than that in the present-day run, the westerlies are intensified over most of Europe (Fig. 2B). This enhances advection of moisture and stormtracks31, and hence precipitation over Europe in the Tortonian run. Additionally there is an increase of pressure over the Caspian and Black Sea regions in July in the Tortonian run (Fig. 2B). This may be due to the presence of the Paratethys and the cooler summer surface temperatures there. Such a high pressure anomaly might have contributed to the decrease of WMP particularly over the eastern Mediterranean in the Tortonian.

Figure 2

(A) 850 hPa wind (vector; m/s) and sea level pressure anomalies (Pa) to global average (contour) in July in the Tortonian global simulation31. (B) Difference of 850 hPa wind (vector; m/s) and sea level pressure (Pa) in July between the Tortonian and present-day global simulation. The coloured vectors denoted the changes in wind speed (m/s), which are significant with a Student's t-test (p < 0.05). Maps were created by the NCAR Command Language.

Both proxy and modeling results indicate that there was a subtropical high pressure system over the Atlantic (south) west to the European mainland in the Tortonian summers (Fig. 2), resulting in the Tortonian European mainland being dominated by westerlies, with an overall (sub)humid climate with the absence of Mediterranean climate type, probably except from its southernmost parts.

Possible shift of atmospheric circulation between Serravallian and Tortonian

The Tortonian wind direction deduced from the present study with a subtropical high in mid-latitudes around 40° N or so (Fig. 2A), is markedly different from conditions in the preceding Serravallian15, suggesting a critical change in atmospheric circulation and hence climatic regime reorganization.

During the Serravallian summer, European fossil snakehead fish data exclusively suggest that a subtropical high pressure zone situated over northeast to east Europe around 47°N15 drove winds (south)westward over Europe1532. If this hypothesis is further confirmed, then the Serravallian–Totonian transition must have seen a nearly orthogonal shift of the atmospheric circulation over Europe, associated with a southwestern shift of the subtropical high (Fig. 1A).

The southwestward subtropical high shift might be the result of ephemeral Tortonian glaciation after the broadly ice-free Serravallian in the northern hemisphere oceans. Biostratigraphical and isotopic records from northernmost Atlantic undoubtedly indicate a major cooling event in the earliest Tortonian (~11 Ma)33, and furthermore coarse-fraction compositions in ocean sediments show a clear signal of nearly coeval seasonal sea-ice cover in the region between Greenland and Svalbard associated with a cold East Greenland current34. On the one hand, the early accumulation of ice caps and glaciers in north polar regions in the Tortonian2 must considerably have, although not totally, offset the hemispherical asymmetry of atmospheric circulation by oceanic currents1 and forced the high pressure system to cooler region in the summer. On the other hand, marine microfossils strongly suggest that the Fram Strait between Greenland and Svalbard was considerably deepened at ~11 Ma35, which together with the East Greenland current may have jointly resulted in a fundamental reorganization of oceanic circulation36, contributing to the palaeo-North Atlantic gyre37 and palaeo-Azores High19. In addition, the remarkable development of Himalayan–Tibetan system in both elevation and extension38 would have enhanced the reign of low pressure over Eurasian land mass in summer by increasing land–sea heat capacity differentiation39, which strikingly reinforced a longitudinally pressure gradient40 and modified global atmospheric circulation26.

The Tortonian subtropical high over the Atlantic deviated southward or southeastward during the late Messinian as evidenced by the significant aridification in the Iberian Peninsula6121821 and the Mediterranean Messinian Salinity Crisis41. However, it appears that the Mediterranean climate type may have not been established until the subtropical high arrived at its near-modern position in the Late Pliocene to Pleistocene4 when ice-sheets were fully developed in both hemispheres2, which finally resulted in the modern atmospheric circulation and climatic distribution pattern42.

It is noted that so far, climate model simulations for the Middle Miocene and Late Miocene have not demonstrated any significant transition of atmospheric circulation over Europe between these two time intervals31434445. In particular, climate models do not show a pronounced northward shift of the Inter-Tropical Convergence Zone (ITCZ) and the subtropical high pressure zone in the Middle Miocene as suggested by proxy data15. Such discrepancy could be either due to the misinterpretation of these proxy records, or possibly to the failure of the climate models to represent the natural climate variability in deep time. It is widely known that current climate models tend to underestimate temperature increase over high latitudes in the warm geological intervals including the Middle Miocene31434647. This can lead to an underestimation of the northward shift of ITCZ and the subtropical high in these stages. In addition, it is still highly uncertain how the presence of Paratethys and Tethys seaways had affected the ocean and atmospheric circulation over Europe in the Middle–Late Miocene48, which would be highly interesting to study this phenomenon using fully coupled climate models in the future.

Conclusions

Deep time atmospheric circulation is one of the hardest palaeoenvironmental factors to be estimated due to the deficiency in records of wind direction. Because of the close relationship between atmospheric circulation and precipitation distribution at the continental scale, this dilemma can now be tackled. In the present study, fossil plant based climate reconstructions show that the Tortonian Europe had mostly humid to subhumid insummers. On the one hand, the absence of (semi-)arid conditions in the Central Mediterranean indicates that a Mediterranean-type climate had not yet established at least until the Tortonian. On the other hand, the spatial pattern of precipitation implies that the Tortonian Europe must have been under the dominance of westerlies in summer, with a higher precipitation in most of the western continent and relatively lower in the eastern parts. The prevalence of Tortonian westerlies field further implies a dramatic shift of the wind field and consequently a critical transition of atmospheric circulation from that in the preceding Serravallian. Such a shift in wind field over the European mainland might be a response to the Late Miocene global cooling. However, further study is needed to reveal pre- and post-Tortonian development of the Azores subtropical high, which is essential for the modern climatic configuration of Europe.

Methods

Palaeoclimate reconstruction

Proxy-based climate data are obtained from a total of 75 European palaeofloras comprising pollen and spores, leaf floras, and carpological records. All floras are dated as Tortonian, based on various sources of evidence (Supplementary Information Table 1)114950. Some of the investigated floras were already published in the frame of research conducted by the NECLIME network and are available in the PANGAEA database (www.pangaea.de)11, while others are analyzed by this study (Supplementary Information Table 1). Climate data for some of the sites may slightly differ with respect to the published values due to revised taxonomy and/or updates in the concept of nearest living relatives (NLRs) of fossil taxa and their climatic requirements. The complete data set used in this study is provided in the supplementary materials. All floras were analyzed using the Coexistence Approach (CA)51. The method is based on the assumption that the climatic requirements of fossil plant taxa are similar to those of their NLRs. With the CA, for each climate parameter the climatic ranges in which a maximum number of NLRs of a given fossil flora can coexist is determined independently and considered the best description of the palaeoclimatic situation under which the given fossil flora lived. In this study, mean warmest month temperature (WMT) and warmest month precipitation (WMP) are considered as variables. The resolution of the CA strongly depends on the number of NLRs contributing with climate data in the analysis (10 to 74 in the present study). Hence, lower diversity of some of the floras may lead to rather broad ranges of climate variables (Supplementary Information Table 1), a data quality to be considered when interpreting spatial patterns.

The palaeohydrological condition of the climatic type is defined by Köppen Aridity Index (AI)7. The AI is calculated by annual precipitation divided by mean annual temperature plus a constant, based on which the humidity/aridity conditions can be classified into 5 categories (humid, subhumid, semi-arid, arid, and hyper-arid, respectively)7. To understand the summer palaeohydrological condition in the concept of AI category, we use the following formula to calculate warmest month AI that defines the summer hydrological condition: The means of CA intervals were used in the calculation, because they are considered as appropriate in visualising climatic trends between regions, especially when interpolating over greater distances11. We further infer the summer wind direction based on the overall distribution of WMP, based on the assumption that the large-scale pattern of continental precipitation is mainly affected by the wind that transporting moisture14.

Tortonian climate model simulation

To better demonstrate the atmospheric circulation changes implied by our WMP reconstruction, a Tortonian climate modelling study based on a fully coupled global climate model (ECHAM5/MPI-OM)31 was employed. The resolution of the atmosphere general circulation model ECHAM5 is T31 (3.75° × 3.75°) with 19 terrain-following vertical layers. The ocean circulation model MPIOM has an approximate resolution of 3° × 3° and the vertical domain is represented by 40 unevenly spaced levels. To better represent the climate in the Tortonian, the physical boundary conditions in the model were modified31. For instance, the orography is generally reduced. In particular, Greenland was much lower than today because of the removal of the modern ice sheet, and the Alps and the Tibetan Plateau are generally reduced to 70% of present- day height. The land–sea distribution in the Tortonian model setup is largely the same as today, except for the presence of the Paratethys Sea and the Pannon Lake. There is also an open Indonesian Seaway and an open Panama Isthmus with a depth of 500 m in the ocean model. The Tortonian vegetation is characterized by a larger-than-present forest cover and less desert. Particularly, the present-day deserts in central Asia and Sahara are replaced by grassland and savanna. The boreal forests extend far more into the high latitudes including Greenland. The CO2 concentration in the Tortonian simulation is set to 360 ppm which lies in the range of the estimated CO2 concentration for the Late Miocene (200–400 ppm) based on different proxy data. The orbital parameters were chosen the same as present-day. More details on the model specifications and model performance are described in ref. 31.

Author Contributions

C.Q., Y.S.L. and T.U. conceived and designed research. C.Q., Y.S.L. and H.T. performed research; C.Q., Y.S.L., H.T. and T.U. analyzed and interpreted data; C.Q. and Y.S.L. wrote the paper.