Warming Hiatus Period Research Articles

Non-symmetric responses of leaf onset date to natural warming and cooling in northern ecosystems.

The northern hemisphere has experienced regional cooling, especially during the global warming hiatus (1998-2012) due to ocean energy redistribution. However, the lack of studies about the natural cooling effects hampers our understanding of vegetation responses to climate change. Using 15,125 ground phenological time series at 3,620 sites since the 1950s and 31-year satellite greenness observations (1982-2012) covering the warming hiatus period, we show a stronger response of leaf onset date (LOD) to natural cooling than to warming, i.e. the delay of LOD caused by 1°C cooling is larger than the advance of LOD with 1°C warming. This might be because cooling leads to larger chilling accumulation and heating requirements for leaf onset, but this non-symmetric LOD response is partially offset by warming-related drying. Moreover, spring greening magnitude, in terms of satellite-based greenness and productivity, is more sensitive to LOD changes in the warming area than in the cooling. These results highlight the importance of considering non-symmetric responses of spring greening to warming and cooling when predicting vegetation-climate feedbacks.

Separating Anthropogenically‐ and Naturally‐Caused Temperature Trends: A Systematic Approach Based On Climate Memory Analysis

AbstractSeparating anthropogenic and natural contributions to global warming is crucial for understanding and predicting climate change, but it is challenging due to the complexity of the climate system. In this study, we try to address this issue by estimating the anthropogenic forcing impacts with its long‐term persistence features properly considered. In this way, a data‐driven approach is proposed to decompose the observed trends from global and regional scales into anthropogenically‐ and naturally‐caused trends. We find that there is a continuous anthropogenic global warming trend since the beginning of the last century, even during the recent global warming hiatus period. On regional scales, the anthropogenically‐forced trends among regions are found at a similar level, while their unevenly distributed warming trends among regions may be attributed to natural causes. Our findings provide new insight into climate warming, and our approach based on persistence analysis provides a new perspective for attribution studies.

Effects of Sea-Surface Temperature, Cloud Vertical Structure and Wind Speed on Temperature Change between Hiatus and Post-Hiatus Periods in Tropical Western Pacific

A region in the tropical western Pacific is selected to study the notable change in temperature between the recent warming hiatus period and the post-hiatus period. In total, three probable factors, namely sea-surface temperature (SST), cloud vertical structure (CVS) and wind speed, which may account for the temperature change are found to exhibit noticeable differences between these two periods. A one-dimensional atmospheric radiative transfer model, incorporating convective adjustment and energy exchange with the ocean, is developed to simulate the diurnal pattern of temperature profile under the influence of the three probable factors in the two concerned periods. Virtual profiles of sea-surface temperature, cloud vertical structure and wind speed in both periods are developed from data available in the literature. Diurnal patterns of temperatures near the air–sea interface are computed with the proposed model over a sufficient number of days. The simulated temperatures under different combinations of factors, in either the hiatus or post-hiatus period, are statistically analyzed to gain insights about the separate and combined effects of these three factors on causing climate change.

Continued spring phenological advance under global warming hiatus over the Pan-Third Pole.

The global surface temperature has witnessed a warming hiatus in the first decade of this century, but how this slowing down of warming will impact spring phenology over Pan-Third Pole remains unclear. Here, we combined multiple satellite-derived vegetation indices with eddy covariance datasets to evaluate the spatiotemporal changes in spring phenological changes over the Pan-Third Pole. We found that the spring phenology over Pan-Third Pole continues to advance at the rate of 4.8 days decade-1 during the warming hiatus period, which is contrasted to a non-significant change over the northern hemisphere. Such a significant and continued advance in spring phenology was mainly attributed to an increase in preseason minimum temperature and water availability. Moreover, there is an overall increasing importance of precipitation on changes in spring phenology during the last four decades. We further demonstrated that this increasingly negative correlation was also found across more than two-thirds of the dryland region, tentatively suggesting that spring phenological changes might shift from temperature to precipitation-controlled over the Pan-Third Pole in a warmer world.

Anomalies in precipitation rather than temperature as the dominant driver of drought stress on vegetation in the Northern Hemisphere

Climate warming has substantially increased drought-induced reduction in terrestrial productivity over recent decades. However, important questions regarding the primary driver of drought stress remain controversial under the context of global warming. The period of warming hiatus between 1998 and 2013 provides a unique opportunity to understand the relative importance of temperature and precipitation anomalies to drought stress on terrestrial ecosystems. Here we analyzed and compared the effects of temperature, precipitation and drought stress on vegetation productivity before and over the warming hiatus period. We found that precipitation anomalies rather than temperature are the primary driver of drought and associated decreased productivity with warming in the Northern Hemisphere. Our study implied that the weight of precipitation and temperature should be adjusted when modeling the drought stress on terrestrial ecosystems under future climate warming scenarios. Data AvailabilityThe data that support the findings of this study are openly available: ORNL DAAC (http://daac.ornl.gov;https://doi.org/10.3334/ORNLDAAC/1789); CRU (https://crudata. uea.ac.uk/cru/data/hrg/; http://doi.org/10.1002/joc.3711).

What caused the interdecadal shift in the El Niño–Southern Oscillation (ENSO) impact on dust mass concentration over northwestern South Asia?

Abstract. Changes in large-scale circulation, especially El Niño–Southern Oscillation (ENSO), have significant impacts on dust activities over the dust source and downwind regions. However, these impacts present an interdecadal pattern, and it remains less clear which factors lead to the interdecadal variability of the ENSO impact on dust activities over northwestern South Asia, although previous studies have discussed the response of interannual dust activities over northwestern South Asia to the ENSO circle. Based on the linear regression model and MERRA-2 atmospheric aerosol reanalysis data, this study investigated the interdecadal variability of the ENSO impact on dust activities as well as the associated possible atmospheric drivers under two different warming phases over northwestern South Asia. Results indicated that the relationship between ENSO and dust column mass density (DUCMASS) experienced an obvious shift from the accelerated global warming period (1982–1996) to the warming hiatus period (2000–2014). The change in Atlantic and Indian Ocean sea surface temperature anomaly (SSTA) patterns weakened the impact of ENSO on dust activities over northwestern South Asia during 1982–1996, while the change in Pacific Decadal Oscillation (PDO) strengthened ENSO's effect when it was in phase with ENSO. Both the Atlantic and Indian Ocean SSTA patterns were modulated by the duration of ENSO events (i.e., continuing and emerging ENSO). This study provides new insights into numerical simulation involving the influence of atmospheric teleconnections on the variability of dust activities and their influence mechanisms.

Global Snowmelt Onset Reflects Climate Variability: Insights from Spaceborne Radiometer Observations

Abstract Snowmelt is a critical component in the cryosphere and has a direct impact on Earth’s energy and water budget. Here, a 40-yr integrated melt onset (MO) dataset over sea ice, ice sheets, and terrestrial snow is compiled from spaceborne microwave radiometers and ERA5, allowing an overall assessment of the cryosphere. Results suggest that MO in both hemispheres shows latitudinal and vertical zonalities. The global cryosphere presented a trend toward earlier MO (−2 days decade−1) with hotpots distributed at the Northern Hemisphere high latitudes where the warming rate is much higher than that at lower latitudes. Overall, variations in MO showed a similar pattern to that in near-surface temperature. The advance of MO has been slowing down since the 1990s and no significant trend was observed during the so-called warming hiatus period (1998–2012). Regionally, climatic linkage analyses suggest the local MO variations were associated with different climate indices. MO in the pan-Arctic region is related with the Arctic Oscillation and North Atlantic Oscillation, while that in the pan-Antarctic region is associated with El Niño–Southern Oscillation and the southern annular mode. Occasionally, abnormal MO occurs as a result of extreme weather conditions. In February 2018, abnormal early melt events that occurred in the Arctic Ocean are found to be linked with the warm southerly flow due to sudden stratospheric warming. These findings suggest the satellite-based MO allows examining the dynamics and extremes in the climate system, both regionally and globally.

Estimation of China’s Contribution to Global Greening over the Past Three Decades

China’s contribution to global greening is regulated by increasing atmospheric CO2 concentrations, climate change, and land use. Based on TRENDY project data, this study identified that the shifts in China’s contribution to the global leaf area index (LAI) trend strongly reduced during the warming hiatus, translating from 13.42 ± 26.45% during 1982–1998 into 7.91 ± 25.45% during 1999–2012. First, significant negative sensitivities of LAI to enhanced vapor pressure deficit (VPD), when only considering the climate effect derived from TRENDY models in China, were found to have shifted substantially after the late 1990s. However, globally, LAI had positive rather than negative responses to enhanced VPD. These opposing shifts in the response of LAI to enhanced VPD reduced the national contribution to global vegetation greening. Second, shifts in land-use change and their effects on the LAI trends in the two periods in China were accompanied by major changes in land cover and land management intensity, including forestry. Consequently, the contribution of land use in China reduced by −47.68% during the warming hiatus period, as compared with the warming period. Such a shift in the impact of land-use change on LAI simulated by ecosystem models might result from the models’ lack of consideration of conserving and expanding forests with the goal of mitigating climate change for China. Our results highlight the need for ecosystem models to reproduce the enhanced negative impact on global LAI and consider the shifts in man-made adaptation policies (e.g., forest management) to improve terrestrial ecosystem models in the future.

Vegetation Browning Trends in Spring and Autumn over Xinjiang, China, during the Warming Hiatus

Satellite-derived vegetation records (GIMMS3g-NDVI) report that climate warming promotes vegetation greening trends; however, the climate impacts on vegetation growth during the global warming hiatus period (1998–2012) remain unclear. In this study, we focused on the vegetation change trend in Xinjiang in spring and autumn before and during the recent warming hiatus period, and their climate-driving mechanisms, which have not been examined in previous studies. Based on satellite records, our results indicated that the summer normalized difference vegetation index (NDVI) in Xinjiang experienced a greening trend, while a browning trend existed in spring and autumn during this period. The autumn NDVI browning trend in Xinjiang was larger than that in spring; however, the spring NDVI displayed a higher correlation with climatic factors than did the autumn NDVI. During the warming hiatus, spring climatic factors were the main controlling factors of spring NDVI, and spring vapor pressure deficit (VPD) had the highest positive correlation with spring NDVI, followed by spring temperature. The larger increase in air temperature in spring than in autumn resulted in increased VPD differences in spring and autumn. In autumn, summer climatic factors (e.g., VPD, WS, RH, and precipitation) were significantly correlated with the autumn NDVI during the warming hiatus. However, the autumn temperature was weakly correlated with the autumn NDVI. Our results have significant implications for understanding the response of vegetation growth to recent and future climatic conditions.

Vulnerability of rice production to temperature extremes during rice reproductive stage in Yangtze River Valley, China

The increasing trend of extreme heat events poses serious implications for rice production in China, especially when it coincides with the heat-sensitive reproductive growth period (RGP) leading to high temperature-induced spikelet sterility. (2) Methods: This study analyzed the long-term meteorological data of Jiangsu province, located at lower reaches of Yangtze River Valley, to ascertain changing trends in temperature extremes. Moreover, stress days and nights were derived from temperature extremes, during RGP as a proxy of rice sterility in the province. The stress days were defined in three ways, with higher daily maximum (≥35 °C, hot day, HD), and minimum temperature (≥28 °C, hot night, HN) and days with both critical limits (severe hot day, SHD). (3) Results: The analysis of data revealed that a definite warming hiatus period in the 1970s and a transitional phase in 1980s were evident for daily maximum temperatures (TMX) in the province. Warm phases since 1990s were evident for both day and night temperature extremes. Asymmetric warming of TMX and daily minimum (TMN) temperatures reduced diurnal temperature range (DTR) over time. The slight negative trend for TMX and HD were observed, while trends for TMN, HN and SHD were strongly positive in the province with regional variations. (4) Conclusion: Observed increasing trends of TMN, HNs, and SHDs, as well as the decreasing DTR during rice RGP can be a threat to rice production in the province. Rice crop in the Southern Jiangsu is more vulnerable to higher temperature extremes while in the northern Jiangsu, decreasing DTR can be crucial. These findings could have useful implications for improving rice yield under changing climate.

Main processes affecting global average surface temperature

<p indent=0mm>Since the industrial revolution, the greenhouse effect caused by greenhouse gas emissions has led to an increase in the net radiative forcing on the Earth’s surface, and the global mean surface temperature (GMST) has continued to rise, accompanied by multi-time scale oscillations including inter-annual decadal and multi-decadal oscillations. Changes in GMST, which reflect the integrated response of the climate system to the combination of external radiation forcing and internal natural variables, are one of the key indicators for understanding the impact of human activities on global climate change. Previous studies have basically determined the contribution of major physical modes of the global surface temperature, such as the El Niño-Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO) based on Empirical Orthogonal Function (EOF) method. In addition to the above climate modes, the physical processes affecting GMST changes also include volcanic eruptions, observation data system errors and so on. Based on the above research progress and these two basic assumptions (there is no long-term changing trend due to factors such as natural variabilities, volcanic eruption events, or system error of observation equipment; a single event does not affect the remotely related structure of the main physical mode), EOF decomposition is applied to the surface temperature of ERSST v3b and HadISST v1.1 ocean observation data in this paper, so as to obtain the TR warming trend item with ENSO, PDO and AMO removed, and thus, by using Ensemble Empirical Mode Decomposition (EEMD) method, to break down each TR time series into components on multiple different time scales and summarize them into low frequencies, high frequency and trend items. The land and ocean data (HadCRUT.4.6) are used to compare ocean data (ERSST v3b and HadISST v1.1) results and conduct a statistical analysis on the errors introduced by EMD methods. This study found that the main physical processes affecting changes in the GMST include global warming caused by the anthropogenic greenhouse gas emissions, the cooling effects caused by aerosols, the internal variables in the global climate system, the system errors caused by changes in the observing system, data noise, and other factors that have not yet been identified. Based on ERSST V3b and HadISST v1.1 observational data, this study also analyzed and determined the impact and contribution of each factor to GMST changes. This could be a key step to identify the effects of human activities and natural variability on global climate change. There are two main conclusive contributions in this paper. One contribution was to quantify the time scale and intensity of the impact of volcanic eruptions on GMST. We estimated the impact of five major volcanic eruptions since 1950 (Agung, 1963; Merapi, 1974; St. Helens, 1980; Pinatubo, 1991; Nabro, 2011). The cumulative effects of these five major volcano events in ERSST and HadISST approximately caused global warming to slow down by 0.32−0.47 and 0.45−0.67°C, respectively. Another major contribution is to find that the TR item of GMST still has a recent global warming mitigation phenomenon after the removal of the three major climate models, that is, there are some processes in the Earth’s climate system that are different from the above three physical models, which could significantly affect the TR changes. Therefore, global warming tends to slow down (warming hiatus). In modern climate conditions, anthropogenic radiative forcing continues to increase, and the heat absorbed by the global climate system continues to increase during global warming hiatus period. That is, even if the amount of heat energy used to heat up the surface of the global surface reduced, this “reduced” amount of heat still remains in the climate system, such as the mid-deep ocean. The results further underscore the necessity for understanding the critical physical processes that lead to global warming from the perspective of the energy balance of the climate system.

The study of changing characteristics of the winter temperature and extreme cold events in China over the past six decades

AbstractBy using winter (December–February) daily temperature at 1629 meteorological stations over China during 1960/61‐2017/18, spatial–temporal changes and probability distribution characteristics of winter temperature and extreme cold events in China were analysed, with a special emphasis on the warming hiatus in China. The results are as follows: (1) the winter temperature in China showed an overall upward trend during the past six decades. However, since the winter of 2006 (so‐called the warming hiatus period), the winter temperature in China has remained stable and even decreased slightly. (2) There is an obvious difference in winter temperature between South China and North China. North China witnessed a more obvious temperature rise during the warm period (1986–2005) than South China did. However, since 2006, winter temperature in most parts of North China has dropped slightly, with a marked increase in extreme cold events. In contrast, South China did not undergo an obvious temperature drop during the hiatus period. (3) The range of temperature change in North China is larger than that in South China. The range of minimum temperature is also wider than that of maximum temperature, with a noticeable increase in extreme low temperature events since 2006. In such situation, North China is more likely to encounter extreme low temperature events, such as cold wave and low temperature weather. (4) China has experienced an increase in extreme cold events during the hiatus period. There is an obvious upward trend in regional long‐lasting extreme cold events (3–5 and 11–20 days duration). Consequently, this study has revealed that winter temperature and frequency of extreme cold events in China was not only influenced by global warming, with a persistent warming, but also showed an obvious inter‐decadal variation which is associated with the natural forcing. It indicates that the natural forcing factor is also a main factor influencing and modulating the winter temperature change at the inter‐decadal scale at the regional scale as China even in the era of the global warming caused by anthropogenic forcing.

An asymmetric variation of hot and cold SST extremes in the China Seas during the recent warming hiatus period

Extreme sea surface temperatures (SSTs) attract much attention in recent years. However, the detailed spatial and temporal pattern of the extreme SSTs in China Seas has not been well understood. Using the daily SST data set of OISST v2 from January 1, 1982 to December 31, 2013, and based on four extreme SST indices, the frequency and intensity of SST extremes in the China Seas were examined. The analysis showed that the annual mean SST exhibited cooling trend, on pace with a trend of − 0.34 °C/decade during 1998–2013, confirming the previous studies that China Seas also experienced the recent global warming hiatus. But during this recent global warming hiatus, there was a notable asymmetric pattern of greater cooling trends in cold SSTs as compared to the hot SSTs in this region. During 1998–2013, the cold days (CDs) frequency increased significantly by 13 days per decade and cold SST extremes which were below the 10th percentile of each year (SST10p) notably decreased by 0.4 °C per decade. Hot days (HD) and hot SST extremes which were above the 90th percentile of each year (SST90p) slowed down, but without any distinct tendency. Meanwhile, the rates of SST10p and CDs were highly heterogeneous in space. Cold extremes in the near-shore areas are much more sensitive to the global warming hiatus than these in the eastern of the Kuroshio Current. Importantly, hot extremes do not reveal any distinct cooling tendency during 1998–2013, there were more frequent hot days and more intense hot SSTs in this region comparing with 1982–1997. These hot extremes could push some marine organisms, fisheries and ecosystems beyond the limits of their resilience, with cascading impacts on economies and societies.

Nonstationary El Ni\xf1o teleconnection on the post-summer upwelling off Vietnam

Summer upwelling has often been observed off Vietnam in the South China Sea (SCS). Occasional disappearance of the upwelling has attracted much attention because it modulates the regional climate and harms surrounding fisheries. Fluctuations of the East Asian monsoon associated with El Niño are considered responsible for the weakened or abolished upwelling. However, analyses of observations performed in the present study were equivocal in terms of the dominant influence of El Niño. Based on long-term sea surface temperature data, we demonstrated that weak upwelling off Vietnam occurs more frequently during periods of accelerated global warming compared to warming hiatus periods. Warming signals in the Indian Ocean vanished relatively quickly during the hiatus period. The accompanying easterly anomalies south of the anomalous anticyclone (AAC) in the northwestern Pacific were also weakened, reducing the impact of the El Niño teleconnection on the SCS summer monsoon and thus preserving the regular post-summer upwelling off Vietnam during warming hiatus periods.

Spatial and temporal variability of the abrupt interannual temperature change and warming hiatus in China, 1951–2016

AbstractThe abrupt temperature change (ATC) and warming hiatus (WH) phenomena impact global resources and the environment. However, information on the spatial and temporal variability in the ATC and WH over large regions, long time scales and densely distributed stations is lacking. In the study, based on average minimum, average and average maximum temperatures data from 1951 to 2016 from 622 meteorological stations in China, the spatial and temporal variability in the timing of the ATC and WH events and the characteristics before and after these events were revealed by using the Mann–Kendall test. In most areas of China, an ATC occurred in the three temperature parameters, and the onset of the changes occurred later at lower latitudes. The ATC in the average minimum temperature occurred earlier than that in the average temperature, and the ATC in the average maximum temperature occurred the latest. After the ATC, a WH occurred at most of the stations that experienced an increase in temperature, whereas a cooling hiatus (CH) did not occur at stations that experienced a decrease in temperature. The regions with decreasing temperatures were concentrated in the hilly and plain areas of southern China and in subtropical and tropical monsoon climate zones. The WH of the average temperature occurred earlier than that of the average maximum temperature, which occurred earlier than that of the average minimum temperature. Overall, the WH began later from east to west and was mainly concentrated in approximately 1998 and 2007. Both the ATC and the WH in the Qinghai–Tibet Plateau area showed hysteresis. The ATC to WH period was between three and 27 years, and the earlier the ATC was, the longer was the period. Before the ATC, all three temperature parameters increased slightly. The average minimum temperature rose faster than the average temperature, and the average maximum temperature rose the slowest. Furthermore, the variation became more dramatic from southeast to northwest. After the ATC, the temperatures in most areas increased rapidly, and the rate of temperature increase increased with decreasing latitude. The average maximum temperature decreased in the area east of 100° E and south of 30° N. In this area, the rates of temperature increase for the average minimum and average temperatures were on a par with the rate of decrease in the absolute average maximum temperature, with the rates being three to five times greater than those before the ATC, but without dramatic variations. After the ATC, a CH did not occur at the stations where the temperature had decreased, whereas a WH occurred after a certain period of time at most of the stations where the temperature had increased. Moreover, such occurrences differed with latitude. A comparison of the temperature after the WH with that both before and after the ATC but before the WH revealed that the temperature did not vary dramatically. Eight to 10 years after the WH, the temperatures at a small number of stations in northern China rose again; however, due to the short length of the time series, it is impossible to determine whether the WH had truly ended. The results of the study enrich the findings of climate change research and provide a reference for addressing resource and environmental issues.

Global warming hiatus contributed weakening of the Mascarene High in the Southern Indian Ocean

The Mascarene High (MH) is a semi-permanent subtropical high-pressure zone in the South Indian Ocean. Apart from its large influence on African and Australian weather patterns, it also helps in driving the inter-hemispheric circulation between the Indian Ocean in the south and subcontinental landmass in the north. Using observations and reanalysis products, this study for the first time investigates recent warming trend observed in the MH region during the Global Warming Hiatus (GWH) period (1998–2016). Significant positive trends are observed in sea surface temperature (SST), sea surface height (SSH) and oceanic heat content (OHC) during this period in the MH region. Mixed layer heat budget analysis reveals the dominant role of heat advection in the observed warming trend. During the GWH period, stronger zonal currents advect the warm waters from the Western Pacific (WP) towards the MH region via the Indonesian Throughflow (ITF). This warming in the MH reduces the sea level pressure therein and establishes a weak pressure gradient between the MH and the northern hemisphere landmass. This in-turn weakens the cross-equatorial winds in the western Indian Ocean.

Sea Surface Temperature Extremes of Different Intensity in the China Seas During the Global Warming Acceleration and Hiatus Periods

Based on the daily OISST V2 with 0.25o horizontal resolutions, the present study looks into the variations of sea surface temperature (SST) extremes in the China Seas for different segments of the period 1982-2013. The two segments include the warming acceleration period from 1982 to 1997 and the hiatus period from 1998 to 2013 when the global mean surface temperature (GMST) did not significantly increase as expected, or even decreased in some areas. First, we construct the regional average time series over the entire China Seas (15°-45°N, 105°-130°E) for these SST extremes. During the hiatus period, the regionally averaged 10th, 1th and 0.1th percentile of SSTs in each year decreased significantly by 0.40℃, 0.56℃ and 0.58℃ per decade, respectively. The regionally averaged 90th, 99th and 99.9th percentile of SSTs in each year decreased slightly or insignificantly. Our work confirm that the regional hiatus was primarily reflected by wintertime cold extremes. Spatially, the trends of cold extremes in different intensity were non-uniformly distributed. Cold extremes in the near-shore areas were much more sensitive to the global warming hiatus. Hot extremes exhibited non-significant trend in the China Seas during the hiatus period. In short, the variations of the SST extremes in the two periods were non-uniform spatially and asymmetric seasonally. It is unexpected that the hot and cold extremes of each year during 1998-2013 were still higher than those extremes during 1982-1997. It is obvious that compared with the warming acceleration period, hot extremes were far more likely to occur in the recent hiatus as a result of a 0.3℃ warmer shift in the mean temperature distribution. Moreover, hot extremes in the China Seas will be sustained or amplified with the end of warming hiatus and the continuous anthropogenic warming.

Reconstructed 3-D Ocean Temperature Derived from Remotely Sensed Sea Surface Measurements for Mixed Layer Depth Analysis

The mixed layer depth (MLD) is generally estimated using in situ or model data. However, MLD analyses have limitations due to the sparse resolution of the observed data. Therefore, this study reconstructs three-dimensional (3D) ocean thermal structures using only satellite sea surface measurements for a higher spatial and longer temporal resolution than that of Argo and diagnoses the decadal variation of global MLD variability. To simulate the ocean thermal structures, the relationship between the ocean subsurface temperature and the sea surface fields was computed based on gridded Argo data. Based on this relationship, high spatial resolution and extended periods of satellite-derived altimeter, sea surface temperature (SST), and wind stress data were used to estimate the 3D ocean thermal structures with 0.25° spatial resolution and 26 standard depth levels (5–2000 m) for 24 years (1993–2016). Then, the MLD was calculated using a temperature threshold method (∆T = 0.2 °C) and correlated reasonably well (&gt;0.9) with other MLD datasets. The extended 24-year data enabled us to analyze the decadal variability of the MLD. The global linear trend of the 24-year MLD is −0.110 m yr−1; however, from 1998 to 2012, the linear trend is −0.003 m yr−1 which is an order of magnitude smaller than that of other periods and corresponds to a global warming hiatus period. Via comparisons between the trends of the SST anomalies and the MLD anomalies, we tracked how the MLD trend changes in response to the global warming hiatus.

Assessment of the interannual variability of local atmospheric and ITF contribution to the subsurface heat content of southern tropical Indian Ocean in GECCO2 and ORAS4 using ROMS

Abstract Atmospheric and Ocean reanalysis products are handy tools for studying the past climate. However, the choice of these products could influence the outcome of the study. In this study, we compared two popular Ocean reanalysis products (ORAS4 and GECCO2) in the context of the Indian Ocean (IO) heat content variability. The region of interest is the southern tropical IO (STIO) as the region features a seasonal upwelling zone and the Indonesian Throughflow (ITF) is nearly aligned with it. The comparison mainly includes the trends in sea surface height anomaly, IO response to Interdecadal Pacific Oscillation (IPO), and the recent changing behavior of Indo-Pacific Oceans. Among the drivers of southern tropical Indian Ocean subsurface heat content (STIOSHC), the local atmospheric wind forcing seems to be contributing more to its variability in ORAS4 where as ITF seems to be contributing more in GECCO2. To get a deeper understanding of the local and remote influences, we distinguish the interannual variability in the STIOSHC due to local atmospheric contribution and ITF contribution by setting up an Indian Ocean model using ROMS and carrying out some experiments. We show that the interannual variability in STIOSHC contributed by local winds is higher in ORAS4 when compared to GECCO2 and it is mainly reflected in the western IO. In the eastern IO, in ORAS4, the local wind and ITF contributions to the STIOSHC variability are found to be comparable whereas in GECCO2, the ITF contribution is greater than that of ORAS4. In GECCO2, the local wind and ITF contributions are found to be more or less same whereas in ORSA4, the local wind contribution is noticeably higher than the ITF contribution. Also, the local wind and ITF contribution to STIOSHC are found to be out of phase with dominant time periods of 8 to 15 years. The high STIOSHC during first (second) half of the warming hiatus period is found to be contributed by ITF (local surface forcing). The sea surface height anomalies and sea surface temperature anomalies (SSTA) in ORAS4 at few key locations in the Indian Ocean are found to be more in line with observations than those of GECCO2. We posit that ORAS4 is more reliable than GECCO2 over the IO.

A link of China warming hiatus with the winter sea ice loss in Barents–Kara Seas

Since the late 1990s, the global warming has ground to a halt, which has sparked a rising interest among the climate scientists. The hiatus is not only observed in the globally averaged surface air temperature (SAT), but also in the China winter temperature trend, which turns from warming during 1979–1997 to cooling during 1998–2013. However, the reasons and the relevant mechanisms of the warming hiatus are far from being understood. Arctic sea ice (ASI), one of the tipping points of the Earth system change, has contributed to the dramatic climate change. It is found that the ASI over the Barents–Kara Seas in the warming hiatus period (1998–2013) has declined about three times faster than that in the previous continuing warming period (1979–1997). The observational analysis and the wave ray trajectories suggest that, for the hiatus period, the ASI over the Barents-Kara Seas explains 61.4% of the decreasing SAT trend and 14.4% of the interannual SAT variation in China. The possible dynamical process is as follows: the anomalous ASI loss associated with the significant warming in Barents-Kara Seas can excite a downstream teleconnection wave train pattern over the Eurasia continent. The significant easterly winds in the subpolar region slacken the polar-front jet, while the anomalous westerly winds over the Tibetan Plateau reinforce the East Asian subtropical jet. As a result, the concurrent out-of-phase variations in the intensity of the jets lead to the cold flow moving southward from the Arctic and converging in China. The northwest–southeast tilted dipole pattern of the height centers over Eurasia, accompanied with the weakened polar-front jet, usually favors the persistence of Ural blocking (UB). Furthermore, in the lower level, the anomalous high pressure belt over the northern Eurasian continent can help cold northeasterly winds blow from the Russian Far East to China and bring about the cold winters. Thus, the accelerated melting of ASI can exert a profound influence on China warming hiatus in recent two decades.