Tropospheric Temperature Anomalies Research Articles

Enhanced ENSO-Unrelated Summer Variability in the Indo–Western Pacific under Global Warming

Abstract El Niño–Southern Oscillation (ENSO) is an important but not the only source of interannual variability over the Indo–western Pacific. Non-ENSO forced variability in the region has received recent attention because of the implications for rainy-season prediction. Using a 35-member CESM1 Large Ensemble (CESM-LE) and 30 CMIP6 models, this study shows that the ensemble means project intensified interannual variability for precipitation, low-level winds, and sea level pressure under global warming, associated with the enhanced large-scale anomalous anticyclone (AAC) over the tropical northwestern (NW) Pacific after the ENSO signal is removed. A decomposition based on the column water vapor budget reveals that enhanced precipitation variability is due to the increased background specific humidity. The resultant anomalous diabatic heating intensifies the AAC, which further strengthens the precipitation anomalies. Over the tropical NW Pacific, the wind-induced evaporative cooling on the southeastern flank of the AAC is countered by the increased shortwave radiation due to the strengthened precipitation reduction. Tropospheric temperature anomalies in the ensemble means show no significant change, suggesting no apparent change of the interbasin positive feedback between the AAC and northern Indian Ocean SST. Intermodel analysis based on CMIP6 reveals that models with a larger increase in ENSO-unrelated precipitation variability over the NW Pacific are associated with stronger background warming in the eastern equatorial Pacific, due to the modulated Walker and Hadley circulations.

Model consistency for the underlying mechanisms for the Inter-decadal Pacific Oscillation-tropical Atlantic connection

Modeling evidence suggests that the Inter-decadal Pacific Oscillation (IPO) can remotely affect the tropical Atlantic sea surface temperature (SST) variability. However, the root causes of the IPO-tropical Atlantic inter-basin teleconnections are not fully understood. Using idealized pacemaker experiments wherein the observed IPO-SST anomalies are specified, we show that a warm-phase IPO-SST anomaly drives a basin-wide SST warming over the tropical Atlantic. The trans-basin IPO-tropical Atlantic connection is established via the tropical tropospheric temperature mechanism and the atmospheric bridge teleconnections. An IPO positive-phase SST initiates the warming tropospheric temperature anomalies, leading to a more humid atmosphere and increasing longwave radiation downward into the tropical Atlantic. By comparison, the reduced Pacific Walker circulation and mid-latitude Rossby wave responses cause the trade winds to strengthen, acting to suppress the tropical Atlantic warming through the Bjerknes positive feedback mechanism. Thus, similar to previous studies, the tropical Atlantic widespread warming is ultimately traced back to the warming effects generated by the water vapor-longwave radiation-SST positive feedback, which, contrary to previous studies, overwhelms the cooling effects associated with the intensified trade winds. Our results highlight the model dependence on the details of mechanisms that connect the tropical Pacific and tropical Atlantic on the decadal timescales.

Weakened feedback of the Indian Ocean on El Niño since the early 1990s

The present paper investigates changes in the feedback of Indian Ocean warming on the western Pacific by using observational data and model experiments. It is found that the relationship between easterly wind anomalies over the equatorial western Pacific and Indian Ocean warming during the mature and decaying phases of El Nino has changed since 1994. Before 1994, when El Nino reached its peak in the winter, significant warming of the entire Indian Ocean caused tropospheric temperature anomalies and emanated Kelvin waves into the western Pacific. Thus, Indian Ocean warming produced easterly wind anomalies over the western Pacific. However, significant warming of the Indian Ocean has been confined to the west of the Indian Ocean and Kelvin waves have been weakened since 1994. Therefore, the Indian Ocean has contributed less to easterly wind anomalies in the western Pacific after 1994. Changes in the intensity and pattern of the Indian Ocean dipole (IOD) mode may be responsible for these results. After 1994, the intensity of the IOD, especially the intensity of the cold component of the IOD mode, has become stronger, resulting in Indian Ocean warming being confined to the west of the Indian Ocean. These results are confirmed by numerical model experiments. The paper also discusses the enhancement in the feedback of the north tropical Atlantic on El Nino after 1994.

Relevance of Relative Sea Surface Temperature for Tropical Rainfall Interannual Variability

AbstractThe coupling between sea surface temperature (SST) anomalies and rainfall is an important driver of tropical climate variability. Observations however reveal inconsistencies, such as decreased convective rainfall in regions with positive SST anomalies in the tropical Indian and Atlantic Oceans during and after El Niño–Southern Oscillation (ENSO) events. The upper troposphere warms during El Niño events, stabilizing the atmosphere. SST anomalies only account for the influence of the surface. Using theoretical arguments, we show that relative SST (RSST; defined as SST minus its tropical mean) anomalies, by also accounting for the influence of upper tropospheric temperature anomalies on gross moist stability, explain tropical convection interannual variations better than SST anomalies in observations and climate models. This relation further improves when RSST anomalies are weighted by the precipitation climatology to account for low and high‐precipitation regimes. Using RSST is thus essential to understand El Niño–Southern Oscillation teleconnections and interactions between modes of tropical climate variability.

Recent Regime Shifts in Mineral Dust Trends Over South Asia From Long-Term CALIPSO Observations

Mineral dust aerosols have significant implications on the regional-hydroclimate, especially over the regions located downwind of dust sources. During the premonsoon season, much of South Asia is characterized by enhanced aerosol loading favored by the transport of mineral dust from the desert regions of West Asia/Northwest India. Vertically resolved backscatter measurements at dual polarization from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) are used to estimate the dust optical depth over the Indian region. Using a decade long data set of CALIOP observations, a shift in long-term trend of dust over the Northwest India, Indo-Gangetic Plain, and West Asia is demonstrated. The decreasing trend in dust loading over the Indian region reversed to increasing after 2013. The interannual variability in premonsoon dust optical depth over Northwest India is found to be associated with winter time rainfall. The interannual variation of tropospheric temperature anomalies over Northwest India did not show a direct correlation with mineral dust loading.

Distinguishing interannual variations and possible impacted factors for the northern and southern mode of South Asia High

Given the large meridional coverage of South Asian High (SAH), the interannual variabilities of SAH and the corresponding impacted factors may differ between mid- to high-latitude and low-latitude components of SAH. This study distinguishes the interannual variations of the mid- to high-latitude and low-latitude components of SAH to obtain further understanding of SAH variations. By means of composite analysis, this study investigates the characteristics of the northern and southern mode of SAH variation (N-SAH, S-SAH). When the SAH is regarded as an entirety (E-SAH) based on numerous previous studies, it strengthens distinctly and expands toward both meridional and zonal directions in strong E-SAH comparing to weak E-SAH. The geopotential height (HGT) and tropospheric temperature (TT) anomalies related to the E-SAH are concurrently observed over the low-latitude and mid- to high-latitude regions. In contrast, when N-SAH and S-SAH are defined in this study, the SAH is distinctly strengthened only over its northern (southern) side and expands northward (southward) in strong N-SAH (S-SAH) comparing to the weak group. The HGT and TT anomalies related to the N-SAH (S-SAH) cases are limited over the mid- to high-latitude (low-latitude) regions. Possible impacted factors related to E-SAH, N-SAH, and S-SAH are further analyzed. The N-SAH is closely associated with precipitation anomalies over Indian Peninsula, meridional circulation over Tibet Plateau (TP), land surface temperature (LST) anomalies over Indian Peninsula and northern TP, meridional LST gradient between Indian Peninsula and northern TP, and circumglobal teleconnection (CGT). For instance, positive rainfall anomalies together with obviously ascending motion over Indian Peninsula contributes to a decreased LST anomalies via cloud-radiation feedback here, which forms a north-to-south LST gradient. The LST gradient will reduce the climatological south-to-north LST gradient here, which in turn contributes to a northward meridional circulation over northern TP, and finally contribute to a strong N-SAH existing over north of 25°N. Furthermore, the positive rainfall anomalies over Indian Peninsula tends to inspire a strong CGT over mid-high latitude through atmospheric response to condensation latent heating source, which in turn contributes to a strong N-SAH. In contrast, the meridional circulation associated with S-SAH is exhibited by southward shifting over southern TP (south of 30°N) due to atmospheric response to heating source generated by significant SST anomalies over Indian Ocean. Obviously warm center is observed in low-upper troposphere from tropical region to southern TP, which finally result in a strong S-SAH, accompanied by shifting southward. However, variation of E-SAH is a result of the combined action of rainfall anomalies over Indian Peninsula and SST anomalies over Indian Ocean. These differences indicate that it is necessary and of scientific significance to distinguish the northern and southern mode of the SAH.

Weakening of Northwest Pacific Anticyclone Anomalies during Post–El Niño Summers under Global Warming

The northwest Pacific anticyclone (NWPAC) anomalies during post–El Niño summers are a key predictor of the summer climate in East Asia and the northwestern Pacific (NWP). Understanding how this will change under global warming is crucial to project the changes in the variability of the northwest Pacific summer monsoon. Outputs from 18 selected coupled models from phase 5 of the Coupled Model Intercomparison Project show that the anomalous NWPAC response to El Niño will likely be weakened under global warming, which is attributed to the decreased zonal contrast between the tropical Indian Ocean (TIO) warming and the NWP cooling during post–El Niño summers. Under global warming, the NWPAC anomalies during the El Niño mature winter are weakened because of decreased atmospheric circulation in response to El Niño–Southern Oscillation (ENSO), which leads to the weakening of local air–sea interaction and then decreases the cold NWP SST anomalies. Furthermore, the decreased surface heat flux anomalies, the weakened anticyclone anomalies over the southeastern Indian Ocean, and the slackened anomalous easterlies over the north Indian Ocean weaken the warm TIO SST anomalies. However, the strengthened tropospheric temperature anomalies could enhance the anomalous TIO warming. Although the changes in TIO SST anomalies are indistinctive, the weakening of the SST anomaly gradient between the TIO and the NWP is robust to weaken the NWPAC anomalies during post–El Niño summers. Moreover, the positive feedback between the TIO–NWP SST anomalies and the NWPAC anomalies will enhance the weakening of NWPAC under global warming.

Association between Arctic Circulation and Indian Summer Monsoon Rainfall

The cluster regions for different meteorological fields at lower, mid and upper troposphere are identified using Shared Nearest Neighbor (SNN) algorithm. The relationships among weather conditions over the Arctic cluster regions, Indian Summer Monsoon Rainfall (ISMR) and Arctic oscillation are studied. It is observed that Arctic oscillation and mid tropospheric zonal wind anomaly over cluster region [60°N-70°N; 5°W-55°W] are directly correlated with each other in winter season and this relationship in turn influences ISMR. During contrasting phases of ESI (Effective Strength Index) tendency, the significant inverse relationship between zonal wind anomaly over Arctic clusters and ISMR show spatial and temporal variability. During positive (negative) ESI-tendency, lower tropospheric (upper tropospheric) temperature and pressure anomalies over respective cluster regions in Arctic have relationship with ISMR.

On the role of extratropical air‐sea interaction in the persistence of the Southern Annular Mode

AbstractUsing the daily atmosphere and ocean reanalysis data, this study highlights the role of extratropical air‐sea interaction in the variability of the Southern Annular Mode (SAM). Our analysis shows that the SAM‐induced meridional dipolar sea surface temperature (SST) anomalies, through surface heat fluxes, can maintain persistent lower tropospheric temperature anomalies, which further results in anomalous eddy momentum forcing enhancing the persistence of the SAM. With the Finite Amplitude Wave Activity diagnosis, we illustrate that response of the eddy momentum forcing to SST anomalies can be attributed to changes in both baroclinic processes as baroclinic eddy generation and barotropic processes as wave breaking thus resultant diffusive eddy mixing, with the former confined at high latitudes and the latter strongest at midlatitudes. Spectral analysis further suggests that the above air‐sea interactions are important for bimonthly and longer time scale SAM variations. The dipolar SST pattern may be an indicator for predicting subseasonal and interseasonal variabilities of the SAM.

The Meridional Displacement of the Summer Asian Jet, Silk Road Pattern, and Tropical SST Anomalies

Abstract The Silk Road pattern (SRP), which depicts the teleconnection pattern along the Asian jet, has been extensively investigated and commonly described as the leading mode of upper-tropospheric meridional wind anomalies in summer. In this study, the SRP is identified as having a significant relationship with the meridional displacement of the Asian jet (JMD), which manifests as the leading mode of upper-tropospheric zonal wind anomalies. This significant relationship is confirmed by the correlation coefficient between the indices for JMD and SRP, which is 0.39 and reaches statistical significance at the 0.01 level. When the Asian jet is in a northward (southward) displacement, the phase of SRP tends to be shown as anticyclonic (cyclonic) anomalies over western Asia and East Asia and cyclonic (anticyclonic) anomalies over Europe and central Asia. The authors propose an internal atmospheric mechanism for this relationship. In addition, it is found that the JMD is significantly affected by the tropical surface temperature anomalies. In particular, the negative (positive) SST anomalies in the tropical central and eastern Pacific of the preceding spring lead to significant cooler (warmer) tropical tropospheric temperatures in summer and may induce the northward (southward) displacement of the Asian jet through modifying the meridional gradient of tropospheric temperatures. The tropical tropospheric temperature anomalies may also affect the SRP through the JMD.

Time Series Construction of Summer Surface Temperatures for Alabama, 1883–2014, and Comparisons with Tropospheric Temperature and Climate Model Simulations

AbstractThree time series of average summer [June–August (JJA)] daily maximum temperature (TMax) are developed for three interior regions of Alabama from stations with varying periods of record and unknown inhomogeneities. The time frame is 1883–2014. Inhomogeneities for each station’s time series are determined from pairwise comparisons with no use of station metadata other than location. The time series for the three adjoining regions are constructed separately and are then combined as a whole assuming trends over 132 yr will have little spatial variation either intraregionally or interregionally for these spatial scales. Varying the parameters of the construction methodology creates 333 time series with a central trend value based on the largest group of stations of −0.07°C decade−1 with a best-guess estimate of measurement uncertainty from −0.12° to −0.02°C decade−1. This best-guess result is insignificantly different (0.01°C decade−1) from a similar regional calculation using NOAA’s divisional dataset based on daily data from the Global Historical Climatology Network (nClimDiv) beginning in 1895. Summer TMax is a better proxy, when compared with daily minimum temperature and thus daily average temperature, for the deeper tropospheric temperature (where the enhanced greenhouse signal is maximized) as a result of afternoon convective mixing. Thus, TMax more closely represents a critical climate parameter: atmospheric heat content. Comparison between JJA TMax and deep tropospheric temperature anomalies indicates modest agreement (r2 = 0.51) for interior Alabama while agreement for the conterminous United States as given by TMax from the nClimDiv dataset is much better (r2 = 0.86). Seventy-seven CMIP5 climate model runs are examined for Alabama and indicate no skill at replicating long-term temperature and precipitation changes since 1895.

Extremes in June rainfall during the Indian summer monsoons of 2013 and 2014: observational analysis and extended‐range prediction

The onset/progression phase of theIndian summer monsoon (ISM) is very crucial for the agricultural sector of the country as it has strong bearing on the sowing of kharif crops, which in turn affects overall food grain production and hence food security. The recent ISMs of 2013 and 2014 exhibited quite distinct progression phases. While 2013 had one of the fastest advancement in the last 70 years, 2014 witnessed a comparatively lethargic progression phase. The major difference was felt in the early monsoon month of June, with 2013 (2014) monthly rainfall being +34% (−43%) of its long period average. Observational investigations reveal that, during June 2013, the monsoon trough was very active in its normal position favouring low‐level positive vorticity generation and moisture convergence, whereas the absence of monsoon trough during June 2014 facilitated the prevalence of a strong low‐level anticyclonic circulation over central India hampering the northward progression of the ISM. It is found that June 2013 (2014) was associated with (i) stronger (weaker) north‐south tropospheric temperature (TT) gradient with positive (negative) TT anomalies over Eurasia and north of 60°N; (ii) negative (positive) SST anomalies over the equatorial Indian Ocean, northwestern Arabian Sea and equatorial eastern Pacific; (iii) stronger (weaker) monsoonal Hadley circulation; and (iv) stronger (weaker) Walker circulation in response to the negative (positive) SST anomalies over the equatorial Pacific. The study also examines the skill of an Ensemble Prediction System (EPS) in predicting the observed contrasting behaviour during June 2013/2014 on extended range (∼15–20 days in advance) in real time. The EPS not only forecasted the observed discrepancy, but also predicted the influential role of the large‐scale meteorological conditions prevalent during June 2013 (2014), thus demonstrating the remarkable skill of the EPS in predicting June extremes.

El Niño–Southern Oscillation Sea Level Pressure Anomalies in the Western Pacific: Why Are They There?*

Abstract Although sea level pressure (SLP) anomalies in the western Pacific have long been recognized as an integral part of the classic Southern Oscillation pattern associated with El Niño–Southern Oscillation (ENSO), there is an unresolved question regarding the dynamics that maintain these anomalies. Traditional studies of the ENSO response in the tropics assume a single deep baroclinic mode associated with the tropospheric temperature anomalies. However, the SLP anomalies in the western Pacific are spatially separated from the baroclinic signal in the NCEP–NCAR reanalysis, CMIP5 models, and an intermediate complexity model [a quasi-equilibrium tropical circulation model (QTCM)]. Separation of ENSO SLP anomalies in the tropical Pacific into baroclinic and barotropic components indicates that the barotropic component contributes throughout the tropics and constitutes the primary contribution in the western Pacific. To demonstrate the roles of baroclinic and barotropic modes in ENSO teleconnections within the tropics, a series of QTCM experiments is performed, where anomalies in the interactions between baroclinic and barotropic modes are suppressed over increasingly wider latitudinal bands in the tropical Pacific. If this suppression is done in the 15°N–15°S band, the pressure signals in the western Pacific are only partly removed, whereas if it is done in the 30°N–30°S band, the anomalies in the western Pacific are almost entirely removed. This suggests the following pathway: interactions with SST anomalies create the baroclinic response in the central and eastern Pacific, but baroclinic–barotropic interactions, arising substantially in the subtropical Pacific, generate a barotropic response that yields the SLP anomalies in the western Pacific.

Centennial-scale teleconnection between North Atlantic sea surface temperatures and the Indian summer monsoon during the Holocene

Proxy records have shown that abrupt changes in the Indian summer monsoon (ISM) are closely linked to cold events in the North Atlantic at centennial timescales during the Holocene. However, mechanisms for these co-occurring phenomena are not fully understood. This study uses simulation results from a coupled atmosphere–ocean–sea-ice general circulation model forced by astronomical variations to investigate how summer (June, July, August and September) North Atlantic sea surface temperatures (SSTs) may have influenced the ISM at centennial timescales during the Holocene (9.5–0 ka BP). Our analyses identified an intimate relationship between the North Atlantic tripole SST (NATS) mode and the ISM. The NATS mode can affect the ISM in several ways. First, air–sea interactions over the tropical Atlantic can induce negative tropospheric temperature (TT) anomalies over the Indian Ocean, resulting in a strengthened meridional TT gradient favorable to a prolonged monsoonal rainy season. Second, a positive NATS mode tends to induce closed zonal vertical circulation over the tropical Atlantic, North Africa and the tropical Indian Ocean, creating anomalous convergence over India, and hence an enhanced ISM. Third, westerly surface wind anomalies, related to the NATS mode and coursing over the Arabian Sea, can increase moisture delivery to the monsoon region, causing enhanced rainfall in India. This mechanism resembles a decadal-scale mechanism that operates in the present-day climate. We also compared the Atlantic multidecadal oscillation (AMO), an alternative North Atlantic SST mode, with the NATS mode to determine their relationships to the ISM. A Holocene transient simulation indicates that the AMO’s trend has diverged from that of the ISM since 5.5 ka BP, due to inverse SST trends over the tropical and extratropical North Atlantic. This latter trend leads to a much weaker relationship between the AMO and the ISM, relative to that observed between the NATS mode and the ISM. We therefore suggest that the centennial relationship between the North Atlantic SSTs and the ISM during the Holocene differs from the decadal to multidecadal relationship they share in the present-day climate system.

Effects of Large Volcanic Eruptions on Global Summer Climate and East Asian Monsoon Changes during the Last Millennium: Analysis of MPI-ESM Simulations

Abstract Responses of summer [June–August (JJA)] temperature and precipitation to large volcanic eruptions are analyzed using the millennial simulations of the earth system model developed at the Max Planck Institute for Meteorology. The model was driven by up-to-date reconstructions of external forcing, including natural forcing (solar and volcanic) and anthropogenic forcing (land-cover change and greenhouse gases). Cooling anomalies after large volcanic eruptions are seen on a nearly global scale. The cooling in the Northern Hemisphere (NH) is stronger than in the Southern Hemisphere (SH), and cooling is stronger over the continents than over the oceans. The precipitation decreases in the tropical and subtropical regions in the first summer after large volcanic eruptions. The cooling, with amplitudes of up to −0.6°C, is also seen over eastern China. East Asia is dominated by northerly wind anomalies, and the corresponding summer rainfall exhibits a coherent reduction over the entirety of eastern China. The tropospheric mean temperature anomalies indicate that there is coherent cooling over East Asia and the tropical ocean after large volcanic eruptions. The cooling over the middle-to-high latitudes of East Asia is stronger than over the tropical ocean. This temperature anomaly pattern suggests a reduced land–sea thermal contrast and favors a weaker East Asian summer monsoon (EASM) circulation. Analysis of the radiative fluxes at the top of the atmosphere (TOA) suggests that the reduction in shortwave radiation after large volcanic eruptions is nearly twice as large as the reduction in emitted longwave radiation, a net loss of radiative energy that cools the surface and lower troposphere.

Different influences of two types of El Niños on the Indian Ocean SST variations

By comparing correlation of sea surface temperature (SST) and vertical circulation with canonical El Nino and El Nino Modoki, we find that El Nino Modoki has an effect on the Indian Ocean different from traditional El Nino. There exists obvious Indian Ocean basin mode (IOBM) after canonical El Nino, while insignificant SST anomalies exist in the Indian Ocean after El Nino Modoki. Anomalous downdraft and updraft appear over the eastern and western Indian Ocean, respectively, during canonical El Nino, while anomalous updraft is weak over the Indian Ocean during El Nino Modoki. Besides, the strength of El Nino Modoki is slightly weaker than that of canonical El Nino. According to previous studies, two mechanisms can explain IOBM after canonical El Nino: tropospheric temperature (TT) mechanism and ocean dynamics. However, both of them do not exist during El Nino Modoki. Comparing with the complicated oceanic processes, it is convenient to verify the observed TT anomalies and test the possible mechanism using the simple model. Therefore, we pay more attention on the question why TT mechanism does not work during El Nino Modoki. Using a linear barocinic model (LBM), we demonstrate that the strength of SST anomalies and cold SST anomalies in the eastern Pacific have an influence on TT anomalies. Especially, cold SST anomalies in the eastern Pacific cancel the effects of warm SST anomalies in the central Pacific on TT anomalies. It suggests that the SST anomalies in the eastern Pacific are important for the TT mechanism in two types of El Nino.

Plausible reasons for the inconsistencies between the modeled and observed temperatures in the tropical troposphere

We hereby attempt to detect plausible reasons for the discrepancies between the measured and modeled tropospheric temperature anomalies in the tropics. For this purpose, we calculate the trends of the upper‐minus‐lower tropospheric temperature anomaly differences (TAD) for both the measured and modeled time series during 1979–2010. The modeled TAD trend is significantly higher than that of the measured ones, confirming that the vertical amplification of warming is exaggerated in models. To investigate the cause of this exaggeration, we compare the intrinsic properties of the measured and modeled TAD by employing detrended fluctuation analysis (DFA). The DFA exponent obtained for the measured values reveals white noise behavior, while the exponent for the modeled ones shows that they exhibit long‐range power law correlations. We suggest that the vertical amplification of warming derived from modeled simulations is weighted with a persistent signal, which should be removed in order to achieve better agreement with observations.

The rendition of the Atlantic Warm Pool in the reanalyses

The Atlantic Warm Pool (AWP) region, which is comprised of the Gulf of Mexico, Caribbean Sea and parts of the northwestern tropical Atlantic Ocean, is one of the most poorly observed parts of the global oceans. This study compares three ocean reanalyses, namely the Global Ocean Data Assimilation System of National Centers for Environmental Prediction (NCEP), the Climate Forecast System Reanalysis (CFSR) of NCEP, and the Simple Ocean Data Assimilation (SODA) for its AWP variation. The surface temperature in these ocean reanalyses is also compared with that from the Extended Range SST version 3 and Optimally Interpolated SST version 2 SST analyses. In addition we also compare three atmospheric reanalyses: NCEP-NCAR (R1), NCEP-DOE (R2), and CFSR for the associated atmospheric variability with the AWP. The comparison shows that there are important differences in the climatology of the AWP and its interannual variations. There are considerable differences in the subsurface ocean manifestation of the AWP with SODA (CFSR) showing the least (largest) modulation of the subsurface ocean temperatures. The remote teleconnections with the tropical Indian Ocean are also different across the reanalyses. However, all three oceanic reanalyses consistently show the absence of any teleconnection with the eastern equatorial Pacific Ocean. The influence of the AWP on the tropospheric temperature anomalies last for up to a one season lead and it is found to be relatively weak in R1 reanalyses. A simplified SST anomaly equation initially derived for diagnosing El Nino Southern Oscillation variability is adapted for the AWP variations in this study. The analysis of this equation reveals that the main contribution of the SST variation in the AWP region is from the variability of the net heat flux. All three reanalyses consistently show that the role of the ocean advective terms, including that associated with upwelling in the AWP region, is comparatively much smaller. The covariance of the SST tendency in the AWP with the net heat flux is large, with significant contributions from the variations of the surface shortwave and longwave fluxes.

The Influence of the MJO on Upstream Precursors to African Easterly Waves

The Madden–Julian oscillation (MJO) produces alternating periods of increased and reduced precipitation and African easterly wave (AEW) activity in West Africa. This study documents the influence of the MJO on the West African monsoon system during boreal summer using reanalysis and brightness temperature fields. MJO-related West African convective anomalies are likely induced by equatorial Kelvin and Rossby waves generated in the Indian Ocean and West Pacific by the MJO, which is consistent with previous studies. The initial modulation of tropical African convection occurs upstream of West Africa, near the entrance of the African easterly jet (AEJ). Previous studies have hypothesized that an area to the east of Lake Chad is an initiation region for AEWs. Called the “trigger region” in this study, this area exhibits significant intraseasonal convection and wave activity anomalies prior to the wet and dry MJO phases in the West African monsoon region. In the trigger region, cold tropospheric temperature anomalies and high precipitable water, as well as an eastward extension of the African easterly jet, appear to precede and contribute to the wet MJO phase in West Africa. An anomalous stratiform heating profile is observed in advance of the wet MJO phase with anomalous PV generation maximized at the jet level. The opposite behavior occurs in advance of the dry MJO phase. The moisture budget is examined to provide further insight as to how the MJO modulates and initiates precipitation and AEW variability in this region. In particular, meridional moisture advection anomalies foster moistening in the trigger region in advance of the wet MJO phase across West Africa.

A multi-data set comparison of the vertical structure of temperature variability and change over the Arctic during the past 100 years

We compare the daily, interannual, and decadal variability and trends in the thermal structure of the Arctic troposphere using eight observation-based, vertically resolved data sets, four of which have data prior to 1948. Comparisons on the daily scale between historical reanalysis data and historical upper-air observations were performed for Svalbard for the cold winters 1911/1912 and 1988/1989, the warm winters 1944/1945 and 2005/2006, and the International Geophysical Year 1957/1958. Excellent agreement is found at mid-tropospheric levels. Near the ground and at the tropopause level, however, systematic differences are identified. On the interannual time scale, the correlations between all data sets are high, but there are systematic biases in terms of absolute values as well as discrepancies in the magnitude of the variability. The causes of these differences are discussed. While none of the data sets individually may be suitable for trend analysis, consistent features can be identified from analyzing all data sets together. To illustrate this, we examine trends and 20-year averages for those regions and seasons that exhibit large sea-ice changes and have enough data for comparison. In the summertime Pacific Arctic and the autumn eastern Canadian Arctic, the lower tropospheric temperature anomalies for the recent two decades are higher than in any previous 20-year period. In contrast, mid-tropospheric temperatures of the European Arctic in the wintertime of the 1920s and 1930s may have reached values as high as those of the late 20th and early 21st centuries.