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John T. Fasullo
,
Julie M. Caron
,
Adam Phillips
,
Hui Li
,
Jadwiga H. Richter
,
Richard B. Neale
,
Nan Rosenbloom
,
Gary Strand
,
Sasha Glanville
,
Yuanpu Li
,
Flavio Lehner
,
Gerald Meehl
,
Jean-Christophe Golaz
,
Paul Ullrich
,
Jiwoo Lee
, and
Julie Arblaster

Abstract

An adequate characterization of internal modes of climate variability (MoV) is a prerequisite for both accurate seasonal predictions and climate change detection and attribution. Assessing the fidelity of climate models in simulating MoV is therefore essential; however, doing so is complicated by the large intrinsic variations in MoV and the limited span of the observational record. Large ensembles (LEs) provide a unique opportunity to assess model fidelity in simulating MoV and quantify intermodel contrasts. In this work, these goals are pursued in four recently produced LEs: the Energy Exascale Earth System Model (E3SM) versions 1 and 2 LEs, and the Community Earth System Model (CESM) versions 1 and 2 LEs. In general, the representation of global coupled modes is found to improve across successive E3SM and CESM versions in conjunction with the fidelity of the base state climate while the patterns of extratropical modes are well simulated across the ensembles. Various persistent shortcomings for all MoV are however identified and discussed. The results both demonstrate the successes of these recent model versions and suggest the potential for continued improvement in the representation of MoV with advances in model physics.

Significance Statement

Modes of variability play a critical role in prediction of seasonal to decadal climate variability and detection of forced climate change, but historically many modes have been poorly simulated by coupled climate models. Using recently produced large ensembles, this work demonstrates the improved simulation of a broad range of internal modes in successive versions of the E3SM and CESM and discusses opportunities for further advances.

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Hamish D. Prince
and
Tristan S. L’Ecuyer

Abstract

Satellite observations reveal that decreasing surface albedo in both polar regions is increasing the absorption of solar radiation, but the disposition of this absorbed energy is fundamentally different. Fluxes of absorbed solar radiation, emitted thermal radiation, and net energy imbalances are assessed for both polar regions for the last 21 years in the Clouds and Earth’s Radiant Energy System record. Arctic absorbed solar radiation is increasing at 0.98 ± 0.69 W m−2 decade−1, consistent with the anticipated response to sea ice loss. However, Arctic thermal emission is responding at a similar rate of 0.94 ± 0.55 W m−2 decade−1. This is surprising since the radiative impact of ice loss would be expected to favor increasing solar absorption. We find however, that clouds substantially mask trends in Arctic solar absorption relative to clear sky while having only a modest impact on thermal emission trends. As a result, the Arctic net radiation imbalance has not changed over the period. Furthermore, variability of absorbed solar radiation explains two-thirds of the variability in annual thermal emission suggesting that Arctic thermal fluxes rapidly adjust to offset changes in solar absorption and re-establish equilibrium. Conversely, Antarctic thermal emission is not responding to the increasing (although not yet statistically significant) solar absorption of 0.59 ± 0.64 W m−2 decade−1 with less than a third of the annual thermal variability explained by accumulated solar absorption. The Arctic is undergoing rapid adjustment to increasing solar absorption resulting in no change to the net energy deficit, while increasing Antarctic solar absorption represents additional energy input into the Earth system.

Significance Statement

The polar regions of Earth are undergoing ice loss through ongoing global warming, reducing the ice cover and decreasing solar reflectivity, which would be expected to warm these regions. We use satellite observations to measure the trends in solar absorption and emitted thermal radiation over the Arctic and Antarctic for the last two decades. Arctic thermal emission is increasing at a compensating rate to solar absorption with a close relationship between these processes. Conversely, Antarctic thermal emission is not responding to solar absorption demonstrating that Antarctic surface temperatures are not significantly influenced by the region’s reflectivity. The Arctic is undergoing rapid adjustment to increasing solar absorption through warming, while increasing Antarctic solar absorption represents additional energy input into the Earth system.

Open access
Jiao Li
,
Yang Zhao
,
Deliang Chen
,
Ping Zhao
,
Chi Zhang
, and
Yinjun Wang

Abstract

Two distinct categories of weather patterns, denoted as Type 1 and Type 2, which show higher-than-expected frequency of summer heavy rainfall days (HRDs) over North China (NC), are selected from nine weather patterns categorized by the self-organizing map algorithm during 1979–2019. The respective HRDs over NC exhibit dissimilar characteristics, with Type 1 showing a northern distribution and Type 2 a southern distribution. The quantitative disparities in terms of moisture content and vertical motion are discussed in reactions to the synoptic-scale patterns associated with HRDs. The outcomes of a 20-day backward tracking, using the so-called Water Accounting Model-2layers, reveal noteworthy contrasts in moisture sources. Type 1 predominantly receives moisture from the western North Pacific, while Type 2 relies more on contributions from the Arabian Sea, Bay of Bengal, and Eurasia. However, the major moisture sources with grid cells contributing more than 0.01 mm show a consistent cumulative contribution of 77% for Type 1 and 80% for Type 2. The finding suggests that the discrepancy between the two types cannot be solely attributed to moisture supply. Further examination of the transverse and shearwise Q-vector components provides insights into how these distinct weather patterns influence HRDs by the alteration of vertical motion. In Type 1, an upper-level jet entrance induces a thermally direct secondary circulation that enhances vertical motion, while a baroclinic trough plays a dominant role in generating vertical motion in Type 2. Moreover, these unique configurations for each type of weather pattern are not only pre-existing but also intensified during HRDs.

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Fouzia Fahrin
,
Alex O. Gonzalez
,
Brett Chrisler
, and
Justin P. Stachnik

Abstract

Longstanding climate model biases in tropical precipitation exist over the east Pacific (EP) Ocean, especially during boreal winter and spring when models have excessive Southern Hemisphere (SH) precipitation near the intertropical convergence zone (ITCZ). In this study, we document the impact of convectively coupled waves (CCWs) on EP precipitation and the ITCZ using observations and reanalyses. We focus on the months when SH precipitation peaks in observations: February–April (FMA). CCWs explain 93% of total precipitation variance in the SH, nearly double the percent (48%) of the NH during FMA. However, we note that these percentages are inflated as they inevitably include the background variance. We further investigate the three leading high-frequency wave bands: mixed Rossby–gravity waves and tropical depression–type disturbances (MRG–TD type), Kelvin waves, and n = 0 eastward inertia–gravity waves (IG0). Compared to their warm pool counterparts, these three CCWs have a more zonally elongated and meridionally narrower precipitation structure with circulations that resemble past observational studies and/or shallow water theory. We quantify the contribution of all CCWs to four different daily ITCZ “states”: Northern Hemisphere (NH) (nITCZ), SH (sITCZ), double (dITCZ), and equatorial (eITCZ) using a new precipitation-based ITCZ-state algorithm. We find that the percent of total precipitation variance explained by each of the CCWs is heightened for sITCZs and eITCZs and diminished for nITCZs. Last, we find that nITCZs are most prevalent weeks after strong CCW activity happens in the NH, whereas CCWs and sITCZs peak simultaneously in the SH.

Significance Statement

Convectively coupled atmospheric waves (CCWs) are a critical feature of tropical weather and are an important source of precipitation near the region of highest precipitation on Earth called the intertropical convergence zone (ITCZ). Given three decades of climate model biases in CCWs and ITCZ precipitation over the east Pacific (EP) Ocean during spring, few studies have examined the relationship between CCWs and the springtime EP ITCZ. We explored the CCWs and EP ITCZ relationship through calculations of the percent of precipitation that comes from CCWs. A significant portion of the tropical precipitation is associated with CCWs during spring. CCWs are even more impactful when the ITCZ is in the SH or on the equator, which are both problematic in climate models.

Open access
Charles Knight
and
Richard Washington

Abstract

Rainfall over the southern African tropical boundary is highly variable, particularly around the onset of the summer rains. However, understanding of the causes of variability in onset timing is poor. A lack of observational data in the region is compounded by the complexity of climate dynamics and variability at the interface of the tropics and the midlatitudes. The key to onset over the region is the Congo air boundary (CAB), a distinguishing circulation feature present in September and declining in frequency until November. The CAB is crucial to early season rainfall; precipitation is broadly inhibited when the CAB is present, while its breakdown allows large-scale rainfall and onset over central southern Africa. This work identifies a remote midlatitude influence on the timing of CAB breakdown using the high-resolution reanalysis dataset ERA5. Propagating and (upstream) breaking low-wavenumber Rossby waves in the Southern Hemisphere westerly jet are shown to be related to >70% of breakdown events. Breakdown is forced by 1) the replacement of subsidence over eastern southern Africa with synoptically forced ascent and 2) wind field modification that leads to positive 850-hPa continental convergence anomalies and enhanced moisture advection from the Congo basin and Indian Ocean. Upper-level anticyclonic vorticity tendency induced by enhanced convection slows Rossby wave propagation aloft, contributing to wave breaking and the persistence of conditions favorable to convection. The identification of this midlatitude influence on CAB breakdown reveals new potential sources of predictability for onset and demonstrates the significant equatorward extent of midlatitude influences to the tropical edge.

Significance Statement

This study aims to improve our understanding of the causes of variability in the start date of the summer rainy season in central southern Africa. The research reveals that certain upper-level waves in the Southern Hemisphere westerly jet are associated with patterns of weather that force a large-scale transition from dry pre-onset conditions to widespread rainfall in a time scale of 2–5 days, and that these waves account for a large proportion of such transitions. This finding is of interest to subseasonal weather forecasters and may help to improve the long-range forecasting of onset date.

Open access
Cameron Bertossa
and
Tristan L’Ecuyer

Abstract

Previous studies have shown the Arctic exhibits two preferred radiative states, one that is regarded as “radiatively opaque” and the other as “radiatively clear”; this presents as bimodality in the surface longwave flux distributions. How frequently these two states occur and what causes them to persist has significant implications for the polar climate. Furthermore, in the presence of multimodality, evaluating models based solely on their ability to resolve the mean and variance of a distribution can lead to a poor representation of the physical evolution of our climate. This study takes a holistic view of this bimodal behavior, seeking to understand to what degree the high latitudes of both hemispheres reside in distinct radiative states. Even when separated into climatologically distinct subregions, many polar regions exhibit bimodality in their longwave flux distributions not observed at lower latitudes, suggesting that the existence of these two states is both common in and unique to polar regions. Bimodality arises due to a tendency for the atmosphere to alternate between transmissive or opaque clouds, with surface longwave radiative effects of approximately 0 and 75 W m−2 (relative to clear-sky values), respectively. Clouds need not contain liquid to lead to the opaque state, as is typically assumed. The presence of solely ice clouds can cause bimodality to arise in downwelling longwave flux distributions. While some regions do not explicitly exhibit multimodal surface longwave radiation distributions, it is found that similar cloud states exist but in disproportionate frequencies.

Significance Statement

Radiation plays an important role in shaping the Arctic and Antarctic climates. Several Arctic expeditions have found that certain regions flip between having a very large energy deficit (“transmissive”) to relatively small energy deficit (“opaque”). The former allows for surface cooling and promotes the formation of ice, while the latter hinders such behavior. This study utilizes satellite observations to understand if this behavior is consistent across the Arctic and Antarctic. Understanding the frequencies of these two states is increasingly important within the context of our rapidly changing climate, and by uncovering the fundamental processes which lead to them, we may be able to model how they will change in the future.

Restricted access
Torben Kunz
and
Thomas Laepple

Abstract

A fundamental statistic of climate variability is its spatiotemporal correlation function. Its complex structure can be concisely summarized by a frequency-dependent measure of the effective spatial degrees of freedom (ESDOF). Here we present, for the first time, frequency-dependent ESDOF estimates of global natural surface temperature variability from purely instrumental measurements, using the HadCRUT4 dataset (1850–2014). The approach is based on a newly developed method for estimating the frequency-dependent spatial correlation function from gappy data fields. Results reveal a multicomponent structure of the spatial correlation function, including a large-amplitude short-distance component (with weak time scale dependence) and a small-amplitude long-distance component (with increasing relative amplitude toward the longer time scales). Two frequency-dependent ESDOF measures are applied, each responding mainly to either of the two components. Both measures exhibit a significant ESDOF reduction from monthly to multidecadal time scales, implying an increase of the effective spatial scale of natural surface temperature fluctuations. Moreover, it is found that a good approximation to the global number of equally spaced samples needed to estimate the variance of global mean temperature is given, at any frequency, by the greater one of the two ESDOF measures, decreasing from ∼130 at monthly to ∼30 at multidecadal time scales. Finally, the multicomponent structure of the correlation function together with the detected ESDOF scaling properties indicate that the ESDOF reduction toward the longer time scales cannot be explained simply by diffusion acting on stochastically driven anomalies, as it might be suggested from simple stochastic-diffusive energy balance models.

Open access
Yujun He
,
Bin Wang
,
Juanjuan Liu
,
Yong Wang
,
Lijuan Li
,
Li Liu
,
Shiming Xu
,
Wenyu Huang
, and
Hui Lu

Abstract

Accurately predicting the decadal variations in Sahel rainfall has important implications for the lives and economy in the Sahel. Previous studies found that the decadal variations in sea surface temperature (SST) in the Atlantic, Mediterranean Sea, Indian Ocean, and Pacific contribute to those in Sahel rainfall. This study evaluates the decadal prediction skills of Sahel rainfall from all the available hindcasts contributing to phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6), in comparison with the related uninitialized simulations. A majority of the prediction systems show high skill with regard to Sahel rainfall. The high skill may be partly attributed to external forcings, which are reflected in good performance of the respective uninitialized simulations. The decadal prediction skills of the key SST drivers and their relationships with the Sahel rainfall are also assessed. Both the hindcasts and the uninitialized simulations generally present high skill for the Atlantic multidecadal variability (AMV) and Mediterranean Sea SST indices and low skill for the Indian Ocean basin mode (IOBM) and interdecadal Pacific variability (IPV) indices. The relationship between the Sahel rainfall and the AMV or Mediterranean Sea SST index is reasonably captured by most prediction systems and their uninitialized simulations, while that between the Sahel rainfall and the IOBM or IPV index is captured by only a few systems and their uninitialized simulations. The high skill of the AMV and Mediterranean Sea SST indices as well as the reasonable representations of their relationships with the Sahel rainfall by both the hindcasts and uninitialized simulations probably plays an important role in predicting the Sahel rainfall successfully.

Significance Statement

Predicting Sahel rainfall on the decadal time scale is of great importance. This study provides a thorough evaluation of the decadal prediction skills of Sahel rainfall in the current decadal prediction systems participating in phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6). A majority of the systems achieve high prediction skill of Sahel rainfall, which probably results from the high prediction skill of some key sea surface temperature (SST) drivers, especially in the Atlantic and Mediterranean Sea SST, and their relationships with Sahel rainfall. This study provides a reference for better understanding the predictability of Sahel rainfall.

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Hanii Takahashi
,
Catherine M. Naud
,
Derek J. Posselt
, and
George A. Duffy

Abstract

Extratropical cyclones (ETCs) produce most of the winter precipitation at midlatitudes and are often associated with the most extreme winter weather events. For climate models to accurately predict the occurrence and severity of these extreme events in a changing climate, they need to accurately represent moist processes in general and ice processes in particular. To provide an observational constraint for model evaluation, because cloud cover and precipitation are prevalent in warm-frontal regions, a compositing method is applied to ice retrievals from satellite observations to explore the ice distribution across warm fronts in both hemispheres. Ice water path (IWP) and its variability are compared between Northern Hemisphere (NH) and Southern Hemisphere (SH) warm fronts for different ETC-wide characteristics, as well as for different ETC origination regions. Results reveal that warm-frontal IWP and its variability tend to be higher in the NH than the SH, even when controlling for the ETC strength and environmental precipitable water (PW). IWP differences between NH and SH are found to be primarily related to where the cyclones originate. As the intertropical convergence zone is shifted north, ETCs that originate close to the northern tropics have more PW than those that originate close to the southern tropics. This, in turn, seems to lead to larger IWP in NH frontal clouds than in the SH frontal clouds at a later time. This highlights the importance, for ice amounts generated in warm-frontal regions, of the environmental conditions that an ETC encounters during its genesis phase.

Significance Statement

Extratropical cyclones (ETCs) are responsible for most of the winter precipitation in the midlatitudes and are often associated with severe winter weather events. In order for climate models to accurately predict these extreme events in a changing climate, they need to correctly represent moist processes, especially those involving ice. To evaluate and improve these models, we apply a compositing method to satellite observations of ice profiles in warm-frontal regions, which are known for having high cloud cover and precipitation. This helps us understand the distribution of ice across warm fronts in both the Northern Hemisphere (NH) and the Southern Hemisphere (SH). We compare the ice water path (IWP) and its variability between NH and SH warm fronts, considering different characteristics of ETCs and their formation regions. Our findings show that NH warm fronts generally contain more ice, and the amount varies a lot more across warm fronts than for SH warm fronts. This is true even when accounting for the strength of the cyclones and the moisture available to them. These differences in IWP between NH and SH are found to be primarily related to the locations where the cyclones originate. As the intertropical convergence zone (ITCZ) is shifted northward, ETCs originating closer to the northern tropics tend to have more moisture available to them than those originating closer to the southern tropics. This leads to greater ice amounts in NH frontal clouds compared to SH frontal clouds at a later time. These results emphasize the importance of understanding the origin of ETCs in order to accurately characterize ice processes in warm-frontal regions.

Open access
Xiaoxuan Zhao
,
Riyu Lu
,
Jianqi Sun
,
Ke Xu
, and
Chaofan Li

Abstract

The duration of cross-equatorial flow (CEF) events over the Maritime Continent (MC) varies greatly, from 3 days to more than 1 month. In this study, we classify CEF events into short- and long-lived ones using a threshold of 7 days, and conduct separate investigations on their statistical and evolution characteristics, as well as the associated physical processes from the perspective of two dominant modes of the intraseasonal oscillation. Results indicate that short-lived events, characterized by westward-propagation southerly anomalies, are largely dependent on the 10–25-day oscillation, while the contribution of the 30–60-day component is negligible. The associated enhanced convection is primarily confined to the western North Pacific (WNP) and moves northwestward, accompanied by an anomalous cyclone. In contrast, long-lived events show consistent changes among different CEF branches, characterized by southerly anomalies dominating the MC, which benefit from the favorable background of the 30–60-day oscillation. Associated convection anomalies show a dipole pattern, with enhanced convection over WNP and suppressed convection over the Indian Ocean. The enhanced WNP convection gradually migrates northward, inducing CEF anomalies via a continuous anomalous cyclone, while the suppressed convection can be found to slightly expand to the MC. Meanwhile, the 10–25-day oscillation shows similar magnitude and evolution with that in short-lived events, but is no longer crucial to the establishment of long-lived events.

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