Recent Northern Hemisphere mid-latitude medium-range deterministic forecast skill

The combined effect of the El Niño–Southern Oscillation (ENSO) and Arctic Oscillation (AO) on the variability of boreal winter (December–February) temperature over South Korea is examined at the subseasonal time scale using subseasonal-to-seasonal (S2S) hindcast data. Daily hindcast data from the European Centre for Medium-Range Weather Forecasts (ECMWF) database is used. We selected the following six composite cases using a threshold of ±0.5 for each index: El Niño and positive AO (EP), El Niño and negative AO (EN), La Niña and positive AO (LP), La Niña and negative AO (LN), positive AO only (PA), and negative AO only (NA). Results from reanalysis data suggest the possibility of using these two climate factors as predictors for 1-month prediction of South Korea up to 4 weeks in advance. Thus, we confirmed that the ENSO plays a statistically significant role in strengthening (weakening) the AO influences on the temperature anomalies in the in phase (out of phase). For example, there is a significant increase (decrease) in mean temperature anomalies through positive (negative) geopotential height (GPH) anomalies and warm (cold) temperature advection over South Korea in the EP (LN) case. The ECMWF S2S hindcast demonstrated an acceptable ability to reproduce circulation patterns over East Asia up to 3 weeks in advance, and sufficiently predicted weekly mean temperature anomalies over South Korea in EP, LN, and PA cases.

This study investigates the combined impact of the El Niño–Southern Oscillation (ENSO) and Arctic Oscillation (AO) on the variability of winter temperatures in South Korea at the subseasonal time scale. This is achieved by analyzing hindcast data from the European Centre for Medium-Range Weather Forecasts (ECMWF) database. Six composite cases are selected based on thresholds for each index: El Niño with positive AO (EP), El Niño with negative AO (EN), La Niña with positive AO (LP), La Niña with negative AO (LN), positive AO only (PA), and negative AO only (NA). Results from reanalysis data indicate that these climate factors can serve as predictors for predicting South Korea's temperature up to 4 weeks in advance. The study confirms that ENSO significantly influences the strength of AO's impact on temperature anomalies, depending on whether they are in phase or out of phase. For instance, during El Niño periods, there's a notable increase in mean temperature anomalies due to positive geopotential height (GPH) anomalies and warm temperature advection over South Korea in the EP case. Conversely, during La Niña periods, there's a significant decrease in mean temperature anomalies due to negative GPH anomalies and cold temperature advection over South Korea in the LN case. The ECMWF S2S hindcast exhibits reasonable ability to replicate circulation patterns over East Asia up to 3 weeks in advance. It also adequately predicts weekly mean temperature anomalies over South Korea in the EP, LN, and PA cases. This suggests that the combination of ENSO and AO indices can contribute to improved subseasonal forecasting of winter temperatures in South Korea.

Pentad (5-day averaged) forecast skill over the Arctic region in boreal winter is evaluated for the subseasonal to seasonal prediction (S2S) systems from three operational centers: the European Centre for Medium-Range Weather Forecasts (ECMWF), the U.S. National Centers for Environmental Prediction (NCEP), and Environment and Climate Change Canada (ECCC). The results indicate that for a lead time longer than about 10 days the forecast skill of 2-m air temperature and 500-hPa geopotential height in the Arctic area is low compared to the tropical and midlatitude regions. The three S2S systems have comparable forecast skill in the northern polar region. Relatively high skill is observed in the Arctic sector north of the Bering Strait in pentads 4–6. Possible sources of S2S predictability in the polar region are explored. The polar forecast skill is found to be dependent on the phase of the Arctic Oscillation (AO) in the initial condition; that is, forecasts initialized with the negative AO are more skillful than those starting from the positive AO. This is likely due to the influence of the stratospheric polar vortex. The tropical MJO is found to also influence the prediction skill in the polar region. Forecasts starting from MJO phases 6–7, which correspond to suppressed convection in the equatorial eastern Indian Ocean and enhanced convection in the tropical western Pacific, tend to be more skillful than those initialized from other MJO phases. To improve the polar prediction on the subseasonal time scale, it is important to have a well-represented stratosphere and tropical MJO and their associated teleconnections in the model.

The present study reveals the changes in the characteristics of cold surges over East Asia associated with the Arctic Oscillation (AO). Based on circulation features, cold surges are grouped into two general types: wave train and blocking types. The blocking type of cold surge tends to occur during negative AO periods, that is, the AO-related polarity of the blocking type. However, the wave train type is observed during both positive and negative AO periods, although the wave train features associated with negative AO are relatively weaker. The cold surges during negative AO are stronger than those during positive AO in terms of both amplitude and duration. The cold surges during positive AO in which the extent of effect is confined to inland China passes through East Asia quickly because of weaker Siberian high and Aleutian low, leading to short duration of these cold surges. In contrast, the cold surge during negative AO, characterized by a well-organized anticyclone–cyclone couplet with high pressure over continental East Asia and low pressure over Japan, brings continuous cold air into the entire East Asian region for more than one week with long-lasting cold advection. It is also found that the tracks of the cold surges during negative AO tend to occur more frequently over Korea and Japan and less frequently over China, compared with those during positive AO. The tracks are related to a west–east dipole structure of the ratio of rain conversion to snow according to AO phase, resulting in freezing precipitation or snowfall events over inland China (Korea and Japan) are likely to occur more frequently during the positive (negative) AO periods.

Using the Arctic Oscillation (AO) index, the exceptional winter (DJF) of 2009 has been analyzed. The middle-to-high latitudes of the Northern Hemisphere suffered from a nearly zonally symmetric anomaly of temperature and pressure. This situation revealed that two negative AO events occurred in the winter of 2009/2010, with unprecedented low values in January 2009 and February 2010. The negative AO event in January 2009 can be further divided into two stages: the first stage was mainly driven by enhanced upward-propagating planetary waves, which led to a weak stratospheric polar vortex associated with a downward-propagating negative AO signal; the second stage was caused by a lower tropospheric positive temperature anomaly in the high latitudes, which maintained the positive geopotential height anomaly of the first stage. The two successively occurring stages interacted and caused the lower troposphere to experience a strong and lengthy persistence of the negative AO event. We consider that the second event of negative AO in February 2010 is related to the downward-propagating negative AO after sudden stratospheric warming. Eleven long-persistence negative AO events were analyzed using reanalysis data. The results suggest that the negative AO in the troposphere might have been caused by stratospheric sudden warming, a downward-propagating weak stratospheric circulation anomaly or dynamic processes in the troposphere. Further study shows that the negative phase of the AO in the winter of 2009/2010 corresponded to a wide range of temperature and precipitation anomalies in the Northern Hemisphere. Therefore, to improve the accuracy of weather forecasting and climate prediction, more attention should be paid to the AO anomaly and its impact.

In this paper, the authors quantitatively investigated the joint impact of the Arctic Oscillation (AO) and El Niño‐Southern Oscillation (ENSO) on vegetation net primary productivity (NPP) over Indo‐Myanmar in boreal winter from 1981 to 2018, and found that there is a significant in‐phase variation between them. When the warm ENSO co‐occurs with the positive AO, the NPP in more than 80% of the grids in the study region significantly increases by approximately 24 gC m−2. For the cold ENSO plus the negative AO, the regionally averaged NPP anomalies are approximately −10 gC m−2, and the local minimum is as low as −60 gC m−2. The combination of AO and ENSO can explain approximately 36% of the total variance of NPP in Indo‐Myanmar. The AO/ENSO linkages to the NPP are very likely due to regional precipitation anomalies through atmospheric circulation. In association with the positive (negative) AO, the Rossby wave, propagating eastward along the subtropical jet stream, brings an anomalous cyclone (anticyclone) over Indo‐Myanmar. This enhances (weakens) Indo‐Myanmar Trough, and resulting in more (less) regional precipitation. During the warm (cold) ENSO, through the Gill‐type response, an anomalous high (low)‐pressure appears in the lower and middle troposphere over Philippine‐South China Sea. The Walker circulation is also weaker (stronger) than normal. These are conducive to anomalous southerly (northerly) winds in Indochina. As a result, in the warm ENSO plus positive AO winters, the mean precipitation anomaly in Indo‐Myanmar increased by 27% as averaged for five data sets. In contrast, when the cold ENSO co‐occurs with negative AO, the precipitation is significantly reduced where the largest anomaly exceeds 100 mm in the ERA5. The precipitation changes are consistent with the local NPP anomalies, whereas the temperature does not seem to be a dominant limiting factor.

In 2010, the Northern Hemisphere, in particular Russia and Japan, experienced an abnormally hot summer characterized by record-breaking warm temperatures and associated with a strongly positive Arctic Oscillation (AO), that is, low pressure in the Arctic and high pressure in the midlatitudes. In contrast, the AO index the previous winter and spring (2009/2010) was record-breaking negative. The AO polarity reversal that began in summer 2010 can explain the abnormally hot summer. The winter sea surface temperatures (SST) in the North Atlantic Ocean showed a tripolar anomaly pattern—warm SST anomalies over the tropics and high latitudes and cold SST anomalies over the midlatitudes—under the influence of the negative AO. The warm SST anomalies continued into summer 2010 because of the large oceanic heat capacity. A model simulation strongly suggested that the AO-related summertime North Atlantic oceanic warm temperature anomalies remotely caused blocking highs to form over Europe, which amplified the positive summertime AO. Thus, a possible cause of the AO polarity reversal might be the “memory” of the negative winter AO in the North Atlantic Ocean, suggesting an interseasonal linkage of the AO in which the oceanic memory of a wintertime negative AO induces a positive AO in the following summer. Understanding of this interseasonal linkage may aid in the long-term prediction of such abnormal summer events.

The North Atlantic Oscillation, referred to herein as the Northern Hemisphere annular mode (NAM), owes its existence entirely to atmospheric processes. In this chapter, we review the structure of the NAM in the atmospheric general circulation, discuss opposing perspectives regarding its physical identity, examine tropospheric processes thought to give-rise to NAM-like variability, and review the role of the stratosphere in driving variability in the NAM. The NAM is characterized by a deep, nearly barotropic structure, with zonal wind perturbations of opposing sign along ∼55° and ∼35° latitude. It has a pronounced zonally symmetric component, but exhibits largest variance in the North Atlantic sector. During the Northern Hemisphere (NH) winter, the NAM is strongly coupled to the circulation of the NH stratosphere. The NAM also affects tropical regions, where it perturbs the temperature and wind fields of both the tropical troposphere and stratosphere. The structure of the NAM is remarkably similar to the structure of the leading mode of variability in the Southern Hemisphere circulation. The processes that give rise to annular variability are discussed. In the troposphere, the NAM fluctuates on timescales of ∼10 days and is associated with anomalous fluxes of zonal momentum of baroclinic waves across ∼45°N. It is argued that the tropospheric component of the NAM exhibits largest variance in the Atlantic sector where the relatively weak thermally driven subtropical flow and the relatively warm lower boundary conditions at subpolar latitudes permit marked meridional excursions by baroclinic waves. In the stratosphere, fluctuations in the NAM evolve on timescales of several weeks. Evidence is presented that long-lived anomalies in the stratospheric NAM frequently precede similarly persistent anomalies in the tropospheric NAM. It is argued that variability in the lower stratospheric polar vortex yields a useful level of predictive skill for NH wintertime weather on both intraseasonal and seasonal timescales. The possible dynamics of these linkages are outlined. The recasting of the North Atlantic Oscillation as an expression of an annular mode has generated a debate over the physical identity of the mode in question. This debate attests to the absence of a unique theory for the existence of annular modes in the first place. Our current understanding of the fundamental processes to which the NAM owes its existence is discussed.

Daily inter-annual simulations of SST and MLD using atmospherically forced OGCMs: Model evaluation in comparison to buoy time series

Atmospheric rivers (ARs) are narrow bands of enhanced integrated water vapor transport, modulated by large-scale and synoptic-scale variability. Here, we investigate how ARs and snowpack are shaped by large-scale variability such as arctic oscillation (AO) by examining the synoptic conditions and characteristics of ARs and snowpack in the different phases of AO. Using Integrated Multi-Satellite Retrievals for Global Precipitation Measurement (IMERG) data, Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA2) reanalysis data, and in-situ observation data over the eastern Pacific and western United States. we found that more precipitation is observed in lower latitudes (35° N–45° N) during negative AO months and farther north (north of 45° N) in latitude during positive AO months. These are associated with wavelike synoptic patterns in negative AO months and more straight-line type synoptic patterns in positive AO months. The different phases of AO also modulate the AR characteristics: 2.6% less intense (5.3% more intense) integrated water vapor transport and total precipitation, and 16.0% shorter (21.1% longer) duration of ARs than the climatological mean (1980–2019) for positive AO (negative AO) phase. AR frequency is also higher (~50.4%) than the climatological mean for negative AO phase, but there is no statistically significant difference between either negative AO or positive AO phase, especially in southern California. In addition, the snow water equivalent (SWE) tends to be reduced in the positive AO phase and under high-temperature conditions, especially in recent years (2010s). The similar relationships are found in the early 1990s and 2000s, but their statistical significances are low. Considering that lower atmospheric temperature keeps increasing over the eastern Pacific and the western U.S., and SWE tends to be reduced in the positive AO phase in recent years, SWE may decrease over northern California if the warming condition persists. These findings highlight how the characteristics of local extreme weather can be shaped by large-scale climate variability.

The variability of the atmospheric circulation described by the AO and NAO indices has a significant impact on the fluctuations of climate parameters in the Baltic Sea region. Many research show, that a decline in Arctic Sea ice extent increases the likelihood of negative phases of the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) in winter, and in the future could lead to more frequent relatively cold weather events during winter months in Baltic Sea region. This research is important for understanding possible teleconnections between Arctic Sea ice and atmospheric circulation (AO and NAO). In addition, this research helps to better understand the dynamics of Arctic Sea ice extent and Baltic Sea region air temperature, as well as AO and NAO circulation impact on the air temperature in the Baltic Sea region. A statistically significant decrease in the Arctic Sea ice extent and a statistically significant increase in the air temperature in the Baltic Sea region have been determined. The obtained results show that the expansion of the Arctic Sea ice in January-February with a delay of one month determines the AO and NAO values (correlation coefficients vary from 0.32 to 0.55). This means that if Arctic Sea ice decreases during winter months, AO and NAO indices are more likely to become negative as well. However, as the obtained correlations are not very strong, the results are insufficient to confirm the hypothesis that the Arctic Sea ice extent impacts negative AO and NAO phases likelihood and requires further research. A strong correlation between the Baltic Sea region air temperature and the values of AO/NAO indices has been established. The obtained results show that the Baltic Sea region air temperature variability in December-March is greatly influenced by the AO and NAO phases. Keywords: teleconnections, Arctic Sea ice, AO, NAO, Baltic Sea region, air temperature.

The Navy Operational Global Atmospheric Prediction System (NOGAPS) has proven itself to be competitive with any of the large forecast models run by the large operational forecast centers around the world. The navy depends on NOGAPS for an astonishingly wide range of applications, from ballistic winds in the stratosphere to air-sea fluxes to drive ocean general circulation models. Users of these applications will benefit from a better understanding of how a system such as NOGAPS is developed, what physical assumptions and compromises have been made, and what they can reasonably expect in the future as the system continues to evolve. The discussions will be equally relevant for users of products from other large forecast centers, e.g., National Meteorological Center, European Centre for Medium-Range Weather Forecasts. There is little difference in the scientific basis of the models and the development methodologies used for their development. However, the operational priorities of each center and t...

This study deals with high-resolution precipitation forecasts for hydropower industry using a statistical downscaling method based on the linear regression of the categorized daily precipitation forecasts taken from the European Centre for Medium-Range Weather Forecasts (ECMWF), Japan Meteorological Agency (JMA), the US National Centers for Environmental Prediction (NCEP), and the United Kingdom Met Office (UKMO), in the TIGGE archive as well as the quality-controlled precipitation data from China Merged Precipitation Analysis (CMPA). To further improve the forecast skill of the daily precipitation, the calibration of the precipitation forecast has been performed by using a statistical postprocessing approach called the frequency matching method (FMM). The results show that the statistical downscaling forecast skill using categorized precipitation scheme is much larger than that of bilinear interpolation and that using uncategorized precipitation scheme in terms of the equitable threat score (ETS), anomaly correlation coefficient (ACC) and root-mean-square error (RMSE), no matter the precipitation is light rain, moderate rain, or heavy rain and the above. The calibration of the precipitation forecasts using FMM can significantly reduce the false alarm of the light rain and the missing rate of the heavy rain and the above. Hence, it can improve the inflow forecast skill in the hydrological models which make use of observed and predicted precipitation as input variables.

This study discovered that strong positive correlations exist between the frequency of tropical cyclones (TC) during the summer around Taiwan and the Arctic Oscillation (AO) during the preceding March to May period. In positive AO years, during the preceding spring to summer period, anomalous cyclone and anomalous anticyclone were strongly developed at low and middle latitudes, respectively. Because of such a distribution of pressure system, in Taiwan, Korea, and Japan during the positive AO years, anomalous southeasterlies, which play the role of anomalous steering flows in transferring TCs to these regions, were strengthened. On the other hand, in southern China and the Indochina Peninsula during the positive AO years, anomalous northwesterlies, which prevent the transfer of TCs to these regions, were strengthened. Moreover, such a distribution of pressure system strengthening during the positive AO years led TCs to occur, move, and recurve more eastward in the western North Pacific in positive AO years as compared with the negative AO years. Contrarily, during the negative AO years, TCs showed the tendency to pass over the South China Sea from the Philippines and move west toward southern China and the Indochina Peninsula. Eventually, the intensity of TCs in these years was lower than that of TCs in positive AO years due to the topographic effects from a high TC passage frequency in mainland China.

Temporal and spatial variability of the Arctic atmospheric moisture budget is investigated using a new 19‐year data set (1980 to 1998) produced from daily precipitable water retrieved from the TIROS Operational Vertical Sounder (TOVS) and upper‐level winds from the NCEP‐NCAR Reanalysis. A companion paper describes the creation and validation of these new moisture budget products [Groves and Francis, 2002]. Seasonal differences in moisture transport arise from distinct winter/summer circulation regimes and meridional moisture gradients. In winter, approximately 80% of the net precipitation (precipitation minus evaporation, P‐E) is transported along well‐defined storm tracks. Summer P‐E is double that of winter and dominates the annual pattern. Decadal differences in winter P‐E reveal statistically significant increases in the Beaufort and eastern Greenland‐Iceland‐Norwegian Seas, decreases in the Canadian Archipelago (islands in far northeast Canada) and Kara Sea, and a slight increase in the Arctic as a whole. Annual differences are dominated by winter changes. When the phase of the Arctic Oscillation (AO) index is positive, the net PW flux across 70°N in winter is 6 times larger than on negative‐index days. Over the entire Arctic, P‐E is 29% larger (20% lower) than the average on days with a positive (negative) AO index. In summer the PW transport is twice as large, and P‐E is 27% higher on positive versus negative AO days. These results suggest that if the AO continues its trend toward a predominantly positive phase, we should expect to observe increasing precipitation in the Arctic overall, and particularly in regions adjacent to the marginal ice zones.