Monthly high-resolution land surface precipitation data from 1901 to 2006 associated with sunspot number (SSN) data is investigated for the relationship between summer precipitation in China and the decadal solar variability. Generally, on a national scale, precipitation is poorly correlated with SSN. However, in many regions, the long-period (>8 years) variability in summer precipitation is significantly (at >95% confidence level) correlated to SSN. Absolute value correlation coefficient can exceed 0.48 (at >99% confidence level) in some regions. If only the decadal (9–13 years) precipitation component is considered, the correlation becomes stronger with a maximum (minimum) correlation coefficient of 0.73 (–0.73) (at >99.9% confidence level). Considering that the decadal component is the most important factor among precipitation’s low-frequency signals in the high correlation areas (because it explains more than 50% of the precipitation’s low-frequency variance), it can be concluded that solar variability seems to dominate the long-period variation of summer precipitation in these areas. Furthermore, in these high correlation areas, temporal variation patterns in the power spectrum of summer precipitation is similar to that of SSN, strongly suggesting that there is a very likely physical link between solar variability and precipitation in these regions. More convincing and direct evidence shows the significant difference of low-level monsoon flow between high and low solar activity years, which may cause the higher precipitation rate for high, rather than low, solar activity years in central China.
Time integration using the Regional Atmospheric Modeling System (RAMS), a non-hydrostatic cloud-resolving model, was performed for 12 days over a low-latitude band (45°S–45°N) circling an aqua planet with 5-km horizontal grid-point intervals. Tropical and subtropical regions with active precipitation and clear sky, respectively, were clearly divided at 10° latitudes. The numerical experiments derived obvious tropospheric mid-level detrainment (TMD) flows near the 0°C level (z ∼ 5 km) out of the tropics into the subtropics. The TMD flows became largest near the border (10° latitude). In this paper, the time-longitudinal mean field was spotlighted and the atmospheric structure accompanying the TMD flow was investigated. When averaged over time and longitude, the subtropical mid-level troposphere, into which the TMD flows move, is approximately in a state of local thermodynamic equilibrium sustained mainly by the balance between the net radiative cooling and adiabatic heating due to mean subsidence flow. Considering the heat balance, a thermodynamic diagnosis of the mean subsidence flow field suggests the following mechanisms for the mean TMD flow: (1) The mean atmosphere near the melting level has stronger radiative cooling and a larger temperature lapse rate than the atmosphere above it. (2) Free subsidence in the mean subtropical mid-level troposphere, which is consistent with the vertical variation of thermal structure and suffers from no direct dynamic forcings, such as buoyancy, involves a vertically mass-divergent layer just above the melting level. (3) The steady poleward mean TMD flow out of the convective tropic atmosphere exists so as to compensate for the vertical mass divergence in the subtropical atmosphere. Because net meridional transports of sensible heat and water vapor in the middle troposphere are influenced by the mean TMD flow, the existence and the maintaining mechanisms of the mean TMD flow could be important elements of the climate system.
El Niño-Southern Oscillation (ENSO) is an important air-sea coupled phenomenon that plays a dominant role in the variability of the tropical regions. Observations, atmospheric and oceanic reanalysis datasets are used to classify ENSO and non-ENSO years to investigate the typical features of its periodicity and atmospheric circulation patterns. Among non-ENSO years, we have analyzed a group, called type-II years, with very small SST anomalies in summer that tend to weaken the correlation between ENSO and precipitation in the equatorial regions. A unique character of ENSO is studied in terms of the quasi-biennial periodicity of SST and heat content (HC) fields over the Pacific-Indian Oceans. While the SST tends to have higher biennial frequency along the Equator, the HC maximizes it into two centers in the western Pacific sector. The north-western center, located east of Mindanao, is strongly correlated with SST in the NINO3 region. The classification of El Niño and La Niña years, based on NINO3 SST and north-western Pacific HC respectively, has been used to identify and describe temperature and wind patterns over an extended-ENSO region that includes the tropical Pacific and Indian Oceans. The description of the spatial patterns within the atmospheric ENSO circulation has been extended to tropospheric moisture fields and low-level moisture divergence during November–December–January, differentiating the role of El Niño, when large amounts of condensational heat are concentrated in the central Pacific, from La Niña that tends to mainly redistribute heat to Maritime Continents and higher latitudes. The influence of the described mechanisms on equatorial convection in the context of the variability of ENSO on longer timescales for the end of the 20th century is questioned. However, the inaccuracy of the atmospheric reanalysis products in terms of precipitation and the shorter time length of more reliable datasets hamper a final conclusion on this issue.
Tropical multi-scale convective organization of the super-cluster kind and convectively coupled gravity waves are investigated by both two and three-dimensional cloud-system-resolving simulations. The experimental setup includes a constant-temperature ocean surface, constant and horizontally-uniform radiative cooling in the troposphere, and a uniform easterly background wind. The objective of this study is to quantify the impacts of dimensionality on the simulated large-scale convective patterns and associated gravity waves. Eastward propagating large-scale coherent precipitating convection occurs regardless of the spatial dimension. The convective organization has a horizontal wavenumber-one structure in the computational domain and travels at about 13–17 m s-1 relative to the ground, equivalent to 19–23 m s-1 relative to the environmental flow. However, the convectively-induced wave signature is much weaker in three dimensions than in two dimensions, as well as a faster translation and a smaller tilt of the vertical. Moreover, a two-dimensional framework generates additional organizational modes compared to the three-dimensional results, including a fast westward-moving system with a mean-flow-relative speed comparable to the eastward-moving wavenumber-1 counterpart and the quasi-stationary (relative to the background flow) higher wavenumber precipitating system. This does not necessarily imply that these additional modes are artifacts of two dimensionality.
Severe downslope winds accompanying turbulence, which are of great concern to aviation safety, often occur on the lee side of Zao mountain range in winter. Special Doppler lidar observations and numerical simulations with a grid spacing of 100 m were carried out focusing on the downslope winds around the Sendai airport on 14 February 2008. The model reproduced stationary lee waves with an approximate 20-km wavelength, consistent with the observed horizontal scale in water vapor images from the Japanese geostationary multifunctional transport satellite (MTSAT-1R). The simulated lee waves are accompanied by strong vertical shear, which leads to the separation of a vortex sheet from the surface, where small-scale vortices are generated successively. The vortex sheet is under favorable conditions for Kelvin-Helmholtz instability and may account for the generation of small-scale vortices. The simulated vortices, which are advected downstream, are accompanied by weak low-level wind patches. The Doppler lidar observation also captured the weak low-level wind patches with a horizontal scale of 1000-m moving downstream near the surface. An observed temporal variability in line-of-site wind speed decreased with increasing altitude from the surface. This feature was reproduced by the model in a qualitative sense, although the model tended to overestimate (underestimate) the variability below (above) a height of approximately 800 m.
The effect of the earth’s rotation on spontaneous inertial gravity wave radiation from an unsteady rotational flow is investigated numerically using a shallow water system on a rotating sphere, changing three parameters, the Rossby number (1 ≤ Ro ≤ 30), the Froude number 0.1 ≤ Fr ≤ 0.7), and the latitudinal positions of the jet (11.25°N ≤θ0 ≤ 78.75 °N). The spectral-like three-point combined compact difference (sp-CCD) scheme is used for numerical calculation to estimate the amplitude of gravity waves with high accuracy and resolution similar to the spherical harmonics model. The jet, which is initially balanced but evolves with time because of barotropic instability, is maintained by relaxation forcing. The time variations of nearly balanced rotational flows continuously radiate gravity waves. While the amount of gravity wave flux for large Ro (≥ 10) in all positions of the jet is almost constant, that for relatively small Ro (< 10) depends considerably on the positions of the jet and the observational latitude. First, there is almost no gravity wave radiation from the jet positioned at a high latitude. Second, even when gravity waves are radiated from the jet positioned at low latitude, they rarely propagate toward a higher latitude. To discuss the results, the source of gravity waves is introduced as an analogy to the aeroacoustic sound wave radiation theory (Lighthill theory) with an f-plane approximation at the position of the jet. Spectral analysis of the sources reasonably explains the waves. Because the effect of the earth’s rotation varies with the latitude, gravity waves are radiated only from an unsteady source with higher frequency than the inertial cut-off frequency at the position of the jet. In a similar way, gravity waves that propagate toward high latitudes should have a higher frequency than the inertial cut-off frequency at that latitude.
In this study, we have investigated contributions of tropical Indian Ocean (IO) sea surface temperature (SST) warming and El Niño—Southern Oscillation (ENSO) to the interannual variability of tropical cyclone (TC) genesis frequency over the western North Pacific (WNP) between 1948 and 2010 and the involved physical mechanisms. Both ENSO and tropical IO Basin Mode (IOBM) warming are found to play important roles in modulating the WNP TC genesis frequency, but their effects are significantly different. The time series of seasonal empirical orthogonal function of tropical IO and tropical Pacific SST with trend and multi-decadal variability removed are defined as the IOBM index and ENSO index, respectively. The results show that the IO warming year is usually the El Niño decaying year. The number of total TCs, especially weak TCs, decreases during the tropical IO warming year when an anomalous anticyclonic circulation is observed over the tropical Northwest Pacific off the equator. On the other hand, the number of intense TCs increases in the El Niño developing year because of the eastward shifts of both the western Pacific monsoon trough and the cyclonic shear of the equatorial westerlies, and thus the eastward shift of the main TC genesis region. It is also found that the relationship between ENSO and the frequency of intense TCs has a decadal variation. During 1968–1987, the number of intense TCs was not related to ENSO with a correlation coefficient of only 0.19, while the correlation coefficient is 0.63 and 0.73 during 1948–1967 and 1988–2007, respectively. The extent to which the WNP anomalous anticyclone is forced by IO warming is investigated by using the global climate model (European Centre Hamburg Model, ECHAM) with imposed SST anomalies over the tropical IO. The results suggest that the WNP anomalous anticyclone that develops from June to September results mainly from the northward shift of the tropical IO warming from boreal spring to boreal summer.
This study estimates the effect of greenhouse gases (GHGs) on future climate projections produced by multi-model ensembles (MMEs) using twenty-one coupled Atmosphere-Ocean general circulation models. In this study, potential predictability (P) is applied to assess the reliability of GHGs effects on future climate projections. P is a ratio of external variance to total variance, which is used to measure the impact of external forcing. When the internal variance is very small, P will be close to 1, and the ensemble is highly predictable, indicating that the variation is largely controlled by external forcing. P reveals that future climate changes in the 21st century due to the GHGs effect are statistically more significant over higher latitudes than lower latitudes, and over land rather than ocean for the surface air temperature (SAT). For precipitation (PCP), most regions show that P is less than 0.5, except for the South African, North Eastern Pacific, North Atlantic, Central South American, and East Asian regions. It is clearly shown that the strong regions of correlation between MME and the observation where P is strong have a small inter-model difference. This study suggests that an analysis method such as P provides credibility to signals in projection of the SAT and PCP changes.