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The atmospheric response to a potential slowdown of the Atlantic meridional overturning cell (AMOC) is examined using the nonlinear analytical approach (for Heinrich events) introduced by Sandal and Nof (Sandal, C. and Nof, D., A new analytical model for Heinrich events and climate instability. J. Phys. Oceanogr. 2008, 38, 451-466; SN, hereafter). Most numerical global climate models predict that the atmosphere should cool in response to the increased freshwater-fluxes ('hosing') that slow the AMOC down and significantly reduce the heat-flux to the atmosphere. Our application of SN to the modern day climate suggests that the answer to the question of how the atmosphere responds to a slowing AMOC is not that simple. Within the (admittedly limited) dynamics which SN invoke, we find that, as the global numerical climate models predict, a slowdown of the AMOC will indeed cause the mean atmosphere of the entire Northern Hemisphere to cool. However, in contrast to the numerical predictions, our analytical approach suggests that a region in the immediate vicinity of the Atlantic convection (up to a distance of ∼O(1000 km)) may warm up, not cool down (roughly 3°C for 50% mass-transport reduction). For some extreme conditions of a constant atmospheric transport independent of the AMOC (which is not a part of the dynamics involved by SN), the atmosphere can indeed locally cool, but the cooling is minimal (less than 0.3°C for a 50% ocean mass transport reduction), and the associated reduction in heat flux from the ocean to the atmosphere is almost totally negligible. We also place the SN results on a somewhat firmer ground by examining in detail about its closure condition and the most critical assumption adapted by SN. The first has to do with the ratio of the atmospheric and oceanic mass transports (assumed unity in SN) and the second involves up-to-date maps of the ocean-atmosphere heat-fluxes. We show that the system of governing equations admits physically relevant solutions only for particular relationships between the atmospheric and ocean mass transports participating in the ocean-atmosphere heat exchange. Still, as the analytics misses critical atmospheric components such as moisture and variability in the heat exchange interaction area, our results can only serve as an indicator of the problem complexity. [ABSTRACT FROM AUTHOR]

A climatic reconstruction published in E&E (Loehle, 2007) is here corrected for various errors and data issues, with little change in the results. Standard errors and 95% confidence intervals are added. The Medieval Warming Period (MWP) was significantly warmer than the bimillennial average during most of the period 820 - 1040 AD. The Little Ice Age was significantly cooler than the average during most of 1440 - 1740 AD. The warmest tridecade of the MWP was warmer than the most recent tridecade, but not significantly so. [ABSTRACT FROM AUTHOR]

A significant interdecadal cooling with vivid seasonality and three-dimensional (3D) structure is first revealed in the upper troposphere and lower stratosphere over East Asia. A robust upper-tropospheric cooling appears in March and has two peaks in April and August, but in June, a moderate upper-tropospheric warming interrupts the cooling, while strong cooling occurs in the lower stratosphere. The seasonally dependent upper-tropospheric cooling leads to a clear seasonality of interdecadal changes in the atmospheric general circulation and precipitation against their normal seasonal cycle over East Asia. In March, precipitation over southern China (south of 26°N) has increased in accordance with the strong upper-tropospheric cooling occurring in northeast Asia. In April and May, following the southward extension and intensification of the upper-tropospheric cooling, the normal seasonal march of the monsoon rainband has been interrupted, resulting in a drying band to the south of the Yangtze River valley in late spring. In June, the moderate upper-tropospheric warming and strong lower-stratospheric cooling over northeast Asia has suddenly enhanced the northward migration of the rainband and resulted in an increase of precipitation in the mid–lower reaches of the Yangtze River and farther north. During July and August, the return of upper-tropospheric cooling has weakened the northward progression of southerly monsoon winds, resulting in a mid–lower Yellow River valley (34°–40°N) drought and excessive rain in the Yangtze River valley. The change of surface temperature is well correlated with the change in precipitation, especially in the spring. The surface cooling is generally collocated with excessive rain, while the warming is generally collocated with droughts. Possible causes for the robust interdecadal change are discussed, and stratosphere–troposphere interaction is suggested to play a crucial role in seasonally dependent 3D atmospheric cooling over East Asia. [ABSTRACT FROM AUTHOR]

During intensive observation period (IOP)-8 of the Mesoscale Alpine Program, there was a stable layer of air in the lowest levels of the Po Valley and just upstream of the Apennines. In this study, the effects of the stable layer on the formation of convection in the southern Alpine region were investigated through a series of two-dimensional, idealized experiments. The goals of this study were twofold: 1) to determine if stable layer strength affected the placement of convection and 2) to test the notion that the stable layer during IOP-8 behaved as an effective mountain. To accomplish the first objective, three simulations were compared in which the strength of the inversion and low-level cooling in the Po Valley and upstream of the Apennines was varied. The results of these simulations show that the stronger the inversion and low-level cooling, the farther south the convection was positioned. Additionally, it was found that convection developed as a result of the formation of a broad region of moist instability over the stable layer. Cellular convection developed in this region of moist instability. The second objective was tested through a simulation where the cold pool was replaced by terrain (MMTN). As in the reference (or STB10) simulation, the upslope of the terrain in the MMTN simulation was characterized by a wide zone of moist instability. However, wave structures to the lee of the Apennines were markedly different in the STB10 and MMTN simulations. This led to different convective and precipitation patterns, with the MMTN simulation exhibiting heavier convection over the Po Valley while the heaviest convection in STB10 was upstream of the Apennines. These results suggest that, at best, the stable layer in the STB10 simulation can only be roughly approximated by terrain. [ABSTRACT FROM AUTHOR]

Reports on a temperature reconstruction from the last glacial and Holocene periods which accounts for a drastic cooling event without the use of external forcing. How previous attempts to explain the climate fluctuations inserted large amounts of fresh water into the northern North Atlantic Ocean; Details of the model which included a persistent northwesterly wind that transported large amounts of cold and fresh water into the northern North Atlantic Ocean.

Motivated by outstanding issues from a previous case study of a midlatitude cold surge that affected Mexico and Central America, the climatology of Central American cold surges is examined in this paper. An independently derived listing of 177 cold-surge events is employed for which the following properties are tabulated: onset date, duration, time between cold-surge events, latitude of maximum equatorward penetration ( fmin ), and 48-h maximum surface temperature change at Merida, Mexico (DT). These data show that 75% of the cold surges have durations of 2--6 days, the same timescale as mobile disturbances in the westerlies. Also, there does not appear to be any relationship between DT and the duration of the event, although cold surges that penetrate to low latitudes ( fmin 5 78--108N) have a weak tendency to persist longer than those that do not penetrate to low latitudes ( fmin 5 158--208N). In addition, the Reding data indicate that the cold surges tend to reach their most equatorward extent where topographic features impede the progress of equatorward-moving cold air; the tem-perature decrease in the postsurge air (as measured by DT) does not appear to be related to the most equatorward extent. To examine the planetary- and synoptic-scale patterns associated with different categories of cold surges, events with similar characteristics from this database were composited: COLD ( fmin # 108N and DT $ 98C), COOL ( fmin # 108N and DT 5 48--58C), and LONG (events lasting at least 8 days). COLD surges are characterized by a persistent upper-level ridge over the western United States, 200-hPa confluence over the Gulf of Mexico, and the migration of a Canadian lower-tropospheric anticyclone equatorward along the Rocky Mountains and the Sierra Madre. [ABSTRACT FROM AUTHOR]