Planktic foraminifera abundances and winter SST reconstruction for sediment core SO90-39KG

The Indian winter monsoon (IWM) is a key component of the seasonally changing monsoon system that affects the densely populated regions of South Asia. Cold winds originating in high northern latitudes provide a link of continental-scale Northern Hemisphere climate to the tropics. Western Disturbances (WD) associated with the IWM play a critical role for the climate and hydrology in northern India and the western Himalaya region. It is vital to understand the mechanisms and teleconnections that influence IWM variability to better predict changes in future climate. Here we present a study of regionally calibrated winter (January) temperatures and according IWM intensities, based on a planktic foraminiferal record with biennial (2.55 years) resolution. Over the last ~250 years, IWM intensities gradually weakened, based on the long-term trend of reconstructed January temperatures. Furthermore, the results indicate that IWM is connected on interannual- to decadal time scales to climate variability of the tropical and extratropical Pacific, via El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO). However, our findings suggest that this relationship appeared to begin to decouple since the beginning of the 20th century. Cross-spectral analysis revealed that several distinct decadal-scale phases of colder climate and accordingly more intense winter monsoon centered at the years ~1800, ~1890 and ~1930 can be linked to changes of the North Atlantic Oscillation (NAO).

Observed Teleconnections in Northern Winter: Subseasonal Evolution and Tropical Linkages

Teleconnections refer to the climate variability links between non-contiguous geographic regions, and tend to be associated with variability in both space and time of the climate’s semi-permanent circulation features. Teleconnections are well-developed in Northern winter, when they influence subseasonal-to-seasonal climate variability, notably, in surface temperature and precipitation. This work is comprised of four independent studies that improve understanding of tropical-extratropical teleconnections and their surface climate responses, subseasonal teleconnection evolution, and the utility of teleconnections in attribution of extreme climate events. After an introduction to teleconnection analysis as well as the major teleconnection patterns and associated climatic footprints manifest during Northern winter, the lagged impact of the Madden-Julian Oscillation (MJO) on subseasonal climate variability is presented. It is found that monitoring of MJO-related velocity potential anomalies is sufficient to predict MJO impacts. These impacts include, for example, the development of significant positive temperature anomalies over the eastern United States one to three weeks following an anomalous convective dipole with enhanced (suppressed) convection centered over the Indian Ocean (western Pacific). Subseasonal teleconnection evolution is assessed with respect to the Pacific-North America (PNA) pattern and the North Atlantic Oscillation (NAO). This evolution is analyzed both in the presence and absence of MJO-related circulation anomalies. It is found that removal of the MJO results only in small shifts in the centers of action of the NAO and PNA, and that in either case there is a small but significant lag in which the NAO leads a PNA pattern of opposite phase. Barotropic vorticity analysis suggests that this relationship may result in part from excitation of Rossby waves by the NAO in the Asian waveguide. An attempt is made to elegantly differentiate between the MJO extratropical response and patterns of variability more internal to the extratropics. Analysis of upper-level streamfunction anomalies is successful in this regard, and it is suggested that this is the preferred method for the real time monitoring of tropical-extratropical teleconnections. The extreme 2013-2014 North American winter is reconstructed using teleconnection analysis, and it is found that the North Pacific Oscillation-West Pacific (NPO/WP) pattern was the leading contributor to climate anomalies over much of North America. Such attribution is cautionary given the propensity to implicate the tropics for all midlatitude climate anomalies based on the El Niño-Southern Oscillation (ENSO) paradigm. A recent hypothesis of such tropical influence is presented and challenged.

Global oceanic precipitation: A joint view by TOPEX and the TOPEX microwave radiometer

The TOPEX/POSEIDON mission offers the first opportunity to observe rain cells over the ocean by a dual-frequency radar altimeter (TOPEX) and simultaneously observe their natural radiative properties by a three-frequency radiometer (TOPEX microwave radiometer (TMR)). This work is a feasibility study aimed at understanding the capability and potential of the active/passive TOPEX/TMR system for oceanic rainfall detection. On the basis of past experiences in rain flagging, a joint TOPEX/TMR rain probability index is proposed. This index integrates several advantages of the two sensors and provides a more reliable rain estimate than the radiometer alone. One year's TOPEX/TMR TMR data are used to test the performance of the index. The resulting rain frequency statistics show quantitative agreement with those obtained from the Comprehensive Ocean-Atmosphere Data Set (COADS) in the Intertropical Convergence Zone (ITCZ), while qualitative agreement is found for other regions of the world ocean. A recent finding that the latitudinal frequency of precipitation over the Southern Ocean increases steadily toward the Antarctic continent is confirmed by our result. Annual and seasonal precipitation maps are derived from the index. Notable features revealed include an overall similarity in rainfall pattern from the Pacific, the Atlantic, and the Indian Oceans and a general phase reversal between the two hemispheres, as well as a number of regional anomalies in terms of rain intensity. Comparisons with simultaneous Global Precipitation Climatology Project (GPCP) multisatellite precipitation rate and COADS rain climatology suggest that systematic differences also exist. One example is that the maximum rainfall in the ITCZ of the Indian Ocean appears to be more intensive and concentrated in our result compared to that of the GPCP. Another example is that the annual precipitation produced by TOPEX/TMR is constantly higher than those from GPCP and COADS in the extratropical regions of the northern hemisphere, especially in the northwest Pacific Ocean. Analyses of the seasonal variations of prominent rainy and dry zones in the tropics and subtropics show various behaviors such as systematic migration, expansion and contraction, merging and breakup, and pure intensity variations, The seasonality of regional features is largely influenced by local atmospheric events such as monsoon, storm, or snow activities. The results of this study suggest that TOPEX and its follow-on may serve as a complementary sensor to the special sensor microwave/imager in observing global oceanic precipitation.