3.5.1 Surface or Sea Level Pressure
Maps of MSLP synthesize the atmospheric circulation status. Hurrell and van Loon (1994) noted MSLP changes in the SH beginning in the 1970s while major changes were also occurring over the North Pacific in association with the 1976–1977 climate shift (Trenberth, 1990; Trenberth and Hurrell, 1994). More recently, analyses of sea level pressure from 1948 to 2005 for DJF found decreases over the Arctic, Antarctic and North Pacific, an increase over the subtropical North Atlantic, southern Europe and North Africa (Gillett et al., 2003, 2005), and a weakening of the Siberian High (Gong et al., 2001). The strength of mid-latitude MSLP gradients and associated westerly circulation appears to have increased in both hemispheres, especially during DJF, since at least the late 1970s.
The increase in MSLP gradients in the NH appears to significantly exceed simulated internal and anthropogenically forced variability (Gillett et al., 2003, 2005). However, the significance of changes over the SH is less clear, especially over the oceans prior to satellite observations in the late 1970s, as spurious trends are evident in both major reanalyses (NRA and ERA-40; Marshall, 2003; Bromwich and Fogt, 2004; Trenberth and Smith, 2005; Wang et al., 2006a; see also Appendix 3.B.5). Consistent changes, validated with long-term station-based data, do however seem to be present since the mid-1970s and are often interpreted in terms of time-averaged signatures of weather regimes (Cassou et al., 2004) or annular modes in both hemispheres (Thompson et al., 2000; Marshall, 2003; Bromwich and Fogt, 2004; see Section 3.6).