There are many important questions about the historical ocean temperature record before 2005 due to a lack of data. Understanding the ocean’s temperature and its changes are important as this is a factor on ocean currents which have a strong effect on global and regional temperatures.  

The IPCC lacks important data on ocean currents but seems to have more data on this than they do with ocean temperatures.

However, the IPCC admits they lack data on past global ocean currents as well. They note in 1992 a satellite was launched to measure oceans waves with a study done in the early 1990s by the World Ocean Circulation Experiment. (AR5 Section 3.6.1 Page 281)

They note models can reproduce general features of large-scale ocean currents. However, they state “be cautious over these near-term projections” stating the models have a tough time with small changes in the NH ocean circulation. (AR5 Section 11.3.2.4.1 Page 988)

One of the reasons for problems in modeling ocean currents comes from natural instabilities like El Nino that can cause long term “upper ocean temperature anomalies” driving the atmosphere. (AR5 FAQ 11.1 Page 964)

El Nino is hard to predict just like hurricanes and storms which makes all of them difficult to model.

However, the IPCC tries to look at long-term circulation anomalies dating as far back as 1000 years ago. For example, they state there is some evidence that “long-term circulation anomalies may help to explain the hemispheric warmth early in the millennium, although results vary dependent on input parameters of the method.” (AR5 Section 10.7.1.1 Page 919)

There are questions about an important driver of ocean currents, the subtropical gyres. On this the IPCC believes it is “very likely” the subtropical gyres in the North Pacific and South Pacific have grown since 1993. They note it is “about as likely as not” that this is “decadal oscillation linked to changes in wind forcing”. (AR5 Section 3.6.3 Page 285)

The IPCC admits there has not been regular sampling of MOC (Meridional Overturning Circulation). However, they state there are indications that the MOC (Meridional Overturning Circulation) has slowed 30% during five samples taken from 1957 to 2004. (AR4 Section 9.5.1.3 Page 707)

Despite this the IPCC notes they believe that the AMOC (Altaic Meridional Overturning Circulation) is showing no indication of changing. (AR5 Section 3.6.3 Page 284)

It may be a surprise but even the sea ice may play an important role in effecting things like ocean currents, which then affects the climate.

For example, the IPCC in AR5 discusses the loss of global ice that “may affect global ocean currents and marine ecosystems”. (AR5 Section 4.1 Page 321)

The IPCC has other questions about sea ice but first the IPCC explains how sea ice is formed by stating sea ice is formed when heavy snow accumulates on the sea ice that then pushes it down causing flooding by sea water, this flooding then freezes the snow into ice. (AR5 Section 4.2 Page 323)

The IPCC distinguishes between Arctic sea ice thickness from the Antarctic sea ice thickness because of different properties of the ice, one over water with the other over the land. So, they note the thickness and reach of sea ice differs between Artic and Antarctica. (AR5 Section 4.2 Page 323)

The IPCC admits they “have much more data on Artic ice than we do on Antarctica ice thickness”. (AR5 Section 4.2 Page 323)